Creep rupture of copper and aluminium alloy under combined loadings-experiments and their various descriptions

Creep tests are carried out under tension, pure torsion, and combined tension and torsion at an elevated temperature of 523 K for pure copper and 423 K for an aluminium alloy. Different creep and rupture properties of the materials are observed throughout the deformation process under the different stress states. The effects of stress states on primary creep, secondary creep, the failure and lifetime of the materials are analysed. A new set of multiaxial mechanisms-based creep damage constitutive equations has been formulated on the basis of analysis of previous creep models. The proposed unified creep damage constitutive equations are determined using experimental data achieved for both materials at three effective stress levels. Creep tests for each stress level were carried out for three stress states. The comparison of experimental and computed effective creep strain curves is carried out for all the stress states and stress levels tested for both materials. In addition, it has been verified that the determined multi-stress-state creep damage constitutive equations can be used to predict the isochronous surfaces of the materials very precisely. To compare with conventional constitutive descriptions, a set of multiaxial constitutive equations is also determined for the experimental data of the two materials using the same optimisation techniques. The quality of the fittings is compared and further discussion is carried out for the stress-state variable α.