Issue 45

L.M. Viespoli et alii, Frattura ed Integrità Strutturale, 45 (2018) 121-134; DOI: 10.3221/IGF-ESIS.45.10 122 When proceeding to the static and fatigue life assessment for components characterized by sharp notches, the use of the Notch Stress Intensity Factors is a feasible approach, instead of the theoretical stress concentration factors [3,22-23] used for blunt notches, where no singularity in the stress field is present. Considered that the welded geometry constitutes, in fact, a point of intensification of the tresses, being a crack or a V-notch, it would be possible to execute the prediction resolving to the computation of the Notch Stress Intensity Factors, using a classic linear elastic fracture mechanics approach [4]. These parameters have two major drawbacks: require an accurate evaluation of the stresses [5], thus an extremely fine discretization and their critical value is not a constant, but a function of the notch opening angle [6]. A reduction in the number of degrees of freedom is allowed by the use of special elements [7] able to follow the asymptotic behavior of the stress field in the proximity of the stress intensification location, although this kind of element is not implemented in commercial codes. Adopting the Strain Energy Density approach for the fatigue life assessment [8] provides a more general tool, which constitutes both a parameter for the critical life evaluation and for the computation a posteriori of the Notch Stress Intensity Factors [9]. These are correlated to the stress field in the proximity of the notch tip by closed-form analytical relations. Moreover, being the SED a function of the stiffness matrix and the nodal displacements, it value it very accurate even with an extremely coarse discretization [10]. This important property allows to use coarse meshes for the SED computation, thus the fatigue assessment, which would normally be unacceptable for an assessment based on the stress field. Another advantage of the SED over the NSIFs is that its dimensions are constant and its critical value does not depend on the notch opening angle. The use of the deformation energy necessary to the crack initiation has been presented as Absorbed Specific Fracture Energy by Gillemot [11]. According to Sih [12], failure can be determined by the critical value Sc of the Strain Energy Density Factor S, being this parameter the product of the Strain Energy Denity and a critical distance from the notch or crack tip. Sih’s theory has been proven fit also in the case of plasticity [13] and blunt tip [14]. Glinka and Molski [15] developed a local SED-based criterion, which fundamental point is the SED constancy around the notch tip, which is suitable to application also to sharp V-notches in plain strain condition and small yielding [16]. The value adopted for the assessment is the mean SED [9], ΔW over a small critic volume of typical, material dependent radius R, containing the notch or crack tip. The material data used as reference for welded structural steel for the fatigue life predictions of the joints tested are shown in Fig. 6 and are the result of an extensive experimental campaign of ca. 900 tests from different details and with main plate thickness between 6 and 100mm [17-20]. Figure 1 : Geometry object of the test. The samples have been realized in S355 structural steel, welded with MIG process using ESAB OK AUTROD 12.51 copper coated wire as filling material. Dimensions in mm. The goal of this work is to provide a critic confront in terms of preprocessing difficulty and accuracy of the results between two of the most diffused fatigue life assessment procedures, the Nominal Stress and the Structural Hot Spot Stress approaches and the local energetic approach based on the mean Strain Energy Density. Some short notes will be reported from the literature on the topic, after which the numerical investigation and the test results are presented and confronted.