PSI - Issue 75

Andrew Halfpenny et al. / Procedia Structural Integrity 75 (2025) 234–244 Author name / Structural Integrity Procedia (2025)

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© 2025 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of the scientific committee of the Fatigue Design 2025 organizers Keywords: Crack initiation, crack growth, crack retardation, strain-life fatigue, welded joints 1. Introduction Fatigue is the leading cause of structural failure under cyclic loading conditions. The process of fatigue failure generally comprises two primary stages: crack initiation, characterized by the formation of one or more cracks, and crack propagation where, under sufficiently high cyclic stress, cracks propagate until structural failure occurs. In general, simulation methodologies tend to focus exclusively on either the crack initiation or crack propagation stage, which can result in inaccurate fatigue life predictions when both stages are not negligible. This issue is particularly relevant for applications such as welded structures, lightweight jointed structures, and lightweight cast components, fundamental for the development of lightweight components and environmentally sustainable transportation solutions. This paper describes a unified fatigue life estimation method designed to address this issue. Known as the 'Total Life' method, this approach unifies crack initiation and crack propagation by combining principles from strain-life and fracture mechanics. Additionally, it incorporates a state-of-the-art multiaxial crack-tip plasticity model to account for mean-stress and overload retardation effects. The paper also describes the implementation of the ‘Total Life’ method into a software solution to make it compatible with the computer-aided engineering (CAE) environment. Finally, the validity of the method is shown by comparing the fatigue lives obtained by testing welded and machined T-shape specimens to the corresponding predicted fatigue lives. 2. The ‘Total - Life’ Method Conventional simulation methodologies typically concentrate on either the crack initiation stage, often employing a strain-life (EN) model, or the crack propagation stage, using well-established crack growth and crack retardation laws. This binary approach can result in inaccurate fatigue life predictions when both stages are not negligible. This issue is particularly pertinent for welded structures, lightweight jointed structures, and lightweight cast components, which are increasingly vital in the pursuit of environmentally sustainable transportation solutions. To address this challenge, Mikheevskiy and Glinka (2009) and Mikheevskiy (2009) introduced a unified fatigue life estimation approach known as the 'Total-Life' method. This method unifies the initiation and propagation stages by combining strain-life and fracture mechanics principles, alongside a state-of-the-art multiaxial crack-tip plasticity model that accounts for mean-stress and overload retardation effects. Th e ‘Total - Life’ method considers the local failure of each idealised grain within the material as local crack initiation failures. Consequently, crack growth can be described as a series of consecutive initiation failures. As the crack propagates, the stress intensity increases, leading to a corresponding rise in strain energy, and the failure of each grain accelerates until final fast fracture. Despite sharing several fundamental principles with the Linear Elastic Fracture Mechanics (LEFM) model, the ‘Total - Life’ method assumes a blunt crack with a tip radius ∗ , comparable to the grain size of the material. This assumption mitigates the stress singularity associated with LEFM and addresses issues related to modelling small fatigue cracks. As a consequence, it allows to use a non-linear elastic-plastic stress distribution ahead of the crack tip. The four elements that constitute the foundations of the ‘Total - Life’ method are: • A crack growth model • A cyclic crack tip plasticity model and crack retardation © 2025 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under the responsibility of Dr Fabien Lefebvre with at least 2 reviewers per paper

• Cycle counting and memory rules • Estimation of material parameters