Low-Cycle Fatigue under Biaxial Strain

The present work aims to obtain some information about the crack initiation criterion under biaxial low-cycle fatigue strain in phase or 180 deg out of phase. The investigation was carried out on A 316 stainless steel. A servohydraulic experimental rig was developed for application of biaxial high-strain reversed cycles. The strains were controlled and the axes of principal strain were fixed during these tests. The test specimens are thin-walled tube on which we apply axial tension-compression and internal-external pressure difference by using two hydraulic servosystems. The results obtained for three different strain ratios are plotted as a function of half of the equivalent Tresca strain. The Tresca criterion has been shown to be nonconservative for cases where the axes of principal strain rotate continuously, but in our tests we observe that torsion is less damaging than plane strain, which is less severe than tension. In other words the Tresca criterion is conservative, which is in agreement with the ASME code. Finally, the fatigue crack growth is studied by a scanning electron microscope, and a propagation law is proposed.