Investigation of the Damage Mechanisms Influencing the Short Crack Behavior of Inconel 625 Under Variable Amplitude Fatigue Loading

Variable amplitude fatigue loading can result in both accelerated and decelerated fatigue damage due to load interaction effects. Short fatigue cracks in particular exhibit a wide range of crack growth behavior due to multiple damage mechanisms contributing to interaction effects. To investigate this variation in fatigue damage behavior and the influence of causative damage mechanisms, variable amplitude fatigue tests were conducted on an Inconel 625 alloy. Periodic overload, high-low, and repeated block loading patterns were applied, and specimens were analyzed with a surface replication technique during testing to capture crack growth. Fracture surface imaging of failed specimens identified crack face morphology. High stress cycles in the overload and repeated block loadings resulted in increased fatigue life, and evidence of plastic crack closure was noted in periodic overload samples. Crack growth deceleration due to overload was identified in crack lengths as short as 65 µm. This increase in fatigue life differs from other research that demonstrated damage acceleration of short cracks during variable amplitude fatigue. This acceleration was attributed to crack closure and microstructural barriers, whereas the deceleration in this study is attributed to the interaction of plastic crack closure and crack extension caused by the application of an overload.