A sheet tension/compression test for elevated temperature

An apparatus was designed, simulated, optimized, and constructed to enable the large-strain, continuous tension/compression testing of sheet materials at elevated temperature. Thermal and mechanical FE analyses were used to locate cartridge heaters, thus enabling the attainment of temperatures up to 350 °C within 15 min of start-up, and ensuring temperature uniformity throughout the gage length within 8 °C. The low-cost device also makes isothermal testing possible at strain rates higher than corresponding tests in air. Analysis was carried out to predict the attainable compressive strains using novel finite element (FE) modeling and a single parameter characteristic of the machine and fixtures. The limits of compressive strain vary primarily with the material thickness and the applied-side-force-to-material-strength ratio. Predictions for a range of sheet alloys with measured buckling strains from −0.04 to −0.17 agreed within a standard deviation of 0.025 (0.015 excluding one material that was not initially flat). In order to demonstrate the utility of the new method, several sheet materials were tested over a range of temperatures. Some of the data obtained is the first of its kind. Magnesium AZ31B sheets were tested at temperatures up to 250 °C with a strain rate of 0.001/s. The inflected stress–strain curve observed in compression at room temperature disappeared between 125 and 150 °C, corresponding to the suppression of twinning, and suggesting a simple method for identifying the deformation mechanism transition temperature. The temperature-dependent behaviors of selected advanced high strength steels (TWIP and DP) were revealed by preliminary tests at room temperature, 150 and 250 °C.