PRACTICAL APPLICATION OF THE N-SIF APPROACH IN FATIGUE STRENGTH ASSESSMENT OF WELDED JOINTS

In the notch stress intensity approach the weld toe is modelled as a sharp V-notch, rho = 0, and local stress distributions in plane configurations are given on the basis of the relevant Mode I and Mode II Notch Stress Intensity Factors (NSIFs). Then, the local strain energy density over a circular sector surrounding the point of stress singularity can be easily calculated as soon as the NSIFs are known. These parameters include not only the influence of the main geometrical ratios, but also the size effect. Welded joints simply scaled in geometrical proportion have different NSIFs. Whilst the local stress-based evaluation of the NSIFs needs very fine meshes in the vicinity of the points of singularity, which is a drawback of the approach in the presence of complex geometries, the mean value of the elastic strain energy density (SED) on the control volume can be accurately determined by using relatively coarse meshes. Thanks to the SED use, the degree of refinement of FE models is not so different from that usually used to determine the 'hot-spot stress' according to the structural stress approach. The present paper deals with this topic, considering both bi-dimensional and three-dimensional welded details and discussing the degree of accuracy and the limits of applicability of the method. An alternative, simplified application of the NSIF approach valid for fillet-welded joints with fatigue failures from the weld toe (the so-called Peak Stress Method - PSM) is also presented.