A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 2 2 4 3 SMJ HIGHLIGHTS of the pertinent literature indicate that pre-strains intensify texture, increase martensite volume fraction, and induce plasticity that may enhance or degrade fatigue performance. The majority of experimental studies demonstrate that fatigue cycling with mean strains within the two-phase region leads to longer lives compared to cycling in the linear elastic region or at mean strains greater than the end of the stress plateau. These effects are described in terms of martensite stabilization, cyclic phase change, and the decrease in composite modulus with increasing mean strain. These factors combine to result in alternating strain between approximately constant upper and lower plateau stresses to induce phase change (Fig. 2). (a) Fig. 2 — In situ experimental characterization using nf-HEDM showing the evolution of geometric necessary dislocation (GND) density with cycling in 3D: (a) spatially resolved orientationmap; (b) spatially resolved GND density map pre-cycling; and (c) spatially resolved GND density map after 20 load-biased thermal cycles (one grain is highlighted to emphasize the 3D nature of the technique). 1 5 (b) (c)
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