Isochronous creep rupture loci for metals under biaxial stress

Theoretical and experimental methods are presented for the determination of creep isochronous loci of copper and aluminium alloy subjected to multiaxial stresses. Creep tests were carried out in a complex stress state at elevated temperature equal to 523K in the case of copper and 423 K for aluminium alloy. Typical creep parameters, such as the duration of primary creep, the duration of secondary creep, the minimum creep rate and the time to rupture are analysed. The preliminary results for copper proved its isotropy during monotonic loading tests. In the case of creep conditions the material demonstrated anisotropic behaviour. A similar effect was observed for aluminium alloy; however, in this case the material exhibited additionally an initial anisotropy during monotonic loading tests. It is shown that for both materials, at the range of selected effective stresses, all typical creep parameters (e.g. minimum creep rate, time to rupture) are functions of the state of stress. The isochronous creep rupture surfaces were determined on the basis of experimental data and compared with those resulting from theoretical considerations. The surfaces describing the time to the end of primary creep and the start of tertiary creep are the same as the isochronous rupture surface. The latter is best described using the Sdobyrev equation with appropriate coefficients.