Issue 37

L. Susmel et alii, Frattura ed Integrità Strutturale, 37 (2016) 207-214; DOI: 10.3221/IGF-ESIS.37.27 209 In these equations, k  (  eff ) is the negative inverse slope and  A,Ref (  eff ) is the reference shear stress amplitude extrapolated at N A cycles to failure (Fig. 1). Constants  ,  , a and b are material fatigue parameters that have to be determined through appropriate experiments [6]. Observing that, in the absence of stress concentration phenomena,  eff is equal to unity under fully-reversed uniaxial fatigue loading and to zero under torsional cyclic loading [6], the constants in Eqs 2 and 3 can be estimated from the fully-reversed uniaxial and torsional fatigue curves as follows [6]:           0 k 0 k 1 k k eff eff eff eff eff         (4)   A eff A A eff fRe ,A 2            (5) In Eq. 4 k  (  eff =1) and k  (  eff =0) are the negative inverse slope of the uniaxial and torsional fatigue curve, respectively, whereas in Eq. 5  A and  A are the endurance limits extrapolated at N A cycles to failure under fully-reversed uniaxial and torsional fatigue loading, respectively. As to calibration relationships 4 and 5, it is important to point out that  A,Ref (  eff ) and k  (  eff ) are assumed to be constant and equal to  A,Ref (  lim ) and to k  (  lim ), respectively, for  eff values larger than an intrinsic material threshold denoted as  lim [6, 8]. To estimate fatigue lifetime according to the MWCM, initially both  a and  eff have to be determined at the assumed critical location by adopting the appropriate algorithms [9, 10]. Subsequently, the corresponding modified Wöhler curve has to be derived from Eqs 2 and 3 through the estimated value for  eff . Finally, the number of cycles to failure under the investigated load history can be predicted as follows [6]: ) (k a eff ref ,A A f eff t ) ( NN             (6) To conclude, it can be recalled that, as far as conventional welded joints are concerned, the MWCM has proven [11] to be accurate in performing the multiaxial fatigue assessment when it is applied not only in terms of nominal and hot-spot stresses, but also along with the reference radius concept as well as the Theory of Critical Distances. (a) (b) (c) Figure 2 : I-STIR FS welding platform equipped with a fourth axis (a) ; Al 6082-T6 FS welded tubular specimen (b) ; transverse macrosection of the weld region (c) . E XPERIMENTAL DETAILS he technology used to manufacture the FS welded tubular samples for testing was developed at the Nelson Mandela Metropolitan University, South Africa. Circumferential FS welds were manufactured by incorporating a fourth axis into a commercial I-STIR platform (Fig. 2a). In order to obtain high-quality welds, an ad hoc retracting T