A notch stress intensity approach to assess the multiaxial fatigue strength of welded tube‐to‐flange joints subjected to combined loadings

Abstract

Full penetration butt weld joints between a tube and its flange are considered, subjected to pure bending, pure torsion and a combination of these loading modes. The model treats the weld toe like a sharp V-notch, in which mode I and mode III stress distributions are combined to give an equivalent notch stress intensity factor (N-SIF) and assess the high cycle fatigue strength of the welded joints. The N-SIF-based approach is then extended to low/medium cycle fatigue, considering fatigue curves for pure bending and pure torsion having the same slope or, alternatively, different slopes. The expression for the equivalent N-SIF is justified on the basis of the variation of the deviatoric strain energy in a small volume of material surrounding the weld toe. The energy is averaged in a critical volume of radius R-C and given in closed form as a function of the mode I and mode III N-SIFs. The value of R-C is explicitly referred to high cycle fatigue conditions, the material being modelled as isotropic and linear elastic. R-C is thought of as a material property, independent in principle of the nominal load ratio. To validate the proposal, several experimental data taken from the literature are re-analysed. Such data were obtained by testing under pure bending, pure torsion and combined bending and torsion, welded joints made of fine-grained Fe E 460 steel and of age-hardened AlSi1MgMn aluminium alloy. Under high cycle fatigue conditions the critical radius R-C was found to be close to 0.40 mm for welded joints made of Fe E 460 steel and close to 0.10 mm for those made of AlSi1MgMn alloy. Under low/medium cycle fatigue, the expression for energy has been modified by using directly the experimental slopes of the pure bending and pure torsion fatigue curves

Développement d'une méthode de prédiction de la durée de vie en fatigue de structures tubulaires soudées en aluminium /

“…Une courbe de fatigue E-N est utilisée pour déterminer la durée de vie des cette méthode est plus difficile à mettre en oeuvre puisqu'elle nécessite bien souvent des essais expérimentaux de propagation de fissure. Les références [3,5,24,30,31,[38][39][40] fournissent plus de détails sur cette méthode, ainsi que des exemples d'application.…”

Fatigue damage criteria classification, modelling developments and trends for welded joints in marine structures

“…An average most likely R can be obtained using MCF and HCF fatigue test data and a (Basquin type of) resistance relation, minimising the error. Typical R values (Lazzarin et al 2004) are comparable to the effective notch stress or strain criterion parameter ρ Loading & response ratio, welding induced residual stress and notch elastoplasticity can be incorporated using dedicated K N formulations (Section 2.4). For uni-axial far field loading & response conditions, the mode-I or mode-III components are typically considered only, like for the NSIF, rather than taking the full stress tensor into account.…”

“…Alternatively an (infinite life Gough-Pollard type of) interaction equation is proposed (Lazzarin et al 2004). The mode-I and mode-III NSIFs replaced the normal and shear stress components, meaning the correct (fracture) plane parameters with respect to the bi-sector are naturally involved.…”

Crack initiation life in notched Ti-6Al-4V titanium bars under uniaxial and multiaxial fatigue: synthesis based on the averaged strain energy density approach

“…where the coefficient c w accounts for the effect of the nominal load ratio R [27], when the range value of the nominal stress is applied to the FE model. It is equal to 1 for R = 0 and to 0.5 for R = 1.…”