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Fatigue strength of adhesive bonded box beams with longitudinal partition was investigate. From results of the fatigue tests, it was seen that the fatigue strength of bonded beams was higher than that of spot welded beams. Fatigue strength of bonded beams was independent of plate thickness and partition. The flexural rigidity of the box beams in the plane of partition can increase without decrease of torsional rigidity and torsional fatigue strength, if the partition is jointed by adhesive bonding instead of spot welding.

Fatigue strength of steel sheet adhesive joints was investigated. From the results of the fatigue tests, it was obtained that the fatigue strength of the tensile-shear adhesive joints was much higher than that of the T-type tension adhesive joints. Stress distributions on adhesive layer have been analyzed by theoretical analysis method. All of the data obtained by the fatigue tests fell almost on one S/N curve, in spite of the differences of joints type and plate thickness, if the equivalent stress(Mises stress) range of the adhesive layer at the periphery of the end of the jointed part obtained by theoretical analysis was taken as the ordinate of S/N curve.

Fatigue strength of typical types of spot-welded joints made of high strength steel sheets was experimentally investigated, as a cooperative program of “Subcommittee on Strength of Spot-Welded Joint” of JSAE. The materials used were rephosphorized high-strength steel sheet (HSS) of 400 MPa class and low-carbon steel sheet (LCS); both were 0.8 mm, 1.2 mm and 1.6 mm thick. The types of loading were tensile-shear, cross-tension and plane bending. The results obtained are as follows; 1) The fatigue strengths of joints of HSS in tensile-shear and plane bending were higher than those of LCS, respectively. But, in cross-tension, the strength of joints of HSS was exceeded by that of LCS beyond 106 cycles. 2) The fatigue strengths of joints of different materials in cross-tension and plane bending were found lower than those of the same materials. 3) The strain range - cycles to fracture (Δε-N) diagrams for the joints of sheets of different steels and thicknesses were obtained.

For the purpose of changing the stress concentration controlling its fatigue strength. An “ELLIPTICAL” spot weld was investigated by the analytical method, the finite element analysis of the stress distribution around the spot weld and related fatigue experiments. The stress around the specimen with a flat elliptical spot weld can be lower than that a circular spot with the same area. Therefor, flat spot weld can be used for reduction of the stress concentration. Using the maximum stress, all of experimental S-N data for the spot welded specimens with various sizes and various radius ratios fell nearly into one scatter band.

Fatigue cracks initiated at the spot-welded points or edges of the flange are observed on spot-welded box beams under repeated bending moment in the plane of the spot-welded flange. This fatigue fracture caused by buckling of the spot-welded flange is an inherent characteristic in discontinuous thin sheet structures, which we designate “buckling assisted fatigue.” A two-dimensional buckling model is analyzed in order to clarify the mechanism of this kind of fatigue fracture. The load-stress curves and stress distribution of the spot-welded flange obtained from this theoretical analysis closely coincide with experimental results. Finally, a new method used on this theory is proposed for predicting the failure mode of spot-welded box beams by applying the plane bending Δε -N curves of a single spot-welded specimen and the base metal.