FATIGUE CRACK-GROWTH IN PLATE SPECIMENS UNDER MODE-III LOADING

Fatigue crack growth in plate specimens of aluminum and an aluminum alloy has been investigated under Mode III loading conditions. It has been found that only when a fully plastic situation exists, does crack extension proceed by a valid Mode III mechanism. When plasticity is restricted to planes of maximum shear there is a strong component of Mode I cracking which results in delamination in the direction of macroscopic growth. Under constant load amplitude cycling the crack growth curve exhibits three stages: a reduction in rate is followed by a period of steady growth prior to acceleration in the final stages. Such a profile is independent of material condition or failure mechanism. Cycling between fixed displacement limits leads to a reducing growth and crack arrest which occurs at a length proportional to the square of the displacement.