Fatigue crack tip deformation

The shape of a fatigue crack tip as influenced by an air or a vacuum environment has been investigated in two stainless steels and an aluminum alloy. Under plane strain conditions and at crack growth rates in the Paris region, the crack tip opening displacement (CTOD) is much larger in vacuum than in air, a circumstance attributed to strain localization in air due to the presence of moisture and the absence of strain localization in vacuum. In type 304 stainless steel, a strain-induced transformation from austenite to martensite occurs at the crack tip, and the extent of this strain-induced transformation in type 304 stainless steel is consistent with the degree of blunting taking place at the crack tip as influenced by the environment. In air, the extent of transformation is a function of the ΔK level, and as a result, the crack opening level is found to differ in a ΔK decreasing test as compared to aAK increasing test. Fatigue striations are observed in air but are absent in vacuum. It is proposed that the greater extent of blunting in vacuum is responsible for the absence of striations in vacuum.