Features of the influence of stress concentration on the multicycle fatigue resistance of ZhS3DK heat-resistant nickel alloy

1. Breaks were obtained in the fatigue curves in the 105−107 cycle area of lives for specimens with stress concentration (ρσ = 2.5 and 3.9) of ZhS3DK cast nickel alloy under high temperature conditions (700 and 850\dgC) with various load cycle asymmetries. On the basis of a fractographic analysis of fatigue fractures the breaks in the fatigue curves may be explained based on a hypothesis verified for steels of the change in the stressed state in front of a developing fatigue crack from the plane stressed state in elastoplastic deformation at high levels of variable stress to plane strain at lower levels of variable stress (below the critical). 2. The effect of a very rapid change in the stress concentration sensitivity factor q of ZhS3DK alloy related to the breaks in the fatigue curves of specimens with stress concentration and the absence of them on the fatigue curves of unnotched specimens was revealed. A higher sensitivity to stress concentration (q=0.3−0.5) for a base of 104−105 cycles for the left branches of the fatigue curves and a weak sensitivity (q=0−0.05) for a base of 107−108 cycles for the right branches of the fatigue curves were obtained. 3. A different sensitivity to stress concentration of ZhS3DK alloy with different stress ratios A = σa/σm for the areas of preferential failure from fatigue or from cyclic creep separated by the critical stress ratio Acr was established.