THE USE OF PEAK STRESSES FOR FATIGUE STRENGTH ASSESSMENTS OF WELDED LAP JOINTS AND COVERPLATES WITH TOE AND ROOT FAILURES

It was shown in previous papers that in plane problems the elastic maximum tangential stress at the weld toe or at the root of fillet-welded joints evaluated by means of a finite element analysis, with a well defined pattern of elements, is proportional to the mode I Notch Stress Intensity Factor (NSIF). Then, in its original formulation the Peak Stress Method (PSM) was adopted to assess the fatigue strength when only mode I stresses are singular (for example at the toe of fillet-welds) or significant (for example at the root of load-carrying fillet welded cruciforms). In welded lap joints and cover plates both mode I and mode II stresses are singular at the weld root (where fatigue cracks are likely to initiate), so that the relevant NSIFs have to be taken into account. In the present paper the PSM is extended to mode II loading conditions; it is shown that the finite value of the elastic shear stress evaluated at a slit tip by means of a finite element analysis is directly proportional to the mode II SIF as soon as an appropriate mesh pattern is drawn. Afterwards an expression linking the tangential and shear peak stresses to the local strain energy density (SED) averaged in a properly defined structural volume is presented. As a result, an equivalent local stress is derived, which can be used to assess either weld toe or weld root fatigue failures by means of a single design scatter band.