Low-Cycle Fatigue Damage Mechanisms at High Temperature

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Abstract
Observed effects of wave shape on low-cycle fatigue resistance and on concomitant fracture appearance at elevated temperature are summarized. Additionally, previously unreported tests on oxygen-free high conductivity (OFHC) copper at 673 K, as well as available published results, are examined using unequal strain rates to produce the wave shape. For OFHC copper, the fatigue lifetimes decreased by an order of magnitude as the tensile-going strain rate was reduced from 1.7 × 10 -3 s -1 to 1.7 × 10 -5 s -1 with a constant cyclic period. Accompanying this reduction in lifetime, the fracture mode changed from a transgranular fracture for the fast-slow wave shape to an intergranular single-crack fracture for equal ramp rates to interior cavitation for the slow-fast test. Based on these findings and related work of other investigators, a qualitative model is presented to predict the ranking in fatigue resistance and in fracture mode as a function of wave shape and environment, for a given temperature, strain range and overall frequency. Essential features of this model include the assumption of grain boundary cavity size variability depending on the magnitudes of the tensile-going and compressive-going strain rates of the hysteresis loop and recognition of the important role of environment in degrading life and affecting fracture mode, especially when tensile- and compressive-going strain rates are equal.