PSI - Issue 79

Santi Marchetta et al. / Procedia Structural Integrity 79 (2026) 224–232 227 The hotspot stress associated to a cyclic load is equal to the difference between the hotspot stress ℎ , associated to the maximum load and the hotspot stress ℎ ,m associated to the minimum load: ℎ = ℎ ,max − ℎ ,m (1) Once evaluated, all the hotspot stresses and their corresponding experimental number of cycles to failure are compared with the FAT Curves (Fig. 2a reports the steel related curves) in order to evaluate their consistency with the fatigue design recommendations provided by the International Institute of Welding (IIW) (Hobbacher, (2016)). The hotspot stress approach can be regarded as safe for design purposes when the experimental data points fall on, or above, the recommended FAT curve for the investigated joint. In the case of welded T-joints, the IIW suggests either the FAT 80 or the FAT 71 design curve as reference. It is important to note that the IIW recommendations provide FAT design curves for welded joints in steel and aluminium alloys, whereas no reference curves are available for titanium. Consequently, it is not possible to directly compare the hotspot stresses obtained for Ti6Al4V joints with the fatigue curves defined for steel or aluminium, since the underlying material behaviour, weld metallurgy, and defect sensitivity are markedly different. A preliminary approach proposed by Corigliano & Palomba, (2025) is to normalize the steel FAT curves with respe ct to the Young’s modulus E (Fig. 2b) . By doing so, the comparison is carried out in terms of strain rather than stress.

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b)

Fig. 2. Steel FAT Curves: a) Original stress-based Steel FAT Curves (Hobbacher, (2016); b) Strain-based FAT Curves.

2.2. Strain Energy Density approach The basic assumption of this method is that, under linear elastic conditions and for brittle failure mechanisms, fatigue failure will occur when the Local Strain Energy Density W evaluated in a control volume of radius R 0 reaches the critical value W C . The present study deals with welded joints characterized by a weld toe angle 2α of 135°. For a V-notch angle at the weld toe 2 > 102.6° , the non-singularity of Mode II crack propagation is assured (Lazzarin & Tovo, (1998)). Consequently, the control volume radius R 0 can be determined by means of the following equation: 0, =( √2 1 1 ) 1− 1 1 (2) Where Δ 1 is the Mode I NSIF fatigue strength and 1 is the Williams’ eigenvalue for Mode I.