Investigation of variability in apparent values of materials properties in thermo-mechanical uniaxial tensile tests on sheet metals

Abstract

Thermo-mechanical uniaxial tensile testing is commonly carried out to characterise the mechanical properties of materials under conditions which mimic advanced industrial forming processes, such as hot stamping of steels and aluminium alloys, and to generate microstructures for metallographic investigation. However, in this type of testing, heat loss to the specimen grips can lead to nonuniform temperature distributions along the gauge length, resulting in challenges in determining absolute values of materials properties at the nominal temperature of interest. The present study investigates the effect of these nonuniform temperature distributions on the variability in the thermo-mechanical properties as measured in the tests, and in the microstructures of the tested specimens. For this purpose, uniaxial tensile tests on the boron steel 22MnB5 and aluminium alloy AA6082 were performed under hot stamping conditions using a Gleeble 3800 thermal-mechanical physical simulation system, in which the specimens were heated using resistance heating and the strain fields were measured using digital image correlation (DIC). The nonuniformity of the temperature distributions along the gauge length was quantified. Both the strains and the strain rates along the gauge length were then computed and the effects of factors such as pre-forming gauge length, post-forming gauge length and specimen design on the spatial distribution of strains and strain rates were investigated. The effects of these factors on the values of thermo-mechanical properties determined from the tests, such as the ductility and the ultimate tensile strength (UTS), were also analysed and quantified. This study reveals the variability in the apparent values of materials properties as determined by thermo-mechanical testing resulting from nonuniform temperature distributions, and provides experimental data for the development of new standards for thermo-mechanical tests in future.