Strain rate effects on medium carbon steel and its effects on autofrettage

Abstract

Most metals are rate-sensitive under high loading speeds. In metal forming, the quasi-static analysis is commonly employed since the strain rate is comparably low. Recently, Krempl shows that the difference between the static analysis and the experimental result is insignificant when the strain rate is less than 10{sup {minus}4}s{sup {minus}1}. Under ambient conditions, the constitutive relationship proposed by Marciniak may be applied in low strain rates ranged from 10{sup {minus}4}s{sup {minus}1} to 10{sup {minus}1}s{sup {minus}1}. However, the effects of strain rate on constitutive relationships under cyclic loading should not be neglected in certain cases. In this study, uniaxial tensile and compressive tests have been carried out to determine the material parameters of medium carbon steel. Hysteresis loops have been obtained under different loading speeds. The effects of strain rate on the elastic modulus, the yield stress, the reverse yield stress and the hardening parameters are determined. Autofrettage is a special forming process by which beneficial compressive residual stresses are introduced into thick-walled tubes. The magnitude of the residual stress field induced by autofrettage depends greatly on the reverse hardening characteristics of the tubing material. Based on the experimental results and the assumption that the difference in the magnitude between themore » tensile and the reverse yield stresses is independent of strain rate, an elasto-visco-plastic model is proposed. Therefore, the influences of two different strain rates on the residual stresses can be predicted. The analysis also shows that the strain rate has a significant effect on the magnitude of the residual stress field of the autofrettage.« less