Tensile Properties of Sheet Metals at Intermediate Strain Rates with Pre-Strain

Abstract

This paper investigates the tensile properties of sheet metals at intermediate strain rates in relation to the pre-strain history induced by the fabrication process. Sheet metals such as SPCEN(DDQ), EZNCD(DQ), SPCC(CQ), SPRC340R, and SPRC440E were selected with the pre-strain of 2, 5, and 10% by tensile elongation with static tensile test machine, which could be equivalent to the plastic strain induced by a sheet metal forming process. The pre-strained specimens were elongated at the strain rate ranged from 1 to 100 s−1. The experimental results reveal that the stress-strain curves are noticeably influenced by the pre-strain at the intermediate strain rate for low yield stress steels such as SPCEN, EZNCD, SPCC, and SPRC340R. It is also noticed that the flow stress does not trace the given stress-strain curve when a sheet metal is pre-strained and the yield stress as well as the ultimate tensile strength increases. This behavior is against the old conjecture that the flow stress follows the stress-strain curve even with the pre-strain. This paper suggests a new flow stress model for pre-strained sheet metals. The flow stress model consists of two terms: the first term is the ratio of the yield stress of a pre-strained specimen to the equivalent flow stress obtained from the test without pre-strain; the second term is the general empirical flow stress model without the pre-strain effect.