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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 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.

The stress distribution in each plates and the adhesive layer of weldbonding lap joints under tension was theoretically analyzed by superposition of the solutions of a plane stress problem and a plate bending problem based on the theory of elasticity. Main results were as follows: (1) theoretical results of stress distribution in the plates corresponded well with the experimental results; (2) the stress concentration at the edges of spot welded part of plates was much relieved by weldbonding; and (3) as the adhesive layer softens with heat, the load shearing at the spot welded part increases, and shear stress at the lap ends decreases.

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.