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Tensile testing technique for the small sample was newly developed. Small tensile specimens with gage length of 1mm were taken from spot welds of high tensile strength steel sheets, and stress-strain relationships and ductility of base metal, heat affected zone including corona bond and nugget were individually measured. Finite element analyses of spot-welded joints under the conditions of static and dynamic tensile-shear loading were carried out with these local mechanical properties to predict the fracture mode and strength of the joints. It was clarified that the effects of both nugget diameter and class of steels were evaluated with good accuracy.

This paper describes the lap welding of Zinc-coated steel sheet using a high power continuous wave YAG laser. The well-known problem of welding the Zinc-coated sheet is related to the low boiling point of zinc compared with the melting point of steel. During lap welding, zinc coating at the interface vaporize rapidly and causes defects1)2). In this study, therefore, lap welding was performed by YAG laser. The effects of type of coating layer, welding conditions, tensile strength and corrosion resistance after electro-deposition was examined. It was found that the weldability of coated steel is different by type of coating. Zn-Ni coated steel showed good weldability, but galvanealed steel inevitably pore pits with no gap set up. These defects not only lower the strength of joint, but also produce irregular bead where easily corroded after electro-deposition.

For the improvement in the frontal crash resistance of automotive body, the application of high-strength sheet steels are considered. At first the authors conducted the high speed tensile test by Split Hopkinson Bar Method to clarify the mechanical property of various sheet steels at a high strain rate. Secondary the authors conducted dynamic and static compression tests of the columns of various sheet steels to evaluate the energy absorption at the frontal crash. Results clarified that the energy absorption by column collapse is estimated only by tensile test. It was also clarified in comparison with the sheet steels with the same tensile strength, that the absorbed energy can be increased 5 - 15% by the adoption of a proper sheet steel. In case of the sheet steels with tensile strength of 590MPa, a tri-phase sheet steel of ferrite, bainite and retained austenite exhibited the largest energy absorption than others of similar tensile strength.