PSI - Issue 57

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Cristian Bagni et al. / Procedia Structural Integrity 57 (2024) 598–610 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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© 2023 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 2023 organizers Keywords: Adhesive joints; Fatigue life prediction; Finite element analysis; Stress-life parameters; Lap shear joints; Coach peel joints 1. Introduction The increasing need and focus on greener ways of transportation has made lighter structures necessary. For this reason, the use of adhesives in structural joints has gained increasing interest in the transportation industry. The main advantages of adhesives, compared to traditional jointing techniques such as welds and rivets, are their reduced weight and the ability to join dissimilar materials, such as metals and composites. These traits create more opportunities for utilizing different materials and design optimisation to achieve lighter structures, without compromising (and often improving) structural rigidity and fatigue performance. Adhesives are knowingly stronger under shear loads than under peel loads. Therefore, adhesive joints should be ideally designed to be subject to predominantly shear loads. However, adhesive joints in real components are subject to complex loading and they often fail due to crack propagation through the joint. The typical failure modes are cohesive (crack propagates through the adhesive, Fig. 1a), interfacial or adhesive (crack propagates at the interface between the adhesive and the adherend, Fig. 1b), or a mixture of the previous two failure modes. Fatigue performance and failure mode are influenced by several factors, including material and thickness of the joined sheets, type of adhesive as well as surface preparation and cleanliness of the adherends. © 2024 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 2023 organizers

Fig. 1. (a) cohesive and (b) interfacial/adhesive failure.

Due to the increasing interest of the transportation industry in the use of adhesives in structural joints as an alternative to traditional joining techniques, and the fact that these adhesively bonded structural components are often subject to fatigue loading, the fatigue behaviour of adhesively bonded joints has been extensively researched over the past forty years. A comprehensive review of the most relevant research efforts carried out between 1975 and 2011 was produced by Abdel Wahab (2012). The fatigue life of a component can typically be seen as the sum of two main processes: crack initiation and crack propagation. Both the numerical modelling and the physical detection of crack initiation present several difficulties. Therefore, the easiest way to deal with these issues is to neglect the initiation phase and consider just the propagation phase in the fatigue analysis. This approach was adopted by research works in the past, as this assumption was more applicable to early generation adhesives, characterised by high brittleness. However, modern adhesives are significantly more ductile and neglecting the initiation phase could lead to a significant underestimation of the fatigue life of adhesive joints. Quaresimin and Ricotta (2006a) proposed a model for the fatigue life estimation of adhesively bonded joints in composite materials, based on the mechanics governing the progression of the fatigue damage. The proposed model considers the fatigue life of the joint as the sum of a crack initiation phase and a subsequent propagation phase. Furthermore, Quaresimin and Ricotta (2006b) investigated the progression of the fatigue damage in single lap adhesively bonded joints. The authors defined the crack initiation as the appearance of a