Abstract
Laser ultrasonic line source methods have been used to study elastic anisotropy in nuclear graphites by measuring shear wave birefringence. Depending on the manufacturing processes used during production, nuclear graphites can exhibit various degrees of material anisotropy related primarily to preferred grain orientation as well as microcracking. This anisotropy can evolve as a result of service-related microstructural changes and the ability to measure graphite anisotropy could be important for monitoring degradation and structural health of graphite in nuclear applications. In this study, laser ultrasonic line source measurements of shear wave birefringence on NBG-25 have been performed to assess elastic anisotropy. Laser line sources allow specific polarizations for shear waves to be transmitted – the corresponding wavespeeds can be used to compute bulk, elastic moduli that serve to quantify anisotropy. In other materials systems, these measurements can be used to assess grain orientation distribution through the orientation distribution coefficients, but for nuclear graphites this type of analysis should be interpreted as representing the combined effects of grain orientation and microcracking on the elastic moduli. Ultrasonic results are compared to and contrasted with measurements of anisotropy based on the coefficient of thermal expansion to show the relationship of results from these techniques.
