Data scatter in multiaxial fatigue: from the infinite to the finite fatigue life regime

Abstract

This paper deals with the data scatter occurring in uniaxial and multiaxial high cycle fatigue. The weakest link concept together with a critical plane damage model based on a microplasticity analysis are combined to describe the distributions of the fatigue limit and of the fatigue life under different loadings. Tension, torsion and combined proportional tension and torsion load conditions are analysed. The application of the Weibull approach is possible under any multiaxial loading by defining a relevant equivalent stress. This probabilistic approach is applied only on the specimen surface to reflect the fatigue damage mechanisms that lead generally to crack initiation on the free surface. The model predictions are compared to some fatigue data from tests on a mild steel C36. This material is first submitted to simple reversed tension and torsion and two reference SN curves are drawn. The scatter around the fatigue limits is estimated from these curves. Some tests under combined proportional tension and torsion loading modes with two different stress amplitude ratios are then conducted. A normalised SN curve can be derived from the probabilistic model and all the possible sinusoidal synchronous multiaxial load conditions can be represented and compared on the same graph giving some information about the cumulative failure probability. All the experimental results under the different loadings are gathered on this curve and it appears that all the points lie in a band from 10 to 90% cumulative failure probabilities.