Acoustic nonlinearity and cumulative plastic shear strain in cyclically loaded metals

摘要

Cyclic loading leads to microstructural changes in metals that result in substantial increases in the material nonlinearity. Two quite distinct approaches for quantifying the nonlinearity via a material nonlinearity parameter assessed directly from acoustic harmonic generation measurements have emerged—the Cantrell model and the model of Kim etal The Cantrell model quantifies the nonlinearity in terms of lattice anharmonicity, dislocation plasticity, and crack growth as independent sources of nonlinearity arising from the accumulated plastic shear strain. The approach of Kim etal links the cumulative plastic shear strain directly to a change in the third-order elastic constants of the material. We show that although the model of Kim etal has the advantage of expediency, the Cantrell model reflects much more accurately the dependence of the nonlinearity parameter on the state of fatigue.