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