Sensitivity of crack-growth lives to sustained thermal gradients

Abstract

This paper investigates the effect of path-dependent material properties on predicted crack-growth lives with material properties set at the instantaneous crack-tip location. Specifically, we focus on cracks advancing through thermal gradients with temperature-dependent crack-growth properties. Here, material properties that describe fatigue crack growth under cyclic load and time-dependent crack growth under sustained load have been set to capture the response of the high-temperature alloy ME3. This study investigates the effect of peak temperature, peak stress, temperature gradient, hold time, and time to reach full loading. To perform this investigation, we design a simple example problem that captures key features of fatigue crack growth without introducing unnecessary complexity. Results indicate that increasing lives are associated with lower crack-tip temperatures due to cooling thermal gradients that reduce crack growth per cycle. Furthermore, these results reveal minimal impacts on lives for temperatures with negligible time-dependent crack growth, and larger increases in lives are expected for cracks with more appreciable time-dependent crack growth. Designers may employ these ideas to extend useful lives of applications with strong spatial gradients in the material response, e.g., recent efforts to design internal cooling passages into gas-turbine engines using additive manufacturing.