Enhanced Fatigue (Crack Initiation and Propagation) Life Prediction of Welded Joints: Integrating Residual Stress and Cyclic Plasticity Effects

Significance 

Predicting the fatigue life of welded joints, especially in the rib-to-deck welded joints used in bridge constructions, is an important aspect of civil engineering and structural design. The process involves understanding the behavior of welded joints under cyclic loading and their eventual failure due to fatigue. Fatigue failure occurs in materials subjected to dynamic and fluctuating stresses. It is one of the most prevalent defects in bridge structures due to the repetitive nature of traffic loads. The fatigue life of rib-to-deck welded joints is influenced by several factors, including weld quality, the type of steel used in the construction of the bridge, stress concentration, loading conditions, and environmental conditions. Several approaches are used to predict the fatigue life of welded joints in bridges, for example, nominal stress approach which uses the S-N curves (stress-life curves) that relate the stress ranges to the number of cycles to failure. It is suitable for preliminary design and assessment but may not capture the complexities of local stress concentrations. Secondly, local stress-strain approach which offers a more detailed analysis by considering the stress and strain behavior at critical locations near the weld. This method requires accurate assessment of the stress or strain distribution at the hot spots which are vulnerable to fatigue cracking. Thirdly, fracture mechanics approach which is used for the assessment of existing cracks and their propagation under cyclic loading, while influence of mean stress and material properties should be considered to ensure accurate prediction of crack propagation life.

A new study was published in the International Journal of Fatigue by Dr. Yixun Wang, Dr. Qiudong Wang from the College of Civil Engineering at Nanjing Forestry University together with Dr. Riccardo Fincato, Dr. Kasumi Morita and Professor Seiichiro Tsutsumi from the Department of Civil Engineering at Osaka University. The researchers conducted a series of simulations to develop and validate their fatigue life prediction method for rib-to-deck welded joints, particularly focusing on the weld roots . These efforts were directed toward understanding the intricate behavior of rib-to-deck welded joints under cyclic loading, and improvement of the accuracy of fatigue life prediction. The prediction method was proposed based on a series of fatigue tests on rib-to-deck welded joints, which were made from Grade 345 steel and joined using submerged arc welding with an 80% penetration rate.

To accurately model the welding process and its effects on residual stress distribution, the researchers conducted thermal-mechanical finite element analysis (FEA). This simulation considered the real bead geometry and welding parameters to estimate the residual stress fields within the welded joints accurately. The authors validated the FEA models against experimental data, ensuring that the simulated residual stress distributions closely matched observed values from experimental investigations in literatures. Next, they employed the Fatigue-SS (FSS) model to predict the cyclic elastoplastic behavior of the welded joints. This model allowed for a precise simulation of the material’s response to cyclic loading in both sub-yield and fully plastic stress states without necessity of using different material characterizations. The parameters of FSS model were calibrated using data from the literature and validated against experimental results to ensure accuracy in predicting the cyclic stress-strain response of the welded joints. The fatigue crack initiation life and propagation life were then assessed by the converged cyclic stress-strain response separately, with the aid of SWT parameters and Paris law. The researchers found that their method, which combined detailed welding simulation with the FSS constitutive model, could predict the fatigue life of rib-to-deck welded joints with high accuracy. The predictions were validated against experimental results, showing good agreement across a range of loading conditions. The study highlighted the critical role of welding residual stress in the fatigue behavior of welded joints. The accurate simulation of residual stress and its relaxation under cyclic loading was essential for precise fatigue life prediction.

In summary, the proposed approach accounted for the distinct mechanisms and driving forces behind crack initiation and propagation, enhancing the prediction’s precision. The study critically addresses the limitations of traditional S-N curve methods, which fail to accurately predict fatigue life due to their inability to distinguish between crack initiation and propagation mechanisms. This understanding is crucial for predicting the fatigue life of welded structures under cyclic loading conditions. The adoption of the FSS constitutive model to predict cyclic elastoplasticity in the sub-yield stress state represents a notable advancement. This model, by accurately simulating the cyclic stress-strain response, allows for a more precise assessment of fatigue life, especially under conditions where traditional plasticity models fall short. The authors’ methodology, which integrates detailed simulations of welding residual stress, local cyclic elastoplastic response, effects of mean stress and multiaxial stress conditions on assessment of fatigue crack initiation and propagation, offers a holistic approach to fatigue life prediction. This comprehensive method provides a more accurate and reliable prediction of the fatigue life of rib-to-deck welded joints. The researchers’ study has significant practical implications, particularly for the design and maintenance of orthotropic steel bridge decks. By enabling more accurate predictions of fatigue life, engineers can design more durable structures and implement more effective maintenance strategies, thereby enhancing the safety and durability of critical infrastructure. The findings of Professor Seiichiro Tsutsumi and colleagues offer valuable insights for the design and maintenance of steel bridge decks. Moreover, the new methodology provides engineers with a powerful tool for assessing the structural integrity of welded joints, potentially influencing future design codes and maintenance practices.

Reference

Yixun Wang, Riccardo Fincato, Kasumi Morita, Qiudong Wang, Seiichiro Tsutsumi, Cyclic elastoplasticity-based life assessment of fatigue crack initiation and subsequent propagation in rib-to-deck welded joints, International Journal of Fatigue, Volume 173, 2023, 107679,

Go to International Journal of Fatigue

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