An improved correlation for the estimation of the yield strength from small punch testing

This study aims at improving the empirical correlation for estimating the yield strength from small punch tests. The currently used procedure in the European standard EN 10371 to determine the elastic-plastic transition force – based on bi-linear fitting – involves a dependency not only on the onset of plastic flow but also on the work hardening of the material. Consequently, the yield strength correlation factor is not universal but depends on the material properties and on the ge-ometry of the small punch set-up – leading to a significant uncertainty in the yield strength estima-tion. In this study, an alternative definition of the elastic-plastic transition force is proposed, which significantly less depends on the work hardening of the material and on the small punch geome-try. The approach is based on extensive elastic-plastic finite element simulations with generic ma-terial properties, including a systematic variation of the yield strength, ultimate tensile strength, and uniform elongation. The new definition of the transition force is based on the deviation of the force-deflection curve from the analytical elastic slope derived by Reissner's plate theory. A signifi-cant reduction of the uncertainty of the yield strength estimation is demonstrated.