Mitsugi Fukahori

Development of a Technique to Strengthen Body Frame with Structural Foam

A technique to strengthen body frame with a polymeric structural foam has been developed with benefits of reducing vehicle weight and improving drivability and fuel economy. The idea of this new technology was evolved from the concept that body frame strength will increase drastically if the body frames are prevented from folding on collision. The energy of a collision impact would be effectively absorbed if weak portions of body frames are reinforced by a high strength structural foam. The new technology composed of the high strength structural foam and a light-weight frame structure with partial foam filling is reported here. INTRODUCTION The objectives of achieving collision performance and light-weight body structure are co-existed with the advanced development of body structure, materials, and manufacturing . Up until now, steel bodies have been lightened mainly by applying high strength steel sheets, devising frame structures and applying new manufacturing technologies such as the tailored welded blank. However, the weight increase is sometime inevitable from the demand of further collision performance improvement. We have developed a body frame strengthening technology that uses a polymeric structural foam to enhance the collision performance with a minimum weight increase. Deformation modes of body frames have both an axial collapse and a bending collapse during crash. The entire frame is distorted during the axial collapse. However, during the bending collapse, the deformation area does not extend and the energy absorption is relatively small because the frame is only partially distorted . Our developed technology spreads the crash energy by preventing the bending collapse by filling a structural foam in the body frame which controls local buckling deformation of the frame. This newly developed structural foam which expands and cures at painting process with outstanding strength, and the partial filled frame structure with just a small amount of foam to improve the frame strength are reported in this paper. REQUIREMENT OF STRUCTURAL FOAM To increase the energy absorption during the frame bending deformation, it is essential to prevent the local buckling deformation and to broaden the deformation area. To investigate the material properties of structural foam to prevent buckling deformation effectively, three-point bending tests were used to simulate the hat section frame with various structural foams. The test configurations and the relationship between the compression strength of the structural foam and the energy absorption of the frame are shown in Figure 1. Compressive strength of material (MPa) A bs or be d en er gy (K J) Wood (0.4) Aluminum (2.7) Aluminum foam (0.3) Epoxy (0.5)

Mechanical Behavior of 980MPa NANOHITENTM at Elevated Temperatures and its Effect on Springback in Warm Forming

High tensile strength steel sheets have large springback after being formend at room temperature. Warm forming can be a solution to reduce springback of high tensile strength steel parts. NANOHITENTM is a high strength ferritic steel precipitation-strengthened by nanometer-sized carbides developed by JFE Steel Corporation. Tensile strength of the steel at room temperature does not change before and after deformation at elevated temperatures up to 873K since the carbides in the steel are stable at high temperatures less than 973K. Therefore, the steel is suitable for warm forming. Springback of 980MPa NANOHITENTM parts warm formed at 873K is the same level of that of cold formed conventional 590MPa steel parts. In this study, two kinds of material testing at room temperature and at elevated temperatures between 573K and 937K were performed to understand the mechanical behavior of 980MPa NANOHITENTM: uniaxial tensile tests and bending tests. The steels flow stress depends on not only material temperature but also strain rate in uniaxial tensile tests. After a bending test, the specimen shows springback measured by the change of an angle between the two sides. Stress relaxation happens while a test specimen is held at the bottom dead point after bending. And the stress relaxation could be used to reduce springback of warm formed parts.

Effect of Stress Relaxation on Springback of Steel Sheet in Warm Forming

Springback of a high strength steel (HSS) sheet of 980 MPa grade was investigated at elevated temperatures ranging from room temperature to 973 K. From U-and V-bending experiments it was found that springback was decreased with increasing temperature at temperatures of above 573 K. Furthermore, springback was decreased with punch-holding time because of stress relaxation. In this work, the stress relaxation behavior of the steel was experimentally measured. By using an elasto-vicoplasticity model, the stress relaxation was described, and its effect on the springback of sheet metals in warm forming was discussed theoretically.

Elasto-Plastic Property of High Strength Steel at Warm Temperature and its Springback

Stress-strain responses of a high strength steel sheet of 980MPa grade under uniaxial tension and its springback in V- and U-bending were investigated at elevated temperatures ranging from 573-973K. The flow stress decreased drastically with the increase of temperature, from which it was expected that springback is reduced by warm forming. In V-bending test, however, the temperature effect on springback was not so clear, while in U-bending springback decreased with temperature rise. It was found that such difference in temperature dependent springback behavior between V- and U-bending was caused by stress relaxation which took place during unloading process.