Fatigue crack propagation for a through thickness crack: a crack propagation law considering cyclic plasticity near the crack tip

A new parameter, which can quantitatively describe fatigue crack propagation, is proposed in this paper. This parameter reflects the cyclic plasticity behaviour near the crack tip, and the resulting built up of layers with the residual plastic deformations on the fatigue crack surfaces. A fatigue crack closure model suitable for an arbitrary applied stress distribution with an arbitrary residual stress distribution based on appropriately modified Dugdale’s concept of crack tip plasticity is developed. A re-tensile plastic zone generated (RPG) load, at which the tensile plastic zone starts to develop ahead of a crack tip, is defined. The effective stress intensity factor range is redefined by replacing the crack opening load with the RPG load. This redefined effective stress intensity factor is termed ΔKRPG. When ΔKRPG substitutes for ΔK or the crack opening load based on ΔKeff the “knee” in the threshold region of the crack growth data is not seen any longer. As a result, it is confirmed that a stopping phenomenon of a fatigue crack propagation can be quantitatively and appropriately described without the threshold value such as (ΔKeff)th. It is also confirmed that the proposed crack closure model can provide quantitative estimates of the fatigue life under various loading conditions and in pre-existing residual stress field.