1 Effective Strain-Fatigue Life of Dual Phase 590 Steel M. El-Zeghayar 1 , T. Topper 1 , J. Bonnen 2 , R. Sohmshetty 2 1 University of Waterloo, Waterloo, Canada; 2 Ford Motor Company, Dearborn, Michigan Abstract Dual phase 590 steel belongs to the family of advanced high strength steels (AHSS) that has gained popularity in the automotive industry as lightweight vehicle components. An experimental study on dual phase (DP) 590 as-received steel was conducted to study the effect of variable amplitude loading on the fatigue life and construct the effective strain-life curve for the material. Overloads were applied in the form of large near yield stress cycles that decreased the crack opening stress for the subsequent smaller cycles. An effective strain-life curve was derived from constant amplitude and overload tests, and a crack opening stress equation was calibrated by a comparison of constant amplitude and effective strain ranges at given fatigue lives. It was also noticed that the material even at a life of 10 7 cycles exhibited significant plastic strain amplitudes that deviated from the single slope behavior of a Coffin-Manson equation. 1. Introduction Consumer preferences have limited the downsizing options available to automakers, and regulatory requirements and performance standards have resulted in a very limited ability to reduce weight further with conventional materials [1]. Several new commercialized advanced high-strength steels (AHSS) that exhibit high strength and enhanced formability are being offered in the automotive industry. These steels help optimize crashworthiness and greatly improve both formability and durability [2]. This paper describes an investigation of the fatigue durability of DP 590 steel one of a class of high strength formable steels. Dual phase steels feature a soft ferrite microstructure which is a soft phase and gives a low yield point, and a matrix containing islands of martensite which increases hardenability [3]. Tests are described that produce “effective strain-life” and crack opening stress curves for use in the fatigue analysis of components that experience severe variable amplitude service loads. The effective strain life curve is derived from constant amplitude and overload tests which contain blocks of one large strain cycle and varying numbers of smaller cycles that result in crack closure free crack growth during the small cycles. The calculations of the steady state crack opening stress are based on an empirical model proposed by DuQuesnay [4]. The constants for the crack opening stress equation are obtained by comparing strains at given lives in a constant amplitude strain-life curve with those in the effective strain-life curve.