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Abstract. While the formability is important and indispensable for success in very complicated sheet metal forming, it seems that few studies has been carried out about the formability of sheet metal at the high strain rate. The present experimental results report that the elongation is dependent on the crosshead speed in tensile tests. In this paper, the tensile elongation has been obtained from various steel sheets for an auto-body at the intermediate strain rate. The strain rate in the experiment is ranged from 0.003/sec to 200/sec. The experimental result demonstrates that the tensile elongation does not decrease as the strain rate increases. This tendency has varieties depending on the microstructure and forming history of sheet metal. Some high strength steels have the tendency that the tensile elongation increases as the strain rate increases, while others not. This phenomenon is very important not only in sheet metal forming but also in the crashworthiness evaluation to predict the fracture and tearing of sheet metal members.

This paper investigates the dynamic tensile characteristics of TRIP600, TRIP800, DP600 and DP800 at the range of strain rate from 0.003 to 200/s. The tensile test acquires stress−strain curves and the strain rate sensitivity of each material. Experimental results show two important aspects for TRIP-type and DP-type sheets quantitatively: the flow stress increases as the strain rate increases; the elongation is not a monotonic function of the strain rate and increases at the same level of the strain rate even when the strain rate increases. In order to investigate the pre-strain effect for two types of metals at the high strain rate, TRIP600 and DP600 were elongated with the pre-strain of 5 and 10% at the strain rate of 0.003/s. Then, dynamic tensile tests were carried out at the strain rate of 0.003, 1, 10 and 100/s. The results demonstrate that the material properties of TRIP600 and DP600 are noticeably influenced by the pre-strain when the strain rate was over 1/s. The ultimate tensile strength as well as the yield stress increases due to the pre-strain effect.

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