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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.

Torsional fatigue tests of: spot-welded box-section beams were conducted to explain fatigue fracture phenomena, and a technique to predict fatigue life of spot welds in beams was developed. When a torsional moment acts on box-section beams that are hat beams with their flanges spot-welded to form closed sections, shearing force is transmitted through the spot welds. Based on theoretical analysis of stress distribution in the vicinity of spot welds and using strain values measured on the outer surface of spot welds, fatigue life of beams was successfully predicted. A technique to predict fatigue life of beams was developed by computing the load acting on spot welds in a beam and using fatigue strength data of a single-point spot-welded joint of the same load coponent.