PSI - Issue 42

Numan Berat Yondu et al. / Procedia Structural Integrity 42 (2022) 1567–1575 Author name / StructuralIntegrity Procedia 00 (2019) 000 – 000

1572

6

scale and SEM images. In Figure7 failure surface of non-dwell fatigue test observed as smooth surface and can be considered as transgranuler fracture, which is more expected in terms of fatigue fracture. After dwell application in Figure8 obvious change in fracture surface can be observed. Performed SEM imagining showed slightly controversial results since the dwell fatigue and non-dwell tests showed similar features along the fracture surface. In Figure9 fatigue striations and secondary cracks are observed in both test conditions.

Figure6. Microstructural imaging of fracture surfaces a) 450 o C Fatigue test 598Mpa 1.08 normalized cycles to failure b) 450 o C Fatigue test 598Mpa 1.18 normalized cycles to failure. By examining NCR values and fatigue curves it can be stated that Inconel718 material is not effected from dwell durations at 450 o C. This observations can be caused by different mechanisms. In literacy works different mechanisms are observed to arrest crack growth and cause crack closure such as; plasticity induce crack blunting, oxide induce crack arrest, mechanical closure. From our observations with softening, increase in plasticity at the crack tips are highly possible to lead crack propagation arrest. Crack closure and local plastically deformed zone which is a high energy zone and hardens the crack to propagate, at lower stress levels this mechanism is most likely to increase the material's pure bending fatigue life.