Search Results

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.

A new type of bake-hardenable high strength hot-rolled sheet steel was developed, which shows a remarkable increase in tensile strength as well as yield strength after strain age hardening without addition of special elements. Then the impact absorption energy of the developed steel was evaluated by the finite element method using a hat square column model. The equivalent thickness and tensile strength of the developed steel assuming use as a substitute for the conventional 440MPa steel were clarified.

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.

To clarify the effect of steel strength on the frictional behavior during press forming, frictional coefficient and galling property were studied with cold rolled steel sheet. Frictional coefficient was measured by single sliding test. Critical contact pressure above which the frictional coefficient decreased sharply was observed to increase with the strength of steel. A repetitive sliding test under the same compressive force was introduced to evaluate galling property. It is shown that the galling property is controlled by the ratio of contact pressure to yield strength of steel, and the galling property of high strength steel is better than that of mild steel if the initial contact pressure is the same. It is also notable that galling property becomes better as the increase of frictional coefficient with the number of slide becomes smaller.

The effects of strain rate on the deformation behavior of steels were investigated to find the most appropriate micro structure of steel for anti-crash parts of automobiles, such as front-side-members. The dual phase steel absorbed a higher amount of energy during dynamic deformation than other steel with the same static yield strength. The increase of volumetric fraction of the austenite phase in the dual phase steel deteriorates the dynamic deformation behavior. The FEM analysis for crash test of member also showed the superior performance of the dual phase steel.