Abstract:To calculate the multi-axis fatigue life of structural steel and its welded connections under complex stress conditions, the stress fields on the unstable propagation area of multi-axial fatigue cracks in the inclined butt-welded joint were calculated theoretically based on an ellipsoidal fracture model. The initiation and stable propagation lengths of multi-axial fatigue cracks, as well as the maximum fracture index (under the maximum fatigue load) and the fracture index amplitude (under the fatigue load amplitude) defined by the ellipsoidal fracture model on the fatigue failure area, were calculated. A unified multiaxial fatigue model was established that satisfies the stress boundary conditions and expresses the model parameters, the maximum fracture factor, and the fracture-factor amplitude as functions governing fatigue-crack initiation and steady-state propagation life. The formulation comprehensively accounts for the contributions of individual stress components to multiaxial fatigue of full-penetration oblique cruciform welds and oblique butt weld connections under combined tension-shear cyclic loading. Closed-form unified multiaxial fatigue-life calculation expressions for full-penetration oblique cruciform welds and oblique butt weld connections subjected to tension-shear cyclic stresses are derived. The calculation results show that the calculation errors of the unified multi-axial fatigue life calculation formulas for the multi-axial fatigue life of inclined full-penetration welded cruciform joint of Q345qC steel and butt-welded joint of SAE1050 steel are from -26.8% to -0.2% and -87.1% to 10.2%, respectively. By comparison, the calculation error of the fatigue life formula recommended in the current code are from -63.5% to -13.3% and -79.2% to 237.33%, respectively. The proposed multi-axis fatigue model can be applied to the multi-axis fatigue life assessment of structural steel.
