Wavespeeds Measurements in Anisotropic Materials by Using Laser Generated Ultrasound

Wavespeed measurements made on the quasi-longitudinal (qL) mode have been achieved in two composite materials (i.e. a 1-D glass/epoxy and a 2-D SiClSiC) by using a correlation algorithm between the measurement signal at a given observation angle, and a reference signal recorded at normal incidence. The wavespeed measurement at normal incidence is obtained, when possible, by correlating the direct qL ray with the 3qL multiple reflection. The qL wavespeeds measured by the laser method are then compared to the phase velocities acquired by standard ultrasonics techniques. The agreement is quite acceptable in most cases. When dealing with laser generated ultrasound, the wavefront of any given ray is propagated at the group velocity. The correlation measurements, should provide wavespeeds which tend to be close to such ray surface velocity. However, the difference between the two wavespeeds is small on the qL mode (generally less than 10 %) and can be as a first approximation safely ignored.