A study of the stress ratio effects on fatigue crack growth using the unified two-parameter fatigue crack growth driving force

An experimental evaluation of fatigue crack growth

One of the basic mechanisms for fatigue crack growth in ductile metals is that depending on crack–tip blunting under tensile loads and re-sharpening of the crack–tip during unloading. In a standard numerical analysis accounting for finite strains it is not possible to follow this process during many cycles, as severe mesh distortion at the crack–tip results from the huge geometry changes developing during the cyclic plastic straining. In the present numerical studies, based on an elastic–perfectly plastic material model, crack growth computations are continued up to 200 full cycles by using remeshing at several stages of the plastic deformation. Three different values of the load ratio R = K min / K max are considered. It is shown that the crack–tip opening displacement, CTOD, typically undergoes a transient behaviour, with no crack closure during many cycles, before a steady-state cycling with crack closure at the tip starts to gradually develop.

On fatigue crack growth in ductile materials by crack-tip blunting

Small time scale fatigue crack growth analysis

Crack growth behaviour of ultrafine-grained AA6063, processed by equal-channel angular pressing (ECAP) via route E at room temperature, was evaluated with special emphasis on the effect of grain size distribution and work hardening. A bimodal, two times ECAPed condition and a monomodal ultrafine-grained condition after eight ECAP passes are compared with the coarse grained peak aged material. Depending on their microstructure, the ECAPed materials show significantly lower fatigue threshold values (ΔKth) and higher crack growth rates (da/dN) than their coarse grained counterparts. Micrographs of the crack propagation surfaces reveal the reduced grain size as major key to increased crack growth rates of the ECAPed material, as it influences roughness-induced crack closure and crack deflections. Furthermore, the effects of other features, such as ductility, work hardening capability and grain boundary characteristics, are discussed.

J.Z Zhang

Papers

A shear band decohesion model for small fatigue carck growth in an ultra-fine grain aluminium alloy