A simplified fracture mechanics method for fatigue life analysis of weld roots

Volvo CE currently uses the effective notch stress method for assessing fatigue life in welded structures. For large structures such as frames, a sub modelling technique with a very fine mesh is necessary using millions of DOF. Stress tensors from unit load cases, estimated with FEA, are combined with time history loading from Multi Body Simulations of the complete machine operating on customer representative digitized test tracks. A critical plane approach (the Modified Wöhler Curve Method) is used to estimate the fatigue life. The usage of the effective notch method implies a very time-consuming design process. An LEFM approach for weld roots would be even more time-consuming but would be a more natural approach since the weld root consists of crack like defects. To overcome the drawbacks with the current design process that involves usage of the effective notch method, a hybrid method is defined using pre-calculated geometry factors for different crack sizes and weld geometries. The geometry factors as a function of crack size, and the weld geometry, are stored in a database. From the unit load cases the structural stress using a coarse mesh in the weld is extracted. Together with the geometry factors, the stress intensity factors may be estimated and used with Paris law to calculate the fatigue life of welds. A comparison between the effective notch method, LEFM and the new hybrid method is carried out for typical fillet welds is presented in this paper.