PSI - Issue 75

Mattias Clarin et al. / Procedia Structural Integrity 75 (2025) 467–473 Clarin et al./ Structural Integrity Procedia 00 (2025) 000 – 000

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weld toe region. This is especially relevant in high-strength steels, where the interaction between high yield strength 30 and localised stress concentration may enable a favorable residual stress state post-preload. 31 Gadallah and Shibahara [1] has shown that the level of overload and the steels yield strength significantly influence 32 fatigue crack growth in welded joints. Their finite element-based study revealed that overload-induced relaxation of 33 tensile residual stresses improved fatigue life, particularly in ultra-high-strength steels (UHSS), when residual stresses 34 were considered. Heyraud et al. [2] investigated proof loading in welded S355 stiffeners and reported up to 240% 35 improvement in fatigue life. They attributed this gain to the induction of compressive residual stress and local strain 36 hardening at the weld toe. Huther et al. [3] showed that overloads exceeding the yield strength of the base S355 steel 37 significantly enhanced fatigue life. Their residual stress analysis confirmed the role of compressive stress 38 redistribution in this improvement. Extending this concept to high-strength steels, Khurshid et al. [4] studied 39 compressive residual stresses introduced by high-frequency mechanical impact (HFMI) treatment in welded UHSS 40 joints. They observed that these stresses remained stable under constant amplitude (CA) loading but could relax under 41 variable amplitude (VA) loading, with overloads in VA conditions showing a potentially detrimental effect. Grönlund 42 et al. [5] have conducted a systematic study on welded S700 and S1100 joints, evaluating the effects of overloads and 43 VA loading using both experiments and a parametric fatigue model. They demonstrated substantial improvements in 44 fatigue strength, up to 60% for S700 and 70% for S1100, when tensile overloads of 0.8fy were applied. The SSAB 45 Design Handbook [6] provides comprehensive recommendations on structural design and fatigue detailing in high- 46 strength, including views of preloading or stress-relief strategies for enhancing fatigue life, particularly in thin, high- 47 strength structural members. 48 However, limited research exists on the systematic effects of preloading or early overloading on the fatigue strength 49 of non-load-carrying welded joints in S960 or similar UHSS grades. The interaction between high yield strength, 50 residual stress redistribution, and weld toe geometry under preloading remains an open question. Moreover, the 51 specific case of circular flat studs, a welded geometry commonly used in structural applications, has not been explored 52 in this context. 53 The present study investigates the effect of nominal preloading (up to the yield strength) on the fatigue life of 54 welded S960 circular flat studs. Through experimental fatigue testing, the influence of preloading on local stress 55 concentrations and fatigue life is quantified. The findings provides a potential for life extension through controlled 56 overload in high-strength steel welded joints. 57 2. Experimental work 58 2.1. Material 59 The material used to fabricate the 40 test pieces used for the experimental work presented herein was the SSAB 60 high strength QT strip product Strenx® 960 Plus in 6 mm thickness. The material meets the requirements of EN 10 61 025-6 for the S960QL grade. Mechanical properties of the material were measured to Rp0.2 = 981 MPa and Rm = 62 1090 MPa along the rolling direction. 63 2.2. Specimen geometry and fabrication 64 Symmetrical, non-load-carrying circular flat stud specimens were fabricated to investigate the effect of preloading. 65 Each specimen had overall dimensions of 600 × 60 × 6 mm, with circular flat studs measuring 40 mm in diameter and 66 6 mm in thickness, as shown in Fig. 1. The symmetrical design helped minimise imbalance during loading and ensured 67 consistent mechanical response across specimens. To avoid stress concentrations, the weld start and stop positions 68 were located at a 45° offset from the loading direction, preventing them from coinciding with the region of highest 69 stress. 70