PSI - Issue 75

Alberto Campagnolo et al. / Procedia Structural Integrity 75 (2025) 564–571 Alberto Campagnolo, Giovanni Meneghetti/ Structural Integrity Procedia (2025)

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1. Introduction Welding remains a widely adopted and cost-effective method for assembling structural components. However, it introduces critical regions susceptible to fatigue damage under cyclic loading, particularly at the weld toe and weld root. These locations are characterized by severe stress concentrations and often high tensile residual stresses, therefore they act as primary sites for fatigue crack initiation (Hobbacher 2016). To mitigate such detrimental effects, various thermal and mechanical treatments applied during or after welding have been proposed in the literature (Kirkhope et al. 1999). Post-weld treatments primarily aim to reduce local stress concentrations and alleviate tensile residual stresses at the weld toe. The effectiveness of post-weld treatments in improving fatigue performance has been extensively documented and is codified in guidelines from the International Institute of Welding (IIW) (Haagensen and Maddox 2013; Hobbacher 2016; Marquis and Barsoum 2016). Among these, High-Frequency Mechanical Impact (HFMI) treatment has emerged as a promising solution for fatigue-critical applications (Statnikov et al. 1977; Kudryavtsev et al. 1994). HFMI involves applying high-frequency impacts, i.e. typically above 90 Hz, via cylindrical indenters to the weld toe region. The resulting localized plastic deformation introduces several beneficial effects: (i) a smoother geometric transition between base metal and weld bead (see Fig. 1), (ii) induction of compressive residual stresses, and (iii) formation of a cold-worked material layer close to the weld toe. Extensive experimental evidence confirms that HFMI can significantly improve the fatigue performance of structural steel welded joints (Yildirim and Marquis 2012; Marquis et al. 2013). The improvement increases with the yield stress of the base material, while increasing the nominal load ratio tends to reduce the benefit (Yildirim and Marquis 2012; Marquis et al. 2013). Moreover, HFMI-treated joints typically exhibit steeper S – N curve slopes under uniaxial loading conditions, namely k = 5 (Yildirim and Marquis 2012), in contrast to k = 3 commonly assumed for as-welded joints (Eurocode 3 2005; Hobbacher 2016). Several approaches (Marquis et al. 2013; Marquis and Barsoum 2016; Leitner et al. 2018) have been proposed to assess the fatigue strength of HFMI-treated joints, including methods based on nominal stress, structural hot-spot stress, notch stress, and Linear Elastic Fracture Mechanics (LEFM). Additionally, local approaches employing stress, strain, or strain energy-based criteria are particularly effective for components with complex geometries or multiaxial loading conditions (Radaj et al. 2006). Amon them, the local models based on the Notch Stress Intensity Factors (NSIFs) (Lazzarin and Tovo 1998), such as the averaged Strain Energy Density (SED) (Livieri and Lazzarin 2005) and the Peak Stress Method (PSM) (Meneghetti and Lazzarin 2007; Meneghetti and Campagnolo 2020), deserve to be mentioned. More in detail, the PSM is a rapid method to estimate the NSIF terms based on FE analyses with coarse mesh patterns. After having been coupled with the averaged SED fatigue criterion, it has been applied for the fatigue assessment of as-welded joints made of structural steels subjected to uniaxial as well as multiaxial loading conditions (Meneghetti and Campagnolo 2020). Recently, the PSM has been extended to HFMI-treated joints (Campagnolo et al. 2022), incorporating tailored analysis procedures and design curves calibrated on experimental data generated from joints tested under uniaxial loading with nominal load ratios R = 0.1 and 0.5. The considered datasets cover structural steels with yield stress in the range 355 ≤ σ y < 750 MPa. The present study expands on these efforts by calibrating new PSM-based design curves using literature data generated from HFMI treated welded joints made of a high-strength steel with yield stress in the range 550 ≤ σ y < 750 MPa. The fatigue performance is evaluated under nominal load ratios spanning the full range 0 ≤ R ≤ 0.8, with the aim of further validating the applicability of the PSM to HFMI-treated joints subjected to different load conditions. 2. Peak stress method (PSM) for HFMI treated welded joints The HFMI treatment not only generates compressive residual stresses and a cold-worked material layer close to the weld toe, but also results in a geometric modification by introducing a finite notch tip radius, ρ > 0 (see Fig. 1) (Yildirim and Marquis 2014) . In scenarios where the notch exhibits a rounded tip (ρ > 0), Lazzarin and Berto (Lazzarin and Berto 2005) suggested a structural volume for local SED-based assessments which assumes a