FATIGUE-CRACK GROWTH UNDER MODE-II LOADING

The behavior of fatigue crack growth for low and medium carbon steels, an austenitic stainless steel and an aluminum alloy under pure Mode I I loading was investigated experimentally, using cruciform specimens. The results show that under pure Mode II loading, fatigue crack propag ation has three possibilities, namely, bifurcation into two branches, propagation along the original Mode II direction, and the mixture of t hese two situations, depending on the material. The growth rate da/dN vs, Delta K-II relation for Mode II propagation is similar to a Paris type law for Mode I growth. Fractographic observations by optical micr oscopy and SEM were made also on all specimens tested. When a crack br anched, striations parallel to the crack front which were often associ ated with Mode I fatigue crack growth were observed and long marks par allel to the crack propagation direction were also found for slanted f racture surfaces. When a crack propagated along the original Mode II d irection, many frictional marks parallel to the crack propagation dire ction were observed.