Issue 9

T. Marin et alii, Frattura ed Integrità Strutturale, 9 (2009) 76 - 84 ; DOI: 10.3221/IGF-ESIS.09.08 80 The mesh sensitivity of the nodal stresses extrapolated to the nodes on the toe line is obvious in Fig. 5. These are the normal stresses to the weld, calculated as weighted average of the stresses (extrapolated to the nodes) given only by the elements in the toe. The convergence of the mesh is evident since the coarse mesh provides lower stresses but, even if the finer mesh provides higher stresses, these values are still far from the structural stress calculated based on nodal forces. Figure 5 : Comparison of the normal stress to the weld calculated from nodal stresses and the structural stress obtained from nodal forces (toe 1 in Fig. 3). One of the major drawbacks of the structural stress approaches in their basic forms is that they usually take into account only the stress component normal to the weld line. In analogy with Eq. (1) , a structural shear stress along the fillet could be calculated as: 2 6 t m t f y x b m s         . (2) As of now only few studies on multiaxial fatigue and structural stress are found in the literature, [10], so a complete understanding of the correct combination of  s and  s is yet to be available. The whole procedure has been implemented by the authors in Matlab routines which act as a post-processor to the FE software Abaqus. Together with the calculations described above, the code detects the toes for all the fillets in the shell mesh of a structure or a component, thus offering an automatic, quick and complete analysis of the welded joints. T HE A SME MASTER S-N CURVE ccording to this approach, the structural stress defined in Equation 1 is consistent with the far-field stress typically used in fracture mechanics to compute the stress intensity factors K for a given crack shape and size. Since the life of welded joints is dominated by crack propagation, the structural stress and its components correlate the actual geometry and loading of any joint to simplified fracture mechanics configurations where crack growth models can be applied. As a result of the analytical procedure developed in [9] for a two-stage growth model, an equivalent structural stress parameter can be defined as: m m m s s rI t S /1 2/) 2( )(       (3) where the structural stress range  s is modified by a loading mode function I(r) and by a thickness correction factor. The polynomial I(r) 1/m is a function of the ratio r : A