Understanding infrared thermography reading 6 chapter 3

NDEQC-V16+1 Thermal Inspection

Charlie Chong/ Fion Zhang

1. Introduction THERMAL INSPECTION comprises all methods in which heat-sensing evices are used to measure temperature variations in components, structures, systems, or physical processes. Thermal methods can be useful in the detection of subsurface flaws or voids, provided the depth of the flaw is not large compared to its diameter. Thermal inspection becomes less effective in the detection of subsurface flaws as the thickness of an object increases, because the possible depth of the defects increases. Thermal inspection is applicable to complex shapes or assemblies of similar or dissimilar materials and can be used in the one-sided inspection of objects. Moreover, because of the availability of infrared sensing systems, thermal inspection can also provide rapid, noncontact scanning of surfaces, components, or assemblies.

Thermal inspection does not include those methods that use thermal excitation of a test object and a nonthermal sensing device for inspection. For example, thermally induced strain in holography or the technique of thermal excitation with ultrasonic or acoustic methods does not constitute thermal inspection.

Charlie Chong/ Fion Zhang

2. Principles of Thermal inspection The basic principle of thermal inspection involves the measurement or mapping of surface temperatures when heat flows (1) from, (2) to, or (3) through a test object.

Temperature differentials on a surface, or changes in surface temperature with time, are related to heat flow patterns and can be used to detect flaws or to determine the heat transfer characteristics of a test body. For example, during the operation of a heating system, a hot spot detected at a joint in a heating duct may be caused by a hot air leak. Another example would be a hot spot generated when an adhesive-bonded panel is uniformly heated on one side. A localized debonding between the surface being heated and the substructure would hinder heat flow to the substructure and thus cause a region of higher temperature when compared to the rest of the surface.

Generally, the larger the imperfection and the closer it is to the surface, the greater the temperature differential.

Charlie Chong/ Fion Zhang

Heat Transfer Mechanisms. Heat will flow from hot to cold within an object by conduction and between an object and its surroundings by (1) conduction, (2) convection, and (3) radiation.

Within a solid or liquid, conduction results from the random vibrations of individual atoms or molecules that is transferred via the atomic bonding to neighboring atoms or molecules.

In a gas, the same process occurs but is somewhat impeded by the greater distance between the atoms or molecules and the lack of bonds, thus requiring collisions to transfer the energy. When a gas or liquid flows over a solid, heat is transferred by convection. This occurs from the collisions between the atoms or molecules of the gas or liquid with the surface (conduction) as well as the transport of the gas or liquid to and from the surface. Convection depends upon the velocity of the gas or liquid, and cooling by convection increases as the velocity of the gas or liquid increases.

Charlie Chong/ Fion Zhang

Radiation is the remaining mechanism for heat transfer. Although conduction and convection are generally the primary heat transfer mechanisms in a test object, the nature of thermally induced radiation can be important, particularly when temperature measurements are made with radiation sensors.

Electromagnetic radiation is emitted from a heated body when electrons within the body change to a lower energy state. Both the intensity and the wavelength of the radiation depend on the temperature of the surface atoms or molecules. For a blackbody, the radiation wavelength and spectral emission power vary as a function of temperature, as shown in Fig. 1. At 300 K (27 °C), the temperature of a warm day, the dominant wavelength is 10 μm (400 μin.), which is in the infrared region (Fig. 2). A surface would have to be much hotter for the dominant wavelength to fall in the visible region below 0.7 μm (30 μin.). Examples of this are red-hot steel in a forge, a light bulb filament, and the sun (indicated as solar radiation in Fig. 1 at a temperature of 5800 K). However, most subjects in thermal inspection methods will be at temperatures near room temperature and will emit in the infrared region.

Charlie Chong/ Fion Zhang

Fig. 1 Spectral blackbody emissive power

Charlie Chong/ Fion Zhang

Fig. 2 Spectrum of electromagnetic radiation

Charlie Chong/ Fion Zhang

Material Heat Transfer Characteristics. The heat transfer mechanisms of conduction, convection, and radiation are affected by material heat transfer characteristics, and thermal inspection depends on local variations in the material heat transfer characteristics. The material characteristics that affect conduction and convection are as follows:  Specific heat, c, is the amount of heat a mass of material will absorb for a given temperature interval  Density, ρ, is the mass per unit volume of the material  Thermal conductivity, k, is the amount of heat that flows in a given direction when there is a temperature difference across the material in that direction  Thermal diffusivity, α, is the speed at which the heat flows away from a region of higher temperature to the surrounding material  Convection heat transfer coefficient, h, is a measure of how efficiently heat is exchanged between a surface and a flowing gas or liquid  Temperature, T, is a measure of the heat energy (local thermal agitation) contained at each point in the test object

Charlie Chong/ Fion Zhang

Thermal inspection depends on differences in these material characteristics to establish a measurable, and usually localized, temperature differential. For example, when a test body with variations in density and specific heat is heated or cooled from a state of uniform temperature, the change in temperature will occur more slowly in the regions with a higher density and/or specific heat. This difference in the rate of change of temperature within the body produces temperature differentials, which may be measurable. The important material characteristic in radiation heat transfer is the emissivity, Îľ, of a test surface. The emissivity indicates the efficiency of a surface as a radiator (or absorber) of electromagnetic radiation.

Blackbodies, the most efficient radiators and absorbers of electromagnetic radiation, have an emissivity of 1.0. All other bodies have an emissivity less than 1.0. Emissivity is a function of several variables, including color and surface roughness. Like other material heat transfer characteristics, variations in emissivity are important in thermal inspection. This is particularly true when surface temperatures are measured with infrared sensors. Variations in emissivity change the power of radiation emitted at a given temperature and thus affect infrared temperature measurements.

Charlie Chong/ Fion Zhang

3. Surface Preparation The surface condition of the test object is important for thermal inspection.

Inspection results can be influenced by variations in surface roughness, cleanliness, foreign material (such as decals), and the uniformity and condition of paint or other surface coatings. A good practice is to clean the surface, remove or strip poorly adhering coatings (if present), and then apply a uniform coating of readily removable flat-black paint. This will allow uniform heat transfer into (or from) the subject and will also produce a reasonably uniform emissivity Îľ.

Charlie Chong/ Fion Zhang

4. Establishing Heat Flow In thermal inspection, the test object can be classified as either thermally active or thermally passive. Thermally active test objects generate the necessary heat flow during their operation, while thermally passive test objects require an external heat source or heat sink.

Thermally Active Test Objects. Some test objects can be inspected without the application or removal of heat because they are involved in a process that either generates or removes heat. When a defect results in an abnormal temperature distribution on the surface, no external heating or cooling is required. When the heat transfer process is transient, the timing of the inspection is important. An example of this would be a fluid-contaminated honeycomb panel on an aircraft that has just landed after a long flight at high altitudes. Although the entire aircraft would be warming up from the cooler temperatures experienced at high altitude, the contaminated regions would not warm as rapidly as the uncontaminated areas and therefore could be detected as cool spots in the structure if the inspection were performed immediately after landing.

Charlie Chong/ Fion Zhang

However, if the inspection were delayed, the entire structure would reach an equilibrium temperature, and the contaminated regions would no longer be detectable. When the heat transfer process is in a steady-state condition, timing no longer becomes critical. An example is an electronic circuit board.

Defective electronic components, in which the defect changes the electrical resistance of the component, will be either hotter or cooler than the same component properly operating on another circuit board. Another example would be a blocked tube in a heat exchanger. Temperatures along the tube would be different from temperatures along adjacent, unblocked tubes.

Charlie Chong/ Fion Zhang

Electronic Circuit Board Thermogram

Charlie Chong/ Fion Zhang

Thermal Excitation of Passive Test Objects. Thermally passive test objects require an external heat source or heat sink to establish the flow of heat to or from the test object. Generally, infrared or thermal measurement techniques become more sensitive as the average temperature of the subject increases.

Consequently, the most common form of excitation is heating. However, in cases where additional heating could cause damage, cooling is used to create the required heat flow.

Key points: Generally, infrared or thermal measurement techniques become more sensitive as the average temperature of the subject increases. Consequently, the most common form of excitation is heating.

Charlie Chong/ Fion Zhang

Precautions. One primary concern in heating or cooling the test object is that the thermal changes must not be intrusive. The rate of heating or cooling must be below the point of producing damaging thermal stresses. For example, a chilled glass placed in hot water may crack because the exterior is expanding rapidly while the interior is still cool and because the induced mechanical stress is sufficient to cause fracture. The degree of heating of metals must be controlled so as not to affect the heat treatment, and excessive heat can degrade the material properties of adhesives and matrices in reinforced resin composites.

Charlie Chong/ Fion Zhang

Point heating can be generated with either a laser or a spherically focused infrared or visible light source to apply heat to a very localized area. For a very thin, homogeneous subject, the heat will flow outward in a circle, and the temperature change will be inversely proportional to the square of the distance from the center of the circle or the irradiated point. For a very thick, homogeneous subject, the heat can also flow in the thickness direction; consequently, the temperature change will be inversely proportional to the cube of the distance to the point of irradiation. Most subjects are somewhere between these two extremes. Because of the rapid change in temperature across the subject, the use of point heating requires rapid-response, continuous time-based measurements, which may be enhanced with pulsed heating techniques (Ref 1, 2, 3).

Charlie Chong/ Fion Zhang

Line heating is similar to point heating and is usually accomplished with a linear heat source such as a quartz-tube lamp or other heat source with a reflector that concentrates the heat into a line on the surface of the test object.

For a thin panel, the heat will flow away from the line on both sides, and the temperature change will be inversely proportional to the distance from the line.

For a very thick, homogeneous subject, the temperature change will be inversely proportional to the square of the distance to the line of irradiation.

Because the temperature changes rapidly with distance, line heating should be monitored with rapid-response, continuously monitored, time-based equipment (tracking the time to reach a given temperature by scanning the detector outward from the line source). Another technique involves scanning the linear heat source across the surface.

Charlie Chong/ Fion Zhang

Area heating typically involves uniform heating on one surface of the test object. A bank of quartz heaters, a hot plate with a high thermal conductivity material such as copper as a face sheet, or other similar devices can be used as area heaters. If heat is applied to the same side of the object that is subsequently monitored for temperature differences, the heat source is usually switched off to prevent interference with the detector from the source.

If the opposite surface is heated, the heat source usually will not interfere with the temperature sensor and can be left on to provide a steady-state condition.

The heat source can also be switched off to produce a transient measurement condition.

Vibration-Induced Heating. Exciting a specimen on a shaker table or with a high-power speaker will cause the specimen to respond with its natural modes of vibration. These vibrations will induce localized areas of stress and strain in the object. An anomalous area will respond differently to these induced stresses and, as a result, may be hotter or cooler than the surrounding areas if the anomaly is not at a vibration node (Ref 4, 5, 6).

Charlie Chong/ Fion Zhang

Mechanically Induced Heating. Mechanically loading the specimen will cause heating in those areas that are plastically deformed. Because deformation of the specimen is normally destructive (except in those cases where deformation is part of the intended processing), mechanically induced heating is probably best suited to research studies. Examples are investigations of fatigue and crack growth, analysis of the response of a component to loading, and optimization of deformation processes such as forging (Ref 7, 8, 9).

Electrically Induced Heating. Passing current through a specimen, or inductive heating, can be applied only to materials that are electrically conductive. The heat produced is a product of the square of the electrical current and the resistance. If the anomaly of interest locally changes the electrical resistance, the local current flow and the local temperature will also change. One shortcoming of this method is that electrically conductive materials often have good thermal conductive properties, thus making the local changes in temperature short in duration (Ref 10).

Charlie Chong/ Fion Zhang

Area cooling can be used when heating may be damaging or impractical.

Partial immersion of a test object in a cold fluid such as ice water or liquid nitrogen may provide sufficient heat flow to reveal anomalous areas when monitoring the surface temperature as the specimen cools or returns to ambient. As with area heating, a steady-state temperature profile obtained by continued cooling of one surface of the specimen may be useful. For example, filling a cooler with ice and water would allow the exterior detection of cold spots caused by faults in the insulation or a defective lid seal. Total immersion of the test object in a cold fluid can also be considered a cooling method. In this case, the transient surface temperatures are monitored while the test object returns to ambient temperature.

Cold-thermal-wave excitation involves preheating the test object, followed by cooling with an air jet (see Example 2). The cold-thermal-wave approach is attractive for the thermal testing of metallic structures for a number of reasons.

It is an inexpensive technique for fast thermal stimulation of large areas; preheating can be performed with low-rate elements such as hot-wire heaters, although in several on-line applications the part may already be above ambient temperature; and finally, the use of a cold thermal source eliminates self- mission noise problems.

Charlie Chong/ Fion Zhang

5. Thermal Inspection Equipment The temperature sensors used in thermal inspection can be separated into two categories: noncontact temperature sensors and contact temperature sensors. Other equipment includes recording instruments and calibration sources.

■ Noncontact Temperature Sensors Noncontact temperature sensors depend on the thermally generated electromagnetic radiation from the surface of the test object. At moderate temperatures, this energy is predominately in the infrared region. Therefore, noncontact measurements in thermal inspection primarily involve the use of infrared sensors.

Infrared imaging equipment is available with a wide range of capabilities.

The simplest systems are responsive to the near-infrared portion of the optical spectrum. These include night-vision devices and vidicon systems with silicon or lead sulfide sensors (Ref 11). Silicon sensors provide sensitivity for temperatures above 425 °C (800 °F), while lead sulfide sensors respond to temperatures above 200 °C (400 °F).

Charlie Chong/ Fion Zhang

Spectral Power Distribution

Charlie Chong/ Fion Zhang

http://ips-innovations.com/low_emissive_wall_coatings_ref.htm

Spectral Power Distribution

Charlie Chong/ Fion Zhang

http://hyperphysics.phy-astr.gsu.edu/hbase/mod6.html#c1

Charlie Chong/ Fion Zhang http://hyperphysics.phy-astr.gsu.edu/hbase/mod6.html#c1

Blackbody Spectral Power Distribution

Hand-held scanners are portable imaging systems capable of responding in the far-infrared portion of the optical spectrum (wavelengths of 8 to 12 μm).

This range is emitted by objects at or near room temperature. In general, handheld scanners have poor imaging qualities and are not suitable for the accurate measurement of local temperature differences. However, they can be useful for detecting hot spots, such as overheated components, thermal runaway in an electronic circuit, or unextinguished fires (Ref 11).

High- resolution infrared imaging systems are required for most part inspection applications. These systems use either pyroelectric vidicon cameras with image-processing circuitry or cryogenically cooled mechanical scanners to provide good-quality image resolution (150 pixels, or picture elements, per scan line) (Ref 11) and temperature sensitivity to 0.1 °C (0.2 °F) (Ref 12). One system is claimed to have a temperature resolution of 0.001 °C (0.002 °F) (Ref 12). In addition to good image resolution and temperature sensitivity, response times of the order of 0.1 s or less facilitate the detection of transient temperature changes or differentials. These imaging systems will use either a gray scale or a color scale correlated to temperature ranges to depict the temperature distribution within the image.

Charlie Chong/ Fion Zhang

Thermal wave interferometer systems combine modulated laser excitation with rapid phase and amplitude sensing that can be scanned across a surface to produce an image (Ref 1, 12). One application for this type of system is the inspection of plasma-sprayed coatings. The system senses the interaction between the thermal waves of the laser and the thermal variations from coating defects and thickness variations.

Radiometers and pyrometers are devices for measuring radiation, or spot or line temperatures, respectively, without the spatial resolution needed for an imaging system. Radiometers, because they usually have slow response times, are most useful for monitoring constant or slowly varying temperatures.

Pyrometers are used as noncontacting thermometers for temperatures from 0 to 3000 째C (32 to 5400 째F). Newer instruments can superimpose a line trace of the temperature on the visible-light image of the surface or scene being viewed (Ref 13). Radiometers and pyrometers are usually rugged, low-cost devices that can be used in an industrial environment for the long-term monitoring of processes.

Charlie Chong/ Fion Zhang

â–

Contact Temperature Sensors

Contact temperature sensors include material coatings and thermoelectric devices. Material coatings are relatively low in cost and simple to apply, but they may have the disadvantage of providing qualitative temperature measurements (the exception is coatings with liquid crystals, which can be calibrated to show relatively small changes in temperature). Another disadvantage of coatings is that they may change the thermal characteristics of the surface.

Charlie Chong/ Fion Zhang

Cholesteric liquid crystals are greaselike substances that can be blended to produce compounds having color transition ranges at temperatures from - 20 to 250 째C (- 5 to 480 째F) (Ref 13). Liquid crystals can be selected to respond in a temperature range for a particular test and can have a color response for temperature differentials of 1 to 50 째C (2 to 90 째F) (Ref 13).

When illuminated with white light while in their color response range, liquid crystals will scatter the light into its component colors, producing an iridescent color that changes with the angle at which the crystals are viewed. Outside this color response range, liquid crystals are generally colorless. The response time for the color change varies from 30 to 100 ms (Ref 13). This is more than adequate to allow liquid crystals to show transient changes in temperature (Ref 14). The spatial resolution obtainable can be as small as 0.02 mm (0.0008 in.) (Ref 14). In addition, because the color change is generally reversible, anomalies can be evaluated by repeating the test as many times as needed.

Charlie Chong/ Fion Zhang

Techniques for applying liquid crystals are relatively straightforward once the proper blend of compounds is selected. Because liquid crystals function by reflecting light, they are more readily seen when used against a dark background. Therefore, if the specimen is not already dark, covering the surface with a removable, flat-black coating is strongly recommended before application. The crystals can then be applied by pouring, painting, spraying, or dipping. Care must be taken that the specimen or the coating is not attacked by the solvent base used with the liquid crystals. The applied film of liquid crystals must be of uniform thickness to prevent color irregularities caused by thickness differences rather than temperature differences. A good film thickness is about 0.02 mm (0.0008 in.) (Ref 13). Successive layers used to build up the film thickness should not be allowed to dry between coats. A coating of proper thickness will have a uniform, low-gloss appearance when viewed with oblique illumination.

Charlie Chong/ Fion Zhang

Thermally quenched phosphors are organic compounds that emit visible light when excited by ultraviolet light. The brightness of a phosphor is inversely proportional with temperature over a range from room temperature to about 400 째C (750 째F), as indicated in Fig. 3. Some phosphors exhibit a change in brightness of as much as 25% / 째C (14% / 째F). An individual phosphor should be selected to cover the temperature range used for a particular inspection. The coating is applied by painting a well-agitated mixture of the phosphor onto the surface to a thickness of about 0.12 mm (0.0047 in.).

Charlie Chong/ Fion Zhang

Fig. 3 Relative brightness of four thermally quenched phosphors (U.S. Radium Radelin phosphor numbers) as a function of temperature

Brightness decrease with raising temperature?

Charlie Chong/ Fion Zhang

Other Coatings. Heat-sensitive paints, thermochromic compounds, heatsensitive papers, and meltable frosts and waxlike substances can also be used to indicate surface temperatures. These coatings are useful for determining when a surface has exceeded a certain temperature. A few of the coatings, such as the photochromic paints and thermochromic compounds, have reversible changes that can be used to evaluate indications by retesting.

Each of the coatings can be applied directly to the surface. After some experimentation, the coatings could be used for specialized thermal inspections.

Charlie Chong/ Fion Zhang

Thermoelectric devices are widely used for measuring temperature. Typical thermoelectric devices are thermocouples, thermopiles, and thermistors.

Charlie Chong/ Fion Zhang

Thermocouples consist of a pair of junctions of two different metals. As the temperature of one of the junctions is raised, an electromotive force (voltage) relative to the other (reference) junction is produced that is proportional to the temperature difference between the two junctions. Thermocouples are usually used in a bridge circuit, with the reference junction maintained at a known and stable temperature. Thermocouples can be placed in contact with the surface of the subject or can be used near the surface to measure the air temperature.

Thermopiles are multiple thermocouples used electrically in series to increase the output voltage. Although thermopiles have a greater output (resulting in greater sensitivity) than individual thermocouples, they also have a slower response time because of the increased mass. Thermopiles are used as a sensing element in radiometers.

Thermistors are electrical semiconductors that use changes in electrical resistance to measure temperature. Thermistors are usually used in a bridge circuit, with one of the thermistors maintained at a known and stable temperature.

Charlie Chong/ Fion Zhang

â–

Recording Equipment

When high-resolution imaging systems are used, recording equipment, such as a videotape recorder, is extremely useful for analyzing transient effects or for reviewing techniques. Videotape recordings showing the time-varying response of a location on the specimen can be used to estimate the size and depth of the anomalies (Ref 2, 5, 15). Such recorded data can be processed with digital data-processing techniques to enhance the detection of temperature differences and to suppress spurious noise signals (Ref 4) (see the section "Digital Image Enhancement" in this article). Photographic techniques can also be used to record the thermal images of specimens.

When coatings are used, the recording equipment usually consists of color photographs of liquid crystals, heat-sensitive paints, thermochromic materials, and wax sticks. Black-and-white photographs are adequate for most heatsensitive papers, melting paint materials, and thermally quenched phosphors.

For time-based measurements with liquid crystals, a video recorder can provide an excellent recording of color changes as the test progresses. The recording equipment for thermoelectric devices is usually time-based chart recorders or digital recorders.

Charlie Chong/ Fion Zhang

â– Temperature Calibration Sources Temperature calibration sources are needed for all devices used to measure temperatures. Some systems have built-in or internal calibration sources.

External sources can vary from very simple devices, such as a container of ice and water or boiling water, to thermocouple-controlled heated plates that can be adjusted to the desired temperature.

Charlie Chong/ Fion Zhang

6.

Inspection Methods

Steady-state methods are used to detect anomalies where temperatures change very little with time.

Many of the thermally active objects or processes can be observed under steady-state conditions. For example, a steam line that regularly carries steam can be inspected for insulation defects by looking for hot spots. These types of anomalies usually produce large differences in temperature, and the resultant images or contact coating indications are easily interpreted.

Steady-state methods are more challenging with the thermal excitation of passive objects. Uniform heating and cooling are essential, and the heating and cooling rates must be adjusted to allow the temperature differences caused by the defects to be maintained.

Charlie Chong/ Fion Zhang

A honeycomb panel can be inspected for liquid intrusion or disbonds by heating one side and cooling the opposite side. The cooled side is viewed for the inspection, and either infrared imaging or contact coatings can be used.

Areas of liquid intrusion will produce warmer temperatures than the surroundings, while disbonds will produce cooler temperatures. In general, anomalies must be large, must be close to the surface, or must create large temperature differences to be detectable with steady-state methods. With active heating or cooling, steady-state measurements will be more effective when made on the surface where the heat transfer is by convection.

Charlie Chong/ Fion Zhang

IR Applications - Aerospace

Charlie Chong/ Fion Zhang

IR Applications - Aerospace

Charlie Chong/ Fion Zhang

Time-based methods are used to detect anomalies where temperatures change during the inspection. Temperature differences may develop and then disappear as a subject changes from one overall temperature to another (Ref 2, 15). The change may be actively or passively produced. For example, a hot forging exiting a forging press or die will cool to room temperature and will produce a temperature difference at a forging lap for only a short time.

An aircraft structure with hidden corrosion can be actively heated from one side and then allowed to cool. Areas with trapped water will cool more slowly and create a temporary warm spot, while thinned or corroded areas will cool slightly faster than the rest of the structure and produce a temporary cool spot.

In general, time-based methods can provide the maximum detection sensitivity and can permit inspection from one side.

For infrared or photothermal imaging methods utilizing active heating, inspection can be performed after the heat source is turned off, thus eliminating the interference in the image from the heat source itself.

Charlie Chong/ Fion Zhang

Multiple time-based measurements are required for quantitative interpretation of thermal images (Ref 5). For example, a video recording will capture transient temperature differences produced with surface coating methods on a continuous basis and can be analyzed frame by frame to evaluate the indications as they appear and disappear.

Image interpretation has proved to be the most difficult part of many thermal inspection applications. Strong indications with large temperature differences are the easiest to interpret. Strong indications will usually provide a truer image of the anomaly than weak indications. An anomaly close to the surface will produce a stronger indication than an identical anomaly far from the surface, and the resultant indication will more accurately portray the size and shape of the source. Thermal images reflect the heat flow in the structure. If the structure has an area with a subsurface support or cavity, the heat will flow faster or slower, respectively, into these areas. The edges of the structure will experience more convective and radiative heat transfer than the remainder. Support of the structure will also affect the image because the support is usually another source of heat transfer.

Charlie Chong/ Fion Zhang

Thermal images can also be influenced by other factors. The heating source, as well as nonuniformities in the emissivity of the specimen caused by variations in surface roughness and color, can produce indications.

Consequently, the first step in image interpretation is to look at the strength of the image and closely examine the surface and structure of the specimen as well as the uniformity of the heating source. When no obvious surface or structural correlation exists between the specimen and the image, an internal anomaly should be suspected. If available or possible, digital image enhancement, such as: â– â– â–

filtering, image averaging, or reference image subtraction, (self or standard referencing)

should be used to eliminate nonrelevant indications. An indication that persists is probably a valid indication of an anomaly.

Charlie Chong/ Fion Zhang

Digital image enhancement can be used to improve the quality of thermal images. Spatial filtering can be used to smooth the temperature data by eliminating the high-frequency noise caused by heating devices and external sources. One technique involves mathematically replacing each pixel value with the average value of its four nearest neighbors (Ref 4). Signal-averaging techniques can also be used to reduce measurement noise if the image does not vary too rapidly with time. Averaging individual pixel values from 100 successive image frames will eliminate 90% of the noise and will increase sensitivity (Ref 4).

Space-domain and time-domain subtraction functions can also be used to enhance flaw detection. Space-domain subtraction functions can eliminate temperature differentials from repetitive sources of noise (such as local variations in surface emissivity), patterns resulting from a nonuniform (or line or spot) heat source, and local variations in heat loss unrelated to anomalies (such as convection patterns or conduction into supporting structures).

Charlie Chong/ Fion Zhang

If self-referencing is not possible, a defect-free reference specimen with similar characteristics can be used to provide a reference image in a spacedomain subtraction algorithm (Ref 15). For example, an image taken when a component is new or when a process is functioning properly can be subtracted from an image taken at a later date to indicate damage, flaw propagation, or changes in the process (Ref 4).

If self-referencing is not possible, a defect-free reference specimen with similar characteristics can be used to provide a reference image in a spacedomain subtraction algorithm (Ref 15). For example, an image taken when a component is new or when a process is functioning properly can be subtracted from an image taken at a later date to indicate damage, flaw propagation, or changes in the process (Ref 4).

Charlie Chong/ Fion Zhang

Fig. 4 Example of time-domain image processing. (a) Image over a defect (10 Ă— 10 mm and 0.5 mm deep, or 0.4 Ă— 0.4 in. and 0.2 in. deep) obtained 3 s after initial heating. (b) Image over the same defect obtained 5 s after initial heating. (c) Image of defect after subtracting image (a) from image (b).

Courtesy of P. Cielo, National Research Council of Canada

Charlie Chong/ Fion Zhang

7. Quantitative Methods In general, thermographic methods do not lend themselves to the rapid derivation of quantitative data on anomalies. One exception is the case of thin coatings, in which anomaly size and shape correspond very closely to the size and shape of infrared images or contact coating indications from optimized inspections. Similarly, quantitative estimates of very thin coating thicknesses are possible with proper time-based techniques (Ref 1).

Quantification of anomaly size following the detection by thermal methods is usually more readily accomplished by application of a second nondestructive evaluation (NDE) method. For example, determining the size and depth of a delamination detected in a graphite/epoxy laminate by thermal methods would be best achieved by ultrasonic techniques. A crack detected in a rocket propellant tank would be best quantified by other methods, such as multiple x- ray films or x-ray computer-aided tomography.

Charlie Chong/ Fion Zhang

Rocket Propellant Tank

Charlie Chong/ Fion Zhang

Rocket Propellant Tank

Charlie Chong/ Fion Zhang

Rocket Propellant Tank - Engine

Charlie Chong/ Fion Zhang

There are some cases in which quantification from thermal indications is always desirable. For example, in a process producing hot specimens, rapid feedback may be necessary because time is too short to wait for the specimens to cool for a subsequent inspection. Another example would be a coating process that needs a rapid thickness measurement to ensure proper thickness. Most quantitative methods are still under development. Nearly all the quantification methods must be tailored to specific applications. Although some methods can be approached from a theoretical basis, most require an empirical development effort involving reference standards, multiple timebased measurements, and interpretive methods or rules (Ref 5, 13).

Charlie Chong/ Fion Zhang

Reference standards are as necessary for thermal methods as they are for other NDE methods. Standards ensure consistent performance of the thermal activation methods and the temperature-sensing materials or equipment.

Reference standards are essential for quantitative techniques. A reference standard should closely represent the thermal and surface characteristics of the components or items inspected. For example, if the subject is a solder joint, the reference should be a solder joint with the same contact area, heat sink, wire sizes, and other characteristics. The standard must also provide an anomaly-free condition and a second unacceptable condition associated with a size or the presence of an anomaly. A standard for quantitative methods requires a progression of anomaly sizes and locations. The standard needs to cover the potential accept/reject conditions that can exist in the components to be inspected. This typically necessitates a test program to establish the anomaly sizes and locations that are unacceptable for proper operation of the components to be inspected (Ref 16).

Charlie Chong/ Fion Zhang

Multiple time-based measurements are needed to establish correlations between a response to thermal activation and anomaly size and location (Ref 5, 16). For pulsed or modulated thermal excitation, the time delay of the temperature response can be recorded as a relative phase change, which may be related to a parameter such as coating thickness (Ref 1) or delamination depth (Ref 3). Alternatively, variations in isotherm position with time can be monitored as a function of anomaly size and depth to establish known time-based response characteristics. Figure 5 shows how the effusivity (which is a function of the pulsed heat input and the time-dependent temperature change) directly over a 20 Ă— 20 mm (0.8 Ă— 0.8 in.) void in a carbon-epoxy laminate varies with time for void depths of 0.3, 1.12, and 2.25 mm (0.012, 0.044, and 0.088 in.).

Charlie Chong/ Fion Zhang

Fig. 5 Experimental apparent-effusivity curves, after correction, of the carbonepoxy laminate sample in the regions with embedded defects of 20 Ă— 20 mm (0.8 Ă— 0.8 in.) area at three depths. A, 0.3 mm (0.012 in.); B, 1.12 mm (0.044 in.); C, 2.25 mm (0.088 in.). Source: Ref 2

Charlie Chong/ Fion Zhang

Quantitative interpretation rules are needed before the time-based data can be used in an evaluation. A simple example is the phase change response as a function of coating thickness of yttria-stabilized zirconium on a nimonic substrate, as shown in Fig. 6. For an acceptable coating thickness of 0.45 to 0.6 mm (0.018 to 0.023 in.), the data show that a 0.5 Hz modulation of the thermal excitation will supply the necessary phase sensitivity and that acceptable coating thickness will be indicated by phase angles ranging from 5 to 13째. This information then provides the accept/reject criteria for tests conducted under these conditions.

For a three-variable situation in which void thickness, width and depth beneath a surface are determined, at least three time-based measurements are needed (Ref 5). The resulting calculations relating defect geometry and location are complicated and can be performed only with computer assistance.

Charlie Chong/ Fion Zhang

Fig. 6 Variations in the phase of temperature changes versus the coating thickness of a yttria-stabilized zirconia coating on a nimonic substrate. The phase of the temperature changes are relative to the modulation (0.5 and 5 Hz) of the thermal excitation. Source: Ref 1

Charlie Chong/ Fion Zhang

Photothermal Radiometry The photothermal measuring method is based on intensity modulated or pulsed heating of surfaces by laser light. During excitation, the surface temperature is monitored by an infrared detector. The generated heat diffuses into the sample. The detector measures the resulting surface temperature, which depends on material parameters and sample geometry. A lockin amplifier extracts the phase information of the infrared signal. Using calibration measurements, the phase can be linked to certain material parameters like hardness or coating thickness.

Therefore, the system needs to be calibrated with destructive measurements on sample material and geometry. The measurement duration depends on the type of application:

Charlie Chong/ Fion Zhang

http://www.thermoconcept-sarl.com/index.php/en/thermalanalysis-ndt-systems/infrared-non-destructive-testingndt/photothermal-radiometry-ndt

8. Applications The applications discussed below provide examples of how thermal methods have been used. For ease of discussion, the applications have been divided into the following categories: · Hot and cold equipment · Process control · Liquid intrusion · Disbonds, delaminations, and voids · Electronic devices · Research

Charlie Chong/ Fion Zhang

Hot and Cold Equipment. Many types of equipment that conduct or generate heat during operation are likely candidates for thermographic methods. Heating ducts, steam lines (Ref 13), radiators (Ref 17), heat exchangers (Ref 15), exhaust systems/chimney stacks (Ref 13), and refrigeration systems are heat transfer devices for which thermal inspection techniques can be used during periodic inspections for leaks, clogged passages, and missing or defective insulation. Furnaces, ovens, salt baths, autoclaves, reaction stacks, and hot manufacturing equipment (such as presses and rolling mills) may also require periodic inspection for unnecessary heat losses. Thermographic techniques are also used to inspect cryogenic tanks. Other equipment, such as bearings, slides, brakes, transmitting antennas, and electrical equipment, generates heat during operation. Localized hot areas are usually a symptom of a mechanical or electrical malfunction, and early detection provides the opportunity to replace the defective components during regularly scheduled maintenance or before more serious damage occurs.

Charlie Chong/ Fion Zhang

Process Controls. Thermal inspection methods are appropriate for certain processes in which the products are above room temperature as they exit a process. Examples are heat-set and heat-shaped plastics, hot-worked metal components, hot coating processes, and weld components. Components can be monitored as they exit the process. Abnormal temperatures would indicate an out-of-control process, and corrections can be made to prevent the production of large numbers of defective parts. An example of an actively heated process control method is the technique for monitoring phosphate coating thickness using infrared absorption characteristics (Ref 18).

Liquid Intrusion. Water or fuel intrusion in honeycomb structures is a significant problem for aircraft maintenance. Inspections can be performed immediately after flight to detect such liquids as the structure warms to ambient temperature. Water intrusion in roofs can also be detected in the evening as ambient temperature drops. This method has been reported to be successful in detecting leaks and retained moisture in insulation (Ref 19). The level of fluids in a sealed tank can also be determined thermographically. This is accomplished by either heating or cooling the tank and then noting the location of a sharp differential in resulting tank temperature.

Charlie Chong/ Fion Zhang

Disbonds, Delaminations, and Voids. Modern fabrication techniques rely heavily on the use of bonding for structures and protective coatings. Thermal techniques are good candidates for the detection of disbonds, delaminations, and voids in thin laminates, honeycomb to thin face sheets, and protective coatings (Ref 15). As the specimen thickness increases, thermal inspection becomes less effective because the possible depth of the defect may be greater. Thermal inspection may also be difficult with bonded metal structures, such as adhesive-bonded aluminum. The radiative heating of aluminum is relatively inefficient because of the low surface absorptivity, and the thermalmission signal is low because of the low emissivity. For such reasons, as well as to avoid reflective noise, the aluminum parts must normally be blackainted prior to thermal inspection. Moreover, the high thermal diffusivity of aluminum requires a high thermal-power injection to produce a visible thermal pattern before thermal uniformization is reached within the structure.

Nevertheless, the possibility of evaluating adhesive-bonded aluminum structures at the rapid pace afforded by thermal inspection is attractive, particularly for online applications (Ref 20). The following two examples describe the thermal inspection of adhesive-bonded aluminum after blackainting.

Charlie Chong/ Fion Zhang

Example 1: Thermal Inspection of Adhesive-Bonded Aluminum Sheets (Ref 15). Figure 7 shows the detection by a transmission configuration of a 1.5 Ă— 1.5 cm (0.6 Ă— 0.6 in.) lack-of-adhesive defect on a lap joint with nearly 1 mm (0.04 in.) thick skin. The hotter portions on the left and right of the thermal image correspond to the single-sheet material, which was heated at a faster rate than the adhesive joint. The heating rate must be sufficiently fast to avoid significant thermal propagation from the hot single-sheet to the adhesive point. Properly shaped shading masks on the lamp side can be used to avoid single-sheet over-heating when the sample geometry is repetitive. A line heating and sample displacement configuration of the type described in Ref 21 can also be used for the fast-response testing of longitudinal joints.

Charlie Chong/ Fion Zhang

Fig. 7 Thermographic inspection of adhesive-bonded aluminum sheets. (a) Experimental configuration. (b) Thermal image of a 15 × 15 mm (0.6 × 0.6 in.) disbonding in an image area of 40 × 50 mm (1.6 × 2.0 in.). Courtesy of P. Cielo, National Research Council of Canada

Charlie Chong/ Fion Zhang

Example 2: Cold-Thermal-Wave Technique for the Thermal Inspection of an Adhesive-Bonded Aluminum Structure (Ref 15). Rapid back heating procedures are difficult to apply when analyzing complex structures of the type shown in Fig. 8. In this case, a cold-thermalave approach was applied by which the whole structure was previously warmed up uniformly by nearly 10 째C (20 째F) above ambient, and an ambient air jet was used to rapidly cool the inspected face. The thermal image shows a relatively warmer central area corresponding to the adhesivebonded strip (oriented vertically in the thermograph). The cooler area in the center of the strip corresponds to a lack-of-adhesive unbond.

Charlie Chong/ Fion Zhang

Fig. 8 Cold-thermal-wave technique for inspecting a preheated, adhesivebonded aluminum structure. (a) Experimental configuration. (b) Thermal image of a 10 Ă— 10 mm (0.4 Ă— 0.4 in.) disbonding. Courtesy of P. Cielo, National Research Council of Canada

Charlie Chong/ Fion Zhang

Electronic devices typically fail by local overheating, corrosion, or poor solder joints. Infrared microscopes have been used to inspect microelectronics for overheating, and laser-generated heat has been used to inspect circuit-board solder joints. More conventional thermal inspection methods have been used to inspect solder joints during cool-down after fabrication or during circuit operation. Solar cells have also been screened for defects with thermal techniques.

Research. Several investigators have used the heat generated by permanent deformation to track the onset of failure in tensile overloading and crack propagation in fatigue testing (Ref 7, 8, 9). Samples of components can be loaded to failure under thermographic monitoring to pinpoint the origin and propagation of deformation (Ref 7).

Charlie Chong/ Fion Zhang

End Of Reading Three

Charlie Chong/ Fion Zhang