Ratchetting strain as a driving force for fatigue crack growth

Ratchetting deformation has significant implications on material damage and fatigue life of components under service loading conditions. In this work, we explore the concept of ratchetting strain as a driving force in controlling crack growth, both time-independent and time-dependent, utilising elasto-plastic, visco-plastic and crystal-plasticity constitutive models. Characteristics of crack tip deformation were examined for both stationary and growing cracks using the finite element method. Whilst the strain range and the stress range remained largely unchanged throughout the loading cycles, distinctive strain ratchetting behaviour near the crack tip occurred in all cases, leading to progressive accumulation of tensile strain normal to the crack growth plane. It seems plausible that this tensile strain, or ratchetting strain, may be responsible for material separation leading to crack growth. The effects of material constitutive model, grain orientation and loading frequency on the development of ratchetting strain were also examined. Ratchetting strain was found to be related to accumulated plastic strain; and both were used in a crack growth criterion with which the crack growth rates of a nickel alloy were predicted. The preliminary results seem to be encouraging. (C) 2012 Elsevier Ltd. All rights reserved.