Effective elastic constants in acoustoelasticity

In the context of studies on ultrasonic wave propagation in stressed media (acoustoelasticity), a classic formalism can be considered (second order) by replacing the elastic constants with effective elastic constants (EECs) for which the influence of stress is taken into account. Up until now, this formalism presented some limitations since certain EEC split, which meant the authors have to consider the pseudo-orthotropy or pseudohexagonality to describe the symmetry of the materials with respect to ultrasonic wave propagation. In this letter, the possibility of avoiding these degenerations by carrying out the developments using the natural coordinates of the material is shown.