Elastic waves used for characterization of
structural media stress states
N.Ye. Nikitina
Nizhny Novgorod Branch of Mechanical Engineering Institute,
the Russian Academy of Science,
Nizhny Novgorod, Russia
Taking into account nonlinear terms in the wave equation yields relation between the shear or longitudinal wave velocity and the values of elastic stresses or strains. This relation can serve as a physical basis of a stress (strain) characterization without the material's destruction.
In accordance to the small values of nonlinear effects as a nonresonance
interactions between elastic waves and stress (strain) quasistatic field
those velocity variations are equal to 
. However the structural
state of solid also provides the elastic wave velocity variations so we sometimes
can not determine the values of stresses as a result of "pure" acoustoelastic
effect.
Theoretical and experimental investigations of this problem have been carried out by the author for many years. The main idea of this researches is that the real solid is not an ideal crystal but the granular medium with different scale parameters along the different directions. The presence of scale parameters in the medium provides the frequency dependence of elastic wave velocity and attenuation. The precise measurement of those parameters must help us to distinguish the effects of stress and structure in the problems of wave propagation in real solids.