Noncontact Determination of Antisymmetric Plate Wave Velocity in Ceramic Matrix Composites

Abstract A 13-mJoule NdYAG 1064-nm, 4-nsec laser pulse was employed to produce ultrasonic plate waves in 20% porous SiC/SiC composite tensile specimens of three different architectures. An air-coupled 0.5-MHz transducer was used to detect and collect the waveforms that contained first antisymmetric plate wave pulses for determining the group velocity (VS). These results were compared to VS values determined on the same specimens with 0.5-MHz ultrasonic transducers with contact coupling. Averages of four noncontact determinations on each of 18 specimens were compared to averages of four contact values. The noncontact VSs fall in the same range as the contact. The standard deviations for the noncontact VS values averaged 2.8%. The standard deviations for the contact measurements averaged 2.3%, indicating similar reproducibility. Repeated laser pulsing at the same location always leads to deterioration of the ultrasonic signal. The signal would recover in about 24 h in air, however, indicating that no permanent damage was produced.