THE INTERACTION OF LAMB WAVES WITH DEFECTS

The interaction of individual Lamb waves with a variety of defects simulated by notches has been investigated using finite-element analysis, the results being checked experimentally. Excellent agreement has been obtained between the numerical and experimental results and it has been shown that a two-dimensional (2-D) Fourier transform method that the authors have developed may be used to quantify Lamb wave interactions with defects in both experimental and numerical investigations. The results have shown that the sensitivity of individual Lamb waves to particular notches is dependent on the frequency-thickness product, the mode type (symmetric or antisymmetric), the mode order, and the geometry of the notch. The sensitivity of the Lamb modes a1, a0, and s0 to simulated defects in different frequency-thickness regions has been predicted as a function of the defect depth to plate thickness ratio (h/2d) and the results have indicated that Lamb waves may be used to find notches when the wavelength to notch depth ratio is of the order of 40. It has also been shown that provided the notch width is small compared to the wavelength, the transmission and reflection amplitudes are insensitive to changes in width, so the ratio of the depth of the notch to the plate thickness is the controlling parameter. Both the numerical and experimental results have shown that the transmission ratios of Lamb waves across defects are highly frequency dependent, particularly at higher frequency-thicknesses. The positions of the maxima and minima in the transmission curves are a function of notch depth that suggests that monitoring the change in transmission ratio with frequency may provide a means of defect sizing.