Fatigue life estimation of pitted specimens by means of an integrated fracture mechanics approach

The synergistic nature of corrosion and fatigue is one of the main reasons for the premature failure of engineering structures and components. The decrease in fatigue life of specimens subjected to aggressive environments is likely to be attributed to local, pit-induced, stress concentrations that cause premature initiation of fatigue cracks. In this work, we have developed a predictive approach to assess the life of specimens containing pits assuming the pit both as a crack and as a smooth notch. The proposed approach assumes that even though the critical place for crack initiation seems to be the pit mouth, once the crack initiates, during propagation, the location of the hot spot shifts according to the location of the crack tip and due to the redistribution of stresses and strains. An integrated fracture mechanics approach that compares the driving force of the crack emanating from the pit and the evolution of the material threshold to crack propagation with crack length is proposed. The material threshold is estimated from the plain fatigue endurance limit, the position d of the strongest microstructural barrier and the SIF threshold for long cracks. The effective driving force is assessed by means of parametric FEA. This approach considers the influence of the pit geometry on the stress field surrounding the crack providing a more realistic estimate of the applied driving force. The maximum applied stress range as a function of number of cycles (S-N curves) have been estimated for different configurations (stress level, initial crack length, location at the crack front) assuming that failure of the component will be given when the critical crack length is reached. The procedure has been first developed and used to assess deep pits, as these are the most detrimental and common configuration encountered in real Oil and Gas applications.