Experimental investigations on extruded 6063 aluminium alloy tubes under complex tension-compression stress states

The yield and plastic flow behaviour of tubes under biaxial tensile stress states has been investigated intensively. However, limited work has been performed that considers the axial feeding force that imparts a tension-compression stress state which could affect the final geometry of hydroformed tubes. In this paper, plastic deformation behaviour of extruded 6063 aluminium alloy tubes under internal pressure and axial compressive force were investigated by using a servo controlled internal pressure – axial compression testing machine. This machine is capable of applying stress paths to tubular specimens via a closed-loop control system. Eleven linear stress loading paths, which were uniformly distributed in the fourth quadrant of the circumferential stress and axial stress plane, were implemented. Initial and subsequent yield loci for different levels of strain-hardening states as well as the flow directions of the plastic strain increments for these loading paths were determined. Several yield and plastic potential functions commonly used in engineering were examined based on the obtained experimental results. It was found that the associated-flow-rule-based YLD2000-2D and Hu2005 yield criteria under the condition of anisotropic hardening can be used for predictions of yield stress whereas the isotropic hardening model is not as representative. However, for predictions of the plastic flow direction, none of these constitutive models can model all loading paths. It was determined that when the ratios of the axial stress to circumferential stress expressed α > −1.19, the associated-flow-rule-based YLD2000-2D and Hu2005 yield criteria were most representative of material behaviour, whereas for α < −1.19, the Barlat89 yield criterion was most representative. The prediction accuracy of the non-associate associated-flow-rule-based Hill48 plastic potential was inadequate for loading paths of α ≤ −1.