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The component being formed experiences some type of prestrain that may have an effect on its fatigue strength. This study investigated the forming effects on material fatigue strength of dual phase sheet steel (DP600) subjected to various uniaxial prestrains. In the as-received condition, DP600 specimens were tested for tensile properties to determine the prestraining level based on the uniform elongation corresponding to the maximum strength of DP600 on the stress-strain curve. Three different levels of prestrain at 90%, 70% and 50% of the uniform elongation were applied to uniaxial prestrain specimens for tensile tests and fatigue tests. Fatigue tests were conducted with strain controlled to obtain fatigue properties and compare them with the as-received DP600. The fatigue test results were presented with strain amplitude and Neuber's factor.

Tensile properties of sheet steels at dynamic conditions are becoming more important for automotives in recent years due to the positive strain rate effect of steels which significantly improves energy absorption capability during crash events. However, several testing techniques are used by different testing laboratories, no testing standards are available, and the quality of data generated by different laboratories is often not comparable. In order to improve the data quality at high strain rate testing conditions and thus to improve the accuracy of crash simulation results, The International Iron and Steel Institute (IISI) initiated a project to develop the “Recommendations for Dynamic Tensile Testing of Sheet Steels”. The document provides guidelines for key elements of high strain rate testing, testing techniques, input methods, specimen geometry and stress/strain measurement instrumentations.

Drop tower axial crush testing was performed on hat section samples of various steel grades ranging in minimum tensile strength from 410 MPa to 1300 MPa. It was demonstrated that the energy absorption capability increases with the tensile strength of the steel. However, steels of very high strength, greater than 980 MPa tensile strength, exhibited a greater tendency for weld button pullout or material fracture, and thus limited energy the absorption capability. The effect of the closeout plate and the yield strength of the steel on energy absorption were also investigated. FEA simulations were performed and correlated to the experimental results. A flow stress based material criterion is introduced based on the analytical approach to compare the crush performance of steels.

Spot weld fatigue performance of dual phase steels is of great interest due to much higher fatigue strength of its base steel. In this study, 0.8mm DP500-EG and 1.4mm DP600-GI were tested for both tensile shear and cross tension conditions. For comparison, tensile shear test was also conducted for 1.6mm HSLA350-GI and 0.8mm DQSK-GI. Although fatigue strength was different due to their different gages, by using the stress index, Ki, a parameter to describe the local stress condition, fatigue strength of all four steels merged to a narrow scatter band, indicating very little dependence of spot weld fatigue on the strength of the base steel. In addition, the effect of weld surface cracking on fatigue strength of dual phase steels is of concern due to their high strength, despite the fact that it can occur to any steels under conditions of high current or electrode misalignment.

Traditional constitutive models can only describe a parallel or divergent stress strain response at different strain rates. This paper presents a new constitutive model that can describe convergent, divergent or parallel stress strain patterns. The new model is a modification to the popular Johnson-Cook model. By comparison with the Johnson-Cook model using high strain rate data of seven high strength steels, the new model is evaluated. The results showed that the new model could adequately describe the stress strain relation at high strain rates for the seven steels. In addition, an empirical relationship between the parameters in the new constitutive model and quasi-static tensile data has been developed based on the analysis of several high strength steels. The equation requires only quasi-static data as the input and is capable of estimating flow stresses at high strain rates.