'Towards a Proper Understanding of Fatigue Crack Growth and Crack Closure

Abstract

Conventional theories to describe and predict FCG are based on a SIF parameter, ΔK. If ΔK is used as similitude parameter to predict FCG, an R-effect arises. This R-effect can be accounted for by means of plasticity induced crack closure: A plastic wake behind the crack tip causes the crack to close before a zero tensile load is applied. As such ΔK is adapted to an effective ΔK, ΔKeff. If ΔKeff is used for similitude the R-effect disappears.These theories have recently been disputed and an alternative, SER dominated FCG description model has been proposed. This research discusses the confusion about using ΔK for similitude and approaches FCG also from a SER perspective. Experiments are designed and executed in air and vacuum environment to obtain a better understanding of fatigue, the R-effect and crack closure. The experimental results show that ΔK is an improper similitude parameter and that the R-effect is an artefact of choosing ΔK for similitude. Instead of that, fatigue should be approached from a SER perspective. Furthermore the research shows that the R-effect is not caused by (plasticity induced) crack closure.