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This paper presents new FE-based fatigue life prediction techniques for spot-welded and arc-welded structures. The nominal structural stress is the fatigue parameter for spot-welded joints and derived from forces and moments acting on spot weld nuggets. The use of forces and moments at shell elements improves the prediction of stress around a nugget. The technique for arc welds is the structural stress approach that considers the proportion of membrane stress component to bending stress component. Proposed techniques for both spot-welded and arc-welded structures are adequate for the fatigue life predictions even when using coarse FE meshes.

In this study, an FEM simulation method was investigated in order to predict the dent resistance and stiffness performance of automotive exterior body panels. The method was based on the combination of a forming simulation and a static dent simulation. The dent simulation was carried out with material models taking forming effects, such as work hardening and thickness variation into account. The modeling method of material properties with the forming effects was discussed to improve the prediction accuracy. For the validation of the accuracy, forming and denting tests were carried out for a door model panel using mild steels or high strength steels. The results of the dent simulations showed good agreement with the experimental results.

Suitability of ultra high strength steels (UHSS) for mechanical joining was investigated. When mechanical joining was applied to conventional 980N/mm2-class UHSSs having dual phase microstructure (ferrite and martensite), surface cracking occurred. Additionally cleaving inside mechanical joints was also observed in cases involving joining of dual phase steel by tools with larger clearance than designed. This cleaving causes a serious decrease in joint strength. Resistance to surface cracking and inside cleaving depends on the metallurgical structure of UHSS. Single martensite phase is superior for preventing the occurrence of defects in mechanical joining. This is because of its excellent local formability, which can be evaluated by the hole-expanding test using a machined hole specimen. Besides superior resistance to cracking and inside cleaving, single martensite phase steel exhibited higher mechanical joint strength than dual phase steel in cross tensile test.

Increase of load capacity for trucks by weight reduction of the body has been required to cut down transportation costs. The weight reduction of truck body is feasible in terms of the application of high-strength steel sheet to body. Especially, the application to truck frame is important because it is heavy. The required properties of steel sheet for truck frame use are press formability and fatigue strength. Press formability can be optimized based on the previous investigation results. On the other hand, investigations on fatigue strength are relatively few, and the value of fatigue strength is not sufficient. Accordingly, the fatigue strength of high-strength steel sheet for truck frame use was investigated in this study. As for fatigue strength, the fatigue strength of steel sheet with scale is significant because the steel sheet is painted without pickling to remove the surface scale.

Finite element modeling based on both 3-D shape measurement and experimental stress-strain relationship was applied to the FEM simulation for estimating the crashworthiness of automobile parts. Compared with the result dynamic crash test using hat-square-column type specimen made of 300 - 590MPa grade steels, the accuracy of the FEM simulation was evaluated for various modeling methods. It was revealed that the modeling of corner radius, bulging of specimen wall and strain rate sensitivity of materials played important roles in predicting the actual dynamic deformation process and the force-stroke relationship.

In order to estimate and improve the crashworthiness of practical automotive parts, dynamic crash test was conducted by using double hat specimen composed of the materials with different strength and thickness. Estimation method of the average force of the specimen was discussed. From the test results, it was clear that absorbed energy of the specimen composed of the materials with different strength and thickness of plate can be evaluated from the linear mixture law for the relations of the absorbed energy and the thickness of the materials used. And the “effective width theory” is useful way to estimate the average force of the parts.

In order to improve the capability of frontal or rear automotive members to absorb impact energy, a basic study was performed to evaluate the effect of strength, micro-structure, and plate thickness on dynamic deformation phenomena using tensile specimens and hat square columns. Tensile strengths of the materials were selected from 340 to 1180 MPa class steels. Five kinds of micro-structure, Ferrite, Ferrite + Bainite, Ferrite + Martensite, Ferrite + Pearlite, Ferrite + Bainite + γ, and Martensite, were investigated. Tensile tests were carried out under strain rates from 10-1 to 103 /sec. and crash tests of hat square columns were conducted under test speeds from 0.1 to 14.0 m/sec.

The effect of a concentration of stress at a welded joint on the fatigue limit was investigated from the point of view of the effective design of automotive parts made of high strength hot rolled steel sheets. The stress concentration at the welded joint was evaluated using FEM analysis and the effect of stress concentration on endurance limit was investigated by fatigue tests using specimens of practical welded joints and specimens with the simulated configuration of welded joints. It is pointed out that another effect besides stress concentration and residual stress should be considered to evaluate the endurance limits for high tensile strength materials of over 590 N/mm2.