Fatigue Life Parameter for Type 304 Stainless Steel Under Biaxial-Tensile Loading at Elevated Temperature

This paper describes a damage Parameter for predicting fatigue life under biaxial-tensile loadings. Several studies have focussed in the past on the situations where the in-plane biaxial stress ratio is negative; however, little attention has been paid for the cases involving both principal stresses in tension. A new testing method is used to carry out biaxial fatigue tests, at room and 427°C, on Type 304 stainless steel for different positive values of the stress ratio. In the experimental procedure, a disk-shaped specimen was used in connection with a spatial-arms mechanism which converts the uniaxial force generated by a conventional testing machine to radial forces extending the disk specimen. A modified virtual strain energy parameter is then suggested to normalize fatigue data obtained under a wide range of stress states. The proposed parameter accounts for the mean stress and the mean strain effects in an explicit form. In addition, the COD equivalent stress and strain concepts are adopted to account for the stress state biaxiality. The predictions of the proposed parameter are compared with the obtained experimental data and the correlation between the applied stress states and the experimental fatigue lives is discussed.