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Crack Detection using Pulsed Eddy Currents-PEC R. Sicard, R&D Manager (TecScan Systems Inc.) Crack detection on multilayered aircraft structures is achieved with two different PEC analysis methods. The PEC analysis method is selected based on the layer thickness and the rivet head physical properties. The designated PEC system requires proper calibration to obtain the desired detection.

Introduction The detection of cracks is of great importance in aerospace structures as they can rapidly grow to cause catastrophic failures. Eddy currents, ultrasounds and radiography are the most common ways of inspecting this type of defect. While radiography has a limited use in tight spaces and because of security reasons, eddy current and ultrasonic inspections fail to detect cracks in all situations. Ultrasonic inspections require a mechanical bonding in order to propagate through multiple layers, which is not always the case for riveted structures. On the other hand, eddy currents can penetrate through unbounded layers, but at limited depths (typically 2 layers). Like eddy currents, Pulsed Eddy Currents have the particular advantage of being able to monitor multiple layers without the need for mechanical bonding. In the case of multilayered aerospace structures, a magnetic field that is strong enough to penetrate all layers of interest must be generated. When this is achieved, pulsed eddy currents are produced on both surfaces of each layer and, from the principles of mutual-inductance, generate an additional magnetic field that interact with the one coming from the driving coil. The presence of cracks affects the pulsed eddy currents and can be monitored in the resulting field. Multiple features can be used to detect cracks from either the transient waveform or its spectral representation.

Experiments C-Scan inspections of multilayered aircraft structures can be done using the ARMANDA scanner (figure 1a), which is a portable scanner that can be fixed on the structure. A PEC inspection was performed using this scanner on a riveted eddy current standard (2 aluminum layers of 0.04” with the bottom layer containing EDM notches of lengths of 0.250”, 0.200”, 0.150” and 0.100” on the rivet holes edge and identified from {1} to {4} on figure 2a). For sample inspection, we selected a conventional reflection eddy current probe (700 Hz – 15 kHz). The PecScan™ driver/receiver unit (figure 1b) is used to drive the probe, generate and receive the PEC signals.

(a) (b) Fig. 1 (a) ARMANDA - Automated utomated scanner for PEC testing (b) PecScan™ Driver/Receiver unit for Pulsed Eddy Current generation and reception.

A picture of the sample is presented in figure 2a. Figure 2b shows the result obtained by analyzing the PEC waveforms using a temporal method (feature: total energy in a time gate) in the form of a C-Scan Scan image. On the other hand, figure 2c shows the C-Scan C obtained through spectral analysis (feature: single frequency component of 10 kHz extracted from the PEC waveforms). This spectral analysis allows displaying the content of the selected frequency on an impedance plane the same way it is performed in conventional eddy current inspections. Based on the impedance response measured on a good rivet, a rotation is applied on the 10 kHz component to minimize the effects of the rivet edge, leading to the result presented in figure 2 (c). 100%

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(d) Fig. 2. Images of the Eddy current standard samples samples. (a) Picture showing the EDM notches of 0.250” {1}, 0.200” {2}, 0.150” {3} and 0.100” {4}. (b) C-Scan Scan obtained from the temporal analysis of the PEC waveforms (energy within a time gate); all scales in mm. (c) C-Scan Scan obtained from the spectral analysis of the PEC waveforms: imaginary part of the 10 kHz component after rotation of the rivet edge signals. (d) Color palette used to display the C C-Scans.