Development of Novel Multi-Material Adhesive Joints

Yousefi Kanani, Armin and Hou, Xiaonan and Ye, Jianqiao (2021) Development of Novel Multi-Material Adhesive Joints. PhD thesis, Lancaster University.

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Abstract

At present, the concept of lightweighting is a hot research topic in the manufacturing sector, as the latest data indicates that the transportation sector is the major contributor of greenhouse gas emissions worldwide, and vehicle lightweighting is widely seen as the most effective short-term solution. With the rapid development of new engineering materials, multi-material structures are now widely used, for which proper joining techniques are critical for the high performance of the overall structures. Among commonly available joining technologies, the use of adhesive joints attracts the most attention due to their advantage of enabling the development of lightweight, cost-effective and highly integrated structures with a better uniform load distribution and improved damage tolerance while protecting surface aesthetics. However, there are still some barriers in using adhesive joining techniques in practice due to a lack of an accepted theory, which describes the fracture mechanism of multi-material joints and summarises the factors affecting the performance of joints. This research aims to provide a better understanding of these joints' behaviour and strength, as well as of their failure mechanisms, to find methods to improve their performance due to the potential for lightweight products. The study starts with the characterisation of materials. Various experimental and numerical methods are performed under tensile and compressive loading conditions to obtain the bulk properties of the adherends/adhesives and fracture parameters of adhesives in mode I and II. The non-contact optical measurement system (Imetrum) is used to measure displacement/strain and to observe the failure mechanism. Due to the complexity of the failure mechanism in adhesive joints, it is challenging to study their behaviour merely by experimental methods. Therefore, a novel FE model is developed to understand the failure performance and validate fracture parameters of adhesives. In all cases, the mixed-mode behaviour of a power law with the average value of normal and shear CZM parameters are used to create CZM laws embedded in the cohesive models. The innovation of the proposed FE models is to use two layers of cohesive elements at the different interfaces between the adhesive bulk and the adherends with different cohesive properties measured from single-mode coupons using the relevant adherends, respectively. The method allows defining different cohesive parameters to the interfaces according to the adjacent adherend, which is especially suitable to simulate interfacial failure in multi-material joints. A comparative numerical and experimental studies that involve several joint shapes, adherends stiffness and overlap lengths (L_0) are carried out to investigate the effect of design parameters on multi-material bonded joints. The relationships between stiffness and specific multi-material joint characteristics are determined through subsequent numerical analysis, and the findings are presented in comprehensive stress analysis for different L_0 values. In addition, the average experimental failure loads (P_m) from the four specimens and estimated failure loads (P_0) using the proposed FE model is utilised to analyse failure load in multi-material joints compared to the conventional joints. The stiffness degradation analysis (SDEG), as well as the failure surface observation, are carried out to improve the understanding of using dissimilar substituents in the joints. Finally, based on the understanding of stress distributions and fracture mechanisms in multi-material joints, two novel designs are developed with material and geometrical modifications to minimise peak stress and asymmetric stress distribution along the bond-line, leading to improved performance. The first novel design uses a combination of the notches and mixed adhesive in the bonding area, and the second novel design uses multi-layers reinforcement, which relies on the local reinforcement of the interface with high strength metal layers. Finite element (FE) models are developed in Abaqus® software to analyse the effects of new multi-material single-lap joint designs on the stress distribution, strength and fracture process. Then, modified single lap joints (SLJs) with different configurations are fabricated and tested to validate the numerical analysis.