Impact of Elevated Temperature Tensile Hold Times on Fatigue Crack Growth: The Concept of a Thermally Affected Zone

Isothermal and thermomechanical fatigue crack growth tests up to and above the maximum usage temperature for Inconel 718, nominally, 650°C, were executed to characterize the impact of time spent at elevated temperature on the fatigue crack growth rate. Various spectra were tested which showed tensile holds executed at a spectrum’s maximum load or immediately following an increase in load increased the fatigue crack growth rate. Additionally, a series of tests were executed to evaluate the impact of the stress intensity, duration, and temperature of a tensile hold on fatigue crack growth rate. The concept of a thermally affected zone to describe an area ahead of the crack tip weakened by the tensile hold that accelerates crack propagation is presented. As the stress intensity, duration, or temperature of a tensile hold increase, the temperature affected zone increases in size leading to an increase in fatigue crack growth rate for subsequent cycling.