The roles of rise and fall time in load shedding and strain partitioning under the dwell fatigue of titanium alloys with different microstructures

•The time for local peak stress to reach equilibrium under different rise and fall time is quantified.•The influence of the rise time is stronger than the fall time.•Both β lath morphology and α variants affect the dependence on the rise and fall time.•The strain partitioning between the α and β phases determines the sensitivity to the rise time. Dwell fatigue failure of titanium alloys has threatened flight safety for over five decades. To quantitatively evaluate the component life, experimental dwell fatigue tests are generally conducted in the lab. However, the loading profile in the lab is generally shorter than those of the realistic in-service conditions by several orders of magnitude, not only for the stress hold but also for the stress rise and fall periods. Although the dependence of fatigue life on the dwell period has been extensively studied, the effect of the rise and fall time has been ignored. Investigating such a topic is extremely time-consuming, especially when the applied stress is lower than the yield strength under the in-service loading. The fatigue tests could require years of loading when the rise and fall periods reach the order of magnitude of 103s, for example, which is experimentally infeasible. Modeling techniques provide a solution to systematically study the effect of the rise and fall on dwell fatigue responses benefit from the high computing power availability. In this study, crystal plasticity models representing equiaxed α and dual-phase α-β lamellar microstructures have been developed and used to study the effects of the rise and fall time over a wide range (10−1-103s) on the load shedding behaviors of IMI834 titanium alloy under dwell fatigue loadings. The time for the peak stress at the soft-hard grain boundary reaching equilibrium under different load profiles is quantitatively investigated. The rise and fall time have been demonstrated to influence the load shedding at different degrees. The microstructural features in the soft grains of dual-phase titanium alloys play a significant role in affecting the local stress evolutions. The correlations between the load shedding and the rise and fall time are also influenced by the microstructure. The strain partitioning between the α and β phases in the soft grains under cyclic dwell fatigue of titanium alloys has been examined to elucidate the underlying mechanisms of the rise/fall time dependence.