A Property Fitting Approach for Improved Estimates of Small Cycle Fatigue Damage

An analysis method is presented for determining the conventional cyclic material properties from uniaxial strain-life fatigue test data. The method focuses on improving fatigue life predictions for variable amplitude histories when a significant amount of damage is associated with small cycles. Under these conditions, it is necessary to extrapolate the strain-life curve below the constant amplitude endurance limit. This extrapolation requires careful attention to the curve fitting process to avoid the influence of endurance limit affected data. A fitting approach has been developed which identifies structure shifts in the data in order to obtain cyclic material properties not influenced by the endurance limit or other transitions that negatively impact life predictions. This approach has been evaluated by comparing test lives of uniaxial specimens using the SAE log-skidder bending history to life predictions made using a fatigue analysis model based on rainflow counting, Smith-Watson-Topper mean stress, and linear damage. Good correlation of predicted to experimental life was achieved.