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The mechanical development of crack and decohesion in the various electrogalvanized coatings during deformation was analyzed by shear lag method. The original shear lag method was modified to allow the plasticity of steel substrate. This modified shear lag method enabled us to calculate not only stress distribution both in the coating and substrate, but also two fundamental fracture properties, viz., tensile strength of the coating and interfacial shear strength from the observed kinetics of microcrack formation. These fundamental properties were measured to compare the calculated values by the modified shear lag method with the experimental values directly obtained from the same coating.

The application of tailored blanks to autobody parts has progressed because of its numerous advantages. The forming of tailored blanks has, however, a lot of technical problems. Among the problems, weld-line movement and formability deterioration are the most significant ones in case of deep drawing. The weld-line movement and formability change were examined experimentally as a function of weld-line location in square cup deep drawing. The weld-line movement of tailored blank consists of two sources. One is the geometrical reason, and the other is due to the hardening of weld bead. The formability of tailored blank is inferior to that of an original blank by the existence of hardened weld region. The mode of fracture changes from wall breakage to a fracture adjacent to punch radius when the weld-line was close to the punch corner.

The stiffness of autobody largely depend on the joint strength of welding spots. Weak welding joints lead to inferior stiffness, which lowers vehicle performance such as ride & handling and NVH(Noise Vibration Harshness). Resistance spot welding, major joining method of autobody, produces superior joint strength but its fatigue limit(fatigue strength at 107 cycle) is inferior, which implies that joint strength may be greatly affected by the fatigue. Mechanical clinching, on the other hand, has lower joint strength but its fatigue limit is higher than that of spot welding. This paper focused on the behavior of joint strength under cyclic loading in both resistance spot welding and mechanical clinching. The deterioration of joint strength during fatigue testing was examined by measuring monotonic shear tension force as a function of fatigue cycles at a given cyclic load.