Prediction of fatigue crack propagation under irregular loading

A method for predicting fatigue crack propagation, both through-thickness and surface growth, is presented which takes into consideration kinetic effects of crack growth following tensile overloading. The fatigue crack propagation prediction is based on the consideration of the contribution of the following factors to the fatigue crack growth retardation after the overload: (a) generation of a residual compressive stress system ahead of the fatigue crack tip and fatigue crack closure due to residual plastic strains occurring in the wake of the growing crack tip; (b) plastic blunting of the fatigue crack tip. It is supposed that the total number of cycles of fatigue crack retardation N-d after overload will be composed by two components: (a) N-d((1)), the number of cycles until the re-establishment of the crack closure level equivalent to that just before overload; (b) N-d((2)), the number of cycles until the initiation of the ''continuation'' fatigue crack from the blunted tip of the crack. Prediction of surface crack propagation is carried out taking into consideration the variable constraint effect along the fatigue crack front. The experimental verification of the proposed method for fatigue crack propagation was carried out on specimens which were fabricated from a 15Kh2MFA steel. Good agreement was obtained between experimental data and crack growth calculations.