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

High-Temperature Fatigue;Thermal Overload;Hastelloy X;Crack Closure;Multiscale digital image correlation (DIC)

Casperson, Mallory ; Lambros ; John

关键词: High-Temperature Fatigue;    Thermal Overload;    Hastelloy X;    Crack Closure;    Multiscale digital image correlation (DIC);   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/30985/Casperson_Mallory.pdf?sequence=4&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

This work investigates, at multiple length scales, the thermal effects on fatigue crack closure of Hastelloy X, a nickel-based superalloy.Using multiscale, digital image correlation (DIC), macroscale experiments were performed at 2x magnification (2 µm/pix), providing full-field crack closure measurements.Microscale experiments performed at 10x magnification (0.4 µm/pix), provided local crack closure measurements at varying locations from the crack tip, along the crack line.Using these techniques, fatigue crack growth experiments were performed on single-edge notch tension (SENT) specimens of Hastelloy X.A ΔK of 19 ±2 MPa√m was maintained constant throughout the experiments along with an R ratio of 0.05.Isothermal experiments were performed at room temperature (RT), 300 °C, and 550 °C.It was found through the microscale experiments that the level of measured crack closure increased as the crack tip was approached, as has been seen in the past.Local closure levels were the same between the room temperature and high temperature cases, to within a threshold of ±10% - the resolution of the microscale method.Through the macroscale experiments, it was shown that regardless of temperature, under isothermal conditions, the measured levels of crack closure were 30% ±10% of peak load.Within the resolution of the measurement methods used here, under isothermal conditions, crack closure was shown to be independent of temperature.Variations in temperature however, caused a strong temperature dependent crack closure response.The thermal jump cases showed that a considerable thermal spike greatly affects the amount of measured crack closure following that increase in temperature.In the case of a 300 °C to 400 °C jump, no change occurred.However, in the cases where a more substantial thermal jump occurred, crack closure was either reduced (as in the 300 °C to 550 °C jump) or completely eliminated (as in the RT to 250 °C jump and the 300 °C to 650 °C jump).In the case of a thermal overload from 300 °C to 650 °C and then back to 300 °C, crack closure was seen to be extremely diminished following the overload, and to then gradually return to nominal levels once the fatigue crack had advanced outside of the enlarged plastic zone caused by the thermal spike.Competing mechanisms including crack tip blunting, the change in temperature, the decrease in yield stress, the decrease in elastic modulus, and the enlarged plastic zone ahead of the crack tip, are thought to be responsible for the changes in closure levels following the thermal jumps and during the thermal overload and were therefore investigated.In all cases where crack closure was eliminated following the thermal jump however, crack tip blunting was observed to be the dominant mechanism affecting closure.The blunted crack needed to reinitiate before further crack growth could occur.