PSI - Issue 57

Cristian Bagni et al. / Procedia Structural Integrity 57 (2024) 859–871

860

2

Author name / Structural Integrity Procedia 00 (2019) 000 – 000

© 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: Hybrid joints; Fatigue life prediction; Finite element analysis; Stress-life parameters; Lap shear joints; Coach peel joints 1. Introduction Lightweight structures are becoming increasingly necessary to reduce emissions and fuel consumption, and to support the development of more environmentally sustainable ways of transportation. Multiple options are available to reduce the weight of components, the most common are the use of lightweight materials and design optimisation. However, the use of hybrid joints is becoming increasingly popular in the transportation industry. The term ‘hybrid joint’ is commonly used to describe a connection where adhesive bonding is combined with tradit ional joining techniques, such as spot welds and rivets, with the aim of exploiting the advantages of the individual joining techniques. The main advantages of adhesives, compared to traditional joining techniques, are their reduced weight and the ability to join dissimilar materials, such as metals and composites. The increasing popularity of hybrid joints in the transportation industry led to a significant research activity in the previous two decades. From the literature review carried out by Abdel Wahab (2012), the common consensus about the benefits of hybrid joints is evident compared to traditional mechanical joints. In particular, it was shown that hybrid joints have better mechanical properties and fatigue performance compared to traditional joining techniques, such as spot welds and self-piercing rivets (SPR). Comparing hybrid joints against purely adhesively bonded joints found that the addition of spot welds to an adhesive joint reduces the fatigue performance of the joint. However, the addition of rivets to an adhesively bonded joint does help to control and slow down the crack propagation in the adhesive. Moroni (2019) studied the fatigue behaviour of lap shear hybrid joints (clinch-bonded and SPR-bonded) made by using sheets of AA5754-H32 aluminium alloy and two-component epoxy adhesive. The results of this study showed that the straight lines fitting the experimental load-life (LN) data obtained by testing the hybrid joints had shallower slopes compared to the joints with the single joining techniques, and showed better fatigue performance only below a certain load. Moroni also studied the evolution of the compliance of the hybrid joints during the tests correlating it to visual observation of the crack initiation and propagation in the adhesive. It was observed that the failure of the adhesive layer corresponded to a sharp increase of the compliance of the joint. Finally, it was concluded that the better fatigue performance of hybrid joints at lower loads was due to the presence of the mechanical joints (clinches or SPRs) that helped to control the crack propagation and to reduce its speed. Similar observations for SPR-bonded aluminium joints were made by Wu et al. (2021); in particular, it was found that the hybrid joints were outperforming the SPR-only and adhesive-only joints at lower loads. Furthermore, by analysing the displacement evolution during the tests it was also concluded that the failure of hybrid joints can typically be split in two stages: crack initiation and propagation in the adhesive layer followed by crack initiation around the rivets and propagation in the base material. Sadowski et al. (2011) showed that the tensile strength of hybrid joints, with adhesive and 5 rivets in a ‘X’ layout, was higher than that of the joints with the single joining techniques. However, in a later work Sadowski and Zarzeka-Raczkowska (2012) observed that the tensile strength of rivets-bonded joints is not always higher than that of the simply bonded joints, but only for layouts where the rivets are homogeneously distributed in the overlap region. By investigating the effect of combining adhesive and friction SPRs to join aluminium sheets, Yang et al. (2022) found that the hybrid joints showed better fatigue performance compared to both adhesive-only and friction SPR-only joints for the full range of loads used in the tests. Lai and Pan (2015), investigated the fatigue behaviour of lap shear specimens made of one sheet of magnesium AZ31B-H24 and one sheet of hot-dipped-galvanised mild steel. The sheets were joined with three different techniques: ultrasonic spot weld (USW), adhesive bonding, hybrid (USW + adhesive). From the tests conducted, it was found that the adhesive and the hybrid joints had comparable fatigue performance, which was also significantly higher than for the USW joints. A change in the failure mechanism was also identified, in both adhesive and hybrid joints, when moving from low-cycle to high-cycle loading conditions. © 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