Graphite anisotropy measurements using laser-generated ultrasound

One of the contributing factors to graphite degradation is oxidation. In order to detect this type of degradation, samples with different levels of oxidation have been studied. Longitudinal velocity measurements were carried out on these samples, using laser ultrasound techniques. These techniques have the advantage that they are non- contact, and therefore do not perturb the sample under investigation. From these measurements, the Young's modulus was estimated, based on a typical Poisson's ratio of 0.20. This was performed for several cylindrical samples; some samples had parallel microstructure orientation in the through- thickness measurement direction, while others had perpendicular orientation. Both orientations had very different longitudinal velocities, with corresponding variations in Young's modulus. Degradation was assessed for both cases of parallel and perpendicular oriented samples. Furthermore, in these perpendicular oriented samples, velocities were dependent on sample rotation about the cylindrical axis. Additional measurements were taken at 90. orientations to confirm this feature. The data positively show polarization of longitudinal velocity components. This may be the first time that acoustic birefringence has been observed in high porosity graphite.