Environmentally Induced Transitions in Fatigue Fracture Mode

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Abstract
The introduction of various gaseous species into the test environment can have significant effects on fatigue crack propagation. This is usually noted as an increase in crack propagation rate in the aggressive environment compared to the baseline data. In this investigation fatigue crack propagation has been conducted in low pressure gaseous environments such as hydrogen and oxygen and a vacuum of 1 × 10−8 torr as reference for several iron and nickel base alloys and a titanium alloy. Other variables investigated include stress intensity level and temperature. Scanning electron fractography and replica fractography have been used to characterize the resulting failure mode. Fracture morphology varies from ductile transgranular fracture in vacuum for all materials to brittle integranular fracture for some of the nickel base alloys in hydrogen gas. Transitions in failure mode caused by oxygen gas and less severe cases of hydrogen embrittlement are more subtle. In some cases the transition in failure mode can be correlated with indications of changes in deformation mode. It is proposed that this transition in deformation mode is the result of an interaction between the adsorbed gas atoms and mobile dislocations within the plastically deforming region at the crack tip.