PSI - Issue 66

Venanzio Giannella et al. / Procedia Structural Integrity 66 (2024) 71–81 Venanzio Giannella et al./ Structural Integrity Procedia 00 (2025) 000 – 000

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3. Fatigue strength assessment according to the PSM The PSM according to Eq. (1) has been applied in Ref. (Pelizzari et al. 2024) to the tested joint geometries in order to estimate (i) the crack initiation location and (ii) the fatigue life. The PSM correctly identified the crack initiation sites in agreement with the experimental observations, namely the tube-side weld toe for the transverse joints and the plate-side weld toe for the longitudinal joints. The detailed analyses are not reported here for the sake of brevity. After that, the experimental fatigue results have been converted from the nominal load range ΔF to the equivalent peak stress range ∆ , , as calculated at the crack initiation site by using Eq. (1). Figure 3 reports the experimental fatigue results expressed in terms of number of cycles to technical crack initiation N i as a function of the equivalent peak stress range ∆ , . Figure 3 includes also the PSM-based fatigue design scatter band characterized by Δσ eq,peak,A,50% = 214 MPa and k = 3, since the local biaxiality ratio calculated at the crack initiation location resulted  = 0.021 and 0.032 for transverse and longitudinal joints, respectively. In other words,  was practically zero in both cases. It can be observed from Figure 3 that the experimental fatigue life for the transverse joints was significantly longer than that predicted by PSM, both for as-welded and stress-relieved joints. On the other hand, the PSM provided an accurate estimation of the fatigue life for the longitudinal joints, regardless of the testing conditions.

2000

N A = 2∙10 6 cycles Δσ eq,peak,A,50% = 214 MPa Scatter index (2.3%-97.7%): T σ = 296/156 = 1.90 Slope k = 3

PSM design scatter band for steel welded joints under mode I

1000

 eq,peak [MPa]

R = 0.05 c w AW= 1 c w SR= 1.105

296

214

Transverse - AW, tube-side weld toe Transverse - SR, tube-side weld toe Longitudinal - AW, plate-side weld toe Longitudinal - SR, plate-side weld toe

 = 0.021

156

 = 0.032

100

N A

1.E+04

1.E+05

1.E+06

1.E+07

Number of cycles to crack initiation, N i

Fig. 3. Comparison between the experimental results and the estimations based on the PSM for transverse and longitudinal joints. The fatigue design scatter band based on PSM and shown in the figure was calibrated in the prior work [22]. A more comprehensive discussion of the PSM analysis can be found in Ref. (Pelizzari et al. 2024). It is worth noting that the adopted technical crack initiation criterion led to fatigue cracks propagated up to several millimetres, as highlighted in Figure 2. However, the averaged SED fatigue criterion, on which the PSM is based, allows to correlate the number of cycles needed to initiate and propagate short fatigue cracks within the material region governed by the local, singular stress field defined by the relevant NSIFs. When cracks undergo a long propagation phase beyond this region, the number of cycles to failure can be estimated only by performing proper fracture