Physical Mechanisms Underlying Ultrasonic Non-Destructive Evaluation of Fatigue

摘要

We have established Dislocation Dynamics (DD) simulations as a practical method to compute the acoustic nonlinearlity parameter, beta, which forms the bases of the emerging metal fatigue detection technique of nonlinear ultrasonics. Our study have uncovered mistakes in two well-known theoretical models in this field and leads to a deeper understanding of the dislocation mechanism to acoustic nonlinearity. For a single dislocation bowing in its glide plane between two pinning points (i.e. the monopole model), our simulations and analytic derivations show a strong dependence of beta on the dislocation orientation, which is missed by the previous model (Hikata et al. 1965). For parallel dislocations forming a multipole configuration (as a model for the vein structure in fatigued metal), our simulations show a pronounced dependence of beta on the applied stress (with beta=0 at zero stress), which contradicts the previous model that predicts a constant beta independent of stress (Cantrell et al. 2001). Our analytic derivations have pinpointed the mistake in the previous model.