The peak stress method for fatigue strength assessment of tube-to-flange welded joints under torsion loading

It was shown in previous papers that in plane problems the elastic tangential stress and the elastic shear stress evaluated at the weld toe or at the root of fillet-welded joints by means of a finite element analysis, with a well-defined pattern of elements, are proportional to the mode I and mode II Notch Stress Intensity Factors (NSIFs), respectively. On the basis of such properties, the so-called Peak Stress Method (PSM) is a simplified, finite element-oriented application of the N-SIF approach to fatigue analysis of fillet-welded joints with un-machined weld seams. In the present paper, the PSM is extended to torsional loading conditions, which induce mode III stresses at the weld toe and at the weld root. First, it is shown that the finite value of the elastic anti-plane shear stress evaluated at the weld toe by means of a finite element analysis is directly proportional to the mode III N-SIF. Afterwards, taking advantage of the strain energy density criterion, an equivalent local stress is derived. Finally, a synthesis of experimental results of fatigue tests on tube-to-flange fillet welded joints subject to torsion loading and failing either from the weld toe or from the weld root is presented.