PREDICTION OF FATIGUE-CRACK PROPAGATION UNDER IRREGULAR LOADING

A method for predicting fatigue crack propagation, both through-thickn ess and surface growth, is presented which takes into consideration ki netic effects of crack growth following tensile overloading. The fatig ue crack propagation prediction is based on the consideration of the c ontribution of the following factors to the fatigue crack growth retar dation after the overload: (a) generation of a residual compressive st ress system ahead of the fatigue crack tip and fatigue crack closure d ue to residual plastic strains occurring in the wake of the growing cr ack tip; (b) plastic blunting of the fatigue crack tip. It is supposed that the total number of cycles of fatigue crack retardation N-d afte r overload will be composed by two components: (a) N-d((1)), the numbe r of cycles until the re-establishment of the crack closure level equi valent to that just before overload; (b) N-d((2)), the number of cycle s until the initiation of the ''continuation'' fatigue crack from the blunted tip of the crack. Prediction of surface crack propagation is c arried out taking into consideration the variable constraint effect al ong the fatigue crack front. The experimental verification of the prop osed method for fatigue crack propagation was carried out on specimens which were fabricated from a 15Kh2MFA steel. Good agreement was obtai ned between experimental data and crack growth calculations.