Issue 42

M. Peron et alii, Frattura ed Integrità Strutturale, 42 (2017) 205-213; DOI: 10.3221/IGF-ESIS.42.22 210 In Eq. (4) 1 A K  is the NSIF-based fatigue strength of welded joints (211 MPa.mm 0.326 at N A = 5×10 6 cycles with nominal load ratio R = 0) and Δ σ A is the fatigue strength of the butt ground welded joint (155 MPa at N A = 5×10 6 cycles R = 0) [29]. Introducing these values into Eq. (4), R 0 = 0.28 mm is obtained as the radius of the control volume at the weld toe for steel welded joints. For the weld root, modelled as a crack, a value of the radius R 0 = 0.36 mm has been obtained by [29], re-writing the SED expression for 2 α = 0. Therefore it is possible to use a critical radius equal to 0.28 mm both for toe and root failures, as an engineering approximation [29]. It is useful to underline that R 0 depends on the failure hypothesis considered: only the total strain energy density is here presented (Beltrami hypothesis), but one could also use the deviatoric strain energy density (von Mises hypothesis) ([30]). The SED approach was applied to a large bulk of experimental data: a final synthesis based on 900 fatigue data is shown in Fig. 5 [17], including results from structural steel welded joints of complex geometries, for which fatigue failure occurs both from the weld toe or from the weld root. Also fatigue data obtained for very thin welded joints have been successfully summarized in terms of the SED ([31]). Recently, the SED approach has been extended to the fatigue assessment of notched specimens made of Ti-6Al-4V under multiaxial loading [32] and to high temperature fatigue data of different alloys [33]–[35]. A new method to rapidly evaluate the SED value from the singular peak stress determined by means of numerical model has been presented by Meneghetti et al. [36]. Some recent applications to creep are reported in [37]. R ESULTS IN TERMS OF SED E analyses of the transverse non-load carrying fillet welded joint have been carried out applying as remote loads on the model the experimental values used for the fatigue tests. A control volume with a radius equal to 0.28 mm was realized in the model, in order to quantify the SED value in the control volume having the characteristic size for welded structural steel. The diagram of the SED range value W  versus the number of cycles to failure N was plotted in a double logarithmic scale, summarizing the fatigue data for both bare and hot-dip galvanized specimens. With the aim to perform a direct comparison, the scatter band previously proposed for welded joints made of structural steel and based on more than 900 experimental data, Fig. 5, has been superimposed to the results of the present investigation (Fig. 6). For the detailed list of the SED values for both bare and HDG specimens corresponding to the stress ranges used in the fatigue tests, please refer to the last columns of Tab. 1. It can be noted that hot-dip galvanized specimens have a lower fatigue strength than the bare specimens, but both bare and HDG data fall within the scatter band previously proposed in the literature for welded structural steel. Figure 5 : Fatigue strength of welded joints made of structural steel as a function of the averaged local strain energy density. F