Fatigue thresholds of Ni-Ti alloy near the shape memory transition temperature

The fatigue thresholds of nickel-titanium alloy with martensite start tempe rature of 80 degrees C were measured in air from room temperature to 150 de grees C and load ratios of 0.1 to 0.9. The Delta K versus K-max fundamental threshold curves are characteristic of ductile metals at all temperatures tested and are consistent with the existence of two threshold parameters, D elta K-th* and K-max,K-th*, for cyclic and maximum stress intensity, respec tively. Delta K-th* increases linearly with temperature, while K-max,K-th*, exhibits a transition between 80 and 100 degrees C, decreasing by half as a function of temperature. Compliance curves suggest the existence of crack closure in the stable martensite regime below 100 degrees C, but little or no crack closure in the stress-induced martensite regime above 100 degrees C up to 150 degrees C. The results suggest that both crack closure and the existence of an intrinsic fundamental threshold curve with both Delta K an d K-max thresholds as a function of load ratio are necessary to fully accou nt for R dependence of Delta K-th. (C) 1999 Elsevier Science Ltd. All right s reserved.