CONSTITUTIVE RELATIONS AND FATIGUE LIFE PREDICTION FOR ANISOTROPIC AL-6061-T6 RODS UNDER BIAXIAL PROPORTIONAL LOADINGS

Cyclic stress-strain curves (CSSCs) and fatigue lives were obtained from fully reversed fatigue tests in strain control on two anisotropic Al-6061-T6 rods. The experiments were conducted at room temperature under three types of loading conditions: tension-compression, torsion and combined proportional tension-torsion. Based on the CSSC data, the anisotropic constitutive relations of the rods were obtained by using Hill's anisotropic plasticity theory. Yield loci and flow behaviour were determined and compared with the theoretical predictions. Two anisotropic effective-stress-effective-strain criteria were evaluated. During the fatigue tests the fatigue cracking behaviour of the rods was observed and found to be shear dominated. Four multiaxial fatigue life prediction models representing three different concepts were used to correlate the fatigue life data. A shear cracking model incorporated with a material anisotropy constant correlated with the test data very well. The other models, however, gave poor correlations.