PSI - Issue 68

M. Mahatab et al. / Procedia Structural Integrity 68 (2025) 815–821 M. Mahatab and R. Ranjan / Structural Integrity Procedia 00 (2025) 000–000

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1. Introduction In welded joint structures, most of the fatigue failure is triggered from the welded location due to a notch effect and high tensile residual stress at the weld toe resulting from the welding process. The fatigue performance of these joints has been extensively studied, leading to the development of various post-weld treatment technologies. High frequency impact treatment (HFMI) is one of the most popular post-weld treatment technologies in the industry due to its practicality and user-friendly features. Various studies have shown the potential of HFMI treatment (Banno et al. (2024); Campagnolo et al. (2022); Hanji et al. (2022); Gkatzogiannis et al. (2021)) to increase the fatigue life of weldments. However, the effectiveness of HFMI treatment on high-strength steel has been the subject of continuous research worldwide. In IIW’s latest guideline on improving the fatigue strength with HFMI (Marquis and Barsoum (2016)), the improvement in fatigue strength is given by two higher FAT classes for high-strength steel (yield strength σ y > 550 MPa) than steel with yield strength 350 MPa. Mikkola et al. (2016) investigated the strain hardening effect due to HFMI on S700MC grade steel and reported the strength and hardness improvement due to HFMI treatment. This study is focused on studying the fatigue behavior of high-strength Indian steel grade S550MC in as-welded and HFMI-treated conditions. Fatigue failure prediction and component design require extensive fatigue test data. However, large-scale testing is not feasible and costly in industries; one must depend upon modelling the fatigue behaviour to quantify the fatigue strength of welded joints under a wide range of loading conditions. Due to variability in the input data and non-linear behaviour of material near the crack tip, modelling crack growth and fatigue degradation is a challenging task. Various fracture mechanics models, such as AFGROW, FASTRAN, NASGROW, UNIGROW, and SBFM, have been developed over time. The crack tip plasticity model ASGROW was developed based on Wheeler (1972) and Willenborg et al. (1971), incorporates the crack retardation and acceleration phenomenon due to load interaction. FASTRAN was developed at NASA by Newman (1992) based on a crack tip closure model to estimate the crack opening stress level with a modified Dugdale strip-yield model. NASGRO is another fatigue crack growth model developed by Mettu et al. (1999) at NASA based on Forman law and Newman’s model for plasticity-induced crack closure. The UNIGROW model utilized the residual stress field ahead of the crack tip to incorporate the plasticity instead of the crack closure model (Noroozi et al. (2005); Noroozi et al. (2007)). The strain-based fracture mechanics (SBFM) model was developed at the University of Waterloo to investigate the fatigue behaviour of newly welded and HFMI-treated components. The SBFM model, which is based on the Paris-Erdogan law widely used in LEFM, incorporates the plasticity ahead of the crack tip by utilizing the inelastic stress-strain response ahead of the crack tip and employs crack closure model to perform the crack growth analysis (Ghahremani et al. (2016); Walbridge (2008); Ranjan and Walbridge (2021)). This work aims to study the impact treatment effect on cruciform welded joints made of Indian steel grade S550MC subjected to constant and variable amplitude loading using the SBFM model. 2. SBFM model description The strain-based fracture mechanics (SBFM) model performs crack growth analysis in two directions (thickness direction and width direction), i.e., at the crack's deepest point and surface points. The basis for the model is the Paris Erdogan crack growth law, commonly used in linear elastic fracture mechanics, which is improved to consider crack closure effects and threshold stress intensity factor range, ∆ K th . To incorporate the local inelastic material behaviour near the crack tip corresponding to nominal stress history, inelastic strain near the deepest and surface point of the assumed semi or quarter-elliptical crack is estimated, which is used to calculate the stress intensity factor ( K ) in the model. (1) ∆ K eff is the effective SIF range considering crack closure effects, and K op is the SIF corresponding to the crack opening strain level at the deepest and surface point of the crack for a given load cycle. K max and K min are the SIFs corresponding to the maximum and minimum load levels and are estimated using the following expression. (2) • •• • •• • ••• • • !"" #$ % % % % ! = " • • • • • • • • • ! " # " $ " " " ! " = # # # # +