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Sheet hydro-forming was applied to hydro-form a door outer panel using different steel grades. The effect of mechanical properties and the forming conditions on panel properties such as thickness profile and cross-sectional shape accuracy were investigated by both experimental sheet hydro-forming and FEM forming analysis. 590MPa T.S. steel grade was successfully formed with improved dent resistance compared to the conventional 340MPa T.S. steel grade. On the other hand, the results of the FEM forming process analysis showed that the pre-forming conditions were important in controlling the fracture formation during forming and to improve dent resistance, which successfully led to the best forming condition.

It has been widely recognized that the accurate estimation of spot welded joint fracture are needed to improve the accuracy of car crash FE analyses. The criteria model must be developed to realize the consideration of spot welded joint fracture under various kinds of load conditions. Conventionally, only tensile shear test and cross-tensile test have been usually performed for estimation of the strength characteristic of spot-welded joints. However, the spot-welded strength under complex loads can not be adequately estimated by using the conventional test methods. In this study, newly developed tests, “Inclined Cross Tensile Tests”, loading both shear and axial force on spot welded joint were conducted to understand fracture strength and build up its criteria. Especially, in the inclination of cross-tensile test, special attachment for the specimen to prevent bending deformation was developed.

The objective of this study is to develop a more precise springback prediction technique by means of finite elements method for the optimization of forming process design. The approach includes a new material modeling method to calculate accurate residual stresses inherent in forming processes, and a new analytical method to calculate springback shape by releasing residual stresses in a sequence of releasing steps. This paper proposed the new analytical method in springback simulation for actual automotive components with complex shapes. A new algorism, Multi-Step-Springback method was developed to improve the accuracy of springback simulation. In order to determine the material parameter, compression-tension tests were carried out. The impact on springback results is shown. The effect of modeling a more complex material behavior is presented.

Forming and strain rate effects on crashworthiness of automotive body components were investigated in this study. Dynamic tensile tests were carried out to establish the stress-strain relationships at elevated strain rates. Dynamic tests of bending and axial crashing at various speeds were conducted using a stamped hat square column. The experimental results indicate that the absorbed energy of the hat square column decreased with the increase of material thinning in case of high strength steels. FEM analyses using material models with both strain rate sensitivity and forming effects were carried out to evaluate the computer prediction accuracy of crashworthiness.

The fatigue strength of the high strength steel sheet for truck frame use was investigated. The fatigue strength of the steel sheet with scale increases with the decrease in the roughness of the steel sheet surface. The fatigue strength of the sheared edge increases with the decrease in the roughness of the fractured surface. In case of the microstructure consisting of pearlite and coarse carbides, the fatigue strength of the sheared edge decreases. Based on these findings, two types of the 780MPa grade high strength hot rolled steel sheet with high fatigue strength as well as good formability have been developed.

From the viewpoint of applying ERW (Electric Resistance Welded) steel tubes for automotive structural parts by means of hydroforming technology, a series of free-bulge experiments is carried out. Four kinds of mild and high tensile strength steel tubes are provided. Initial stress ratio, αm that represents an axial force level is adopted as an experimental parameter. Expansion limit increases with decrease of αm. The bulge shape and the expansion limit are also affected by the mechanical properties of tubes. The strain hardening property in bulge deformation is approximated by a parabolic equation.