Development of Novel Multi-Material Adhesive Joints

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

Improving the Fatigue Life Prediction of Automotive Components Using Simulated Strain Signal Methods

This study aims to determine a suitable approach for generating strain signal leading to fatigue damage estimation using a significant acceleration model. It was hypothesised that the simulated model could reproduce a characteristic strain signal in similar to the actual strain signal. Three strain signals, all at 120 seconds, measured at the McPherson frontal coil spring of a Proton sedan had been used as a case study. The strain signals were acquired from a data acquisition involving car movements on various types of road surfaces at different speeds. The strains were caused by accelerations of the tyre while the car was being driven on rough road surfaces. Using a mathematical expression that was developed for car movements, the measured strain signals yielded acceleration signals usually used to describe the bumpiness of road surfaces. Furthermore, the fatigue-based acceleration signals were considered as disturbances acting on the automotive suspension system. These disturbances on the car body had an effect on generating strain signals via computer-based simulation, as responses of the coil spring, in the form of strains. Based on the simulations, all the simulated strain signals showed similar patterns to the actual strain signals. The simulated results also gave low fatigue damage deviations, which were less than 7.5 % for all the strain signals, with a root-mean square error of 0.011 % and a coefficient of determination of 0.9995. Furthermore, the extractions of higher amplitude cycle based on the energy of the wavelet transform were performed. From the extraction results, it was found that the wavelet transform was able to shorten the strain signal time up to 95.3 % and that 96.1 % of lower amplitude cycles were reduced, which these cycles theoretically contribute to a minimum fatigue damage. Thus, maintenance of fatigue damage by more than 92.7 % was produced. The segments that resulted from the extraction processes had been clustered using the Fuzzy C-means. The clustering results showed that the simulated strain signals had a significant coefficient of determination to the actual strain signals, reaching 0.8904 with a root-mean square error of only 0.5 %. Based on the cyclic testing results, the fatigue lives were distributed in a range of 1:2 or 2:1 correlation with a significant coefficient of determination of 0.9056. The testing time was successfully reduced by more than 85.1 % using the edited actual strain signals. In addition, using the edited simulated strain signals reduced the testing time up to 95.1 %. Indirectly, the use of modified strain signals could reduce device operating costs. The current study results are believed to provide a new knowledge towards generating simulated strain signals. Thus, the results bring greater meaning to the field of fatigue research. This work helps engineers in automotive industries involved in collecting road surface profiles, which are the main input for vehicle structures