PSI - Issue 75

Martin Edgren et al. / Procedia Structural Integrity 75 (2025) 555–563 Martin Edgren et Al. / Structural Integrity Procedia (2025)

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1. Introduction Steel bridges depend on welded connections with high fatigue strength to sustain performance under various loads. However, many existing bridges now show signs of fatigue damage, with some exceeding their designed service lives. Replacing entire bridges is costly and disruptive, makingthe repair of fatigued welded structures a more practicaland economical solution [1]. Two main degradation mechanisms affect these bridges: corrosion and fatigue. Corrosion, driven by humidity, oxygen, and chemicals, gradually degrades both the surface and internal structure of steel, reducing its strength. Meanwhile, repetitive traffic loads and environmental changes cause fatigue damage, initiating and propagating micro-cracks — especially at vulnerable welded joints [2]. Aldén et al [3] demonstrated that HFMI treatment can improve the fatigue life of welds, even for lower quality welds, quality level D, according to ISO5817 [4] showing fatigue life improvements consistent with the IIW recommendations for HFMI [5]. Furthermore, the study showed that the scatter of the fatigue life results varied among the HFMI operators and in-spite of the scatter the HFMI operators could produce consistent treatment results. Edgren et al [6] show that HFMI can be effectively applied to pre-fatigued structures, providing a viable alternative to traditional repair methods that require removal and replacement of damaged sections. Furthermore, Banno et al [7] combined numerical and experimental investigations clarified how crack depth in HFMI-treated areas affects the compressive residual stresses in the vicinity of the HFMI treated region. The studies by Banno et al [7], [8] showed that the crack opening-closing behavior FEA was consistent with the experimental tests on pre-fatigued gusset welded joints. More importantly, Banno et al [7] showed that for the HFMI treated pre-fatigued specimens, the cracks remained closed even for higher nominal stress cyclic loading. Leitner et al [9] analysis of T-joints under variable amplitude block loading highlighted the complexities in fatigue assessment, noting significant differences between the results of nominal stress methods and those obtained using effective notched stress approaches. The IIW HFMI recommendations, Maddox & Hagenseen [10] highlight the importance of post-weld improvement techniques for enhancing the fatigue resistance of welded joints, especially those prone to weld toe cracking. They recommend strategies such as refining the weld profile, optimizing residual stress distribution, and adjusting the surrounding geometry to achieve improved fatigue performance. Al-Karawi et al [11] showed that TIG remelting and high-HFMI have proven particularly effective. The methods used in conjunction have shown to not only restore the fatigue strength of welds but can also enhance it to levels comparable to those of newly fabricated components. HFMI is highly effective for welds exhibiting no fatigue cracks or only shallow cracks ( ≤ 1.5 mm ) in the weld toe [12]. The current study focuses on addressing the effect of shallow cracks (depths <0.5 mm) in welded structures and the potential life extension of these after rehabilitation with HFMI. This is carried out by an experimental approach by measuring strain drop during cyclic loading to estimate the crack depth prior HFMI treatment. 2. Welded detail and quality assessment Non-load carrying in-plane gusset plate is used for the fatigue testing and evaluation. The dimensions of the specimen are according to Figure 1 a), and the material is low-grade steel (S355J2+N, σ y = 355 MPa) with a plate thickness of t=16mm, and corresponding to Structural Detail No. 526 according to IIWs recommendations [13]. See Figure 1 b) for a photo of the test specimen.

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Figure 1 a) Geometry of non-load carrying in-plane gusset plate, b) Photo of manufactured test specimen.