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. The test results fell within a narrow band by using σns determined according to the above theoretical equations.
D-diameters for cross tension and bending moment, that represent the diameter of the constrain position, are required for σns calculation. A way to decide the value of D-diameter was examined by employing a simple model and guideline to determine it for actual structures was proposed.
According to this method and determining optimal D-diameter, fatigue strength of spot-welds can be estimated simply and accurately.