In situ nonlinear ultrasonic characterization of slip irreversibility and material hardening in stainless steel 316L

Abstract

This work uses in situ nonlinear ultrasound measurements to study the relationship between the acoustic nonlinearity parameter β and the low cycle fatigue behavior of stainless steel 316L. The measured β shows a rapid decrease during hardening followed by a transition to a slower decrease in β as a function of fatigue cycles. Measurements show this trend is consistent at two different strain amplitudes. By comparing our results with prior work on dislocation characterizations in the same material, we hypothesize that the transition in slopes of β coincides with the planar-to-wavy transition that occurs at the end of hardening. Further, measurement results show that the parameter Δβ_t-c, the difference between β measured after the tension and compression portions of the fatigue cycle, depends on strain amplitude. The dependence of Δβ_t-c on strain amplitude is related to fatigue life through a power law relationship, similar to slip irreversibility. Overall, the results provided in this work suggest that β correlates with characteristics of low cycle fatigue, and thus supports the idea that in situ NLU measurements can eventually be used as a quantitative measure to predict fatigue life.