Defect Detection by the Scattering Analysis of Flexural Waves

Abstract

The propagation and scattering of flexural waves by obstacles in plates is studied experimentally and theoretically. When the guided wave hits a discontinuity like a hole, a typical scattered displacement field is obtained. Defects like a notch or a fatigue crack at the hole boundary change the scattered field significantly. The first antisymmetric Lamb wave mode A0 is excited selectively by means of a piezoelectric transducer. The scattered field around undamaged and damaged holes is measured on a grid around the hole using a heterodyne laser interferometer. The out-of-plane displacement is measured with good spatial resolution, accuracy, and repeatability. Applying fast Fourier transform, the amplitude and phase values of the scattered field are extracted, overcoming the typical problems associated with the measurement of dispersive waves. The experimental results are compared with theoretical calculations. The wave propagation is studied using classical plate theory and Mindlin’s theory of plates. The scattered field around the circular hole is calculated analytically and good agreement is found for the range of validity of the used theories. The scattering at the hole with a defect is calculated numerically, using a finite difference scheme. The method can be applied for the detection of cracks at rivet holes in aircraft fuselage, an important nondestructive testing application. Guided waves allow a fast inspection of large areas, reducing the need for time-consuming scanning. The minimum detectable crack length and the sensitivity of the method are discussed.