PSI - Issue 79

Santi Marchetta et al. / Procedia Structural Integrity 79 (2026) 224–232

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methodologies have been developed over the years to assess the fatigue life of welded joints, ranging from global approaches to more refined local methods. The literature in marine environments was recently reviewed by Corigliano et al., (2024). Among the global approaches, those most widely considered in international design codes are the nominal stress method and the hotspot stress approach (British Standard Institution, (2015); (DNV) Det Norske Veritas, (2024); EN 1993-1-9: Eurocode 3: Design of Steel Structures - Part 1-9: Fatigue, (2005); Hobbacher, (2016)). The nominal stress method evaluates fatigue life based on the stresses associated with the critical cross section while the hotspot stress approach allows for a more accurate assessment, since it also incorporates the influence of structural discontinuities introduced by the welded joint detail. Both the approaches are widely adopted because of their relative simplicity and because they do not require an excessively detailed modelling of the weld geometry. Nevertheless, these methodologies can, in certain cases, lead to an overestimation or underestimation of the fatigue life of welded joints, since they either partly account for or completely neglect the influence of some crucial design parameters (Foti et al., (2021)). This limitation is largely related to one of the major challenges in welded joints assessment, namely the proper definition of the weld bead geometry. Parameters such as weld bead shape and size, weld toe radius, and the extent of the heat affected zone (HAZ) may significantly vary from joint to joint, even under highly controlled manufacturing conditions (Taylor, (2002)). As a result, global approaches and even local ones, such as the Notch Stress Intensity Factor (NSIF) method, may face limitations when results are transferred from one case to another. Furthermore, since the NSIF method treats the weld toe as a sharp notch, it often underestimates fatigue life, whereas, in reality, some welding procedures can assure a finite weld toe radius different from zero (LAZZARIN et al., (2003)). In this context, the Strain Energy Density (SED) approach provides a well consolidated alternative: by averaging the strain energy density over a control volume around the weld toe (LAZZARIN & ZAMBARDI, (2002)), defined by a radius that is independent of the geometry of the welded joint, the approach allows for a unified treatment of different structural details. While these methodologies are now well established and widely validated for welded steel joints (Dong et al., (2019); Foti et al., (2022); Foti & Berto, (2020); Niemi et al., (2018)), the application to titanium alloys is still in an embryonic stage. Titanium possesses a set of mechanical and physical properties that make it particularly attractive for marine applications — chiefly its excellent corrosion resistance in saline environments (Liu et al., (2022)). However, conventional welding processes can significantly affect its mechanical performance (BALASUBRAMANIAN et al., (2011)), and, unlike steel, the lack of experimental data, standardized procedures, and reference fatigue design curves makes the assessment of welded titanium joints far more complex (Casavola et al., (2009)). This study aims to validate, through finite element analysis, the applicability of the hotspot stress and Strain Energy Density (SED) approaches to the fatigue life assessment of welded T-joints. The investigation is based on two experimental configurations from the literature: a steel T-joint, manufactured by manual metal arc welding and originally proposed by Taylor, (2002), and a Ti-6Al-4V titanium alloy T-joint, introduced by Corigliano & Palomba, (2025), which was produced using an innovative laser welding process without the addition of filler material. It is worth noting that the work by Corigliano & Palomba is, to date, the only study extending the hotspot approach to titanium. In this context, the present work aims to confirm the reliability of both methods for steel joints and to further develop their applicability to titanium configurations. This work aligns itself with the subjects promoted by the cultural association Italian Group of Fracture (IGF), with special reference to the energy approaches applied to new materials.

Nomenclature a

Vertical plate thickness Weld bead length Material’s Young Modulus

b E

E S0

Strain Energy accumulated within the control volume

FAT Fatigue Strength Classes HAZ Heat affected zone L1

Horizontal plate support span

L2

Vertical plate length