A set of tests for studying the effect of the stress state on ultimate metal plasticity at high temperature

A set of tests that can be used tostudy the effect of the stress state on ultimate metal plasticity at high temperature is proposed. Ultimate plasticity can be characterized quantitatively by the strain accumulated to failure. Dimensionless invariant parameters, such as the triaxiality parameter k and the Lode-Nadai parameter μ σ, are used as a characteristic of the stress state. A set of the parameters k and μ σ unambiguously characterizes the stress state under plastic deformation. Being dimensionless, these parameters can be used to compare the stress state of materials with different levels of strength properties. The triaxiality parameter k characterizes the level of normal stresses. If k > 0, then normal tensile stresses dominate. If k σ characterizes the type of the stress state. The value of μ σ = +1/-1 corresponds to the stress state of axisymmetric compression/tension, and μ σ = 0 under the plane stress state. The set of tests consists of conventional tension tests of cylindrical specimens, tension tests of notched cylindrical specimens, newly developed tests of “bell” specimens and tests of a thick-walled cup with a thinned bottom. This types of tests has allowed us to investigate ultimate plasticity under monotonic deformation in the range of parameters -1,2 σ оC. The shear strain rate was H = 0,1; 0,3; 0,5 s -1. The fracture locus of the 01570 aluminium alloy has been obtained from the test results. The fracture locus can be used in damage mechanics models to predict alloy fracture under metal forming processes.