Detection of cracks in plates using guided waves

Abstract

This research combines laser ultrasonic techniques with the two-dimensional Fourier transform (2-D-FFT) to investigate the effect of cracks on the dispersion of Lamb waves propagating in thin aluminum plates. The high fidelity and broad bandwidth of these optical techniques are critical elements to the success of this work. The experimental procedure consists of measuring a series of equally spaced, transient waves in aluminum plates containing notches; a crack is simulated with a 1-mm-thick saw-cut notch. The frequency spectrum (dispersion curves) for each plate is obtained by operating on these transient waveforms with the 2-D-FFT; this procedure extracts steady-state behavior from a series of transient waveforms. This study quantifies the effect of notch depth (two notch depths are examined: one-fourth and one-half of plate thickness) on the dispersion curves of three different plate thicknesses (nominal thicknesses of 1, 1.5, and 3 mm). These dispersion curves show that a notch reduces the transmitted energy by an amount that is directy proportional to a notch’s depth. In addition, scattering by a notch causes definitive reductions in energy (evident in all modes) at certain frequency–wave-number combinations, thus providing experimental evidence of the relationship between crack size and the scattered Lamb wave field.