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Pulsed eddy current technique for defect detection in aircraft riveted structures Yunze He n, Feilu Luo, Mengchun Pan, Feibing Weng, Xiangchao Hu, Junzhe Gao, Bo Liu College of Mechatronics and Automation, National University of Defense Technology; Changsha 410073, China a r t i c l e i n f o Article history: Received 1 July 2009 Received in revised form 29 October 2009 Accepted 30 October 2009 Available online 12 November 2009 Keywords: Nondestructive testing Pulsed eddy current Differential probe Defect detection Riveted structure a b s t r a c t The Pulsed Eddy Current (PEC) technique is an effective method of quantifying defects in multi-layer structures. It is diffi cult to detect defects in riveted structures of aging aircraft. Based on theoretical analysis of PEC technique, three different probes, including a differential hall probe, a differential coil probe, and a two-stage differential coil probe are designed to detect this kind of defects. The averaging method and wavelet analysis method are used to de-noise the hall response signals. By selecting peak amplitude and zero-crossing time of response signal in time domain as key features, defects in riveted structures can be detected effectively. The experimental results indicated that the differential coil probe acted as effectively as the differential hall probe. The defects between third layer and fourth layer in riveted structures can be detected by utilizing the two-stage differential coil probe. The PEC technique has a promising application foreground in the fi eld of aeronautical nondestructive testing. Crown Copyright the vertical coordinates represent the amplitude of peak waves. Fig. 7(a) shows the original results. It can be seen that the hall response signals are disturbed by unknown noise, which leads to the inaccuracy in detecting the defects. Therefore, the averaging method and wavelet de-noise method are used to process the hall responses. Wavelet analysis is a relatively new technique in signal processing. The fundamental idea behind wavelet analysis is to analyze according to scale, therefore both coarse and fi ne features of a data signal can be probed 17.The analysis is done in both time and frequency domains, while the similar and widely used Fourier analysis only provides a frequency aspect. This extra ability makes wavelet analysis suitable to analyze transient phenomena in signal 18. Fig. 7(b) shows the results after data processing. As the probe scans over thedefect, peak waves present a broad crest. Apparently, with the increase of the depth of defects whose width and length remain constant, peak values of crests increase clearly, which provides an effective way to detect subsurface- breaking defects in riveted structure. 5.2. Differential coil probe Based on principle that has been discussed in Section 2, the time domain response signals are analyzed to get the information of defects. In the experiment, the peak amplitude and the zero- crossing time are extracted and used as key features to detect defects. Three defects with different depth and the same length and width on surface are detected with the differential coil probe. The length?width?depth of three defects, are respec- tively, 8?0.8?1 mm3, 8?0.8?1.2 mm3, 8?0.8?1.4 mm3. As shown in Fig. 6, the amplitude of the exciting pulse is 10 V, the repetition rate of the excitation is 100 Hz and the duty ratio is 50%. The experimental results are shown in Fig. 8. The horizontal coordinates represent the scanning time; the vertical coordinates represent the amplitude of transient res- ponse signal. As shown in Fig. 8(a), the peak amplitude value is related with the depth of defects. Apparently, the peak amplitude value increases clearly with the increment of the depth of defects whose width and length remain constant, which provides an effective means to evaluate the depth of surface- breaking defects. As shown in Fig. 8(b), the zero-crossing time is only related with defect depth whose width and length remain constant, which also provides an effective means to evaluate the depth of surface-breaking defects. Therefore, we can evaluate the depth of surface-breaking defects in riveted structures by extracting the peak amplitude and zero-crossing time of response signals. 5.3. Results comparison To compare the performance of two differential probes proposed in Section 3, three defects on surface with different width and the same length and depth are detected with two differential probes. The length?width?depth of three defects are 8?0.6?1 mm3, 8?0.8?1 mm3, 8?1.0?1 mm3. As shown in Fig. 6, the amplitude of the exciting pulse is 10 V, the repetition rate of the excitation is 100 Hz and the duty ratio is 50%. Fig. 9 shows the detecting results of different differential probes. Fig. 9(a) shows the original response signals of differential hall probe. The hall response signals are disturbed by noise, which leads to the inaccuracy in extracting the feature. Therefore, the wavelet de-noise method are used to de-noise the hall response signals. Fig. 9(b) shows the hall response signals after processing. Fig. 9(c) shows the differential coil response signals. Apparently, the differential coil probe acted as effectively as the differential hall probe in detecting defects in riveted structures. Furthermore, the response signals of coil do not need complicated data processing and the cost of making coil is lower. 5.4. Two-stage differential coils probe In detection of deeper defects, to verify the performance of the two-stage differential probe proposed in Section 3, an aluminum specimen whose schematic diagram shown in Fig. 10 is designed. A slot whose length, width and height, respectively, are 10, 1.5 and 1 mm, is machined by the side of rivet hole on the bottom of the third aluminum plate with the thickness of 2 mm. The thickness of fi rst and second aluminum plate is 1 mm. The thickness of fourth aluminum plate is 1.5 mm. These aluminum plates are fastened by snapped rivet and screws to simulate riveted structure of aircraft. In this experiment, the amplitude of the exciting pulse is 12.5 V, the repetition rate of the excitation is 60 Hz and the duty ratio is 70%. To compare the performance of reducing lift-off effect between the two-stage differential probe and regular probe, the same specimen designed above is detected respectively utilizing two- stage differential coil probe and regular probe. Fig. 11 shows the rotating results. The horizontal coordinates represent the rotating angle; the vertical coordinates represent the amplitude of peak waves. It can be seen that the result of two-stage differential probe is obviously better than that of regular probe. Therefore, we Fig. 10. The defect between third and fourth layer. Fig. 11. The scanning peak waves of two-stage differential coil probe and regular probe. Y. He et al. / NDT Fig. 12(b) shows the zero-crossing time of response signals of defect and defect-free. Apparently, the peak amplitude of defect will increase by 164 mV (from 3076 to 3240 mV); the zero-crossing time will decrease approximately by 45ms. Therefore, the two-stage differential probe is effective to detect the deeper defects in riveted structures. 6. Conclusions Defects often appear in airframe riveted structures and seriously threat the safety of aviation. In this paper, three kind of differential hall/coil probes are proposed to detect the defects in riveted structures. The experiment results show that the defects on surface and subsurface can be detected effectively and the differential coil probe act as effectively as the differential hall probe. In addition, defects between third layer and fourth layer of specimen can be detected easily by using the two-stage differential coil probe, which is more effective than the regular probe. The PEC testing can also be used to identify and evaluate the defects in multi-layer structures in ageing aircraft 16. Therefore, PEC testing will play an important role in the fi eld of aeronautical nondestructive testing. Future research of the authors will include defect classifi cation, the real-time defect identifi cation, fi eld defect evaluation, and in-service imaging detection. Acknowledgements The authors would like to extend their appreciation to BinFeng Yang in University of Air Force Engineering for his contributions to the work. References 1 Moulder JC, Bieber JA, Ward WW, et al.Scanned pulsed eddy current instrument for non-destructive inspection of aging aircraft. SPIE 1996;2945: 213. 2 Smith RA, Hugo GR. Transient eddy current NDE for aging aircraft-capabilities and limitations. 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