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

The influence of notching and mixed-adhesives at the bonding area on the strength and stress distribution of dissimilar single-lap joints

With the rapid development of new engineering materials, multi-material structures are now widely used to achieve desired performances instead of conventional ones. The increased use of dissimilar adherends such as composites and metals for joining structural parts in aerospace, maritime and civil and transport structures in the past decades make it essential to find methods to improve the performance of this type of joints due to the potential for lightweight products. The first aim of this research is to minimise peak stress concentration by introducing notches in the bonding area to increase the performance of single-lap joints with epoxy adhesive. This is done by utilising the finite element method (FEA) in Abaqus® software to model a series of single lap joints (SLJ) with various notch designs to find the optimum. Experimental tests are carried out to verify the designs. The optimal design is used then to model various SLJs with mono-adhesive and mixed-adhesives to optimise single-lap joints with dissimilar adherends. The novel geometrical modification reduces peak stresses significantly in the joints with dissimilar adherends, which leads to smaller asymmetric stress distribution along bond-line. The experimental results show significant improvement in the dissimilar joint strength. Compared with using a single material as the adhesive, it is found that using both epoxy and polyurethane as adhesive offers a higher failure load. This can be explained as the polyurethane adhesive provides more uniform stress distribution by transferring stress concentration to the interior part of the overlap length

A novel dissimilar single-lap joint with interfacial stiffness improvement

The increased use of hybrid joints such as bonding composites to metals in aerospace, hull, civil and automotive structures in the past decades makes it essential to find methods to improve the performance of the joints. This study presents both experimental and numerical investigations into a novel dissimilar single-lap joint (SLJ) with interfacial stiffness improvement. The main objective of this research is to minimise the peak stress concentration by reinforcing the lower stiffness adherend’s interface through embedding discrete AL patches to increase the performance of the dissimilar single-lap joint with epoxy adhesive. Finite element models (FEA) were developed in Abaqus® software to analyse the effects of thickness and length of the patches, and the failure mechanism due to the reinforcement. Dissimilar single lap joints with different configurations were fabricated and tested using single lap shear tests to validate the numerical analysis. Both the experimental and numerical results show that the strength of the reinforced joint is significantly enhanced by using the aluminium patches

The effect of joint configuration on the strength and stress distributions of dissimilar adhesively bonded joints

The recent increase in the use of adhesively bonded joints (ABJs) made from dissimilar adherends demands the acquisition of a better understanding of the strength and behaviour of these joints, including their failure mechanisms. Several studies have reported on such joints individually, however few have compared the performances of dissimilar ABJs with varying configurations and design parameters, in order to determine the optimal design configuration for hybrid structures. In this work, a comparative study using experimental methods and finite element analysis was conducted, focusing on four joint configurations (scarf joints, stepped-lap joints, half-lap splice joints and single-lap joints), with the aim of evaluating the ways in which their performances differ. In addition, the effects of overlap length (L0) and the mechanical properties of the adherends on the overall success of each joint were particularly closely analysed and compared. The results showed that the scarf joint provided the best performance of all the designs discussed, and it was found that increasing the overlap length was only significantly beneficial for certain joint configurations and adherend combinations. When the overlap length was increased from 12.5 mm to 25 mm, the failure load increased by 47.50% and 21.25% for the scarf and the stepped-lap joints, respectively. In comparison, the percentage increases for the half-lap splice and single-lap joints under the same conditions were less than 10%. Moreover, the mechanical properties of the adherends considerably affected the failure mechanisms of the dissimilar joints, and for all four joint configurations, the failure was initiated by a crack at the adherend-adhesive interface adjacent to the adherend with a lower modulus

Double lap adhesive joint with reduced stress concentration:effect of slot

Stress distributions at interfaces of adhesive lap joints have been widely studied to optimize overall structural strength. However, these studies focussed mainly on the mechanics of adhesive layers. In this paper, a novel concept for a double lap adhesive joint is proposed by introducing a slot in its inner adherend. Numerical simulations employing a finite-element method are used to validate the proposed concept. The results show that the introduction of the slots can smooth the stress distributions along the edges of the interfaces between adhesive and adherend and reduce stress concentration near the cut-off ends of the joint. The results also show that the height of the slots has significant effects on alternating the interfacial stresses. Thus, the proposed concept provides a promising way to optimize double lap adhesive joints for enhanced strength with reduced weight

The effect of joint configuration on the strength and stress distributions of dissimilar adhesively bonded joints

The recent increase in the use of adhesively bonded joints (ABJs) made from dissimilar adherends demands the acquisition of a better understanding of the strength and behaviour of these joints, including their failure mechanisms. Several studies have reported on such joints individually, however few have compared the performances of dissimilar ABJs with varying configurations and design parameters, in order to determine the optimal design configuration for hybrid structures. In this work, a comparative study using experimental methods and finite element analysis was conducted, focusing on four joint configurations (scarf joints, stepped-lap joints, half-lap splice joints and single-lap joints), with the aim of evaluating the ways in which their performances differ. In addition, the effects of overlap length () and the mechanical properties of the adherends on the overall success of each joint were particularly closely analysed and compared. The results showed that the scarf joint provided the best performance of all the designs discussed, and it was found that increasing the overlap length was only significantly beneficial for certain joint configurations and adherend combinations. When the overlap length was increased from 12.5 mm to 25 mm, the failure load increased by 47.50% and 21.25% for the scarf and the stepped-lap joints, respectively. In comparison, the percentage increases for the half-lap splice and single-lap joints under the same conditions were less than 10%. Moreover, the mechanical properties of the adherends considerably affected the failure mechanisms of the dissimilar joints, and for all four joint configurations, the failure was initiated by a crack at the adherend-adhesive interface adjacent to the adherend with a lower modulus

Fracture mechanisms of hybrid adhesive bonded joints:effects of the stiffness of constituents

In this study, different single-lap hybrid joints are used to analyse the effects of the stiffness of the adherends and the adhesive on the failure mechanism. The hybrid joints include a combination of (a) different adherends: aluminium (6082 T6) and PolyPhtalamide (PPA) reinforced with 50% of glass fibre (grade HTV-5H1 from Grivory) and (b) different adhesives: epoxy-based adhesive (Loctite EA 9497) and silane-modified polymer-based adhesive (Teroson MS 9399). Six different single-lap joints are fabricated and analysed. The cohesive parameters of different adhesives against different adherends are determined respectively using single-mode coupons and validated with finite element modelling. Single-lap shear tests are conducted to understand different fracture mechanisms of the joints. Finite element (FE) models using the Cohesive Zone Method (CZM) are developed to simulate the failure of the joints and validated by the testing results. Different failure processes obtained from different hybrid joints combinations are discussed further by analysing the stress distributions along the interfaces of the joints. Finally, the relationship between the stiffness of the constituents of a hybrid adhesive joint and its failure mechanism is summarised. The load vs displacement behaviour of the single-lap joints demonstrate that the stiffness of adherends affects the maximum failure load of the joints with rigid adhesive (epoxy). However, the joint with flexible adhesive (polyurethane) is not sensitive to the stiffness of the adherends. In addition, higher shear stress distribution occurs in the interface adjacent to the adherend with lower stiffness, leading to the failure initiation at the PPA side regardless of adhesive types

Analysis of failure mechanisms of adhesive joints modified by a novel additive manufacturing-assisted method

The research presented in this paper used an innovative method to modify the configuration of adhesively bonded joints for improved mechanical performance. Additive manufacturing was employed to produce sacrificial support structures with a water-soluble filament (Polyvinyl Alcohol). The design freedom offered by additive manufacturing makes it easy to tailor fixtures to any geometry, which can be used to accurately make the desired fillet shape at the end of the adhesive bond line. In addition to the experimental tests, the finite element method (FEM) was used to study the stress distribution along the bond line for four different modified bonded joints, whilst the discrete element method (DEM) was used to estimate the joint failure load and crack path in the adhesive bond line due to its strength in describing the initiation and progression of micro-cracks. The results show that the novel manufacturing method can produce an accurate fillet at the end of the bond line, regardless of the adhesive type. The mechanical performance of the joints with the modified features increased significantly. Furthermore, the failure load and crack path obtained from the DEM model is in close agreement with experimental and finite element (FE) results. Hence, the failure mechanism of the hybrid joints is then summarised

Estimating microscale DE parameters of brittle adhesive joints using genetic expression programming

Particle-based model has strength and flexibility in modelling the microstructures of adhesives and interface in adhesive joints. In this work, a procedure with genetic expression programming (GEP) technique to calibrate the microscale parameters of discrete element (DE) model was proposed for brittle adhesives. Two categories of adhesive properties, the bulk property of thick adhesive and interlaminar-like property of thin adhesive, were discussed. For the bulk property, three target properties of adhesives, i.e. tensile strength, peak strain, secant modulus, were set as the reproduced features. 300 sets of adjustable microscale parameters were produced to run the numerical tests and generate datasets. GEP was then employed to find regression formulas for predicting the target properties as a function of the microscale parameters. For the interlaminar-like property, fracture energies of the cohesive failure of thin adhesives were approximated. A similar procedure of combined DE modelling and GEP was performed to find the regression models to estimate the fracture energy. The developed regression formulas can cover a general range of brittle adhesives. Loctite EA 9497 adhesive was selected to perform a series of lab tests, of which the results were subsequently used to examine the applicability of the DE model with calibrated parameters. The numerical results exhibit good agreements with testing data and observation

A novel genetic expression programming assisted calibration strategy for discrete element models of composite joints with ductile adhesives

Discrete element (DE) model has a great feasibility in modelling the microstructural behaviours of adhesive composite joints. However, it demands a sophisticated calibration process to determine microscale bond parameters, which involves massive efforts in both experimental and numerical investigations. This work developed a novel calibration strategy based on DE models and genetic expression programming (GEP) approach for predicting the behaviours of hybrid composite joints encompassing the material nonlinearity, large ductile deformation and multiple fracture modes. In the developed strategy, both the bulk and interlaminar-like properties of ductile adhesives were concerned to suit various joint configurations. GEP modelling was performed based on the datasets from virtual DE experiments. Symbolic regression models were subsequently developed to facilitate the parameters determination. A series lab tests were conducted to validate the numerical results. It shows that the calibrated DE model can adaptively simulate the featured behaviours of both the ductile adhesive and composite joints with different configurations well in most examined occasions. Therefore, it could be suggested to generalize the developed strategy in the development of other DE models for saving the massive efforts in the calibration process of microstructural parameters