Ultrasonic lamb waves in a thin plate

The behavior of the wave field produced in a thin unidirectional graphite/epoxy composite plate by a dynamic point load is studied using an approximate shear deformation plate theory (S.D.P.T) and a finite element analysis (F.E.A). Comparisons are made for propagation at 0degrees, 45degrees, and 90degrees directions relative to the fibers showing excellent agreement between the two model approaches. The approximate method is then used to calculate the response of a composite plate as well as of an aluminum plate to a uniform dynamic surface load distributed in a circular region. A periodic reversal in the phase of the signal with propagation distance is observed. It is found that this is caused by the strong dispersion of the first antisymmetric waves at low frequencies. For clarification, the steepest descent method is applied to obtain a closed form analytical expression for the far field response in the aluminum plate for a Dirac delta source. It is shown that the waveform carries a singularity that reverses its phase at regular intervals. The present work should be helpful in understanding the nature of waveform signals produced by impact loads and in the detection and characterization of impact damage in composite structures.