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A method of fatigue life prediction of spot welded joint under multi-axial loads has been developed by fatigue life estimation working groups in the committee on fatigue strength and structural reliability of JSAE. This method is based on the concept of nominal structural stress ( σ ns) proposed by Radaj and Rupp, and improved so that D value is not involved in stress calculation. The result of fatigue life estimation of actual vehicle with nominal structural stress which was calculated through newly developed method had very good correlation with the result of multi-axial loads fatigue test carried out with test piece including high strength steel.

In this paper, a new theory to obtain the nominal structural stress to the cross tension and the bending moment is described and, the method of the fatigue life estimation for the spot welding structure is proposed using this theory. Proposed method can do away with D value decision problem that is the current problem in D.Radaj's method for fatigue life estimation. Moreover, a nominal structural stress that is the fatigue strength evaluation parameter can be estimated in accuracy good, because the deformation around the nugget to which it pays attention in the spot welding structure is introduced into the analysis.

We have investigated into the torsional rigidity of spot-welded box section beam and reported on the torsional rigidities of double and single hat beams, the warping of which is restrained at both ends perfectly, using warping torsion theory of spot-welded box section beam we have proposed. In this paper, as a further analysis, we report on following three cases, 1) A case where the warping is restrained elastically at both ends. 2) A case where torsional moment varies longitudinally. 3) A case where pitch of spot-welds varies longitudinally.

The new method, which can quickly and accurately estimate the stiffness of the jointed part of the automobile body structure by using neural network, is proposed. The non-linear relation between the rotation vectors of the end section of the jointed part and the stiffness of the jointed part is constructed by using neural network. Once the relation of both is constructed, the values of joint stiffness can be estimated by only inputting the some elements of rotation vectors into the trained neural network. And the ability of this method was verified by using some examples.

A method of fatigue life estimation for the spot-welds of vehicle body structures by means of Finite Element Analysis (FEA) was studied. 6 general forces applied to a nugget of spot-weld under multiaxial loads were determined and the Nominal Structural Stress (σns) was calculated from them. It was confirmed that fatigue strength of the spot-welds under various multiaxial loads could be estimated universally by using σns. Based on the theory of elasticity of plates, stress of spot-weld nugget was analyzed. The theoretical equations for determining the principal stress at the nugget edge from6 general forces acting on a nugget were derived. And the principal stress was defined as the σns. The value of σns was determined by FEM that used a solid model and compared with the theoretical calculation value. They agreed quite well. Fatigue tests of DC specimens under various multiaxial loads (shear plus cross tension and tensile shear plus torsion) were conducted.

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.

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.

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.

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.

In automobile frame work, box members formed by spot welding two thin walled members of hat section together, or by welding a flat plate to one hat member are widely used as members. (The former are called double hat members, and the latter single hat members.) When such members are subjected to repeated torsion, cracks would occur at the spot weld point. This is due to a large shearing force transmitted through the weld point. Since fatigue strength is directly required in the aspect of design, many studies are being made in this connection. Considering that the basis of fatigue strength is primarily the stress distribution around the weld point, its problem must be clarified by all means. In this report, then, the stress distribution of welding flanges was theoretically analyzed as a plane stress problem and a bending problem of plates by the theory of elasticity and obtained by superposition of these solutions.