journal of materials processing technology 209 ( 2 0 0 9 ) 3525–3531 journal homepage: www.elsevier.com/locate/jmatprotec Modified strain-life equation to consider the effect of different prestrain paths for dual phase sheet steel Q. Le a , H. Kang a,* , G. Kridli a , A. Khosrovaneh b , B. Yan c a The University of Michigan-Dearborn, 4901 Evergreen Road, Dearborn, MI 48128-1491, United States b General Motors Corporation, 30200 Mound Road, Warren, MI 48090-9010, United States c Arcelor Mittal Steel USA Inc., 3001 East Columbus Drive, East Chicago, IN 46312, United States article info Article history: Received 28 January 2008 Received in revised form 30 July 2008 Accepted 17 August 2008 Keywords: Fatigue Prestrain DP600 Strain-life curve Metal forming abstract Fatigue tests of dual phase sheet steel (DP600) were conducted with strain controlled mode of fully reverse loading conditions at room temperature. The specimens were fabricated with three different prestrain paths such as as-received, uniaxial prestrain and plane strain prestrain. Fatigue properties for the all prestrain paths were determined from the fatigue test results. The strain-life curve equations were depended on the prestrain levels and paths. In this study a universal strain-life equation was proposed based on the experimental data to predict the fatigue life of dual phase sheet steel with different prestrain paths. The pro- posed model utilized fatigue properties of the as-received condition coupled with prestrain levels in the length, width, and thickness of the test specimen. The effectiveness of the proposed universal strain-life equation was demonstrated with a simple metal formed part that experienced various prestrain paths. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Traditionally, stamping is a major manufacturing process to produce the structural components of a vehicle body. Recently, hydroforming processes have also become widely employed for producing automotive structural components to reduce weight but increase strength of the components. Numerous researches have been conducted for the formability stud- ies of automotive materials, such as high strength low alloy (HSLA) steels and dual phase (DP) steels. Bleck et al. (1998) conducted forming limit studies for transformation-induced plasticity (TRIP), DP, austenitic stainless, bake-hardening and interstitial-free (IF) steels. Mamalis and Haidemenopoulos (1992) showed that transformation plasticity can enhance duc- tility, toughness, and formability of automotive sheet steels. Sriram et al. (2004) presented the mechanical behaviors, ∗ Corresponding author. Tel.: +1 313 593 1878; fax: +1 313 593 3851. E-mail address: htkang@umich.edu (H. Kang). splitting limits, and spring back behaviors of mild steels, con- ventional high strength steels, DP, and TRIP steels. Chen et al. (2004) conducted experimental studies to assess the formabil- ity of DP and TRIP steels using a T-channel. During the forming processes, the component being formed experiences different types of prestrain that will have an effect on the fatigue strength of the component. It is well known that the prestrain affects the fatigue properties and therefore, it is considered in the structural designs. In most cases, durability analysis of vehicle components is based on fatigue properties generated from the material in the as- received condition. However, Yan et al. (2001) showed that the fatigue behaviors of prestrained and as-received IF steel components were different. Le et al. (2009) also showed that the fatigue behaviors were different for prestrained and as- received DP600 steel coupons. For better prediction of the 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2008.08.011