Errors in creep-cyclic plasticity testing: their quantification and correction for obtaining accurate constitutive equations

Inter-laboratory comparative cyclic plasticity testing has shown that data collected on the same material varies widely between laboratories across the world. The research reported here examines two testpiece designs using a combined creep-cyclic plasticity damage interaction theory, and compares theoretical results with plain bar data. The testpieces have been selected to examine two geometrical features: firstly, the extensometer ridges, and secondly, the loading shank-gauge section blend radii. For copper at 500 °C, undergoing creep-cyclic plasticity damage interaction, and at 20 °C, undergoing cyclic plasticity damage only, the dominant geometrical feature is shown to be the blend radii. Techniques are reported which enable errors in strain range to be identified and quantified using simple relations. It is shown how these relations can be used to correct data obtained from the different testpiece geometries to achieve unified data from homogenous uni-axial stress conditions. It is proposed that inter-laboratory comparative studies should make use of such techniques to help unify material data, and so avoid variations often wrongly attributed to `material scatter'.