Using miniature cuboidal specimens to determine compression Stress–strain curve of sheet metal under static and dynamic loading

Characterizing the stress–strain response of sheet metal under both tension and compression is important to provide accurate predictions of spring-back in sheet metal forming. Data on compression is scarce as anti-buckling devices typically need to be used to attain large strains in compression experiments on sheet metal. As a technically simpler alternative, we propose upsetting experiments with miniature cuboidal specimens as these can be conveniently extracted from sheet metal. In addition to static experiments, we design a Dropkinson bar system for the dynamic compression testing of miniature specimens. We demonstrate through finite element simulations that upsetting specimens with square cross-section are equally-suitable for measuring the stress–strain curve as traditional cylindrical specimens with circular cross-sections. Even though the local surface strains can be conveniently measured with planar DIC, detailed analysis revealed that using the average strain along the full specimen height yields the most accurate stress–strain curve estimation. Static and dynamic demonstration experiments are performed on miniature cuboidal specimens extracted from DP780, DP980 and DP1180 steel sheets. The comparison with the results from uniaxial tension experiments reveals a systematic strength differential effect with a yield stress for compression that is up to 4% higher than that for tension. The dynamic experiments at strain rates of about 750/s suggest a yield strength increase of up to 25% (as compared to static experiments).