Question

Why do materials fail at lower loads if stress is cyclic? Describe fatigue mechanisms for alloys with pre-cracked structures.

Answer 1

Material under cyclic loading is subjected to cyclic or variable stress. This result in decrease resistance of material due to Bossinger Effect.

For alloys with precracked structure, stress concentration take place at crack end. Crack growth caused by fatigue effect is attributed to plastic deformation at a crack-tip plastic zone, wherein the plastic deformation occurs via dislocations.which dislocation motion occurs; the slip plane. The axial (tension or compression) loading is decomposed into a normal stress, which is perpendicular to the slip plane, and a shear stress, which is parallel to the slip plane. This shear stress then results in the movements along the slip plane. This implies that shear stress contributes to plastic dislocation, and then results in crack growth. In this case, the axial loading is transformed into the shear stress to produce fatigue propogation