Electromagnetic testing q&a 001

Electromagnetic Testing Questions & Answers -001 EC&FL Testing-Book(E) 1995 My ASNT Level III Pre-Exam Preparatory My Self Study Notes 10th April 2015

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Petrochemical Applications

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Petrochemical Applications

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Petrochemical Applications

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Petrochemical Applications

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Petrochemical Applications

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Petrochemical Applications

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Petrochemical Applications

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Petrochemical Applications

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Petrochemical Applications

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Fion Zhang at Shanghai 10th April 2015

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IVONA TTS Capable.

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My Mangoes

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Keys: 1. A test specimen used as a basis for calibrating test equipment or as a comparison when evaluating test results is referred to as a: A. Null�Balancer B. Phase Shifter Tallied choice & standard answer C. Reference Standard D. High Pass Filter 2. In an eddy current testing situation, which of the following can provide sources of noise?

My Choice of A. Instrumentation Electronic Circuits wrong answer B. Nonspecific Variations With The Test Object C. Electrical interference D. All Of The Above Standard answer

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Eddy Current Testing Method

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Level I Q&A

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Level I - Standard Answers 1. C 2. D 3. B 4. D 5. C 6. A 7. B 8. A 9. A 10. A 11. D 12. B 13. D 14. C 15. D 16. B 17. A 18. C 19. C 20. B 21. B 22. D 23. A 24. C 25. B 26. D 27. B 28. C 29. D 30. A 31. D 32. B 33. D 34. C 35. D 36. B 37. A 38. D 39. A 40. B 41. B 42. C 43. D 44. B 45. D 46. A 47. B 48. C 49. D 50. B 51. B 52. C 53. B 54. D 55. B 56. C 57. D 58. B 59. B 60. A 61. C 62. C 63. B 64. C 65. B 66. D 67. B 68. A 69. D 70. B 71. A 72. D 73. C 74. B 75. A 76. D 77. C 78. D 79. B 80. A 81. B 82. C 83. B 84. A 85. A 86. A 87. B 88. C 89. C 90. B 91. D 92. B 93. D 94. B 95. A 96. A 97. C 98. D 99. C

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1. A test specimen used as a basis for calibrating test equipment or as a comparison when evaluating test results is referred to as a: A. Null�Balancer B. Phase Shifter C. Reference Standard D. High Pass Filter 2. In an eddy current testing situation, which of the following can provide sources of noise?

A. Instrumentation Electronic Circuits B. Nonspecific Variations With The Test Object C. Electrical interference D. All Of The Above 3. The ratio of the response or amplitude from signals of interest to the response or amplitude of the indications that contain no useful information for the test being conducted is referred to as: A. Poisson’s Ratio B. Signal To Noise Ratio C. The Conductivity To Permeability Ratio D. The Reactance To Resistance Ratio XL/R ?

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4. IACS is recognized abbreviation for: A. Induced Alternating Current System B. Inductively Activated Comparison System C. Internal Applied Current System D. International Annealed Copper Standard 5. Which of the following is not basic component of eddy current equipment?

A. Amplifier B. Coil C. Liquid Couplant D. Detector 6. The process of comparing the reading or output of an instrument, device, or dial with a standard to determine the instrument’s accuracy, capacity, or graduations is referred to as: A. Calibration B. Differentiation C. Integration D. Phase Shifting Charlie Chong/ Fion Zhang

7. When conducting an eddy current test using a differential comparison coil arrangement that compares an external reference standard with the test specimen, the system should be nulled or balanced: A. With Only The Reference Standard In One Coil B. With The Reference Standard In One Coil And An Acceptable Test Specimen In The Other Coil C. With The Reference Standard In One Coil And An Unacceptable Test Specimen In The Other Coil D. With Only The Test Specimen In One Coil 8. The impedance of an eddy current test coil will increase if the: A. Test Frequency Increases XL=2Ď€fL B. Inductive Reactance Of The Coil Decreases C. Inductance Of The Coil Decreases D. Resistance Of The Coil Decreases 9. Some of the products commonly tested using encircling coils are: A. Rods, Tubes, And Wire B. Plate When Volumetrically Inspected C. Sheets And Metalized Foil D. All Of The Above

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XL

XL

FIGURE 8-44 Impedance plane before normalization. (a) Movement of air point with change in frequency. (b) Variation of conductance with frequency

XL= 2Ď€fL

FIGURE 8-45 Effect of test variables on impedance curve operating point.

 Factors that increase current flow (of induced eddy current?) result in clockwise movement of the impedance operating point along the semicircle,  while those that restrict current flow result in counterclockwise movement of the operating point， as seen in Figure 8-45.

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10. Which of the following would normally be considered the best fill factor when testing straight tubing with an encircling or feed through coil?

A. 0.95 (95%) B. 1.75 (175%) C. 0.50 (50%) D. 0.25 (25%) 11. When a magnetic metal part is placed in an eddy current test coil, the impedance of the coil will be change by which of the following properties of the part?

A. Conductivity B. Dimensions C. Permeability D. All Of The Above Electric (conductivity) /Magnetic (permeability, dimensions) Factors 12. An eddy current is circulating electrical current induced in a conducting article by: A. Gamma Rays B. An Alternating Magnetic Field C. A piezoelectric Force D. Any Of The Above Charlie Chong/ Fion Zhang

13. The conductivity of a material can be changed by changing the: A. Alloy Of The Specimen B. Heat Treatment Of The Specimen C. Temperature Of The Specimen D. All Of The Above 14. Figure 1 is an illustration of the material’s: A. Phase Analysis Loop B. Sine Wave C. Hysteresis Loop D. None Of The Above 15. When testing ferromagnetic materials, the depth to which eddy currents are induced in the material will be determined by the : A. Conductivity Of The Material B. Permeability Of The Material C. Geometrical Shape Of The Material D. All Of The Above δ = (2/ωμσ)-½

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Figure 1

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16. When testing plate with a probe coil, it is noted that the eddy current output indication varies as the distance from the coil to the surface of the test part varies.

The term used to describe this action is: A. Fill Factor B. Lift Off C. Phase Differentiation D. Edge Effect 17. The main purpose for spring loaded eddy current probe coils is to: A. Minimize Lift Off Variations B. Minimize Wear On The Probe C. Reduce Operator Fatigue D. Eliminate Edge Effect 18. Lift off is utilized in: A. Measuring Permeability changes B. Measuring Conductivity Changes C. Measuring The Thickness Of Nonconductive Coatings D. Determining Proper Test Frequency

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19. In eddy current test systems where encircling coils are used, coupling efficiency is referred to as: A. Lift Off B. Edge Effect C. Fill Factor D. Phase Differentiation 20. When inspecting products with a uniform cross section, an eddy current signal is produced when the leading end or trailing end of the product approaches the test coil.

This phenomenon is referred to as: A. Lift Off B. End Effect (edge effect?)

C. Fill Factor D. Phase discrimination 21. Which of the following conditions would be the most difficult to detect when eddy current testing rod with an encircling coil?

A. A Deep Surface Crack That Has A Depth Of 30% Of The Rod diameter B. A Small Inclusion In The Center Of The Rod C. A 5% Change In Diameter D. A 10% change In conductivity

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22. When testing tubing with an inside diameter coil, most of the eddy currents: A. Flow In A Longitudinal Direction Down The Tubing B. Flow Radially In The Tube C. Flow Around Only The Outside Diameter Of The Tubing → D. Flow Around The Inside Diameter Of The Tubing 23. Which of the following test frequencies would produce eddy currents with the largest depth of penetration?

A. 100 Hz B. 10 kHz C. 1 MHz D. 10 MHz 24. An eddy current test coil’s opposition to the flow of alternating current is called: A. Resistance B. Inductive Reactance (XL) C. Impedance (Z) [Z = R + jωL = R+ j2πfL = R+ jXL = √(R2+XL2)] D. Capacitive Reactance

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Eddy Current Bobbing Coil

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http://www.ndt.net/search/docs.php3?id=14618&content=1

Eddy Current Bobbing Coil

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Eddy Current Impedance Diagram

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https://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/Physics/impedance.htm

25. An increased in the impedance of an eddy current test coil will: A. Cause An Increase In The Current Flow Through The Test Coil B. Cause A Decrease In The Current Flow Through The Test Coil C. Not Affect Current Flow In The Test Coil D. Decrease The Voltage Applied To The Coil

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1. C 2. D 3. B 4. D 5. C 6. A 7. B 8. A 9. A 10. A 11. D 12. B 13. D 14. C 15. D 16. B 17. A 18. C 19. C 20. B 21. B 22. D 23. A 24. C 25. B 26. D 27. B 28. C 29. D 30. A 31. D 32. B 33. D 34. C 35. D 36. B 37. A 38. D 39. A 40. B 41. B 42. C 43. D 44. B 45. D 46. A 47. B 48. C 49. D 50. B 51. B 52. C 53. B 54. D 55. B 56. C 57. D 58. B 59. B 60. A 61. C 62. C 63. B 64. C 65. B 66. D 67. B 68. A 69. D 70. B 71. A 72. D 73. C 74. B 75. A 76. D 77. C 78. D 79. B 80. A 81. B 82. C 83. B 84. A 85. A 86. A 87. B 88. C 89. C 90. B 91. D 92. B 93. D 94. B 95. A 96. A 97. C 98. D 99. C

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26. Which of the following will affect the impedance of an encircling eddy current test probe?

A. Conductivity Of A Test Specimen In The Coil B. Permeability Of A Test Specimen In The Coil C. Fill Factor D. All Of The Above 27. When a test coil consists of a double winding arrangement, and one winding is referred to as a primary winding, the other winding is referred to as the: A. Absolute Winding B. Secondary Winding C. Phase Winding D. None Of The Above 28. Eddy current test coils which are wound to form a wide coil would normally be used to detect: A. Pitting B. Small Inclusions C. Change In Conductivity D. Porosity Charlie Chong/ Fion Zhang

29. Eddy current test coils, which are wound to form a narrow coil, would normally be used to detect: A. Slow Changes In Dimensions B. Gradual Changes In Conductivity C. Variation In Heat Treatment D. Small Surface Defects 30. Direct current saturation coils would most likely be used when testing: A. Steel B. Aluminum C. Copper D. Brass 31. The single absolute coil arrangement can be used in: A. Encircling Coil Only B. Probe Coils Only C. Probe And Encircling Coils Only D. Probe, Encircling, And Inside Diameter Coils

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32. The primary purposes of the secondary winding in an eddy current coil is to: A. Induce Eddy Currents In The Test Specimen B. Detect Changes In The Eddy Current Flow C. Induce Eddy Currents In The Test Specimen And To Detect Changes In The Eddy Current Flow D. Provide Desaturation 33. Which of the following eddy current test coil arrangements uses one of the test specimens as a reference standard against which another area on the same specimen is simultaneous compared?

A. Single Absolute Coil B. Double Absolute Coil C. Desaturation Coil D. Differential Coil 34. The readout mechanism used when testing by the ellipse method is a: A. Meter B. Strip Chart Recorder C. Cathode Ray Tube D. Any Of The Above Charlie Chong/ Fion Zhang

35. Reference standards used for eddy current testing: A. Must Contain Artificial Discontinuities Such As Notches And Drilled Holes B. Must Contain Natural Discontinuities Such As Cracks And Inclusions C. Must Be Free Of Measurable Discontinuities D. May Contain Artificial Or Natural Discontinuities Or Be Free Of Discontinuities Depending On The Test System And The Type Of Test Being Conducted 36. A reference standard used to ensure that the amplitude and phase characteristics of an eddy current system do not drift during continuous testing is called a: A. DGS Standard B. Calibration Standard C. Reference Block D. None Of The Above 37. Which of the following could not be tested by the eddy current testing method?

A. A 102 mm (4 in.) Thick Plate To be Tested For Small Discontinuities Throughout The Plate B. Tubing To Be Tested For Surface Cracks C. Rod To Be Tested For Laps And Seams D. Tubing To Be Tested For Variations In Outside Diameter

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I

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38. Which of the following is not applicable to eddy current testing?

A. Can Be Used For High Speed Testing B. Can Accurately Measure Conductivity C. Can Be Set Up To Provide High Sensitivity To Small Discontinuities D. Can Penetrate Up To 152 mm (6 in.) In A Test Part 39. The entire circumference of a test part is evaluated at one time when using: A. An Encircling Coil B. A Probe Coil C. A Secondary Winding D. None Of The Above 40. When attempting to determine the exact point location of a discontinuity, the test system should include: A. A Phase Shifter B. A Probe Coil C. An Attenuator D. A Pentameter Charlie Chong/ Fion Zhang

41. It is often possible to sort various nonmagnetic alloys of a metal by means of an eddy current test when: A. There Is A Unique Range Of Permeability Values For Each Alloy B. There Is A Unique Range Of Conductivity Values For Each Alloy C. The Direction Of Induced Eddy Current Varies For Each Allow D. The Magnetic Domains For Each Alloy Are Different 42. Which of the following products would be most applicable to a test using an inside coil?

A. Sheet B. Rod C. Bolt Hole D. Coating Thickness 43. When eddy current testing rod for discontinuities, which of the following conditions could produce extraneous indications?

A. Improper Adjustment Of The Hold Down Rollers Used To Center The Rod In The Coil B. Rod Is Fed Through The Coil At Varying Speeds C. All Of The Above D. None Of The Above

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74. Material must be reasonably centered in the test coil of flaw detection system because: A. The Coil Would Otherwise Be Out Of Balance B. Parts of the material furthest away from the coil radially, may receive a less sensitive inspection C. The Flow Of Eddy Current Around The Product Will Be Disturbed, Resulting In Improper Penetration D. Lack Of Proper Centering May Change The Phase Adjustment Of the Instrument

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44. One method of reducing nonrelevant indications when eddy current testing ferromagnetic material is to: A. Decrease The Pressure On The Hold Down Rollers B. Use A Direct Current Saturation Coil To Magnetically Saturate The Test Specimen C. Readjust The Gain Control D. Increase The Rate Of Feed Through The Coil 45. Eddy currents are circulating currents induced in a conducting material by a: A. Varying Piezoelectric Field B. Standing Wave Front C. Direct Current D. Varying Magnetic Field 46. In order to generate measurable eddy currents in a test specimen, the specimen must be: A. A Conductor B. An Insulator C. Either A Conductor Or Insulator D. A Ferromagnetic Material Charlie Chong/ Fion Zhang

47. All ferromagnetic materials that have been magnetically saturated to suppress permeability variations may retain a certain amount of the magnetization called the: A. Coercive Force B. Residual Magnetism C. Hysteresis Loop D. Hysteresis Loss 48. An AC current flowing in a conductor will set up: A. An Alternating Current Field Around The Conductor B. A Periodically Changing Voltage Tangent To The Conductor C. An Alternating Magnetic Field Around The Conductor D. None Of The Above 49. The characteristics of the alternating magnetic field in the vicinity of the AC coil are affected by: A. The Coil Parameters B. The Magnitude Of The Applied AC Current C. The Frequency Of The Applied Current D. All Of The Above Charlie Chong/ Fion Zhang

50. When using an encircling coil with both primary and secondary windings, the excitation alternating current is applied to: A. The Secondary Winding B. The Primary Winding C. Either The Primary Or Secondary Winding Depending On The Setting Of An Instrument Control D. Both The Primary And Secondary Coils

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1. C 2. D 3. B 4. D 5. C 6. A 7. B 8. A 9. A 10. A 11. D 12. B 13. D 14. C 15. D 16. B 17. A 18. C 19. C 20. B 21. B 22. D 23. A 24. C 25. B 26. D 27. B 28. C 29. D 30. A 31. D 32. B 33. D 34. C 35. D 36. B 37. A 38. D 39. A 40. B 41. B 42. C 43. D 44. B 45. D 46. A 47. B 48. C 49. D 50. B 51. B 52. C 53. B 54. D 55. B 56. C 57. D 58. B 59. B 60. A 61. C 62. C 63. B 64. C 65. B 66. D 67. B 68. A 69. D 70. B 71. A 72. D 73. C 74. B 75. A 76. D 77. C 78. D 79. B 80. A 81. B 82. C 83. B 84. A 85. A 86. A 87. B 88. C 89. C 90. B 91. D 92. B 93. D 94. B 95. A 96. A 97. C 98. D 99. C

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51. Which of the following is not a read out system used to present eddy current test information?

A. Calibrated And Uncalibrated Meters B. Litmus Paper C. Cathode Ray Tube D. Strip Chart Recorders 52. Which of the following statements best describes the selection of eddy current test frequency?

A. The Frequency Must Equal The f/fg Ratio To Give An Accurate Test B. The Frequency Must Be Within + or ‐ 3% Of The f/fg Ratio To Give An Accurate Test C. There Is A Range Of Suitable Frequencies Centered Around The Optimum Frequency D. The Frequency Should Be Within + or ‐ 25% Of The Characteristic Frequency 53. Which Of the Following Is Not A Method That May Be Used To Improve The Signal To Noise Ratio?

A. Change To A Test Frequency That Will Decrease The Noise B. Increase The Amplification Of The Test Instrument C. Improve The Fill Factor D. Add Filter Circuits To The Instrument Charlie Chong/ Fion Zhang

54. When conducting an eddy current test on tubing, the magnitude of the indication caused by a discontinuity is dependent on: A. The Depth Of The Discontinuity B. The Width Of The Discontinuity C. The Length Of The Discontinuity D. All Of The Above 55. A coil’s magnetic field may be viewed as a distribution of lines of flux around the coil. The number of lines in a unit area is defined as: A. Magnetic Density B. Flux Density C. Magnetic Coupling D. Hysteresis Density 56. Which of the following discontinuities would be easiest to detect with an eddy current test?

A. A Crack That Lies parallel To The Direction Of The Eddy Current Flow B. A Discontinuity Located In The Center Of A 51 mm (2 in.) Thick Diameter Rod C. A Radial Crack That Extends To the Outer Surface Of A Rod D. A Subsurface Radial Crack Located At A Depth Of 13 mm (0.5 in.) In A 51 mm (2 in.) Diameter Rod

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57. Eddy current test techniques can be used to: A. Measure Coating Thickness B. Measure Cladding Thickness C. Gauge The Thickness Of Sheet D. All Of The Above 58. Eddy currents cannot be induced in: A. Aluminum B. Latex Paint C. Steel D. Copper 59. Which of the following characteristics do not apply to magnetic materials?

A. High Permeability Value B. No Hysteresis loop C. Definite Saturation Point On Hysteresis Loop D. Appreciable Residual Magnetism

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60. Which of the following is not an eddy current testing method?

A. Pulse Echo Testing B. Impedance Testing C. Phase Analysis Testing D. Modulation Analysis Testing 61. Eddy current always travel: A. In Nonconductive Materials B. Radially When Testing Rod With An Encircling Coil C. In Closed Paths D. In A Gas 62. For the purposes of eddy current testing, saturation of ferromagnetic material is reached when the current is at such a value that for any further increase in current the signal to noise ratio will: A. Start To Increase B. Start To Decrease C. Show No Significant Change D. Suddenly Drop To Zero Charlie Chong/ Fion Zhang

63. When sorting a known mix of two alloys with a comparator having a quantitative readout device, it is desirable to hold all indications within the bounds of the readout device in order to: A. Eliminate Overloading B. Detect The Possible Existence Of A Third Alloy C. Make Proper Balancing Possible D. Phase The Readings Correctly 64. A term used to define a condition of balance in a device or system which results in zero output is: A. High Frequency Standardization B. Integration C. Null Balance D. Differentiation 65. A term used to define a condition of balance in a device or system which results in zero output is: A. A Maxwell B. A Gauss C. An Ohm D. A Mho

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66. Changes in the hardness of age hardenable aluminum and titanium alloy will normally be accompanied by change in: A. Retentivity B. Permeability C. Magnetostriction D. Conductivity Hardness ≥ conductivity 67. A term used to describe holes, grooves, notches, etc., that are introduced into a reference standard to provide accurately reproducible quality level is: A. A Natural Discontinuity B. An Artificial Discontinuity C. An Ellipse D. None Of The Above 68. A term used to define one or more turns or loops of a conductor wound in such a manner as to produce an axial magnetic field when current passes through the conductor is: A. A Coil B. A Resistor C. A Capacitor D. An Oscillator

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69. Nondestructive tests are commonly conducted to: A. Determine The Quality Of Finished Products B. Locate Defective Material Before Excessive Fabrication Is Performed On The Material C. Monitor Production Techniques D. All Of The Above 70. The region around a magnet that attracts other pieces of iron or steel is called: A. A Maxwell B. The Magnetic Field C. Retentivity D. Alternating Current 71. An eddy current coil arrangement that does not make a comparison with either two sections of the test specimen or one section of the test part and a reference standard is called: A. An Absolute Coil Arrangement B. A Self Comparison Differential Coil Arrangement C. An External Reference Differential Coil Arrangement D. None Of The Above

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72. Which of the following are common applications for eddy current testing?

A. Measurement Of Conductivity Or Combination Of Conductivity And Permeability B. Measurement Of The Thickness Of Thin Metal Sections, Cladding, Or Coating C. Detection Of Surface And Subsurface Discontinuities D. All Of The Above 73. Which of the following is not primary purpose for using an eddy current standard?

A. Determine If The Eddy Current Test System Is Capable Of Performing The Test Properly B. Determine If A discontinuity Is Cause For Rejection C. Determine The instrument Control Settings Needed To pass The Highest Percentage Of Test Specimens Run D. Determine If The Sensitivity Of A Test System Has Drifted With Time

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74. Material must be reasonably centered in the test coil of flaw detection system because: A. The Coil Would Otherwise Be Out Of Balance B. Parts of the material furthest away from the coil radially, may receive a less sensitive inspection C. The Flow Of Eddy Current Around The Product Will Be Disturbed, Resulting In Improper Penetration D. Lack Of Proper Centering May Change The Phase Adjustment Of the Instrument

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75. When you determine that a flaw detection instrument has been set up incorrectly, or is defective, all material: A. Should Be Retested Since The Time Correct Set Up Or Proper Operation Was Last Verified B. Accepted Should Be Retested C. Rejected Should Be Retested D. None Of The Above

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1. C 2. D 3. B 4. D 5. C 6. A 7. B 8. A 9. A 10. A 11. D 12. B 13. D 14. C 15. D 16. B 17. A 18. C 19. C 20. B 21. B 22. D 23. A 24. C 25. B 26. D 27. B 28. C 29. D 30. A 31. D 32. B 33. D 34. C 35. D 36. B 37. A 38. D 39. A 40. B 41. B 42. C 43. D 44. B 45. D 46. A 47. B 48. C 49. D 50. B 51. B 52. C 53. B 54. D 55. B 56. C 57. D 58. B 59. B 60. A 61. C 62. C 63. B 64. C 65. B 66. D 67. B 68. A 69. D 70. B 71. A 72. D 73. C 74. B 75. A 76. D 77. C 78. D 79. B 80. A 81. B 82. C 83. B 84. A 85. A 86. A 87. B 88. C 89. C 90. B 91. D 92. B 93. D 94. B 95. A 96. A 97. C 98. D 99. C

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76. An eddy current system (60 cycle comparator type) will measure variations caused by differences in: A. Size Or Shape B. Grade Or Chemistry Of The Material C. The Way In Which The Material Has Been Processed D. All Of The Above 77. A length of tubing containing a notch running from end to end and having uniform width and depth, when tested with an encircling self reference differential coil system, will produce: A. Erratic Signals B. A Continuous, Sustained Signal C. No Signal D. Excessive Signals 78. In eddy current testing, optimum sensitivity to a known defect placed at random on the surface of a nonferrous length of tubing is obtained by: A. Proper Choice Of Frequency B. Centering The Tubing In The Coil C. Proper Adjustment Of Phase D. All Of The Above

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79. Spinning probe type eddy current instruments are most useful in the detection of: A. Surface And Subsurface Inclusions B. The Smallest Surface Blemishes C. Internal Piping Or Burt D. All Of The Above 80. One principal advantage of spinning probe eddy current instruments is: A. The Ability To Locate The Circumferential Position Of Defects B. Relative Insensitivity To Vibration C. The Variety Of Internal And Surface Defects That May Be detected D. All Of The Above 81. At any given instant, a spinning probe eddy current instrument should be inspecting: A. One Complete Circumference Of The Product B. An Area Defined By The Size Of The Probe Coil C. One Longitudinal Line The Length Of The Product D. One Of The Above Charlie Chong/ Fion Zhang

82. A spinning probe eddy current instrument would be most useful in: A. Measuring Hardness Of Ferromagnetic Steel Products B. Flaw Detection In Hexagonal And Shaped Material C. Detecting And Precisely Locating Surface And Subsurface Discontinuities D. Inspection Of Copper Bus Bars For Electrical Conductivity 83. Which of the following is not a nondestructive testing method?

A. Ultrasonic Testing B. Fatigue Testing C. Eddy Current Testing D. Radiographic Testing 84. Which of the following is not applicable to spinning probe type instruments?

A. The Size Of Product To Be Inspected Is Limited By The DC Saturation Capability of The System B. The Frequency May Be Varied To Accommodate Various Metals And Alloys C. They May Be Used On Straight Or Coiled Wire Products D. They Are Useful In Seam Detection

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85. In a feed through encircling coil eddy current system, a calibration standard may be used to: A. Insure Repeatability And Reliability Of The Setup B. Precisely Calibrate The Flaw Depth C. Reduce Sensitivity To Vibration D. Measure The Test Frequency 86. What causes phase shift in an eddy current test coil?

A. A Change In The Ratio Of Inductive Reactance (XL) To Resistance {R} B. A Change In The Sensitivity Setting Of The Instrument C. Use Of Modulation Analysis D. None Of The Above 87. In a feed through encircling coil flaw detection eddy current system, what would be the purpose of running a calibration defect several times but in various positions (such as top, bottom left and right)?

A. To Check The Phase Selectivity B. To Ensure Proper Centering Of The Material In The Test Coil C. To Select The Modulation Analysis Setting D. To Select The Proper Operating Speed

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88. In a feed through encircling coil eddy current system, DC saturation would probably be most helpful in testing: A. Copper Water Tubing B. Titanium Tubing C. Ferromagnetic Steel Tubing D. Brass Rivet Wire 89. In a feed through encircling coil eddy current system, the test frequency is controlled by the: A. Oscilloscope B. Sensitivity Setting C. Oscillator D. Modulation Analysis Setting 90. Alloy variations in nonmagnetic material would most likely affect: A. Permeability B. Conductivity C. Diameter D. Frequency Charlie Chong/ Fion Zhang

91. Which of the following frequencies will provide the greatest eddy current penetration in aluminum?

A. 1 kHz B. 10 kHz C. 3 kHz D. 300 Hz 92. The coercive force of a soft iron electromagnetic core would be the ______________ coercive force of a permanent magnet.

A. Greater Than B. Less Than C. Twice D. Equal To 93. Metals that do not react or react only slightly to magnetic fields are called: A. Diamagnetic Metals B. Paramagnetic Metals C. Nonmagnetic Metals D. All Of The Above Charlie Chong/ Fion Zhang

94. A symbol commonly used to express conductivity is: A. μ B. σ C. XL D. R 95. A symbol commonly used to express permeability is: A. μ B. σ C. XL D. R 96. The method used to generate eddy currents in a test specimen by means of a coil can most closely be compared with the action of a: A. Transformer B. Capacitor C. Storage Battery D. Generator

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97. When eddy current testing a nonferrous specimen, a discontinuity will: A. Increase The Effective Conductivity Of The Specimen B. Increase The Effective Permeability Of The Specimen C. Decrease The Effective Conductivity Of The Specimen D. None Of The Above 98. Demagnetization is generally needed when the residual field in a specimen: A. May Affect The Operation Or Accuracy of Instruments When The Specimen Is Placed In Service B. May Interfere With The Proper Functioning Of The Part C. Might Cause Particles To Be Attracted And Held To The Surface Of Moving Parts D. Any Of The Above Reasons Could Make Demagnetization Necessary 99. A term used to define testing that requires that the test article be loaded and or sectioned to verify and or establish mechanical or physical properties is: A. Impedance Testing B. Phase Analysis Testing C. Destructive Testing D. Nondestructive Testing

Charlie Chong/ Fion Zhang

Recalling the mistakes

Charlie Chong/ Fion Zhang

Charlie Chong/ Fion Zhang

Lesson Learned

Charlie Chong/ Fion Zhang

Charlie Chong/ Fion Zhang

Eddy Current Testing Level I (20 Questions) 1.B2.B3.C4.B5.D6.B7.C8.D9.B10.D11.D12.C13.B14.B15.D16.B17.D18.D19.C20.A

Charlie Chong/ Fion Zhang

http://ndt-samplequestions.blogspot.com/2011/06/eddy-current-testing-level-i-20.html

Q.1 Generation of eddy currents depends on the principle of: A. wave guide theory.

B. electromagnetic induction.

C. magneto-restrictive forces.

D. all of the above.

Q.2 A secondary field is generated by the test object and is: A. equal and opposite to the primary field.

B. opposite to the primary field, but much smaller.

C. in the same plane as the coil is wound.

D. in phase with the primary field.

Q.3 When a non-ferromagnetic part is placed in the test coil, the coil's voltage: A. increases.

B. remains constant because this is essential.

C. decreases.

D. shifts 90 degrees in phase.

Charlie Chong/ Fion Zhang

http://ndt-samplequestions.blogspot.com/2011/06/eddy-current-testing-level-i-20.html

Q.4 Eddy currents generated in a test object flow: A. in the same plane as magnetic flux.

B. in the same plane as the coil is wound.

C. 90 degrees to the coil winding plane.

D. Eddy currents have no predictable direction.

Q.5 The discovery of electromagnetic induction is credited to: A. Arago.

B. Oersted.

C. Maxwell.

D. Faraday.

Q.6 A standard depth of penetration is defined as the point in a test object where the relative eddy current density is reduced to: A. 25 percent.

B. 37 percent.

C. 50 percent.

D. 100 percent.

Charlie Chong/ Fion Zhang

http://ndt-samplequestions.blogspot.com/2011/06/eddy-current-testing-level-i-20.html

Q.7 Calculate the standard depth of penetration at 10 kHz in copper; σ = 5.7•107 mhos per meter.

A. 0.1 mm B. 0.02 mm C. 0.66 mm δ = √ (2/ωμσ) = √(2/πfσμ) = (πfσμ)-½ , D. 66 mm (for non-magnetic material μ = 4π x 10-7 N∙A-2) (remember this!)

Q.8 Differential coils are usually used in: A. bobbin coils.

B. probe coils.

C. OD coils.

D. any of the above.

Q.9 When using a probe coil to scan a test object,______ A. the object must be dry and polished.

B. the object must be scanned carefully to insure inspection coverage.

C. the object must be scanned in circular motions at constant speeds.

D. the probe must be moving at all times to get a reading.

Charlie Chong/ Fion Zhang

http://ndt-samplequestions.blogspot.com/2011/06/eddy-current-testing-level-i-20.html

Q.10 A "spinning probe" would most likely be a (an): A. bobbin coil.

B. ID coil.

C. OD coil.

D. probe coil.

Q.11 A "feed-through" coil is: A. a coil with primary/secondary windings connected so that the signal is fed through the primary to the secondary.

B. an encircling coil.

C. an OD coil.

D. both B and C. Q.12 When inspecting a tubular product with an encircling coil, which statement is not true?

A. OD discontinuities can be found.

B. Axial discontinuity locations can be noted.

C. Circumferential discontinuity locations can be noted.

D. ID discontinuities can be found.

Charlie Chong/ Fion Zhang

http://ndt-samplequestions.blogspot.com/2011/06/eddy-current-testing-level-i-20.html

Q.13 An absolute coil measurement is made A. by comparing one spot on the test object to another.

B. without reference to or direct comparison with a standard.

C. only with probe coils.

D. by comparative measurement to a known standard.

Q.14 When coils in a differential arrangement are affected simultaneously with the same test object variables, the output signal A. is directly proportional to the number of variables.

B. is "0" or near-"0."

C. is indirectly proportional to the number of variables.

D. is primarily a function of the exciting current.

Q.15Which coil type inherently has better thermal stability?

A. Bobbin B. Absolute C. OD D. Differential

Charlie Chong/ Fion Zhang

http://ndt-samplequestions.blogspot.com/2011/06/eddy-current-testing-level-i-20.html

Q.16 A hybrid coil is composed of two or more coils. The coils A. must be aligned coplanar to the driver axis.

B. may be of widely different dimensions.

C. must be impedance-matched as closely as possible.

D. are very temperature sensitive.

Q.17 Proper selection of test coil arrangement is determined by: A. shape of test object.

B. resolution required.

C. sensitivity required.

D. all of the above.

Q.18 A coil's resistance is determined by: A. wire material.

B. wire length.

C. wire cross-sectional area.

D. all of the above.

Charlie Chong/ Fion Zhang

http://ndt-samplequestions.blogspot.com/2011/06/eddy-current-testing-level-i-20.html

Q.19 Inductance is analogous to: A. force.

B. volume.

C. inertia.

D. velocity.

Q.20 The unit of inductance is the: A. henry.

B. maxwell.

C. ohm.

D. farad.

Charlie Chong/ Fion Zhang

http://ndt-samplequestions.blogspot.com/2011/06/eddy-current-testing-level-i-20.html

Recalling the mistakes

Charlie Chong/ Fion Zhang

Charlie Chong/ Fion Zhang

Level II Q&A

O TW

Charlie Chong/ Fion Zhang

Level II – Q&A 1. A 2. D 3. A 4. B 5. D 6. A 7. D 8. B 9. A 10. C 11. C 12. B 13. A 14. C 15. D 16. D 17. B 18. B 19. A 20. C 21. D 22. B 23. B 24. D 25. C 26. A 27. D 28. B 29. B 30. C 31. B 32. C 33. C 34. C 35. D 36. D 37. B 38. A 39. A 40. B 41. C 42. A 43. C 44. C 45. D 46. C 47. D 48. C 49. A 50. D 51. B 52. D 53. C 54. C 55. D 56. C 57. D 58. A 59. C 60. D 61. A 62. D 63. A 64. B 65. C 66. B 67. D 68. B 69. D 70. A 71. B 72. B 73. D 74. B 75. D 76. C 77. A 78. B 79. C 80. B 81. A 82. D 83. A 84. C 85. A 86. B 87. C 88. D 89. D 90. A 91. A 92. C 93. C 94. C 95. D 96. C 97. A 98. A 99. C 100. A 101. B 102. A 103. E 104. D

Charlie Chong/ Fion Zhang

Level II – Q&A 1. A 2. D 3. A 4. B 5. D 6. A 7. D 8. B 9. A 10. C 11. C 12. B 13. A 14. C 15. D 16. D 17. B 18. B 19. A 20. C 21. D 22. B 23. B 24. D 25. C 26. A 27. D 28. B 29. B 30. C 31. B 32. C 33. C 34. C 35. D 36. D 37. B 38. A 39. A 40. B 41. C 42. A 43. C 44. C 45. D 46. C 47. D 48. C 49. A 50. D 51. B 52. D 53. C 54. C 55. D 56. C 57. D 58. A 59. C 60. D 61. A 62. D 63. A 64. B 65. C 66. B 67. D 68. B 69. D 70. A 71. B 72. B 73. D 74. B 75. D 76. C 77. A 78. B 79. C 80. B 81. A 82. D 83. A 84. C 85. A 86. B 87. C 88. D 89. D 90. A 91. A 92. C 93. C 94. C 95. D 96. C 97. A 98. A 99. C 100. A 101. B 102. A 103. E 104. D

Charlie Chong/ Fion Zhang

1. A method of minimizing the effect of indications caused by gradual dimensional changes while retaining the indications caused by discontinuities is to: A. Include a high pass filter in the eddy current test equipment 高通滤波器 B. Include a low pass filter in the eddy current test equipment C. Increase the band pass of the amplifier D. Use the impedance method of testing 2. Which of the following methods may be used to improve the signal to noise ratio of an eddy current test system?

A. Filtering or differentiation B. Phase discrimination C. Integration D. All of the above 3. In order decrease the effect on conductivity readings due to variations in test part thickness: A. The test frequency should be increased B. The test frequency should be decreased C. The fill factor should be decreased D. There is no practical method for decreasing this effect

Charlie Chong/ Fion Zhang

High Pass Filter 高 “通” 滤波器

Charlie Chong/ Fion Zhang

http://www.cchrispeters.com/SynthesisTerms/HighPassFilter.htm

4. When testing by the eddy current method, discontinuities will be most easily detected when the eddy currents are: A. Coplanar with the major plane of the discontinuity B. Perpendicular to the major plane of the discontinuity C. Parallel to the major plane of the discontinuity D. 90 degrees out of phase with the current in the coil 5. A term used to describe the effect observed due to a change in the electromagnetic coupling between a test specimen and a probe coil when the distance of separation between them is varied is: A. Fill factor B. Edge effect C. End effect D. Lift off 6. A term used to define the phenomenon where, at very high frequencies, the eddy current flow is restricted to an extremely thin outer layer of the conductor is: A. Skin effect B. High frequency filtration C. Low frequency filtration D. Any of the above

Charlie Chong/ Fion Zhang

7. The fill factor when a 13 mm (0.5 in) diameter bar is inserted in a 25 mm (1 in) diameter coil is: A. 0.5 (50%) B. 0.75 (75%) C. 1.0 (100%) Ρ = (D1/D2)2 = (.5)2 D. 0.25 (25%) 8. The term used to define the difference between actual instrument output and expected output as defined by a straight line calibration curve is: A. Phase shift B. Nonlinearity C. Lift off D. Skin effect 9. Which of the following could be used to suppress unwanted high frequency harmonics?

A. Low pass filter B. Oscillator C. Phase discriminator D. High pass filter Charlie Chong/ Fion Zhang

10. The impedance change of an eddy current test coil due to a change in test part characteristics can be most easily analyzed as a combined change in: A. Capacitive reactance and resistance B. Harmonic frequencies and inductive reactance C. Signal amplitude and phase D. Retentivity and harmonic frequencies 11. The inductive reactance of a test coil can be calculated using the formula: A. XL = 2πL B. XL = 0.5πL C. XL = 2πfL D. XL = 1R 12. When conducting an eddy current test, variations in the test material can be detected as variations in: A. Test speed B. Impedance of a pickup coil C. None of the above D. All of the above

Charlie Chong/ Fion Zhang

13. A term used to define two or more coils electrically connected in opposition such that any electromagnetic condition which is not common to both a test specimen and a reference specimen will produce an unbalance in the system is: A. Differential B. Absolute C. Laminar D. Flying probes 14. Tubing is generally inspected using: A. U shaped coils B. Gap coils C. Encircling coils D. None of the above 15. When testing small parts in an eddy current encircling coil for permeability Ďƒ measurements, which of the following test parameters may be considered important?

A. Length of the test sample B. Thickness of the test sample C. Cross sectional area of the test sample D. All of the above

Charlie Chong/ Fion Zhang

16. In a test coil consisting of both a primary and secondary winding, the voltage across the secondary is a function of: A. Test material permeability B. Test frequency C. Geometry of the test object D. All of the above 17. Eddy current testing relies on the principle of: A. Magnetostriction B. Electromagnetic induction C. Piezoelectric energy conversion D. Magnetomotive force 18. When the H field strength around a material is increased and then decreased to zero, the induction field B within the part does not return to zero. The term to define the B value when H has returned to zero is: A. Coercive force B. Residual magnetism (retentivity, remanence) C. Saturation value D. Hysteresis loss

Charlie Chong/ Fion Zhang

Residual Magnetism – Retentivity, Remanence

Charlie Chong/ Fion Zhang

http://hyperphysics.phy-astr.gsu.edu/hbase/solids/magperm.html

Coercivity and Remanence in Permanent Magnets A good permanent magnet should produce a high magnetic field with a low mass, and should be stable against the influences which would demagnetize it.

The desirable properties of such magnets are typically stated in terms of the remanence and coercivity of the magnet materials.

When a ferromagnetic material is magnetized in one direction, it will not relax back to zero magnetization when the imposed magnetizing field is removed.

The amount of magnetization it retains at zero driving field is called its remanence. It must be driven back to zero by a field in the opposite direction; the amount of reverse driving field required to demagnetize it is called its coercivity. If an alternating magnetic field is applied to the material, its magnetization will trace out a loop called a hysteresis loop. The lack of retraceability of the magnetization curve is the property called hysteresis and it is related to the existence of magnetic domains in the material. Once the magnetic domains are reoriented, it takes some energy to turn them back again. This property of ferrromagnetic materials is useful as a magnetic "memory". Some compositions of ferromagnetic materials will retain an imposed magnetization indefinitely and are useful as "permanent magnets".

The table below contains some data about materials used as permanent magnets. Both the coercivity and remanence are quoted in Tesla, the basic unit for magnetic field B. The hysteresis loop above is plotted in the form of magnetization M as a function of driving magnetic field strength H. This practice is commonly followed because it shows the external driving influence (H) on the horizontal axis and the response of the material (M) on the vertical axis.

Besides coercivity and remanence, a quality factor for permanent magnets is the quantity (BB0/Îź0)max. A high value for this quantity implies that the required magnetic flux can be obtained with a smaller volume of the material, making the device lighter and more compact.

The alloys from which permanent magnets are made are often very difficult to handle metallurgically. They are mechanically hard and brittle. They may be cast and then ground into shape, or even ground to a powder and formed. From powders, they may be mixed with resin binders and then compressed and heat treated. Maximum anisotropy of the material is desirable, so to that end the materials are often heat treated in the presence of a strong magnetic field.

The materials with high remanence and high coercivity from which permanent magnets are made are sometimes said to be "magnetically hard" to contrast them with the "magnetically soft" materials from which transformer cores and coils for electronics are made.

Charlie Chong/ Fion Zhang

http://hyperphysics.phy-astr.gsu.edu/hbase/solids/magperm.html

Rare Earth Magnets The permanent magnets which have produced the largest magnetic flux with the smallest mass are the rare earth magnets based on samarium and neodymium. Their high magnetic fields and light weight make them useful for demonstrating magnetic levitation over superconducting materials.

The samarium-cobalt combinations have been around longer, and the SmCo5 magnets are produced for applications where their strength and small size offset the disadvantage of their high cost. The more recent neodymium materials like Nd2Fe14B produce comparable performance, and the raw alloy materials cost about 1/10 as much. They have begun to find application in microphones and other applications that exploit their high field and light weight.

The production is still quite costly since the raw alloy must be ground to powder, pressed into the desired shape and then sintered to make a durable solid.

Charlie Chong/ Fion Zhang

http://hyperphysics.phy-astr.gsu.edu/hbase/solids/magperm.html

δμσ∝∞∙ωΩπθ■√ρφ°≤ewwwwwww

Charlie Chong/ Fion Zhang

Charlie Chong/ Fion Zhang

Charlie Chong/ Fion Zhang

19. The term used to define the value of H field required to decrease the remanence of a material to zero is: A. Coercive force B. Magnetizing force C. Back emf D. The overlap value 20. The three major specimen parameters that influence eddy current testing are: A. Electrical conductivity, frequency, and material geometry B. Density, permeability, and frequency C. Electrical conductivity, permeability, and material geometry D. Thermal conductivity, electrical conductivity, and permeability 21. Doubling the number of turns on a coil will: A. Double the inductance B. Halve the inductance C. Decrease the inductance by a factor of four D. Increase the inductance by a factor of four L= (Îź0ÎźrN2A)/l Charlie Chong/ Fion Zhang

21. Doubling the number of turns on a coil will: A. Double the inductance B. Halve the inductance C. Decrease the inductance by a factor of four D. Increase the inductance by a factor of four

L= (μ0μrN2A)/l , L= (μN2A)/l

Charlie Chong/ Fion Zhang

Self Inductance of a Coil Inductance is the name given to the property of a component that opposes the change of current flowing through it and even a straight piece of wire will have some inductance. Inductors do this by generating a self-induced emf within itself as a result of their changing magnetic field.

When the emf is induced in the same circuit in which the current is changing this effect is called Self-induction, ( L ) but it is sometimes commonly called back-emf as its polarity is in the opposite direction to the applied voltage.

When the emf is induced into an adjacent component situated within the same magnetic field, the emf is said to be induced by Mutual-induction, ( M ) and mutual induction is the basic operating principal of transformers, motors, relays etc. Self inductance is a special case of mutual inductance, and because it is produced within a single isolated circuit we generally call self-inductance simply, Inductance.

The basic unit of measurement for inductance is called the Henry, ( H ) after Joseph Henry, but it also has the units of Webers per Ampere ( 1 H = 1 Wb/A ).

Lenz’s Law tells us that an induced emf generates a current in a direction which opposes the change in flux which caused the emf in the first place, the principal of action and reaction. Then we can accurately define Inductance as being: “a coil will have an inductance value of one Henry when an emf of one volt is induced in the coil were the current flowing through the said coil changes at a rate of one ampere/second”.

In other words, a coil has an inductance, ( L ) of one Henry, ( 1H ) when the current flowing through it changes at a rate of one ampere/second, ( A/s ) inducing a voltage of one volt, ( VL ) in it. This mathematical representation of the rate of change in current through a coil per unit time is given as:…….

Read more on: http://www.electronics-tutorials.ws/inductor/inductance.html Inductance (measured in henry) is an effect which results from the magnetic field that forms around a current carrying conductor. Current flowing through the inductor creates a magnetic field which has an associated electromotive field which opposes the applied voltage. This counter electromotive force (emf) is generated which opposes the change in voltage applied to the inductor and current in the inductor resists the change but does rise. This is known as inductive reactance. It is opposite in phase to capacitive reactance. Inductance can be increased by looping the conductor into a coil which creates a larger magnetic field…….Read more on http://www.raftabtronics.com/TECHNOLOGY/ElectromagneticBasics/InductorBasics/tabid/109/Default.aspx Basic inductance formula:

L = (μN2A)/l = (μ0μrN2A)/l Where: L = Inductance in henries μ0 = permeability of free space = 4π × 10-7 H/m μr = relative permeability of core material N = number of turns A = area of cross-section of the coil in square metres (m2) l = length of coil in metres (m)

Charlie Chong/ Fion Zhang

http://www.raftabtronics.com/TECHNOLOGY/ElectromagneticBasics/InductorBasics/tabid/109/Default.aspx

22. Frequency ratio, f/fg, can be defined as: A. The argument of the mathematical function describing the electromagnetic field within the test specimen B. Test frequency divided by limit frequency C. Neither of these D. Both of these 23. When testing tubing, using an encircling (OD) coil, what is the phase relationship of the output signals from identical ID and OD discontinuities A. The signals are in phase B. The phase of the OD discontinuity leads the phase of the ID discontinuity C. The phase of the OD discontinuity lags the phase of the ID discontinuity D. Indeterminate 24. A term used to define an instrumentation technique that discriminates between variables in the test part by the different phase angle changes which these conditions produced in the test signal is: A. Phase distortion B. Phase shifting C. Phase discrimination D. Phase analysis

Charlie Chong/ Fion Zhang

Note: Subsurface indication lag the near surface indication, (as it’s signal comes later to show up).

Charlie Chong/ Fion Zhang

Phase Analysis – OD & ID Phase Angles

Charlie Chong/ Fion Zhang

http://innospection.com/pdfs/Eddy%20Current%20Theory.pdf

In eddy current testing, what is impedance plane analysis?

Impedance plane analysis or phase analysis is used because the signals generated in an eddy current test are vector and not scalar quantities. That is to say, they are properly described by two quantities, their amplitude and phase rather than just by their amplitude. Phase analysis of the test signal using a cathode ray oscilloscope or LCD screen instead of mere amplitude measurements with a meter allows a greater level of differentiation between relevant signals and unwanted noise. The effect of eddy currents on the coil impedance is described on the impedance plane diagram.

Eddy currents have surprisingly well ordered effects on the amplitude and phase of coil voltages. Thanks mainly to the efforts of Dr Forster in the immediate post-war years, these effects have been rationalised using mathematics. Computers can now be used with a good level of accuracy to predict eddy current test responses to simulated defects.

Charlie Chong/ Fion Zhang

http://www.twi-global.com/technical-knowledge/faqs/ndt/faq-in-eddy-current-testing-what-is-impedance-plane-analysis/

Phase Lag Phase lag is a parameter of the eddy current signal that makes it possible to obtain information about the depth of a defect within a material. Phase lag is the shift in time between the eddy current response from a disruption on the surface and a disruption at some distance below the surface.

The generation of eddy currents can be thought of as a time dependent process, meaning that the eddy currents below the surface take a little longer to form than those at the surface. Disruptions in the eddy currents away from the surface will produce more phase lag than disruptions near the surface.

Both the signal voltage and current will have this phase shift or lag with depth, which is different from the phase angle discussed earlier. (With the phase angle, the current shifted with respect to the voltage.)

Note: Subsurface indication lag the near surface indication, (as it comes later to show up).

Charlie Chong/ Fion Zhang

https://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/Physics/phaselag.htm

Phase lag is an important parameter in eddy current testing because it makes it possible to estimate the depth of a defect, and with proper reference specimens, determine the rough size of a defect. The signal produced by a flaw depends on both the amplitude and phase of the eddy currents being disrupted.

A small surface defect and large internal defect can have a similar effect on the magnitude of impedance in a test coil [ standard depth, δ = (πfμσ) -½ , intensity at depth jx =jo∙e –x/δ ].

However, because of the increasing phase lag with depth, there will be a characteristic difference in the test coil impedance vector.

Phase lag can be calculated with the following equation. The phase lag angle calculated with this equation is useful for estimating the subsurface depth of a discontinuity that is concentrated at a specific depth. Discontinuities, such as a crack that spans many depths, must be divided into sections along its length and a weighted average determined for phase and amplitude at each position below the surface.

θ = x/ δ Charlie Chong/ Fion Zhang

where: x = depth, δ = standard depth

https://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/Physics/phaselag.htm

At one standard depth of penetration, the phase lag is one radian or 57º. This means that the eddy currents flowing at one standard depth of penetration (δ) below the surface, lag the surface currents by 57º. At two standard depths of penetration (2δ), they lag the surface currents by 114º. Therefore, by measuring the phase lag of a signal the depth of a defect can be estimated.

On the impedance plane, the liftoff signal serves as the reference phase direction. The angle between the liftoff and defect signals is about twice the phase lag calculated with the above equation. As mentioned above, discontinuities that have a significant dimension normal to the surface, will produce an angle that is based on the weighted average of the disruption to the eddy currents at the various depths along its length.

In the applet below, the relationship between the depth and dimensions of a discontinuity and the rotation produced on the impedance plane is explored. The red lines represent the relative strength of the magnetic field from the coil and the dashed lines indicate the phase lag of the eddy currents induced at a particular depth.

Charlie Chong/ Fion Zhang

https://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/Physics/phaselag.htm

25. If the characteristic frequency (fg) of a material is 125 Hz, the test frequency required to give an f/fg ratio of 10 would be: A. 1.25 Hz B. 12.5 Hz C. 1.25 kHz D. 12.5 kHz

Charlie Chong/ Fion Zhang

More Reading on Phase Analysis

Charlie Chong/ Fion Zhang http://eddy-current.com/another-thing-not-so-easy-about-eddy-current-testing-of-tubes/

Another inconvenience that arises during the eddy current testing of heat exchanger tubes is that although phase analysis (measuring defect depth based on the angle on the screen) is usually the best measure of a defect’s depth, it is not perfect.

The two eddy current screens shown in the diagram are a bit cluttered because you can see the dent and five defects of an ASME calibration tube that we’ve used for examples in previous articles. The green horizontal signal is, of course, the dent signal. Then, rotated clockwise from this at about 45 degrees, are the through wall hole, followed by the 80, 60, 40, and 20 per cent O.D. pits, all of similar amplitude.

Charlie Chong/ Fion Zhang

http://eddy-current.com/another-thing-not-so-easy-about-eddy-current-testing-of-tubes/

More Reading on Phase Analysis – Flaw Angles

Charlie Chong/ Fion Zhang

http://www.ndt.net/apcndt2001/papers/224/224.htm

More Reading on Phase Analysis – Flaw Depths

?

Charlie Chong/ Fion Zhang

http://www.ndt.net/apcndt2001/papers/224/224.htm

More Reading on Phase Analysis – Flaw Length

Charlie Chong/ Fion Zhang

http://www.ndt.net/apcndt2001/papers/224/224.htm

Level II – Q&A 1. A 2. D 3. A 4. B 5. D 6. A 7. D 8. B 9. A 10. C 11. C 12. B 13. A 14. C 15. D 16. D 17. B 18. B 19. A 20. C 21. D 22. B 23. B 24. D 25. C 26. A 27. D 28. B 29. B 30. C 31. B 32. C 33. C 34. C 35. D 36. D 37. B 38. A 39. A 40. B 41. C 42. A 43. C 44. C 45. D 46. C 47. D 48. C 49. A 50. D 51. B 52. D 53. C 54. C 55. D 56. C 57. D 58. A 59. C 60. D 61. A 62. D 63. A 64. B 65. C 66. B 67. D 68. B 69. D 70. A 71. B 72. B 73. D 74. B 75. D 76. C 77. A 78. B 79. C 80. B 81. A 82. D 83. A 84. C 85. A 86. B 87. C 88. D 89. D 90. A 91. A 92. C 93. C 94. C 95. D 96. C 97. A 98. A 99. C 100. A 101. B 102. A 103. E 104. D

Charlie Chong/ Fion Zhang

26. A term used to define any change in a sinusoidal signal when the output is not a linear reproduction of the input is: A. Distortion B. Linearity C. Phase shifting D. Dynamic range 27. At a fixed test frequency, in which of the following materials will the eddy current penetration, be greatest?

A. Aluminum (35% IACS conductivity) B. Brass (15% IACS conductivity) C. Copper (95% IACS conductivity) D. Lead (7% IACS conductivity) 28. The heating effect in magnetic material caused by the work required to rotate magnetic domains (AC?) can be eliminated by: A. Increasing the rate of displacement of the specimen through the coil B. Magnetically saturating the test specimen C. Decreasing the rate of displacement of the test specimen through the test coil D. Testing in an air conditioned room

Charlie Chong/ Fion Zhang

Magnetic Domain – Hysteresis Curve

Charlie Chong/ Fion Zhang

29. If the test frequency increases while the field strength is held constant, the surface eddy current density: A. Decreases B. Increases C. Remains the same D. Could do any of the above 30. As the fill factor decreases, the impedance variation of a pickup coil produced by a given change in conductivity will: A. Increase B. Remain the same C. Decrease D. Could do any of the above Note on Q30 – The impedance increase similar to lift off effect. The impedance variation by a given change in Ďƒ decrease, due to reduce sensitivity by the lift off effect.

Charlie Chong/ Fion Zhang

31. The abscissa values on the impedance plane shown in Figure 2 are given in terms of: A. Absolute conductivity B. Normalized resistance C. Absolute inductance D. Normalized inductance

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32. In Figure 2 (an impedance diagram for solid nonmagnetic rod), the fg or characteristic frequency is calculated by the formula: A. fg= σμ/d² B. fg= δμ/d C. fg= 5060/σμd² D. fg= R/L 33. In Figure 2, a change in the f/fg ratio will result in: A. A change in only the magnitude of the voltage across the coil B. A change in only the phase of the voltage across the coil C. A change in both the phase and magnitude of the voltage across the coil D. No change in the phase or magnitude of the voltage across the coil

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34. In Figure 3, the solid curves are plots for different values of: A. Heat treatment B. Conductivity C. Fill factor D. Permeability

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35. Figure 4 illustrates the fact that eddy current responses: A. Can be caused by several parameters B. Differ in phase and magnitude with different parameter changes C. Can be made selective with regard to the variable of interest D. All of the above

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Figure 4

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Figure 4 Charlie Chong/ Fion Zhang

36. Which of the following would be easier to detect in tubing by the eddy current method using the self comparison differential coil arrangement?

A. Gradual changes in diameter B. Gradual changes in conductivity C. Changes in temperature D. Short flaws 37. The magnetic domains in an un-magnetized ferromagnetic material are: A. Arranged to facilitate the conduction of electrons B. Randomly oriented and neutralize each other C. Uniformly oriented D. Create a major north and south pole in the material

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38. When a magnetic material is placed in a region of an applied magnetizing force (H), magnetic field (B) is developed in the material by means of: A. Induction B. Conduction C. Heat transfer D. Magnetic domain transfer 39. Figure 5 illustrates the fact that the limit frequency, fg, equations and impedance diagrams for long solid rods and long thin walled tubing are: A. Different B. Identical C. Similar D. Unrelated 40. To eliminate any dependence upon the number of turns of a test coil, the inductance values of an impedance diagram are: A. Disregarded B. Normalized C. Corrected to a value of 1 D. None of the above

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Figure 4

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Figure 5

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ωLo = Eo, for ωL = Eimg, R = Ereal

3.1.2 Limiting Frequency fg of Encircling Coils Encircling coils are used more frequently than surface-mounted coils. With encircling coils, the degree of filling has a similar effect to clearance with surface-mounted coils.

The degree of filling is the ratio of the test material cross-sectional area to the coil cross-sectional area. Figure 3.7 shows the effect of degree of filling on the impedance plane of the encircling coil. For tubes, the limiting frequency (point where ohmic losses of the material are the greatest) can be calculated precisely from Eq. (3.2): fg = 5056/(σ∙ di ∙ w∙ μr)

(3.2)

Where: fg = limiting frequency σ = conductivity di = inner diameter w = wall thickness μr (rel) = relative permeability For Solid Rod: fg = 5060/(σ∙ μr∙ d 2)

(3.2)

Where: d= solid rod diameter

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Introduction to Nondestructive Testing: A Training Guide, Second Edition, by Paul E. Mix

41. A term used to define a system that indicates only the magnitude of variations in the total coil impedance regardless of the phase or direction in which it occurs on an impedance plane is: A. Inductive reactance magnitude system B. Feedback controlled system C. Impedance magnitude system D. Impedance vector analysis system 42. When eddy current testing tubing with a system that includes a frequency discriminating circuit, which of the following variable would b classified as high frequency variable?

A. Small discontinuities B. Conductivity changes C. Diameter changes D. Wall thickness variations 43. A major problem associated with the eddy current test method is the: A. Inability of eddy current testing to accurately measure conductivity B. Need to test at low speeds to prevent skipping C. Large number of known and unknown variables that appear in the output indication D. Inability of eddy current testing to detect small discontinuities Charlie Chong/ Fion Zhang

44. The atoms of a metal showing magnetic characteristics are grouped into regions that are the smallest known permanent magnets. The regions are called: A. Lattice structures B. Cells C. Domains D. Planetary spins 45. When a coil’s magnetizing force is applied to a ferromagnetic material, the flux density in the material is: A. Less than the flux density generated by the test coil because of heat losses B. Less than the flux density generated by the test coil because of the resistivity C. The same as the flux density generated by the test coil D. Greater than the flux density generated by the test coil 46. The Flux density in a magnetic material is usually designated by the: A. Symbol Îź B. Symbol Ďƒ C. Letter B D. Letter H

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47. The magnetizing force of an eddy current test coil is usually designed by the: A. Symbol μ B. Symbol σ C. Letter B D. Letter H 48. In eddy current testing, the specimen is coupled to the test coil by: A. Core coupling B. Magnetic saturation C. The coil’s magnetic field D. Magnetic domains 49. The thickness of nonconductive coatings on a conductive base can be most simply measured by: A. Observing the lift off effect caused by the coating B. Testing both sides of the specimen C. Varying the current in the test coil D. Varying the test frequency over a given range during the test

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50. The use of magnetic shielding around the exciting coil will generally: A. Increasing the magnetic field extension but decrease the eddy current penetration B. Increase the magnetic field extension and eddy current penetration C. Decrease the magnetic field extension but increase the eddy current penetration D. Decrease the magnetic field extension and eddy current penetration

eddy current magnetic field extension

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Shielded and unshielded probes Probes are normally available in both shielded and unshielded versions; however, there is an increasing demand for the shielded variety. Shielding restricts the magnetic field produced by the coils to the physical size of the probe or even less. A shield can be made of various materials, but the ones mostly used are: ferrite (like a ceramic made of iron oxides), Mumetal速, and mild steel.

Ferrites make the best shields because they provide an easy path for the magnetic field but have poor conductivity. This means that there are few eddy current losses in the shield itself. Mild steel has more losses but is widely used for spot probes and ring probes due to its machinability and when ferrites are not available in certain sizes or shapes. Mumetal速 is used sometimes for pencil probes as it is available in thin sheet; however, it is less effective than ferrite.

Shielding has several advantages: first, it allows the probe to move in (or close to) geometry changes, such as edges, without giving false indications; next, it allows the probe to touch ferrous fastener heads with minimal interference; last, it allows the detection of smaller defects due to the stronger magnetic field concentrated in a smaller area.

On the other hand, unshielded probes allow somewhat deeper penetration due to the larger magnetic field. They are also slightly more tolerant to lift-off. Unshielded probes are recommended for the inspection of ferrous materials (steel) for surface cracks, and in particular with meter instruments. The reason for this is that the meter response is too slow to allow the signal from a shielded probe to be displayed at normal scanning speeds due to the smaller sensitive area.

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http://www.olympus-ims.com/en/resources/white-papers/eddy-current-probes-and-application-guide/

Level II – Q&A 1. A 2. D 3. A 4. B 5. D 6. A 7. D 8. B 9. A 10. C 11. C 12. B 13. A 14. C 15. D 16. D 17. B 18. B 19. A 20. C 21. D 22. B 23. B 24. D 25. C 26. A 27. D 28. B 29. B 30. C 31. B 32. C 33. C 34. C 35. D 36. D 37. B 38. A 39. A 40. B 41. C 42. A 43. C 44. C 45. D 46. C 47. D 48. C 49. A 50. D 51. B 52. D 53. C 54. C 55. D 56. C 57. D 58. A 59. C 60. D 61. A 62. D 63. A 64. B 65. C 66. B 67. D 68. B 69. D 70. A 71. B 72. B 73. D 74. B 75. D 76. C 77. A 78. B 79. C 80. B 81. A 82. D 83. A 84. C 85. A 86. B 87. C 88. D 89. D 90. A 91. A 92. C 93. C 94. C 95. D 96. C 97. A 98. A 99. C 100. A 101. B 102. A 103. E 104. D

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51. Which of the following is not a factor that affects the inductance of an eddy current test coil A. Diameter of coils L=ÎźoN2A/l B. Test frequency C. Overall shape of the coils D. Distance from other coils 52. The formula used to calculate the impedance of an eddy current test coil is: D

53. An out of phase condition between current and voltage: A. Can exist only in the primary winding of an eddy current coil B. Can exist only in the secondary winding of an eddy current coil C. Can exist in both the primary and secondary windings of an eddy current coil D. Exists only in the test specimen

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Inductance The increasing magnetic flux due to the changing current creates an opposing emf in the circuit.

The inductor resists the change in the current in the circuit. If the current changes quickly the inductor responds harshly.

If the current changes slowly the inductor barely notices. Once the current stops changing the inductor seems to disappear.

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http://sdsu-physics.org/physics180/physics196/Topics/inductance.html

RL Circuit

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http://sdsu-physics.org/physics180/physics196/Topics/images_196/32_ind1.swf http://sdsu-physics.org/physics180/physics196/Topics/inductance.html

RL Circuit

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http://sdsu-physics.org/physics180/physics196/Topics/images_196/32_ind2.swf http://sdsu-physics.org/physics180/physics196/Topics/inductance.html

Oscillation in LC Circuit

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http://sdsu-physics.org/physics180/physics196/images_196/32_oscillation2.swf http://sdsu-physics.org/physics180/physics196/Topics/inductance.html

54. A term used to define an eddy current test coil which uses magnetic material to purposely shape the magnetic field is: A. Bobbin coil B. Encircling coil C. Spinning coil D. Gap coil 55. A term used to define an eddy current test coil which uses magnetic material to purposely shape the magnetic field is: A. Bobbin coil B. Encircling coil C. Spinning coil D. Gap coil Gap Coil ≥ horse shoe coil?

56. The vector point, ellipse, and linear time base methods are all subdivisions of the: A. Impedance method of testing B. The modulation analysis method of testing C. The phase analysis method of testing D. None of the above

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57. It is frequently possible to separate the conductivity variable from the permeability and dimensional variables when using: A. The vector point method of testing B. The ellipse method of testing C. The linear time base method of testing D. Any of the above methods of testing 58. When eddy current testing by the ellipse method, the normal indication when the test specimen and reference standard are the same is approximately: A. A straight horizontal line on a cathode ray tube B. An ellipse on a cathode ray tube → C. A null reading on a meter D. A clean base line on a strip chart 59. When eddy current testing by the ellipse method, one variable may be indicated by the angle of the ellipse or straight line while another variable may be indicated by the: A. Brightness of the ellipse B. Horizontal length of the straight line C. Size of the ellipse opening D. Sinusoidal shape of the waveform

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60. When eddy current testing by the linear time base method, _________ is applied to the vertical deflection plates of a cathode ray tube when the test specimen and reference standard are the same and the balance controls are properly adjusted.

A. A sinusoidal wave in phase with the timing voltage B. A sinusoidal wave 90 degrees out of phase with the timing voltage C. A sawtooth waveform D. Zero net voltage 61. When eddy current testing by the linear time base method, what is applied to the vertical deflection plates of the cathode ray tube when a condition of unbalance exists between the test specimen and the reference standard?

A. A sinusoidal wave B. A sawtooth wave C. A squarewave D. No voltage 62. The conductivity value for a metal is a function of the: A. Heat treatment give the metal B. Cold working performed on the metal C. Aging process used on the metal D. All of the above

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63. The ratio of the specific diameter of a probe to the minimum discontinuity of interest should be: A. Less than 2 B. Greater than 2 C. Greater than 4 D. Greater than 10 64. Which of the following materials would be more likely used as a mounting material for a probe coil?

A. Aluminum B. Fiberglass C. Copper D. Steel 65. A term used to define a surface or internal rupture that is attributed to processing at too low a temperature or excessive working or metal movement during the forging, rolling, or extruding operation is: A. A cold shut B. Microshrinkage C. Burst D. An inclusion

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66. A term used to define the timing relationships involved in alternating current signals is: A. Magnitude B. Phase C. Impedance D. Reactance 67. When eddy current testing a nonferrous material, the choice of test frequency is determined by the: A. Degree of phase discrimination required θ = x/δ B. Eddy current penetration needed δ = (πfμσ) -½ C. Rate of response required D. All of the above 68. When testing ferrous materials, a small but detectable portion of the magnetic flux in the material will pass outside the metal when a surface discontinuity is encountered.

This flux is called: A. Air flux B. Leakage flux C. Induced flux D. None of the above

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69. A term used to define a property of a test system which enables the separation of signals due to discontinuities in the test specimen that are located in close proximity to each other is: A. Dynamic range B. Sensitivity C. Linearity D. Discontinuity resolution 70. A term used to define a material having a permeability less than that of a vacuum is: A. Diamagnetic B. Ferromagnetic C. Paramagnetic D. Magnetic 71. The inductive reactance of a coil is measured in units of: A. Mhos B. Ohms XL= ωL C. Henrys D. Gauss

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72. Which of the following coil arrangements would eliminate or decrease the effects of minor variations in diameter, chemical composition, harness, etc., which occur gradually along the length of a wire?

A. External reference differential B. Self comparison differential C. Single coil absolute D. Double coil absolute 73. Which of the following conditions are not important when selecting specimens to be used as reference standards?

A. The specimen should be of the same size and shape as the piece to be tested B. The specimen should have the same heat treatment C. The surface finish of the specimen should be the same as the piece to be tested D. If the material is aluminum, the surface should be anodized

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74. An eddy current test coil’s magnetic field intensity in air: A. Increase with distance outside the coil and varies across the diameter inside the coil B. Decreases with distance outside the coil and varies across the diameter inside the coil C. Decreases with distance outside the coil and is assumed to be constant across the diameter inside the coil D. Increases with distance outside the coil and is assumed to be constant across the diameter inside the coil

75. The circuits which make it possible to minimize the effects of signals caused by variables of no interest on the read out mechanism are based on differences in: A. Amplitude B. Phase C. Frequency D. Any one or a combination of the quantities listed above

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Level II – Q&A 1. A 2. D 3. A 4. B 5. D 6. A 7. D 8. B 9. A 10. C 11. C 12. B 13. A 14. C 15. D 16. D 17. B 18. B 19. A 20. C 21. D 22. B 23. B 24. D 25. C 26. A 27. D 28. B 29. B 30. C 31. B 32. C 33. C 34. C 35. D 36. D 37. B 38. A 39. A 40. B 41. C 42. A 43. C 44. C 45. D 46. C 47. D 48. C 49. A 50. D 51. B 52. D 53. C 54. C 55. D 56. C 57. D 58. A 59. C 60. D 61. A 62. D 63. A 64. B 65. C 66. B 67. D 68. B 69. D 70. A 71. B 72. B 73. D 74. B 75. D 76. C 77. A 78. B 79. C 80. B 81. A 82. D 83. A 84. C 85. A 86. B 87. C 88. D 89. D 90. A 91. A 92. C 93. C 94. C 95. D 96. C 97. A 98. A 99. C 100. A 101. B 102. A 103. E 104. D

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76. When a nonmagnetic rod is placed inside an eddy current test coil: A. The magnetic field generated by the coil is increased in intensity B. The distribution of eddy current is uniform through the rod’s across section C. The distribution of eddy currents is a maximum at the rod’s surface, or near the surface and decreases to essentially zero at the rod’s center D. The temperature of the rod decreases 77. The symbol commonly used to signify impedance is: A. Z B. Xjε C. R D. Xc 78. A decrease in conductivity is equivalent to: A. An increase in permeability B. An increase in resistivity R = ρl/A , σ = 1/ρ C. A decrease in permeability D. A decrease in resistivity

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79. If the conductivity of a test part in an eddy current test coil decreases, the magnitude of the eddy currents at a given depth in the test specimen: A. Increases B. Remains the same C. Decrease D. May increase or decrease 80. When the voltage applied to a circuit and the current though the circuit both reach their maximums and minimums at the same time, the voltage and current are: A. Additive B. In phase C. Regenerative D. Out of phase 81. Which of the following is not a commonly used eddy current testing readout mechanism?

A. Signal generator B. Meter C. Cathode ray tube D. Strip chart recorder

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82. Which of the following materials would have the highest resistivity value?

A. Aluminum with a 42% IACS rating B. Magnesium with a 37% IACS rating C. Cast steel with a 10.7% IACS rating D. Zirconium with a 3.4% IACS rating

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83. The two voltages (V1 and V2) shown in figure 6 are: A. In phase B. 45 degrees out of phase C. 90 degrees out of phase D. 180 degrees out of phase

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84. The two voltages (V1 and V2) in figure 7 are: A. In phase B. 45 degrees out of phase C. 90 degrees out of phase D. 180 degrees out of phase

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85. In figure 8, H represents: A. The coil’s magnetizing force B. The material’s flux density C. The material’s permeability D. The material’s conductivity 86. In figure 8, B represents: A. The coil’s magnetizing force B. The material’s flux density C. The material’s permeability D. The material’s conductivity 87. In figure 8, the distance represented by A is a measure of the material’s: A. Permeability B. Conductivity C. Residual magnetism D. Coercive force

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88. In figure 8, the distance represented by C is a measure of the material’s: A. Permeability B. Conductivity C. Residual magnetism D. Coercive force 89. The ratio of a material’s flux density to a test coil’s magnetizing force, (B/H), can be used to determine the material’s: A. Conductivity B. Resistivity C. Lift off D. Permeability 90. The magnetic field generated by eddy currents induced in a test specimen: A. Opposes the magnetic field that induced the eddy currents B. Reinforces the magnetic field that induced the eddy currents C. Cancels the magnetic field that induced the eddy currents D. Has no effect on the magnetic field that induce the eddy currents

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91. The impedance of a test coil usually can be represented by the vector sum of: A. Inductive reactance and resistance B. Capacitive reactance and resistance C. Inductive reactance and capacitive reactance D. Inductance and capacitance 92. For age hardenable aluminum and titanium alloys, change in hardness are indicated by changes in: A. Retentivity B. Permeability C. Conductivity D. Magnetostriction 93. The penetration of eddy currents in a conductive material is decreased when: A. Test frequency or conductivity of the specimen is decreased B. Test frequency is decreased or conductivity of the specimen is increased C. Test frequency, conductivity of the specimen, or permeability of the specimen are increased δ = (πfσμ) -½ D. Permeability of the specimen is decreased

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95. When testing magnetic materials in an AC field, increasing the field strength, (H?) A. Has not effect B. Increases eddy current penetration C. Decreases eddy current penetration D. Decreases eddy current penetration to some minimum value, and then increases it to its maximum value when the specimen becomes magnetically saturated δ = (πfσμ)-½

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94. When electrical current in an eddy current coil reverses direction: A. The direction of the eddy currents in a part remains the same B. The eddy currents in the test part will change phase by 45 degrees C. The direction of the eddy currents in the part also reverses D. The Eddy currents in the part remain the same 95. When testing magnetic materials in an AC field, increasing the field strength: A. Has not effect B. Increases eddy current penetration C. Decreases eddy current penetration D. Decreases eddy current penetration to some minimum value, and then increases it to its maximum value when the specimen becomes magnetically saturated

δ = (πfσμ)-½

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96. Impedance diagrams have been mathematically derived and experimentally verified for materials of any conductivity and permeability having rectangular, cylindrical, or spherical symmetry. To test all specimens of similar geometry under the same condition, it is only necessary to choose a test frequency, f, so that frequency ratio f/fg lies at the same point on the impedance diagram for each specimen. This principle is based upon: A. Ohm’s law B. Kirchoff’s law C. The similarity law D. None of the above

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97. To be useful, limit frequency equations must be: A. Used with their impedance diagrams B. Used to determine their impedance diagrams C. Both of these D. Neither of these 98. When a rod is placed in an encircling type of coil, the density of eddy currents will be the greatest: A. At the surface B. At the center C. Midway between the surface and the center D. None of the above 99. The term fill factor applies to: A. A surface coil B. Coaxial cable C. An encircling coil D. All of the above

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100. An eddy current system using handling equipment with automatic marking of the defective areas primarily makes it possible to: A. Remove defective areas of the product, if desired B. Eliminate test surface contamination C. Allow an inexperienced operator to establish the defect cause D. Pass rejectable material 101. Two test coils are often used in a bridge circuit to: A. eliminate skin effect B. Determine the difference between a known standard sample C. Increase the conductivity of the circuit D. Decrease the system sensitivity 102. In inspecting ferromagnetic materials, relatively low frequencies are normally used because of the: A. Low penetrability of these materials B. Higher resolution C. Lower resolution D. Higher penetrability of these materials

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103. The actual frequencies used in any specific eddy current inspection should be selected on the bases of the: A. Thickness of the material B. Desired depth of penetration C. Degree of sensitivity or resolution required D. Purpose of the inspection E. All of the above 104. In modulation analysis testing, which of the following would not modulate the test frequency applied to the test coil?

A. Discontinuities in the test specimen B. Dimension changes of the test specimen C. Chemical composition of the test specimen D. A filter in the test circuit

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Recalling the mistakes

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Charlie Chong/ Fion Zhang

Level III Q&A

e e r Th

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Level III – Q&A 1. b 2. c 3. d 4. b 5. c 6. d 7. b 8. b 9. c 10. b 11. b 12. b 13. b 14. c 15. d 16. b 17. d 18. b 19. d 20. b 21. c 22. d 23. b 24. c 25. a 26. a 27. e 28. b 29. d 30. e 31. a 32. a 33. b 34. a 35. b 36. b 37. c 38. a 39. e 40. d 41. d 42. c 43. e 44. d 45. c 46. c 47. a 48. c 49. b 50. a 51. b 52. d 53. b 54. c 55. c 56. d 57. a 58. c 59. a 60. d 61. c 62. c 63. b 64. a 65. c 66. d 67. b 68. d 69. a 70. a 7 1. c 72. c 73. d 74. b 75. b 76. b 77. c 78. b 79. d

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Q1. In a feedthrough encircling coil eddy current system, what would be the purpose of running a calibration defect several times but in various positions (such as top, bottom, left and right)? a. to check the phase selectivity b. to ensure proper centering of the material in the test coil c. to select the modulation analysis setting d. to select the proper operating speed Q2. In a feedthrough encircling coil eddy current system, a calibration standard may be used to: a. insure repeatability and reliability of the setup b. calibrate the approximate depth of detectable flaws c. both a and b d. measure the test frequency

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Q3. A calibration standard may be used with a spinning probe eddy current instrument to: a. produce an indication relative to the depth of the flaw b. check the instrument for repeatability and freedom from drift c. check probe coil for possible damage d. all of the above Q4. Spinning probe-type eddy current instruments are most useful in: a. detection of surface and subsurface inclusions b. detection of surface defects such as overlaps and seams c. detection of internal piping or burst d. all of the above

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Q5. A product can be viewed in terms of electrical, magnetic effects. A diameter change of the product in an encircling coil is: a. an electrical effect b. a conductivity effect c. a magnetic effect d. all of the above Q6. In Figure 9, ac flowing through a primary coil set-up a magnetic field and causes a flow of eddy currents in the rod. The voltage of the secondary coil is dependent upon: a. these eddy currents b. the primary coil c. the generator d. all of the above

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Figure 9

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Q7. Which of the following is not a method that may be used to improve the signal-to-noise ratio? a. change to a test frequency that will decrease the noise b. increase the amplification of the test instrument c. improve the fill factor d. add filter circuits to the instrument Q8. In eddy current testing, the theoretical maximum testing speed is determined by the: a. magnetic flux density b. testing frequency c. conveyor drive d. test coil impedance

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Q9. In eddy current testing of ferromagnetic materials, the desaturating field may be provided by: a. an encircling solenoid b. a magnetic yoke c. both a and b d. none of the above Q10. Which of the following is a property of eddy currents induced in a conductor by an encircling coil? a. the magnitude of eddy current flow is large compared to the current flow in the coil b. the eddy current flow is affected by permeability variations in the sample c. the eddy current flow dissipates no power in the conductor d. none of the above

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Q11. Which of the following is a property of eddy currents induced in a homogeneous conductor by an encircling coil? a. they are weakest on the conductor surface b. the phase of the eddy currents varies throughout the conductor c. they travel in straight lines d. they are maximum along the coil axis Q12. Which factor does not affect the phase shift between the transmitted signal and the reflected signal for a reflection-type coil (assuming the part is nonferromagnetic)? a. the conductivity of the sample b. the magnitude of the transmitted signal c. the thickness of the sample d. the presence of defects in the sample

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Q13. Lift-off certainly reduces the amplitude of the flux leakage signal. The other significant effect it has on the signal is a change in: a. phase b. frequency c. increasing lift-off which reduces the apparent width of the defect d. none of the above Q14. The tubular product parameter having the greatest influence on the flux density of the magnetic field in the part (assuming the magnetizing force, H, remains constant) is the: a. surface roughness of the product b. diameter of the product c. wall thickness of the product d. length of the product

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15. Any handling of equipment used in an eddy current system must take into consideration: a. the operator's abilities b. the use of the product being tested c. speed, frequency of test, sorting speed, and physical control of the product d. all of the above 16. An eddy current system lends itself to quality ratings such as "Quality Numbers" where the product being inspected: a. is not defective b. does not allow defective areas to be removed c. is of inferior quality d. has inconsistent quality

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17. When inspecting material with eddy currents in an automatic handling system, it is advisable to calibrate and adjust the sensitivity levels to: a. some electronic source b. another NDT method c. an NBS standard d. an actual test part being inspected 18. A distinct advantage of using handling equipment in an eddy current test system is to reduce the error caused by: a. instrument drift b. lift-off c. skin effect d. all of the above e. none of the above

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19. Decreased coupling or fill factor results in decreased test sensitivity because: a. reduced coupling between the driver coil and the specimen induces less eddy current flow in the specimen b. reduced coupling between the specimen and the pickup coil results in smaller voltages across the pickup coil c. electrical circuits designed to provide fill factor compensation may prove to be inadequate, depending upon the extent of fill factor loss d. all of the above 20. Why is it desirable to hold the fill factor or lift-off constant? a. to avoid arcing between the coil and the specimen b. to minimize tester output signal changes that are not relevant to conditions within the specimen to be tested c. a fill factor or lift-off change will shift the operating frequency d. to minimize the load on the constant current ac excitation circuits

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21. The reactance component is decreased by placing a conducting object in the coil's electromagnetic field. Why is this so? a. the secondary field is exactly in phase with the primary field b. the secondary field is at precisely 90 degrees with the primary field c. the phase angle between the two field components is always greater than 90 degrees which partially cancels the primary field d. the secondary field is 180 degrees out of phase with the primary field which causes a large phase shift

Charlie Chong/ Fion Zhang

Charlie Chong/ Fion Zhang

22. Test coils may be shielded with conducting material or magnetic material to: a. shape field b. increase sensitivity c. increase resolution d. all of the above e. none of the above 23. When a magnetic bar is placed in the coil's electromagnetic field, the coil's reactance is increased. What causes this phenomena? a. the coil becomes magnetically saturated b. the permeability raises the inductance of the test coil c. the magnetic test sample's conductivity increases the reactance value of the coil d. this effect is described mathematically by the equation Îź = B/H

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24. When an excitation voltage is applied to a primary winding, only the magnetic flux is in phase and the secondary magnetic flux is minor. When a test object is inserted in this coil, what action takes place? a. the object gets hot and no information is available b. insertion of the object cancels all information c. the insertion of the test object intensifies the secondary magnetic flux producing a new total magnetic flux which can be used to supply test information d. by subtracting the primary voltage from the secondary voltage, the net voltage is obtained

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25. The test coil excitation current should be held constant so that the test piece information obtained by an eddy current system will: a. contain only flaw information and not indicate variations in magnetic field strength b. not contain signals generated by cross talk c. not contain electrical noise d. all of the above 26. Eddy currents flowing in the test object at any depth produce magnetic fields at greater depths, which oppose the primary field, thus reducing its effect and causing what kind of change in current flow as depth increases? a. a decrease b. an increase c. a frequency change d. none of the above

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27. Skin effect causes eddy currents to tend to flow near the surface of the test piece. Which of the following factors alter the skin effect? a. testing frequency b. test piece temperature c. test piece hardness (affect conductivity?) d. test piece permeability e. all of the above δ = (πfμσ) -½ Conductivity

Magnetic (Permeability & Dimensions)

A. Heat treatment give the metal

A. Length of the test sample

B. Cold working performed on the metal

B. Thickness of the test sample

C. Aging process used on the metal

C. Cross sectional area of the test sample

D. Hardness E. Crack & discontinuities

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28. Which of the following is not a common undesirable effect to the test coil caused by the testing environment? a. temperature variation b. crack in test sample c. test object making contact with test coil d. foreign object in the test coil field e. test coil vibration 29. There is one function that responds to variations in eddy current flaw and magnetic field conditions. This function actually produces the output signal from the coil. What is this function? a. phasing b. resistance c. reactance d. impedance

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30. The inductive reactance of a test coil, which is one of the most important impedance quantities, depends upon which of the following? a. frequency, coil inductance, coil resistance b. coil inductance only c. coil resistance and inductance only d. frequency and coil resistance only e. frequency and coil inductance only XL = ωL

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31. An ac current produces eddy currents in a test object. The vector Hp represents the secondary ac field in the test piece. what function occurs to produce a workable test situation? (See Figure 10.) a. changes in the test specimen such as a crack, metallurgical or dimensional change alter the secondary field phase and amplitude b. the primary ac current must be 60 cycles to produce this effect c. a temperature rise in the specimen d. a mismatch of the HP and Hs fields produces a change in the output

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32. To separate cracks and diameter effects for steel cylinders, the optimum frequencies correspond to f/fg ratios of less than (see Figure 11): a. 10 b. 15 c. 50 d. 100 e. 150

Figure 11

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33. Thin-walled tubes should be tested for cracks, alloy or wall thickness at frequency ratios between (see Figure 12): a. 0.1 and 0.4 b. 0.4 and 2.4 c. 2.4 and 4.0 d. 4.0 and 10

Figure 12 Charlie Chong/ Fion Zhang

34. Figure 13 indicates that the largest eddy current indications from subsurface cracks will occur when the frequency ratio (f/fg) is: a. 5 or less b. 15 c. 50 d. 150 or more

Figure 13 Charlie Chong/ Fion Zhang

35. Figure 13 indicates that the magnitude of a signal from a subsurface crack will increase when the frequency ratio (f/fg): a. remains the same b. decreases c. increases d. none of the above

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36. Figure 14 indicates that when inspecting for surface cracks in nonferromagnetic cylinders, the optimum frequency ratio (f/fg) is between: a. 5 and 10 b. 10 and 50 c. 50 and 100 d. 100 and 150 Figure 14

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37. An operating frequency of 100kHz will have the deepest penetration in: a. titanium b. copper c. stainless steel d. Aluminum δ = (πfσμ)-½ 38. As the operating frequency is increased, the impedance of the empty coil: a. increases b. decreases c. remains the same d. none of the above XL = 2πfL

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39. Disadvantages of using a surface probe coil for the inspection of small diameter tubing include: a. inability to detect small discontinuities b. slow inspection speed c. inherent mechanical problems d. both a and c e. both b and c

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40. Differential coil systems can be of which of the following types? (see Figure 15) a. sketch no. 1 b. sketch no. 2 (reflection differential coils) c. sketch no. 3 d. all of the above e. both a and b

Figure 15

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41. The sensing element in an eddy current system can be which of the following? a. the secondary winding of a differential coil b. an absolute coil c. a single winding coil d. all of the above e. both a and b 42. Of the following test coil systems, which would be considered best for definitive detection of small discontinuities in tubing? a. encircling coils b. inside probe (ID) coils c. surface probe coils d. both a and b

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43. In a test coil system, when the sensing coil is placed near the excitation coil, which of the following statements is true? a. almost the same magnetic flux threads both coils b. signals from either coil can be used to provide information about the test piece c. the arrangement is never sensitive to small discontinuities d. all of the above e. both a and b

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44. In linear time-base equipment: a. the balance control will not affect the phase of the signal on the CRT b. the balance control changes the horizontal position of the signal displayed on the CRT c. the phase control permits initial smoothing of the input signals d. the phase control shifts the signal to the left or right on the CRT 45. Filters are used in modulation analysis to: a. amplify crack or other discontinuity signals b. reduce the signal-to-noise ratio c. eliminate effect of small random changes in conductivity and permeability of test sample d. separate conductivity variations from permeability variations

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46. In modulation analysis, the type of display usually used is: a. cathode ray tube b. analog meter c. chart recorder d. digital voltmeter 47. When the slit technique is used with the linear timebase method: a. a shift in phase will move the waveform on the CRT to the left or right b. after calibration, the waveform on CRT will move up or down when a discontinuity appears c. the equipment can be adjusted so that permeability and conductivity changes will show little or no change in the slit value d. dimension effects will not be displayed at the slit. when the conductivity effect is on the horizontal line

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48. In linear time-base equipment, which type of signal is applied to the horizontal deflection plates? a. sinusoidal voltage b. square-wave voltage c. sawtooth voltage d. amplitude-modulated sinusoidal voltage

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49. In the ellipse method, the following (see Figure 16) indication should be interpreted as: a. change in dimension b. change in conductivity and dimension c. change in conductivity d. no change

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50. The ellipse test method, used with feed through coils: a. can test rods, tubes, and wires for surface and subsurface cracks b. cannot be used for measuring diameter of rods that have surface cracks c. has limited value in the alloy sorting of nonferrous rod d. is not affected by lift-off variations

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51. In cathode ray tube ellipse testing: a. a straight line trace will appear when the two voltages applied to the deflection plates are 90 degrees out of phase b. the presence of a crack in a test specimen will generally produce a phase shift c. changes in diameter of a rod cannot be separated from crack effects d. a circle will appear on the screen whenever no voltage is applied to the vertical plates

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Ellipse Method the display on the CRT will be a straightline.

This condition indicates that there is an acsignal on the horizontal plates and zerodifferential voltage 'standard and test speci-men are the same) from the test coils onthe vertical plates.

Now let's see what happens when we get a change in dimension. After all of our dis-cussion you probably expect to see an ellipse on the CRT - but not yet.

A change indimension will just tilt the horizontal line asshown in this illustration.

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NASA Nondestructive Testing Eddy Current Equipments Methods & Applications Volume-2

A dimension change will not noticably changethe phase of the signal on the test coil, but itwill give a difference in voltage.So now weno longer have a zero differential from thetest coil. We have a voltage difference, butthe test signal is still in phase with the signal on the horizontal plates.Thus ourstraight line remains straight but is inclined. A straight line results because, whentwo voltages having the same phase are applied simultaneously to thhorizontal andvertical plates of the CRT, a straight line will result.

Dimension change (tube diameter, thicknessor shape) will give an inclined straight line on the cathode ray tube.

Suppose that rather than a dimension change we have a crack or discontinuity in thespecimen that gives us a conductivity change in the coil.

Ah ha! Now we get an ellipse finally.

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NASA Nondestructive Testing Eddy Current Equipments Methods & Applications Volume-2

An ellipse is displayed on the cathode ray tube when the specimen discontinuitygives a conductivity change in the coils. However, unlike the dimension change, wenow get a phase change on the signal to the vertical plates rather than a voltage change.This phase change causes the horizontal straight line to open up into an ellipse.

No Defect

Conductivity Defect

Both Dimension &Conductivity Change

Small Change Charlie Chong/ Fion Zhang

Large Change NASA Nondestructive Testing Eddy Current Equipments Methods & Applications Volume-2

52. Eddy current test instruments that indicate only the magnitude of variation in total impedance of the test coil are: (total impedance method?) a. unable to sort aluminum rods having different diameters, but the same chemical composition b. normally used to measure wall thickness variations in as-cast pipe ? c. not used with cathode-ray tube indicators d. more sensitive to dimensional changes than variations in electrical conductivity (more sensitive to magnetic effects than electric effects) 53. When using an inspection system having an "ellipse display" readout, the signal applied to the vertical input of the scope is related to that applied to the horizontal input as follows: a. the vertical signal is demodulated; the horizontal signal is not b. the vertical and horizontal signals are of the same frequency c. the vertical and horizontal signals are always in phase d. none of the above

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54. Which material should be selected to provide the most effective shielding of a test coil from an external RF field? a. fiberglass b. austenitic stainless steel c. copper d. Brass 55. To allow maximum current to flow through the coil, the capacitor should be adjusted so that: a. the capacitive impedance is equal to the generator impedance b. the capacitive impedance is minimum c. the capacitive reactance is equal to the inductive reactance d. the capacitive impedance is maximum Xc = 1/ωC, XL= ωL

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56. A primary-secondary coil inspection system is being used. To suppress the primary coil signal induced in the secondary coil, one might: a. use a differential-type secondary coil b. add a 180 degree out-of-phase signal to the induced signal c. subtract an in-phase signal from the induced signal d. any of the above C.2-40, 2-41 C.* Eddy Current Testing, Classroom Training Handbook ( CT-6-5 ). San Diego, CA: General Dynamics/Convair Division. 1967. (Now published by The American Society for Nondestructive Testing, Inc.)

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57. Which type of readout oscilloscope would be best for examining a nonrepetitive flaw indication during high speed testing? a. a storage type b. a sampling type c. a type having a short persistance screen d. a dual beam type G.38.36 58. An ink-writing strip chart recorder generally provides an accurate representation of its input signal: a. if the signal frequency is 2 000 Hz or greater b. if the signal frequency is at least 200 Hz c. if the signal frequency is limited to 20Hz d. only if the signal frequency is limited to 2 Hz V.88

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59. A probe coil inspection system is being used to detect cracks in bars. If the cracks have a minimum length of 6 mm (0.25 in.), reliable inspection is best achieved by making the coil diameter: a. 6 mm (0.25 in.) b. 13 mm (0.5 in.) c. any multiple of 6 mm (0.25 in.) d. coil diameter is irrelevant G.38.31 60. To ensure reliable flaw detection, the maximum velocity of tubing through an encircling coil must be limited. This limitation is determined by: a. the inspection coil length b. the desired flaw size resolution c. the test frequency d. all of the above C.4-29

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61. Normal test speeds for modulation analysis testing is: a. 0-1.2 m (0-4 ft)/min b. 1.2-9 m ( 4-30 ft)/min c. 9-91 m (40-300 ft)/min d. 91-1219 m (400-4 000 ft)/min C.4-29 62. When performing modulation analysis testing, the type of test coil: a. must be of the differential type b. must be of the absolute type c. can be any basic type d. a and b are the best answers C.4-29

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63. A major limitation of modulation analysis is that the system is based on: a. static tests b. moving tests c. absolute coil arrangements d. differential coil arrangements C.3-26 64. Chemical composition, alloy changes, and heat treat changes usually have a frequency modulation that is: a. low b. medium c. high d. very high C.3-25

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65. Modulation would be defined as: a. a process of comparing an instrument reading with a standard b. a process of grouping articles by their response to eddy currents c. a process of applying a variable effect to something that is constant d. the point where increases in the coil's magnetizing force does not increase the material's flux density D.S-116 66. In modulation analysis, the coil's excitation frequency is being modulated by: a. test article conductivity b. test article dimensions c. test article defects d. all of the above D.S-126

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67. Impedance changes produced by small lift-off variations are greatest when: a. there is a nonconductive coating on the test material b. the coil is usually in contact with the test material c. lower test frequencies are used d. large diameter coils are used U.280 68. When testing for the unknown conductivity of a metal, the measured conductivity is strongly affected by: a. I2R b. material thickness c. coil diameter d. specimen temperature V.78

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69. In selecting a coil for multifrequency tests: a. bandwidth is of major importance b. frequency response is of minor importance c. the Q of the coil should be less than 1 d. the Q of the coil should be less than the inductance A.216 70. In developing a multifrequency test, the parameter separation limitations are greatest for those parameters producing: a. nearly similar signals b. 0 degrees to 90 degrees phase shifts c. signals having no electrical relation d. 90 degrees to 110 degrees phase shifts A.216

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71. Mutual inductance describes: a. the effect two coils have on each other b. the coupling between the test coil and test part c. a and b d. none of the above R.29 72. Leakage flux testing is most applicable for the inspection of: a. shallow surface seams on finished roller bearings b. heavy-wall copper tubing c. surface and subsurface cracks in hot-rolled rough surface ferromagnetic pipe d. soft spots on polished steel bar stock N.122

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73. Increasing the heat-treating temperature of nonferrous metals: a. increases electrical conductivity b. will have no effect on electrical conductivity c. will decrease electrical conductivity d. may either increase or decrease electrical conductivity, depending upon the alloy and the nature of the heat treatment G.42.19 74. Limit frequency, fg , is defined when the argument of the mathematical function describing the electromagnetic field within a test specimen is set equal to: a. 6.25 b. 1 c. 0.37 d. 0 G.36.13

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75. Referring to Figure 17, suppose a test is being conducted at an f/fg ratio of 1. Due to a mix in alloys, a new test piece with a conductivity value four times as great as the original material is inserted in the coil. The new f/fg ratio will be: a. 0.25 Figure 17 b. 4 c. 16 d. unchanged S.l0l Rod: fg = 5060/σμd2 Tube: fg = 5056/σdiwμ

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76. Figure 18 illustrates that the separation of diameter effects and conductivity effects: a. can be accomplished at any f/fg E b. can best be accomplished at f/fg larger than 4 c. can best be accomplished at f/fg less than 4 d. cannot he accomplished at any f/fg

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Figure 18

77. Figure 19 illustrates: a. variations in fill factor for a solid rod b. variations in fill factor for a thin-walled tube c. transition between a solid rod and thin-walled tube d. all of the above G.38.9

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Figure 19

78. Figure 20 illustrates the impedance diagram for tubing where the ratio of the inside diameter to outside diameter is 80 percent. It is possible to calculate an approximate limit frequency, f , by using only the outside diameter. Thus the maximum test sensitivity for effects of conductivity, wall thickness and cracks occurs at a frequency ratio of: a. 6.25 b. 13 c. 25 d. 400 G.38.11, 38.12

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Figure 20

79. The electromagnetic field in a test specimen during eddy current testing is described by: a. Bessel functions b. exponential functions c. algebraic functions d. all of these

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Recalling the mistakes

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Flux Leakage Testing Method

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Level I Q&A-Flux Leakage Testing Method

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Level I - Standard Answers 1. c 2. a 3. e 4. c 5. c 6. b 7. d 8. a 9. c 10. d 11. a 12. b 13. d 14. d 15. d 16. d 17. a 18. d 19. d 20. e

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1. In flux leakage testing of a ferromagnetic object, magnetized close to the saturation point using an encircling coil as a sensor, what is a primary condition which needs to be met in order to obtain a response from the flaw? a. using a coil with a high number of turns b. providing electrical connection between coil and tested object c. moving the tested object in relation to the coil d. shielding the cable which connects the coil with the recorder e. using a strip chart recorder EE.62

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2. What will be the relative flux leakage level on the surface of the I X 7 strand cable (Figure FL-1) caused by an outer wire break compared with that caused by an inner wire break? (Assume that the gap between the ends of broken wire is the same in both cases.) a. greater b. smaller c. the same, if diameters of both wires are the same d. the same, even if diameters of both wires are different EE.63

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3. What method of magnetization can be used in flux leakage testing? a. permanent magnet b. high frequency ac c. dc d. a and b e. a and c Z.43

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4. Flux leakage may occur at a surface defect in a ferromagnetic material: a. when the magnetizing force is present b. after the magnetizing force has been removed c. a and b d. none of the above Z.51 5.Flux leakage inspection can normally be applied to: a. ferromagnetic and nonmagnetic material b. nonmagnetic materials only c. ferromagnetic materials only d. nonconductors only U.330

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6. Rotational transverse magnetization in bar or pipe testing is used for the detection of: a. transverse defects b. defects that have a longitudinal component c. only longitudinal defects d. both transverse and longitudinal defects e. only holes AA.16 transverse magnetization ≥ circumferential magnetization Level II Q&A 22. In the examination of tubular products, a circumferential (transverse) magnetic field can be established by: a. properly positioning north and south poles of a magnet with respect to the tube b. using a central conductor positioned in the tube c. a and b d. neither a nor b DD.629 Charlie Chong/ Fion Zhang

Rotational Transverse Magnetization by Coil

transverse magnetization ≥ circumferential magnetization ≥ circular magnetization?

B Pipe axis

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7.With rotational flux leakage detection systems, distinctions between similar OD and ID defects are accomplished by: a. observing a difference in amplitude b. measurement of impedance change c. observing the signal to noise ratio d. frequency analysis e. only by visual observation of test piece L.54

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8. Using Figure FL-2, what is the vertical component of the leakage flux at the middle of the defect gap of a surface defect? a. zero b. maximum c. equal to horizontal component d. divergent e. convergent Y.91

Figure FL-2

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Vertical component of the leakage flux at the middle of the defect = 0

At maxima only horizontal component

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9. In circular magnetization, the recommended magnetizing current in amperes per linear inch of section thickness is: a. 200 to 400 A/in b. 400 to 600 A/in c. 600 to 800 A/in d. 800 to 1000 A/in e. 1000 to 1200 A/in G.30.13 circular magnetization → circumferential magnetization?

10. Flux leakage techniques can normally examine for: a. surface flaws only b. subsurface defects only c. defects at any location d. surface and near-surface defects G.30.1

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11. A relative motion between specimen and a ___ _ is needed in order to pick-up leakage flux. a. search coil b. hall probe c. magneto probe d. magneto resistor e. piezoelectric crystal 0.302 12. For a given crack with a given magnetic field, more magnetic lines of force are deflected out of the magnetic path when the: a. length of the crack is parallel to the magnetic lines of force b. length of the crack is perpendicular to the magnetic lines of force c. length of the crack is diagonal to the magnetic lines of force d. edges of the crack are polarized G.30.3-4

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13. The curve shown in Figure FL-3 is called a: a. hysteresis loop b. minor loop c. recoil curve d. magnetization curve e. permeability curve G.33.3

Figure FL-3

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14. A Hall effect probe measures: a. permeability b. conductivity c. tangential field strength d. field strength perpendicular to the probe surface e. reluctance G.33.11 15. The ratio BIH is equivalent to a material's: a. field strength b. reluctance c. permittivity d. permeability e. relative permeability G.33.15

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Permittivity In electromagnetism, absolute permittivity is the measure of the resistance that is encountered when forming an electric field in a medium. In other words, permittivity is a measure of how an electric field affects, and is affected by, a dielectric medium. The permittivity of a medium describes how much electric field (more correctly, flux) is 'generated' per unit charge in that medium. More electric flux exists in a medium with a low permittivity (per unit charge) because of polarization effects. Permittivity is directly related to electric susceptibility, which is a measure of how easily a dielectric polarizes in response to an electric field. Thus, permittivity relates to a material's ability to resist an electric field and "permit" is a misnomer.

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https://en.wikipedia.org/wiki/Permittivity/

16. Which discontinuity in Figure FL-4 would show the strongest indication? a. A b. B c. C d. D U.46 Figure FL-4 (for FLT the detection principle is interference with the magnetic fields not the eddy current induced)

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17 The optimum angle at which the magnetic field must intersect a discontinuity in a test specimen is: a. 90째 b. 60째 c. 45째 d. 30째 U.46 18. In the flux leakage examination of tubular products, which of the following discontinuities can be detected? a. longitudinally oriented b. transversely oriented c. slivers d. all of the above DD.627

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19. In the flux leakage testing of tubular products, the amplitude of the signal received from a discontinuity will depend upon: a. the depth of the discontinuity b. the orientation of the discontinuity c. the distance between the flux leakage sensor and the tube d. all of the above e. a and c DD.629 20. Flux leakage inspection can reveal the presence of: a. discontinuities b. voids c. inclusions d. either b or c e. a, b, and c C.l0-2

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Recalling the mistakes

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Level II Q&A-Flux Leakage Testing Method

O TW

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Level II - Standard Answers 1. D 2. d 3. c 4. b 5. a 6. c 7. b 8. d 9. a 10. d 11. a 12. c 13. a 14. d 15. b 16. c 17. a 18. c 19. b 20. c 21. d 22. c 23. d 24. e

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1. In flux leakage inspection of wire ropes using an encircling coil as a sensor, the response of the coil depends on what parameters of the wire break? a. cross-sectional area of broken wire b. location of broken wire within the cross-section c. gap between the ends of the broken wire d. all of the above BB.144-46 2. The highest sensitivity of a Hall generator is obtained when the direction of the magnetic field in relation to the largest surface of the Hall probe is: a. parallel b. at an angle of 46째 c. at an angle of 30째 or 60째 d. perpendicular e. none of the above G.33.11

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3. The best flaw detection sensitivity is obtained when the magnetizing flux is: a. parallel to the flaw's longest dimension b. perpendicular to the flaw's shortest dimension c. perpendicular to the flaw's longest dimension d. none of the above U.330 4. In flux leakage inspection for flaws, the part being inspected should be magnetized: a. beyond saturation b. to saturation or near saturation c. well below saturation d. using a combination of ac and de fields U.330

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5. An advantage that flux leakage testing has in comparison with eddy current testing is that flux leakage testing is: a. less sensitive to interferences due to surface roughness b. useful on products at temperatures above the Curie point c. useful on austenitic steels d. capable of much higher inspection rates N.122-123

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6. Using Figure FL-5, flux leakage strength decreases with distance (d) from the defect surface and is approximately proportional to: a. d b. 1/d c. 1/d2 d. 1d3 e. 1/d4

Figure FL-5

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7. Using Figure FL-6, the relation between the depth (D) of defects and signal amplitude (A) of leakage flux is approximately (K = constant of proportionality): a. A =k 1/D b. A =kD c. A =kD2 d. A =kD3 e. A =kD4 L.54 Amplitude � Depth, Amplitude � 1/W, Amplitude = kD/W Figure FL-6

W

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8. What particular type of defect is not indicated by flux leakage techniques? a. overlap b. grain boundary crack c. slag inclusion with crack d. surface contamination e. longitudinal seam AA.16 9. Electric current flowing in a magnet will alter the direction of magnetic fields in a magnet a. true b. false G.30.30-34

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Grain Boundary Crack

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Grain Boundary Crack

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10. The strength of the magnetic field in the interior of a long solenoid is determined by: a. the number of turns in the solenoid b. the strength of applied current c. the length of the solenoid d. all of the above e. none of the above (for eddy current interior of exciting coil was assumed constant) 0.307 11. In longitudinal magnetization by a coil, for most effective magnetization, the part to be magnetized should be: a. held against the inside wall of the coil b. positioned in the center of the coil c. held near the outside wall of the coil d. held half way between the wall and center of the coil G.30.14

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12. Due to the unique construction and great lengths of wire ropes, a practical and reliable NDT technique for testing is: a. acoustic emission b. ultrasonics c. magnetic techniques d. radiation e. none of the above EE.62 13. In dc magnetic testing of steel wire ropes, one type of magnetizing method is preferred over the other for the following reasons: a. permanent magnet: constant magnetic field intensity, no magnetizing current b. electromagnet: fluctuating magnetic field intensity, heavier, more rugged construction c. electromagnet: constant magnetic field intensity, lighter-weight construction d. permanent magnet: fluctuating magnetic field intensity, heavier, more rugged construction FF.4 Charlie Chong/ Fion Zhang

14. The number of broken wires at a particular location on a wire rope cannot always be determined because: a. the broken ends overlap b. a visual inspection for verification is impossible because the breaks are inside the rope c. the strip chart recorder speed was too slow d. the conditions stated above are all possibilities e. the strip chart recorder speed was too fast CC.ll, 16, 18, 19 15. As a general rule, hard materials have: a. high coercive force and are easily demagnetized b. high coercive force and are not easily demagnetized c. low coercive force and are easily demagnetized d. low coercive force and are not easily demagnetized e. none of the above G.30.22

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16. The point (P) shown on the hysteresis loop in Figure FL-7 is called the: a. coercive force b. initial permeability c. residual field (Remanence) d. leakage flux e. demagnetization point

Figure FL-7

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17. The field strength over a crack is directly proportional to the relative permeability of the steel and the ratio: a. crack depth/crack wtdth b. crack width/crack depth c. crack length/crack depth d. crack length/crack wtdth X.194 B � D/W ?

D W

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18. A calibration standard is prepared using holes. With respect to the reference size, it is considered good practice to: a. include holes larger in diameter than the reference size b. include holes smaller in diameter than the reference size c. include holes both larger and smaller in diameter than the reference size d. include holes of the reference size only DD.630 19. When testing tubes, if the sensor bounces along the surface this may: a. make it difficult to estimate fault severity b. cause false signals c. decrease speed of inspection d. distort the magnetizing system DD.628

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20. In flux leakage testing, the greatest tube wall thickness for which maximum sensitivity can be maintained in mm (in.) is: a. 0.08 mm (0.003 in.) b. 0.8 mm (0.03 in.) c. 8 mm (0.3 in.) d. 76 mm (3 in.)

U.330

21. The following are used in determining magnetic flux leakage: a. coil b. magnetic tape c. Hall element d. all of the above e. a and b 0.316-317

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22. In the examination of tubular products, a circumferential (transverse) magnetic field can be established by: a. properly positioning north and south poles of a magnet with respect to the tube b. using a central conductor positioned in the tube c. a and b d. neither a nor b DD.629

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23. In the examination of tubular products where the flux sensor measures the leakage field at the outside surface of the tube: a. OD discontinuities are detected b. both OD and ID discontinuities may be detected c. both OD and ID discontinuities can be detected but generally cannot be distinguished from each other d. both OD and ID discontinuities can be detected and can generally be distinguished from each other DD.625

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24. Calibration standards used in the flux leakage examination of tubular products should be carefully prepared since the flux leakage signal response from the notch will be affected by: a. notch orientation b. notch width c. notch length d. notch depth e. all of the above DD.630

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Recalling the mistakes

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Level III Q&A-Flux Leakage Testing Method

e e r Th

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Level III - Standard Answers 1. B 2. d 3. d 4. a 5. c 6. b 7. c 8. b 9. b 10. c 11. a 12. c 13. e 14. a 15.

2. b 16. c 17. e 18. b 19. e 20. d 21. b

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1. In flux leakage testing of wire rope, the system of two different diameter search coils is frequently used. What is the main reason for using such a system? (See Figure FL-8.) a. to compensate the influence of testing speed b. to find the radial location of wire breaks c. to detect corrosion d. to detect small cracks inside the rope e. to detect gradual decreasing of rope diameter GG.378 Figure FL-8

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2. In flux leakage testing of wire rope, two separate outer broken wires will produce two separate indications if there is sufficient distance between the breaks. What is the maximum rope speed in feet per minute that would produce two separate indications when the cutoff frequency of the recorder is 60kHz and the breaks are 25 mm (1 in.) apart? (See Figure FL-9.) a. 60 fpm b. 120 fpm c. 240 fpm d. 300 fpm 300x12/60= 60 in/s, 60Hz will detect the indication? e. 360 fpm for 60kHz the speed = 300 x 103 fpm?

GG.378 Figure FL-9

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3. Which devices are used to detect flux leakage? a. coils, Hall probes, and transistors b. piezoelectric crystals, Hall probes, and magnetic diodes c. piezoelectric crystals, transistors, and magnetic diodes d. coils, Hall probes, and magnetic diodes N.123

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4. In Figure FL-10, the flux leakage at slot A will be____ than that at slot B. a. greater b. Smaller A � D/W c. broader d. less readily detected z.so FL-10

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5. In a properly operating flux leakage system, pipe defects occurring at increased depths from the surface will generate signals with: a. increased phase differences b. higher frequency characteristics c. lower frequency characteristics d. increased signal noise ratios L.54 6. Lift-off reduces the amplitude of the flux leakage signal. The other significant effect it has on the signal is a: a. change in phase (?) b. change in frequency c. change in signal noise ratio d. all of the above P.31A

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3.9 COUPLING In eddy current testing, the test piece is coupled to the test coil by the coil’s magnetic field, which is quite similar to transformer action. Coupling efficiency is intimately related to lift-off; coupling efficiency is 100% when lift-off is zero.

Lift-off describes the change in electromagnetic coupling as a function of probe clearance. As lift-off or probe clearance increases from the test surface, coupling efficiency and eddy current probe output decreases.

Lift-off changes both the (1) amplitude and (2) phase of the eddy current signal.

Impedance changes produced by small lift-off variations are greatest when the coil is in contact with the test material. For this reason, spring-loaded probes and self comparison coil or differential coil arrangements are frequently used. With eddy current testing, lift-off is a complex variable that can be detected and compensated for through frequency selection to achieve a desirable operating point on the complex impedance plane

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Introduction Nondestructive Testing-PAUL E. MIX

7. Eddy current shielding, the name given to the unidirectional eddy current flow in products inspected by the flux leakage method, is caused by: a. the interaction between the test magnetic field and a residual field in the product b. fluctuations in the de magnetizing current c. rapidly occurring flux changes in the product created by the rotation of the magnetic field d. rapidly occurring impedance changes in the pickup coils AA.16 12. Too high a rotational test speed or too high active pole rotating head speed can cause the loss of an indication from an ID defect. What can this be attributed to? a. excessive generated surface noise b. limitations of the flux sensor elements c. eddy current shielding d. reverse magnetization effect e. none of the above AA.16 Charlie Chong/ Fion Zhang

8. A flux leakage pipe inspection system with two inspection heads, each having 152 mm (6 in.) long scan paths and rotating at 180 rpm on a 178 mm (7 in.) diameter tube, can have a maximum throughput speed of ______per minute for 100% inspection coverage. a. 201 m (660ft) b. 55 m (180ft) c. 49 m (162ft) d. 27 m (90 ft) e. 25 m (81 ft) U.330,338

6 in.

7 in.

180rpm x 6� = 1080 in/min, 2 head thus coverage = (1080 x2)/12 ft/min = 180’/min.

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9. A flux leakage pipe inspection system with two inspection heads, each having 152 mm (6 in.) long inspection areas and rotating at 180 rpm on a 178 mm (7 in.) diameter pipe, would require a throughput speed of _____per minute to provide a 110% inspection coverage. a. 60 m (198ft) b. 49 m (162ft) c. 45 m (146ft) d. 25 m (81 ft) e. 22 m (73 ft) P.22A 10. The tubular product parameter having the greatest influence on the flux density of the magnetic field in the part (assuming the magnetizing force (H) and the material's permeability Îź remains constant) is the: a. chemistry of the product b. diameter of the product c. wall thickness of the product d. length of the product U.330 Charlie Chong/ Fion Zhang

11. A defect having an inclined angle to the surface has a flux leakage that is (see Figure FL-11): a. lower than a similar normal defect (B∙cos θ?) b. equal to a similar normal defect c. higher than a similar normal defect d. all of the above Y.91

Figure FL-11

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12. Too high a rotational test speed or too high active pole rotating head speed can cause the loss of an indication from an ID defect. What can this be attributed to? a. excessive generated surface noise b. limitations of the flux sensor elements c. eddy current shielding d. reverse magnetization effect e. none of the above AA.16

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13. A current carrying conductor is surrounded by a tube (see Figure FL-12).

Magnetic fields exist in the tube, if the tube is made of: a. steel b. copper c. aluminum d. lead e. all of the above G.30.6 Figure FL12

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14. A tube is magnetized by passing a uniform current through the tube.

There are inside and outside defects in the tube (see Figure FL-13). The two defects have the same dimension and geometry. The defect signal strength: a. is stronger for the outside defect than for the inside flaw b. is stronger for the inside defect than for the outside flaw c. is the same for both defects d. is an uncertainty e. depends on the permeability of the tube G.30.6

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14. A tube is magnetized by passing a uniform current through the tube. There are inside and outside defects in the tube (see Figure FL-13). The two defects have the same dimension and geometry. The defect signal strength: a. is stronger for the outside defect than for the inside flaw b. is stronger for the inside defect than for the outside flaw c. is the same for both defects d. is an uncertainty e. depends on the permeability of the tube G.30.6

B= 0 at internal space

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https://www.nde-ed.org/EducationResources/CommunityCollege/MagParticle/Physics/CircularFields.htm

Circular Magnetic Fields Distribution and Intensity

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https://www.nde-ed.org/EducationResources/CommunityCollege/MagParticle/Physics/CircularFields.htm

Circular Magnetic Fields Distribution and Intensity

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https://www.nde-ed.org/EducationResources/CommunityCollege/MagParticle/Physics/CircularFields.htm

Circular Magnetic Fields Distribution and Intensity

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https://www.nde-ed.org/EducationResources/CommunityCollege/MagParticle/Physics/CircularFields.htm

15. A discontinuity in a steel wire rope may be detected with a dc magnetic instrument if: a. the rope is magnetized to saturation, the discontinuity is polarized, the speed of the rope is great enough to induce a detectable voltage across the discontinuity b. the area surrounding the discontinuity is magnetized to saturation, the discontinuity is polarized, the speed of the rope is great enough to induce a detectable voltage across the discontinuity c. the rope is magnetized to saturation, the speed of the rope is great enough to induce a detectable voltage across the discontinuity d. the rope is magnetized to saturation, the discontinuity is polarized CC.10

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MFL of Wire Rope Principle  the area surrounding the discontinuity is magnetized to saturation,  the discontinuity is polarized,  the speed of the rope is great enough to induce a detectable voltage across the discontinuity

MFL of Wire Rope Principle

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16. What is the SI unit for magnetic flux density B ? a. Weber ( magnetic flux, Ф) b. Gauss (B in CGS) c. Tesla (B in SI) (1 teslas = 10000 Gauss) http://www.magnii.com/magnetic-units-converter.html d. none of the above e. all of the above W.71 B is measured in teslas (symbol:T) and newtons per meter per ampere (symbol: N·m−1·A−1 or N/(m·A)) in the SI. https://en.wikipedia.org/wiki/Magnetic_field 17. What is the SI unit for magnetic field strength? a. Weber b. Tesla c. Ampere d. Henry e. Ampere per meter W.71

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A magnetic field is the magnetic effect of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude (or strength); as such it is a vector field. The term is used for two distinct but closely related fields denoted by the symbols B and H, where ■ H is measured in units of amperes per meter (symbol: A·m-1 or A/m) in the SI. ■ B is measured in teslas (symbol:T) and newtons per meter per ampere [symbol: N·m-1·A-1 or N/(m·A)] in the SI. (1 teslas = 10000 Gauss) B is most commonly defined in terms of the Lorentz force it exerts on moving electric charges. Magnetic fields can be produced by moving electric charges and the intrinsic magnetic moments of elementary particles associated with a fundamental quantum property, their spin. In special relativity, electric and magnetic fields are two interrelated aspects of a single object, called the electromagnetic tensor; the split of this tensor into electric and magnetic fields depends on the relative velocity of the observer and charge. In quantum physics, the electromagnetic field is quantized and electromagnetic interactions result from the exchange of photons.

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https://en.wikipedia.org/wiki/Magnetic_field

Weber (Magnetic Flux Ф)

In physics, specifically electromagnetism, the magnetic flux (often denoted Φ or ΦB) through a surface is the surface integral of the normal component of the magnetic field B passing through that surface.

■ The SI unit of magnetic flux is the weber (Wb) (in derived units: voltseconds), and the CGS unit is the maxwell.

Magnetic flux is usually measured with a fluxmeter, which contains measuring coils and electronics, that evaluates the change of voltage in the measuring coils to calculate the magnetic flux.

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https://en.wikipedia.org/wiki/Magnetic_flux

Gauss (Magnetic Flux Density) gauss, unit of magnetic induction in the centimetre-gram-second CGS system of physical units. One gauss corresponds to the magnetic flux density that will induce an electromotive force of one abvolt (10-8 volt) in each linear centimetre of a wire moving laterally at one centimetre per second at right angles to a magnetic flux. One gauss corresponds to 10-4 tesla (T), the International System Unit. The gauss is equal to 1 maxwell per square centimetre, or 10-4 weber per square metre. Magnets are rated in gauss. The gauss was named for the German scientist Carl Friedrich Gauss.

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http://global.britannica.com/science/gauss

Electrical Inductance - Henry The henry (symbol H) is the unit of electrical inductance in the International System of Units.The unit is named after Joseph Henry (1797–1878), the American scientist who discovered electromagnetic induction independently of and at about the same time as Michael Faraday (1791–1867) in England.

The magnetic permeability of a vacuum μo is 4π×10-7 H m-1 (henries per metre).

The National Institute of Standards and Technology provides guidance for American users of SI to write the plural as "henries". The inductance of an electric circuit is one henry when an electric current that is changing at one ampere per second results in an electromotive force across the inductor of one volt: v(t) = L di/dt where v(t) denotes the resulting voltage across the circuit, i(t) is the current through the circuit, and L is the inductance of the circuit.

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https://en.wikipedia.org/wiki/Henry_(unit)

Magnetic Permeability (B/H) In electromagnetism, permeability is the measure of the ability of a material to support the formation of a magnetic field within itself. Hence, it is the degree of magnetization that a material obtains in response to an applied magnetic field. Magnetic permeability is typically represented by the Greek letter μ. The term was coined in September 1885 by Oliver Heaviside. The reciprocal of magnetic permeability is magnetic reluctivity.

In SI units, permeability is measured in henries per meter (H·m-1), or newtons per ampere squared (N·A-2). The permeability constant (μ0), also known as the magnetic constant or the permeability of free space, is a measure of the amount of resistance encountered when forming a magnetic field in a classical vacuum. The magnetic constant has the exact (defined) value µ0 = 4π×10-7 H·m-1≈ 1.2566370614…×10−6 H·m-1 or N·A-2).

A closely related property of materials is magnetic susceptibility, which is a dimensionless proportionality factor that indicates the degree of magnetization of a material in response to an applied magnetic field.

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https://en.wikipedia.org/wiki/Permeability_(electromagnetism)

18. What is the SI unit for magnetic permeability? a. Ampere per meter b. Henry per meter c. Henry d. Gauss per meter e. Tesla Îź = B/H

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19. A ferromagnetic part can be demagnetized by: a. raising its temperature above the curie point b. withdrawing the part from an ac coil c. alternately reversing and reducing the applied field d. none of the above e. all of the above G.30.22-23

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More Reading: Magnetic Flux & Gauss law http://info.ee.surrey.ac.uk/Teaching/Courses/EFT/dynamics/html/magnetic_flux.html

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20. Figure FL-14 shows an arrangement of probe-coils in a Forester probe used for measuring: a. the static field component b. the normal component of the tangential field c. the tangential component of the normal field d. the field gradient e. all of the above G.33.1 Figure FL-14

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21. In flux leakage testing the maximum permissible speed of probe or tube movement with respect to the other is: a. 914 m/min (3 000 ft/min) b. 91 m/min (300 ft/min) c. 9 m/min (30 ft/min) d. 0.9 m/min (3 ft/min) U.338

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Recalling the mistakes

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making Charlie Chong/ Fion Zhang

■ωσμ∙Ωπ∆º≠δ≤>ηθφФρ|β≠Ɛ∠ ʋ λ α ρτ√ ≠≥ѵФ

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Charlie Chong/ Fion Zhang

Good Luck

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Good Luck

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https://www.yumpu.com/en/browse/user/charliechong Charlie Chong/ Fion Zhang