Issue 41

F. Chebat et alii, Frattura ed Integrità Strutturale, 41 (2017) 447-455; DOI: 10.3221/IGF-ESIS.41.56 450 strength of welded joints having a V-notch angle at the weld toe constant and large enough to ensure the non singularity of mode II stress distributions. A convenient expression is [33]: 1 1 1 1 1 2 N A C A e K R               (11) where both λ 1 and e 1 depend on the V-notch angle. Eq. (11) will be applied in the next sections of the paper taking into account the experimental value 1 N A K  at 5 million cycles related to transverse non-load carrying fillet welded joints with 2 α = 135 degrees at the weld toe. The hypothesis of constancy of R C under mixed mode loads had been validated by Lazzarin and Zambardi [33] by using experimental data mainly provided by Seweryn et al. [35] and Kihara and Yoshii [36]. From a theoretical point of view the material properties in the vicinity of the weld toes and the weld roots depend on a number of parameters as residual stresses and distortions, heterogeneous metallurgical micro-structures, weld thermal cycles, heat source characteristics, load histories and so on. To device a model capable of predicting R C and the fatigue life of welded components on the basis of all these parameters is really a task too complex. Thus, the spirit of this approach is to give a simplified method able to summarise the fatigue life of components only on the basis of geometrical information, treating all other effects only in statistical terms, with reference to a well-defined group of welded materials and, for the time being, to arc welding processes. Eq. (11) makes it possible to estimate the R C value as soon as 1 N A K  and Δσ A are known. At N A = 5  10 6 cycles and in the presence of a nominal load ratio R equal to zero, a mean value 1 N A K  equal to 211 MPa.mm 0.326 can be assumed [37]. For butt ground welds made of ferritic steels Atzori and Dattoma [38] found a mean value Δσ A = 155 MPa (at N A = 5×10 6 cycles, with R =0). That value is in very good agreement with Δσ A =153 MPa recently obtained by Taylor at al. [5] by testing butt ground welds fabricated of a low carbon steel. Then, by introducing the above mentioned value into Eq. (11), one obtains for steel welded joints with failures from the weld toe R C =0.28 mm. The choice of 5 million cycles as a reference value is due mainly to the fact that, according to Eurocode 3, nominal stress ranges corresponding to 5 million cycles can be considered as fatigue limits under constant amplitude load histories. It is worth noting that the simplified hypothesis of a semicircular core of radius R C led to the assessment of a fatigue scatter band that exactly agreed with that of Haibach’s normalised S - N band [39]. In the case 2 α = 0 and fatigue crack initiation at the weld root Eq. (11) gives R C = 0.36 mm, by neglecting the mode II contribution and using e 1 = 0.133, Eq. (8), 1 N A K  = 180 MPa mm 0.5 and, once again, Δσ A = 155 MPa. There is a small difference with respect to the value previously determined, R C = 0.28 mm. However, in the safe direction, the proposal is to use R C = 0.28 mm also for the welded joints with failures from the weld roots which is the case considered in the present manuscript. As opposed to the direct evaluation of the NSIFs, which needs very refined meshes, the mean value of the elastic SED on the control volume can be determined with high accuracy by using coarse meshes [40-43]. M ODELLING OF THE ROLLERS AND EVALUATION OF THE LOCAL SED he rollers considered in the present investigation belong to the series PSV which offer the highest quality and the maximum load capacity of Rulmeca’s production (see Fig. 1) [26]. Rollers PSV are particularly suited to conveyors that operate in very difficult conditions, where working loads are high, and large lump size material is conveyed; and yet, despite these characteristics, they require minimal maintenance. Typical types of application are: mines, caves, cement works, coal-fired electric utilities and dock installations. The effectiveness of the PSV roller sealing system provides the solution to the environmental challenges of dust, dirt, water, low and high temperatures. Roller is made of the following main components: - A mantel, constituted by a tube cut and machined using automatic numerically controlled machines, which guarantee and maintain the tolerances and the precision of the square cut. - Two bearing housing made by a steel monolithic structure (in agreement with UNI EN 10111 characterized by a yield strength 170< σ y <330 MPa), deep drawn and sized to a forced fixed tolerance (ISO M7) at the bearing position. The thickness of the housings is proportional to the spindle diameter and to the bearing type, with thicknesses that are up to 5 mm, to guarantee the maximum strength for each application, including the heaviest. T