Developments in the estimation of tensile strength by small punch testing

Abstract

The Small Punch (SP) test has shown in a number of applications that it can be successfully used for material ranking and material property estimation, especially where standard tests cannot be applied due to sampling location or the amount of material available. The most sought material properties are the ultimate tensile strength (Rm) and proof strength (Rp02) for the classical SP tests and the equivalent creep stress σ for the SPC creep testing. In the case of SP testing the force to stress conversion is classically done by correlating the Rm to descriptive (test set-up dependent) variables such as measured maximum force divided by the product of displacement at the maximum and the disk thickness. Naturally, if the test set-up or the test samples are not according to the standardized dimensions or low material ductility imposes crack growth instead of plastic deformation, these formulations cannot be applied. In this paper the classical formulations are studied and modifications in the formulations and in the extraction of the best descriptive variables for estimating Rm are proposed. The assessments are done on a range of materials using both standardized flat SP samples as well as curved (tube section) samples. It is claimed that the equivalent stress in both SP and SPC tests can robustly be estimated with the same type of equation, at least for ductile and semi-ductile ferritic/martensitic and austenitic steels. The same equations can further be applied on non-standard test samples and test set-ups using FEA determined conversion factors correcting for curvature. The tensile strengths of ductile P91 steel and 46% cold worked 15-15Ti cladding steel, with clearly reduced ductility, are successfully estimated in a broad temperature range. The determination of tensile strength by small punch testing of engineering steels in general and for nuclear claddings in specific has successfully been shown to give robust and accurate estimates.