An investigation of a fatigue model with two competing failure mechanisms

A new combined fatigue model that considers a global and a local fatigue mechanism is presented. The global mechanism is assumed to describe fatigue failure from internal defects. It is modeled using weakest link theory. The local mechanism describes fatigue failure occurring in a very clean material (i.e. almost defect-free). This mechanism is described by the normal distribution, where the stress is the largest occurring stress value, the point stress, or the point stress adjusted with the stress gradient, denoted the gradient adjusted point stress. Experiments have been performed on notched specimens with different notch root radii. At high failure probabilities, the local model dominates in the combined model. At low failure probabilities, results are less clear. There, the local model is not accurate and the global model dominates. Using the gradient adjusted point stress as the local model and the volumetric weakest link integral as the global model gives the best overall fit to the probability of failure. It is noted that for design with respect to high failure probabilities (>20%), it is enough to use only the gradient adjusted local model.