This study discusses multiaxial low cycle fatigue strength of Mod.9Cr-1Mo steel under proportional and non-proportional loadings at room and high (823K) temperatures. Strain-controlled multiaxial low cycle fatigue tests were perfoemed using a hollow cylinder specimen including interruption tests. Strain paths employed were a push-pull straining, a reversed torsion straining and a circle straining. The former two are the proportional loadings where principal directions of stress and strain are unchanged. The latter one is the non-proportional loading where axial and shear strains have 90 degree phase difference and principal directions of stress and strain are changed continuously in a cycle. Behaviors of cyclic deformations and failure lives and an evaluation of the failure life are discussed. Surface cracks and microstructures in fatigued specimens are also observed by a digital microscope and a transmission electron microscope (TEM) in order to investigate mechanisms of cyclic deformation, failure and reduction in failure life due to non-proportional loading.
