Failure process and optimal design of hybrid adhesive joints - a microscale experimental and numerical approach

With development of new engineering materials, hybrid structures are now widely used to achieve desired performances by applying new materials instead of conventional ones. This is especially so in the automotive, aerospace and defence industries, since it is a major method to achieve enhanced performance, better fuel efficiency and to minimise greenhouse gas emissions. This technique is being rapidly exploited in other high-value manufacturing-based economies such as Germany and the US. To fabricate hybrid structures, adhesive joining technique attracts more attentions due to their advantage of enabling cost-effective, highly integrated structures with a uniform load distribution and improved damage tolerance. However, there are still some limitations to the use of adhesive joining in hybrid structures. The concept of "hybrid joint" involves a combination of two or more different constituents, with different material properties. To date, there is no well-established theory, which can describe the relationships among the combination of constituents, geometric configuration (microstructures of interfaces and adhesive layer, thickness and overlap of adhesives, etc.) and the overall performance of a hybrid joint. Furthermore, optimization of hybrid adhesive joints is another challenge, since the number of possible effective factors for hybrid adhesive joints is significantly higher than conventional joints and the factors are interactional. In practice, there is no effective optimization method for hybrid joints. Those limitations have resulted in the tendency to "overdesign" hybrid-joining structures. Therefore, the project will focus on a fundamental study of failure mechanisms of a hybrid adhesive joint, and the development of an optimization strategy, which aligns with perspectives from both academia and industry.

To achieve the objectives, the project has four main work packages (WPs). In WP1, the failure mechanism of the joint will be analysed using advanced testing facilities, with consideration of the effects of microstructure. Then, in WP2, a DEM model will be developed to describe the mechanical properties of the joint and generate essential date points for optimization. Based on the results of WPs 1 and 2, an optimization algorithm will be developed in WP3 using DoE and Genetic Programming techniques for generating optimal design for hybrid adhesive joints according to design requirements. Finally, case studies will be carried out based on real-world applications for validation. It is intended that the outcomes of the project will substantially overcome the current limitations of the researches in this field and allow a step change in development of high-performance hybrid structures in high value manufacturing.

The immediate impact of the project will be on the academic community as described in the section "Academic Beneficiaries". However, further impacts will also be on the economy, society and environment.

The UK is one of the largest industrial nations in the world. As one of the most important parts of its economy, manufacturing industry contributes significantly in GVA and employment. To maintain the UK's position as a leader in high-value manufacturing, more fundamental studies and innovation researches are requested. The outcomes of this project will enable the development of both new hybrid structures with desired performances and a "smart" manufacturing process for such structures. Because of multiple applications of the hybrid structures, it is foreseeable that the impact will be applicable in a range of industrial sectors, such as automotive, shipping and aerospace industries, within the context of Industry 4.0. Hence, the productivity, efficiency and creativity of related industries will be upgraded. So far, the proposal has been discussed with several UK based industrial companies, such as Henkel, members of Northern Automotive Alliance and WMG High Value Manufacturing Catapult Center to ensure that the outcomes are exploitable in real world manufacturing. As a result of these discussions, the developed testing methods in this project will be used by Henkel to develop new testing standards for hybrid adhesive joints, which are currently not available in any ISO standard. Moreover, the WMG Catapult Center will adopt the proposed numerical model and optimization algorithm in their real-world project, such as light rail project "LightJoin" (Innovate UK) in WMG.

Besides the impact on economy, the society and environment will also be benefited by the outcomes of the project. For instance, the knowledge and methodology developed in the project could enable recently developed lightweight and eco-friendly engineering materials to be used in hybrid structures delivering a step change in the mass of vehicles, aircrafts and ships. Moreover, optimal designs of the structures will be determined autonomously based on the proposed optimization algorithm to reduce the material and energy costs in R&D phase. Hence, better energy efficiency and less greenhouse gas emissions will be achieved through the design, manufacturing and usage phases. This is in alignment with the government's environment policy and the global strategies, such as Paris Treaty. Therefore, the social benefits will be in a sustainable environment and a higher quality of life.