Influence of Microplasticity and Crack Initiation on the Fatigue Behaviour of Precipitation Hardened Alloys

Abstract

The influence of microscopic inhomogeneity of plastic strain in precipitation hardened alloys on the surface morphology and the crack initiation during fatigue was investigated. Fatigue cracks are formed in consequence of high stress concentrations at steps on the surface. A quantitative correlation between the height of the steps and crack initiation can be derived. By understanding these crack initiation mechanisms the number of cycles at which cracks occur can be influenced. On the one hand, a localized accumulation of plastic deformation should be avoided, because due to the high frequency of the occurrence of high steps, the probability of crack initiation would be enhanced. This would require a microstructure with homogeneous slip distribution. On the other hand, it is possible to prevent the initiation of crack nuclei by suitably influencing the condition of the surface. By shot peening or surface diffusion coatings the surface yield stress can be raised, a procedure by which a residual compressive stress underneath the surface is achieved. This stress causes a delay of surface cracks.