Some advancements in the ultrasonic evaluation of initial stress states by the analysis of the acoustoelastic effect

The acoustoelastic effect - that is the correlation between the acoustic properties and the stress state for a solid body - can be profitably employed for experimental measurements of applied and/or residual stress, for example starting from the results of ultrasonic tests. Usually, the interpretation of the results of acoustoelastic experiments is performed in the theoretical framework of the so-called third-order elasticity. In the recent past, more general theoretical models aimed at overcoming some limitations of the third-order elasticity for acoustoelastic stress measurements have been developed. In particular, here we refer to a model developed within the linearized finite elasticity theory and describing the propagation of small amplitude waves in prestressed elastic materials. In this model, no assumption on the origin of initial stress neither on the initial anisotropy of the material is made, but the only hypothesis is that ultrasonic waves superimposed on the stressed reference configuration behaves elastically; then, this theoretical approach is also applicable for the experimental stress analysis of plastically deformed bodies and of anisotropic materials. Moreover, this model leads to "universal relations" relating ultrasonic velocities to the stress in a prestressed configuration of a body. Ultrasonic acoustoelastic tests on specimens under known stress states allows us for showing that the application of the above mentioned theoretical model, together with the use of a suitable experimental setup developed at our laboratory, may lead to stress measurements with an high level of accuracy.