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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 Δβt-c, the difference between β measured after the tension and compression portions of the fatigue cycle, depends on strain amplitude. The dependence of Δβt-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. 

The accumulation of dislocations, which are atomic defects in materials subjected to plastic deformation, can cause structural failures. Early detection of such dislocation-related damage is essential to prevent these failures. The acoustic nonlinearity parameter β has been shown to be sensitive to the nonlinearity of dislocation motions, and prior research has shown a relationship between β and dislocation parameters in various damage mechanisms. While most work thus far reports that β generally increases with increased plastic deformation, recent research showed that β can decrease during monotonic tensile loading in stainless steel 316L characterized by in situ nonlinear ultrasonic measurements. The objective of this research is to examine the correlation between the decrease of β with plastic strain as reported in this recent study, and the initial microstructure and strain hardening rate. The initial microstructure, characterized with electron backscatter diffraction (EBSD), shows an increase in dislocation density and a reduction of grain area, which can possibly result in a decrease in β. Further, it is shown that the decrease rate of β monotonically decreases with hardening rate, providing a evidence that the decrease in β may relate to the shift from planar slip to wavy slip. These results help interpret the underlying mechanisms for the decrease in β during tensile loading.