Determination of the instantaneous strain rate during small punch testing of 316 L stainless steel

Abstract

The Small Punch Test, SPT, has been widely used by scientists for the mechanical characterization of nuclear materials and different applications. This technique is able to provide an approximation of mechanical properties from a very small amount of material. However, the inhomogeneous distributions of local stresses and strains, coupled to the response of the material hinder accurate predictions of the mechanical properties in terms of tensile tests parameters. Additionally, the determination of the instantaneous strain rate, as the test proceeds, remained a pending issue. This article analyses the predictions of SPT on 316 L steels by a FEM model, validated by an additively manufactured 316 L SLM (Selective Laser Melting) stainless steel, in terms of thickness evolution, local deformation and instantaneous strain rate during SPT testing. This alternative analysis shows that the deformation behaviour and, thus, the final form of the sample is affected by the strain hardening behaviour, which it has an influence on the instantaneous local strain rate. The instantaneous local strain rate was calculated for two dissimilar 316 L austenitic stainless steel, one “hardened”, characterized by low ductility but very high yield strength (YS) and ultimate tensile strength UTS, and another “annealed”, characterized by very low YS and large ductility, typical of annealed 316 L steel. The calculated central and minimum thicknesses, strain and instantaneous local strain rate, are not linear as SPT testing proceeds. Therefore, they are fitted to polynomial equations for both annealed and hardened 316 L steels. These functions allow, for instance, LVDT measurement corrections for appropriate SPT curve analysis, and proper true strain rate measurements for creep (SPCT) data analysis.