Ultrasonic Evaluation of Thin, Fiber-Reinforced Laminates

This paper reports on the characterization of one-or two-ply, unidirectional and [0,90,0], cross-ply graphite/epoxy laminates whose thicknesses were approximately 145 μm, 275 μm and 400 μm, respectively. The measurements are based on the application of the ultrasonic point-source/pointreceiver technique (PS/PR) using either a focused laser beam operating as a dipole source or a small aperture, piezoceramic shear transducer serving as a monopole source. Detection of the signals is with a piezoceramic sensor that directly responds to the lateral or shear motions of the specimen surface. The detected time signals represent the detailed spatial and temporal characteristics of the elastic wave field. The experimental group velocity data is analyzed using a simple plane-wave, plane-stress model that describes the propagation of quasi-longitudinal and quasi-transverse membrane waves in the plane of the plate. We demonstrate in this paper the strength of the PS/PR measurement technique when coupled with appropriate signal processing to recover the stiffness tensor of thin, fiberreinforced laminates. Additionally, preliminary experimental results on the detection of flaws in thin specimens from in-situ measurements are included