Issue 9

T. Marin et alii, Frattura ed Integrità Strutturale, 9 (2009) 76 - 84; DOI: 10.3221/IGF-ESIS.09.08 77 of taking into account the stress concentrations due to macro- and micro-geometric effects of the joints. It employs several empirical S-N curves that are associated with detail categories and corrective factors. The selection of a detail class for a welded joint type and loading mode is often subjective and, in many common situations, difficult even for a skilled engineer. This is especially true when the geometry of the structure is complex or when the stress state is not reducible to a simple main component. Moreover it must be added that the real structures can develop cracks in locations different to those indicated in the details present in the standards so this method has severe limitations. The group of “local” methods comprises many different strategies, ranging from the notch stress and notch strain approaches to the fracture mechanics approach. A brief generalization of them is not possible since they differ in the local parameter (being a stress, a strain or a stress intensity factor) and in the phase of the fatigue damage where they can be applied (for example local notch stress is suitable for the crack initiation while fracture mechanics is ideal for crack propagation), [5] . Even if these approaches are sophisticated and have a significant theoretical foundation, the applicability is very often confined to specific cases and therefore they cannot be easily generalized to cover the variety of situations typically found in engineering. This is the main reason why they have not seen a straightforward acceptance in the standard codes, [3]. An intermediate approach between “global” and “local” methods uses a definition of a representative stress, in proximity of the weld toes, which is based on an idealized stress distribution in the thickness of the joined members. Different terms have been adopted for this stress depending on the field of application and on the way it is calculated (geometric stress, structural stress, hot-spot stress). Here the term structural stress is adopted. The fundamental idea is to consider the stress component orthogonal to the weld line and to reduce it to a linearized distribution, Fig. 1a. The structural stress approach is suited for the assessment of fatigue failures occurring at the weld toes; accordingly it is the stress component normal to the crack plane, i.e. normal to the weld line that is the driver for crack propagation (Fig. 1b) . Figure 1 : a) Decomposition of the through thickness stress at the weld toe; b) stress component acting normal to the weld fillet. The structural stress can be inferred by surface measurements and extrapolations, leading to the traditional hot-spot technique. The procedure can be replicated by numerical simulations using finite element models and is present in standard codes (i.e. Eurocode3). Linearization of the stress over the section thickness can be achieved only through FE simulations and usually interrogating nodal stresses. Such practiced has also been introduced in pressure vessel standard EN 13445. As a result of the linearization, the structural stress  s at the weld toe is composed by a membrane part  m , constant in the thickness, and a bending part  b , as depicted in Figure 1a. The remaining self-equilibrated non-linear  nl is not considered; therefore the structural stress includes only the effects of gross structural discontinuities but disregards the local notch effect due to the weld geometry. The notch-induced complex stress state at the weld toe can then be simplified and only the two components  m and  b are taken into account. The finite element simulations required for this approach are linear elastic and the fatigue assessment is performed using structural stress S-N curves that are in limited number with respect to the nominal stress S-N curves. S TRUCTURAL STRESS APPROACH BASED ON NODAL FORCES he finite element framework allows the calculation of a structural stress based on forces and moments at the nodes of the mesh. This method has the distinctive advantage of providing a structural stress fairly insensitive to the mesh features (element size and element type) in the areas corresponding to the weld toes. The typical mesh- dependence that is found in the traditional surface extrapolation method and in the through thickness linearization, is T