Investigation of thermal effects on fatigue crack closure using multiscale digital image correlation experiments

Hastelloy X, a nickel-based superalloy, has been extensively used for high temperature applications. In this work, Hastelloy X notched samples were used to investigate fatigue crack growth and crack closure at elevated temperatures. Isothermal, thermal jump, and thermal overload experiments at varying temperatures (up to 650 °C), were performed. Macroscale (2 μm/pixel) digital image correlation was performed on images taken at various stages of crack growth and microscale (0.4 μm/pixel) digital image correlation was used on images obtained directly behind the crack tip to quantify the local effects of crack closure. Experiments focused on the effects of isothermal conditions and thermal overloads on measured crack closure levels. Each isothermal experiment showed steady state crack closure levels of 0.30 while thermal jumps and thermal overloads created significant decreases (or, in some cases, complete elimination) in closure levels immediately following the temperature change. Similar to the case of mechanical overloads, as crack growth was continued beyond the plastic zone enlargement created by the thermal spike, closure levels were reestablished near the original steady state values. Competing mechanisms, including crack tip blunting, crack bifurcation, change in temperature, yield stress, elastic modulus, and plastic zone size, thought to be responsible for the changes in closure levels following the thermal jumps and during the thermal overload, were investigated.