Mixed Adhesive Joints Research Articles

An investigation on the mechanical behavior of mixed adhesively bonded composite joints subjected to transverse pre-impact following by axial post-tensile

In this study, mixed adhesive joints were formed to create a more homogeneous stress distribution in order to increase the failure load that the joints could bear. Single-lap joints (SLJs) made of glass fiber-reinforced plastic (GFRP) composites were formed using a rigid adhesive (Araldite AV138) in the middle and a more flexible adhesive (3 M DP8005) at the ends of the joint. Rigid and flexible adhesives were applied to the surface with lf/lr = 1 and lf/lr = 0.5 bond-length ratio (lf is bond length for the flexible adhesive and lr is bond length for the rigid adhesive). Mono adhesive joints were also created using the same adhesives. Tensile tests at 1 mm/min were carried out without applying pre-impact to some joints and applying 2.5, 3.5, 7.5, and 10 J transverse impacts to other joints to reveal how joint strength changes with the potential impacts to which adhesive joints might be exposed. Mixed adhesive joints bore more load under impact and non-impact conditions compared to mono adhesive joints. The impacts applied in these tests increased the load-bearing capacity of mono DP8005 adhesive joints. The largest decrease of strength after impact was observed for the mixed adhesive joints with lf/lr = 0.5 bond-length ratio.

Effect of cure temperature on the mechanical properties of epoxy-aluminium single lap joints

ABSTRACT Mixed adhesive joints combining different stiffnesses and strengths in the same overlap is a good strategy to increase the mechanical properties. For this purpose, different adhesives with distinct curing temperatures are used, whose optimization is crucial to maximize the mechanical performance. In this context, this study considers Araldite® AV 4076–1/HY 4076 and Araldite® AW 106/HV 953 U adhesives to evaluate the effect of curing temperature on tensile strength, creep and stress relaxation response and fatigue strength of single lap joints. It was possible to conclude that both adhesives exhibited higher static strength, lower stress relaxation and lower creep displacements with increasing curing temperature, but while for the brittle adhesive this trend occurred up to 150°C for the ductile one it was only up to 70°C. After this value, all properties regressed due to the degradation of the ductile adhesive with temperature. Considering values corresponding to 25% of the tensile strength, for example, it was noted that the ductile adhesive relaxes 54.7% more than the fragile one and presents a creep displacement around 95.1% higher. On the other hand, while the fatigue life was shown to be insensitive to the curing temperature for the brittle adhesive, in the case of the ductile one it decreases after 70°C. Therefore, when these adhesives are combined into a mixed adhesive joint, the curing temperature must never exceed 70°C.

Topology optimization of adhesively bonded double lap joints

Current studies show great, but unutilized potential for mechanical optimization of adhesively bonded joints. There are different approaches, e. g. mixed adhesive joints; tapered adherends or joints with spew fillets, all of which try to redistribute the stress peaks, occurring at the ends of adhesive layer overlap, in favor to the load capacity. However, these investigations fail to take a general approach to find optimal geometric solutions to the problem of joining two materials by means of adhesive bonding. Instead the geometry is changed by predefined parameters such as tapering angles, spew fillet radii or the number of adhesives in mixed adhesive joints. To allow a non-parametric optimization, this paper proposes and investigates an extended bi-directional evolutionary structural optimization methodology based on a failure criterion to account for othotropic materials, which are often used in the context of bonding. The optimized joint geometry, generated by the proposed algorithm, resulted in a fairly constant distribution of elemental failure probability compared to the non-optimized state, thus eliminating underutilized spots of material and increasing the efficiency of the connection. Furthermore the influence of different parameters such as the overlap-length to adherend thickness ratio; the stiffness between adherend and adhesive as well as the adherend degree of orthotropy were investigated for optimal joint geometry.

Functionally graded adhesive joints under impact loads

The industrial application of adhesively bonded joints has increased significantly in the last few years, driven by benefits such as the increased design flexibility, high vibration damping, the capability of joining dissimilar materials and the possibility of being used in combination with other joining techniques. However, the presence of stress concentrations at the overlap ends, especially in single lap joints, is one of the major issues associated with this technique, reducing joint strength. To solve this drawback, several techniques have been proposed, such as the use of adhesive spew, adhesive and adherend shaping, mixed adhesive joints and functionally graded adhesive joints. Functionally graded adhesive joints use an adhesive layer where the properties gradually change along the bondline, which results in the reduction of stress concentration peaks at the ends of the overlap, leading to a more uniform stress distribution. Multiple techniques for the creation of a functionally graded bondline have been presented in the literature, such as the inclusion of particles and nanoparticles and the use of functionally graded curing. However, the experimental works available in the literature only report results for quasi-static loading conditions, with the impact behaviour of these joints being an unstudied topic. The main objective of the present work is to fill this gap and study the mechanical behaviour of functionally graded adhesive joints loaded under impact conditions, using both experimental testing and numerical modelling. The results obtained show that, unlike what is found for quasi-static loads, graded joints do not offer significant strength improvement under impact loads. In contrast, energy absorption is significantly increased. This behaviour is explained by the completely different stress distribution on the adhesive layer for quasi-static and impact conditions, leading to the lower effectiveness of functionally graded adhesive joints under impact loads.

Intensity of singular stress field (ISSF) variation as a function of the Young's modulus in single lap adhesive joints

Employing mixed adhesive joints has been proven to be very useful. This type of joint leads to improved performance by increasing strength and decreasing stresses in critical areas of the joint. In the same way, the use of the Intensity of Singular Stress Field (ISSF) has been shown to be suitable for adhesive joint calculation, since the adhesive strength can be controlled by the ISSF at the interface end. Four finite element models have been created by combining two epoxy adhesives with different mechanical properties, and therefore with different Young's moduli. New mixed adhesive joints have been compared with respect to only-one adhesive joints in terms of the ISSF. The results show a clear improvement with one of the configurations of mixed adhesive joints. A significant decrease of 35.64% in the ISSF is obtained compared to the only-one adhesive configuration.

Surface toughening – a concept to decrease stress peaks in bonded joints

ABSTRACTBy using fiber-reinforced plastics as the material for lightweight structures, adhesive bonding becomes an increasingly focused solution for joining these components. In comparison to bolted joints, adhesive bonding has a lot of known benefits, e.g. sealing, no weakening of adherends and a homogeneous load transfer. However, the stress distributions in single-lap joints are not homogeneous all over. Both the shear and peel stress distribution have a peak at the overlap ends of the joint and a lower loaded part in the middle, as shown by Volkersen and Goland and Reissner. In order to reduce these stress concentrations, there are known concepts for the modification of the joint. In this paper, adherend chamfering, different adhesive spew fillet geometries, and the mixed adhesive joint are investigated and compared to a novel local adherend surface toughening concept by using a thermoplastic PVDF layer. A high-resolution digital image correlation is used to visualize the deformation in the bondline. While the state-of-the-art used concepts do not show a high increase of joint strength for single lap joints, the joint strength for the surface toughening specimens can be increased by up to 84%.

Experimental investigation on environmental degradation of automotive mixed-adhesive joints

In this paper, environmental strength degradation of 180 different adhesive single lap joints (SLJ), including mono-adhesive Araldite 2015, mono-adhesive Araldite AV138, and a mixed-adhesive of Araldite 2015 and Araldite AV138 subjected to moist conditions are experimentally studied. Four different moist conditions, i.e. dry, 75.3, 84.2 RH% and immersion in tap water, have been taken into consideration and the specimens are tested after exposing to these environments at room temperature for 0, 35, 80 and 270 days. The specimens have been tested in two different strain rate, i.e. 1 mm/min and 100 mm/min. The results reveal that although, in a dry environment, mixed-adhesive joints have higher failure loads in comparison to mono-adhesive SLJs, in a moist environment, they have the highest reduction in static failure load with regard to the mono-adhesive ones. Moreover, despite the finally brittle trend in failure load, mixed-adhesives manifest a behavior very similar to ductile mono-adhesives regarding elongation. Analytical predictions of failure load are also consistent with the experimental observations in dry condition.

Influence of low and high temperature on mixed adhesive joints under quasi-static and impact conditions

The increasing use of composite structures in the automotive industry is due to strict regulations regarding fuel efficiency and safety standards. The main advantage of the use of adhesives is the possibility of joining dissimilar materials, particularly composites. The technique studied was the mixed adhesive joints, as two or more adhesives can be used in a single lap joint combining the properties of those adhesives to attain mechanical performance superior to that obtained using those adhesives individually. The main goal was to assess if a previously validated combination of mixed adhesives in a composite joint, tested under quasi-static and impact conditions, offers the same advantage over the use of a single adhesive when subjected to low (−30 °C) and high (80 °C) temperatures. The influence of temperature on the behaviour of the composite joints was assessed in quasi-static and impact conditions allowing to infer the effect on each adhesive. Another important aspect of the use of this type of technique is to avoid the early delamination of the composite substrates. This method was found to improve the performance under quasi-static and impact conditions although its behaviour under the wide range of temperature tested was found to vary significantly.

Numerical study of impact behaviour of mixed adhesive single lap joints for the automotive industry

The automotive industry is now extensively using multi-material structures to reduce vehicle weight. Joining these dissimilar materials effectively in a single structure demands the use of adhesive bonding, but the presence of stress concentrations leads to low mechanical performance. Mixed adhesive joints have been proposed as a solution for this issue, using two adhesives in the same joint, combining stiffness and flexibility on the same overlap, improving performance. This work focuses on the numerical modelling of the mechanical behaviour of these joints, using previously published experimental data for validation. The proposed models use cohesive laws with strain rate dependent data to simulate quasi-static and impact loads on several configurations of mixed adhesive joints, using four different adhesives. The models were able to predict the behaviour of most configurations under study, especially for the stiffer adhesives. The model was found to be less effective for modelling high ductility adhesives under impact.

Influence of Mechanical surface treatment on the strength of mixed adhesive joint

Adhesive bonding is a promising joining technique in modern transport industries which provides for lightweight connection of dissimilar materials. However, available adhesive types have reached their limits in terms of durability and strength. A potential solution to this problem is the utilization of mixed adhesives. However, the influence of the surface preparation method on the performance of mixed adhesive joints has not been investigated thoroughly yet. The objective of this study is therefore to analyze such influence on the strength of mixed adhesive single lap joints. In this regard, five mechanical surface treatment methods were examined on aluminium alloy AA6082 using combinations of three different types of adhesives by means of static tensile tests. The results show significant increase in joint strength and ductility for DP 8005-AV138 compared to AV138 alone for sanding surface preparation. The roughness value in the range of Ra=1.7 to 3.1µm and Rm=36.5 to 44.60µm provide the highest shear strength.

Numerical study of the behaviour of composite mixed adhesive joints under impact strength for the automotive industry

The increasing use of composite structures in the automotive industry is due to strict regulations regarding both fuel efficiency and safety standards, since this kind of structures allow to produce strong yet light vehicles. The main advantage of the use of adhesives is the possibility of joining dissimilar materials, particularly composite materials, representing an optimal method in comparison with more traditional ones such as fastening or welding. This work focused on the development and validation of numerical models able to simulate the performance of previously experimentally tested joints. The experimental tests were performed to assess the improvement of quasi-static and impact strength of composite adhesive joints, and with the focus of avoiding early delamination of the composite substrates. The technique selected for this purpose was the use of mixed adhesive joints. Mixed adhesive joints combine two or more adhesives in a single lap joint and combine these properties to attain mechanical performance superior to that obtained using those adhesives individually. The numerical results demonstrated to be able to simulate the experimental results with reasonable accuracy.

JOINT STRENGTH OF MIXED SILYL MODIFIED POLYMER-EPOXY ADHESIVE ON SINGLE LAP JOINT ETCHED ALUMINUM

Currently, aluminum alloy is widely used in an automotive structure such as bus body materials due to decreasing the structure weight and fuel consumption. In this structure, there are tens or even hundreds of joints to meet the engineering design. Because of its advantages, the adhesive joint is usually chosen. Aluminum is stiff enough, but it can deform easily. Due to its deformability, cracks will occur easily on the aluminum adhesive joint if it is joined using strong and stiff adhesive. Otherwise, it will be very weak if it is joined using flexible adhesive. The mixture of the strong adhesive, epoxy (EP) and the ductile adhesive, silyl modified polymer (SMP) was proposed for joining aluminum. This study aims to evaluate the effect of the composition of mixed adhesive and its thickness on the strength of single lap joint (SLJ) etched aluminum. Before joining process, the aluminum surface was treated by sandpapering (SDP), and then chromic-sulphuric acid (CSA) etch. The composition variations of mixed adhesive were 100%EP, 75%SMP:25%EP, 50%SMP:50%EP, 25%SMP:75%EP, and 100%SMP. Mixed adhesives were prepared by using a stirrer with a spatula for 4-6 minutes at 60 rpm. SLJ specimens were manufactured through bonding of two aluminum sheets surface with an adhesive and giving 0.1 MPa pressure. The thickness adhesive was varied of 0.2 mm, 0.4 mm, and 0.6 mm. Specimens got a post-curing process at 100oC for 100 minutes. The shear tests were performed to evaluate the strength of SLJ. The experimental result showed that the surface treatments of SDP and CSA give higher porosity on the adherend surface. The highest strength and elasticity modulus were achieved by joint of 25%SMP:75%EP mixed adhesive joints. The addition a few of SMP adhesive in EP adhesive and adherend surface treatment increases the joint strength. All of the joints have mixed failure mode except the 100%EP adhesive joint has an adhesive failure mode.

Analysis of mixed adhesive joints considering the compaction process

Incorporating a material properties variation along the bondlines has proved to be a useful method for improving adhesive joints performance. In this work, the potential of the technique is analysed for a single lap joint using the mixing adhesives approach. In order to include the compaction process effect in the structural analysis during the joint assembly, a computational fluid-dynamic model capable of integrating different resins along the bondline has been developed. Then, the results obtained from this model are mapped into a finite element model through an application developed for this purpose. Several parametric studies have been carried out comparing different configurations in terms of maximum load capacity of the joints. Finally, one of these joints configurations has been manufactured using a special device developed for assembling these mixed adhesive joints and tested. This banded configuration have shown both numerically and experimentally an ultimate load improvement of over 70%.

Study of the singular term in mixed adhesive joints

In the numerical modelling of mixed adhesive joints, where different materials are disposed along the bond-lines, trimaterial singularities may arise at the adhesive bands transition points, resulting in mesh-dependent models. This work is focused on the study, characterization and treatment of these type of singularities. Firstly, the selection of an intermediate adhesive material for generating a tailored transition between bands is addressed, using analytical solutions and then corroborating by means of a novel technique for determining the influence distance of the singularity with the finite element method as calculation tool. Furthermore, it is proved how a continuum material field transition between bands produces a convergent finite element solution applying the considerations of this framework (analytical asymptotic solutions). Finally, in order to achieve the best numerical convergence rate in the models, the optimum material transition function is obtained and checked through FEM.

Behaviour under Impact of Mixed Adhesive Joints for the Automotive Industry

The automotive industry is increasingly using adhesive joints bonding advanced lightweight materials to reduce vehicle weight. Strength under impact loadings is a major concern for this application and mixed adhesive joints can effectively improve the joints by combining stiffness and flexibility on the same overlap. This work introduces and studies several configurations for static and impact tests of mixed adhesive joints with four adhesives in different combinations. The main purpose of this work is the development of a strong adhesive joint using a mixed adhesive layer and perform a series of mechanical to study its mechanical behaviour. It is concluded that the use of the mixed adhesive technique improves both static and impact strength by introducing flexibility to the joint which subsequently allows more energy absorption when introduced in crash resistant structures.

Geometrical study of mixed adhesive joints for high-temperature applications

The use of adhesives for high-performance structural applications has significantly increased in the last decades. However, the use of adhesive joints in adverse environmental conditions is still limited due to the reduced capability of adhesives to withstand large thermal gradients. Dual adhesive joints, which contain two adhesives with remarkably different mechanical behaviours, are a technique suitable for being used in extreme temperatures. The object of this study is a ceramic–metal joint, representative of the thermal protection systems of some aerospace vehicles. In this paper, several joint-mixed joint geometries are presented, studied with recourse to finite element analysis. In a first phase, the three-dimensional finite element models and the material properties are validated against experimental data. In a second phase, the model geometry is modified, with the aim of understanding the effect of several changes in the joints’ mechanical behaviour and comparing the merits of each geometry. The models’ presented good agreement was found between experimental and numerical data and the alternative geometries allowed the introduction of additional flexibility on the joint but at the cost of lower failure load.

Testing and simulation of mixed adhesive joints for aerospace applications

An important aerospace application of adhesives is in heat shields, bonded with room temperature vulcanizing silicone adhesive, which has high temperature resistance but low strength. Previous works proposed mixed adhesive joints as a solution and an investigation of this technique was performed. Three adhesive joint configurations were tested, including a mixed joint. The aim of the research was to simulate the load on a heat shield and predict the joint strength. Ceramic properties were obtained with an inverse method. There was a good agreement between experimental and numerical data, showing that this technique could be used for prediction and optimization.

Geometrical Optimization of the Overlap in Mixed Adhesive Lap Joints

The aim of this research is to optimize the geometry of the overlap in mixed adhesive single- and double-lap joints using a modified version of Bees and Genetic Algorithms (BA and GA). Accounting for adherends Poisson's ratio in the deduced equilibrium equations, the proposed shear lag model gives a more accurate approximation of joint failure load in comparison with Volkersen's solution. The objective functions used in this work are used separately to maximize the load bearing capacity f and the specific strength (f/w) of the joint. This procedure is applied to optimize aeronautical adhesively bonded assemblies, while taking manufacturing constraints into account. The employed constraints are the application of yield criterion on adherends as well as geometrical constraint on the overlap length. The proposed straightforward procedure provides 18 optimal configurations amid a wide range of changes for optimization variables, among which the designer can take a choice, depending on his/her goal. The efficiency of the two employed algorithms, BA and GA, in searching for the optimum geometrical design of the mixed adhesive joints have also been investigated. The results show the more robust and efficient performance of the modified version of BA over GA in such kinds of engineering problems.

Effect of mixed adhesive joints and tapered plate on stresses in retrofitted beams bonded with a fiber-reinforced polymer plate

This paper focuses on the reduction of interfacial stresses when using bonded laminates in strengthening existing structures. The presence of high interfacial stresses that develop near the end of composite known as edge effect may compromise the résistance to failure of strengthened structure. It is known that the decrease of plate thickness and fitness of adhesive (Young modulus) reduces the stress concentration at plate ends. Another way to tackle the problem is proper design of the plate end shape (tapered plate) and using mixed adhesive joints (MAJs) between the adherents. In this paper, a comprehensive finite element (FE) study has been conducted to investigate the effect of mixed adhesive joints (MAJs) and tapering plate on the interfacial stress distribution in the adhesive layer in retrofitted steel beam with fiber reinforced polymer (FRP) plate, This results indicate that using the correct combination of tapering plate at the end and mixed adhesive joints can reduce the magnitude of the interfacial stresses significantly.

Analysis of Mixed Adhesive Bonded Joints Part I: Theoretical Formulation

An analytical model was developed to study a mixed adhesive single-lap joint and a mixed adhesive double-lap joint to perform from low to high temperatures. A high temperature adhesive can probably be used alone, however it is brittle at low temperatures. On the other hand, a low temperature adhesive cannot be used alone at high temperatures since it is too flexible to support the applied load. To overcome this problem, one possibility is to use a mixed adhesive joint, which consists of a high temperature adhesive in the middle of the overlap and a low temperature adhesive at the ends of the overlap. After an extensive literature review on the existing analytical models for both single- and double-lap joints, it was concluded that the Frostig et al.'s model was easy to partition for the inclusion of more than one adhesive and could include geometric features such as the adhesive spew-fillet. First, this analytical solution was extended to account for the shear deformation in the adherends and the nonlinear geometric effect characteristic of single-lap joints. Then, it was further developed to model mixed adhesive joints. The accuracy of this analytical model was assessed by comparison with a finite element analysis. It was observed that both analyses were in very close agreement. A parametric study of various joint parameters on the stress distribution of mixed adhesive joints is presented in an accompanying paper (Part II).