Mode analyses of laser-generated transient ultrasonic lamb waveforms in a composite plate by wavelet transform

The mode analyses of the transient ultrasonic Lamb waveforms excited by pulsed laser in a composite plate are presented by employing the wavelet analysis. The time vs, frequency representation plots of the waveforms (depending on the propagating directions and the source to receiver distances) show a clear exposition of the anisotropic behavior of the composite plate and the temporal evolution of spectral components of the Lamb waveforms during the propagation process. The analyses indicate also that the waveform mainly consists of three or four lower order Lamb wave modes, and that the other higher order modes can be neglected. The group velocity of each mode is calculated according to the plots. The results demonstrated the effectiveness of the wavelet transform to analyze the transient Lamb waveforms for characterizing the composite materials by the laser generated Lamb wave technique.