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Corrosion Mapping of Aircraft Structures Pulsed Eddy Current Technologies: Corrosion orrosion detection and mapping within multilayered aircraft structures is achieved with PEC inspection/ inspection/analysis methods. The PEC method and probe are selected based on the sample layers configuration and thickness. The configured PEC system successfully achieves corrosion detection etection and localization in multilayered aluminum samples.

Introduction Due to its ease of use and relative precision, ultrasounds have been used for corrosion mapping for a long time. However, when it comes to multilayered structures, ultrasounds can fail the task, especially especi when there is a weak or no mechanical bonding between the layers. The presence of insulation can also represent an obstacle to ultrasounds if it cannot be removed.

Benefits of PEC technology Pulsed Eddy Currents have the advantage of being able to monitor mul multiple tiple layers without the need for mechanical bonding. In the case of multilayered aerospace structures, a magnetic field that is strong enough to penetrate the layers of interest has to be generated. When this is achieved, pulsed eddy currents are produced on both surfaces of each layer which, from the principles of mutual mutual-inductance, inductance, generate an additional magnetic field that interact with the one coming from the driving coil. The presence presen of corrosion, i.e. thickness changes or flaws, can be detected by monitoring this field. Calibration can be used to determine the depth of the corrosion under certain conditions.

Fig. 1. Military airplane.

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The configured PEC system for corrosion mapping uses TecScan’s ARMANDA scanner (figure 2a). It represents an automated portable scanner that can be fixed on the structure. For this technical application note, PEC inspections inspection were performed using this scanner on various configurations of aluminum plates containing simulated corrosion (machined thinning). Different probes are used depending on the sample arrangement. The PecScan™ driver/receiver unit (figure 2b) is used to drive the probe to generate and receive the PEC signals.


Fig. 2. (a) ARMANDA - Automated scanner for PEC testing (b) PecScan™ Driver/Receiver unit for Pulsed Eddy Current urrent generation and reception reception.

Experimental Case 1 A first sample was prepared with 8 aluminum plates (7 plates of 1.9 mm thick and one plate of 4 mm for a total thickness of 17.3 mm in the thickest section of the assembled plates). The layers were assembled to obtain a step sample, each step being equally separated by 40 mm. The inspection as done using a standard eddy current reflection probe (driver/pickup coils) with a nominal bandwidth of 100Hz to 5kHz (diameter of 0.75”) and a spectral analysis was done to extract the 100Hz component from the signal. CC Scan imaging was then performed using the phase of this component, as illustrated in figure 3. This result illustrates the thickness mapping potential of Pulsed Eddy Current.

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Fig. 3. (a) Sketch of the sample: seven plates of 1.9mm on top of a 4 mm one. (b)) PEC inspection from which the phase of the 100Hz component was extracted by spectral analysis and displayed as a C-Scan. C

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Experimental Case 2 A second sample containing machined areas corresponding to thickness changes of 25, 50 and 75% was inspected in two different configurations: as a 2mm thick single plate (figure 4a) and as a multi-layered multi sample below multiple layers (figure 4b) of 1.9 mm each. The inspection of this sample is done using a 100 Hz – 50 kHz sliding reflection probe.

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Fig. 4.. Sketch of the second sample. The percentages indicate the material loss (inspection was performed from the other side. (a) Sample as a single plate; (b) Sample in a multi multi-layer layer arrangement (picture on the left shows an additional simulated corrosion located in the top layer).

The detection of all three thinned areas in a single layer (figure 4a) is done by a simple temporal analysis of the signals (balance point of the mean square amplitudes of a time gate), as shown in figure 5.

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Waveform

B-Scan

B-Scan

C-Scan

Fig. 5. Screenshot of TecViewTM PEC showing the C C-Scan Scan obtained on the sample of figure 4a with a mean square amplitude feature.

The sample presented in figure 4b is inspected using a conventional 100 Hz – 5 kHz reflection probe. In that case, a more advanced analysis is required to discriminate between the defect machined in the top layer and the thinned areas of interest.

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Fig. 6. PEC C-Scan obtained from a spectral analysis of the 200Hz component on sample of figure 4b (a) Amplitude C-Scan Scan of the 200Hz component; (b) Amplitude C C-Scan Scan of the 2.5kHz component showing the imperfections of the first layer; (c) Calculation of the 200 Hz component based on a 0.1ms time gate; (d) Subtraction of the indications of (c) after rotation and gain adjustments.

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Three results are therefore obtained: a C-Scan representing all layers (figure 6a), a C-Scan mainly representing the first layer (figure 6b) and a C-Scan corresponding to the defects of interest (figure 6c). In order to achieve such layer separation, a combination of temporal and spectral analysis is performed on the waveforms. Figure 6a shows the spectral analysis of the 200Hz of the whole signal, which corresponds to the whole sample thickness. Figure 6b is obtained by performing the spectral analysis at the same frequency but on the first 100 Âľs of the signals only. Figure 6c is obtained following a subtraction of the results of a) and b) after gain and rotation manipulations on the outcome of b). Clear separation and good resolution is obtained for the defects of each layer.

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http://www.pecscan.ca/PDFs/Technote_corrosion