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Small Defect Detection Through Local Analysis of Acoustic Spatial Wavenumber



We present some preliminary results in utilizing laser-acquired steady-state ultrasonic response measurements for detecting small defects such as out-of-plane cracking. In our approach, we exploit the local discontinuity in the material impedance, which leads to sharp spatial gradients in the steady-state wave field and manifests as high local wavenumber content in the spatial Fourier domain. We demonstrate the ability to detect, localize, size, and orientate cracks using two techniques: curvature estimation and high-frequency wavenumber imaging. For curvature estimation, we make robust estimates of the Laplacian at each point in two-dimensional steady-state response. In the wavenumber imaging, we high-pass filter the response measurement just above the nominal guided wave wavenumber, and calculated the two-dimensional envelope using the monogenic signal. Both techniques proved effective on two aluminum plate samples, one with 100% through-thickness cracking at a fastener, and one with a 50% through thickness notch on the rear face.

doi: 10.12783/SHM2015/326

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