Competing Failure Modes in Fatigue and the Consequent S-N Curve Shapes

摘要

It has recently been discovered that competing fatigue failure modes lead to two separate S-N curves, one for surface-initiated cracks and the other for interior-initiated cracks. The competing failure modes and the associated two different fatigue life distributions seem to occur in high strength materials that either have sparsely populated defects or are processed to minimize the inclusion content. Typically, there exists a shorter-life-distribution that is associated with surface-crack-initiated failures. The longer-life-distribution generally occurs due to internal-crack-initiated failures. There can be a complete separation of the two failure distributions or they can dominate at high and low stress ranges with a discontinuity (step) in the mid-stress-range, depending on the material. Examples of competing fatigue failures in titanium alloys and in nickel-base superalloys are presented. We illustrate the metallurgical and environmental conditions that would trigger this type of behavior in structural materials. We also develop a broad rationale based on which different shapes of S-N curves, including the very-high-cycle-fatigue segments, can be explained based on the presence or absence of defects and the environmental effect. The competing failure modes phenomenon challenges the prevailing notions about S-N fatigue curves and requires a new paradigm in material design and fatigue life considerations.