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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.

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.

Regeneration capability of a wall-flow monolith type diesel paticulates filter with an electric heater was studied. To prevent filter crack generation and unburned particulates accumulation, a precision controller was added to the test equipment to reduce thermal load. In order to control the supply of oxygen to potentially prevent cracking, a second air feeder was also added. Furthermore, to ignite the accumulated particulates uniformly and propagate extensively to burn accumulated particulates completely a newly improved heater unit was employed. Repeated regeneration tests were conducted with cars on a chassis dynamometer. Though crack generation and unburned particulates accumulation were reduced considerably, satisfactory prevention could not be achieved. Therefore a parameter study using regenerative burning and thermal stress analysis model was carried out.

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.