Adherend Materials Research Articles

Evaluation of stress distributions in trimaterial bonded joints with nano-resin adhesive using machine learning models

Adhesive bonded joints hold significant importance across various industrial sectors in modern engineering, owing to their lightweight nature and myriad advantages. The rising demand for trimaterial joints underscores their utility and versatility. In these joints, the choice of materials for both adherends greatly influences their strength, structural reliability, and overall characteristics. While numerous researches have extensively analyzed stress distributions, their effects, and behaviors, many have relied on a one-factor-at-a-time approach, focusing solely on individual design variables' effects. However, recognizing the intricate interplay among various material combinations and their collective impact on overall performance, this study employs various types of White-box, Black-box, and Grey-box machine learning algorithms to identify an optimized ML model as well as predict stress distributions for any random combinations of upper and lower adherend materials. Dataset of total 178 random material combinations were utilized for the training phases with 5-fold cross validation and model tuning. However, the decision tree regressor emerged as the optimized model by comparing the quantitative metrics of accuracy benchmark as well as the prediction outcomes obtained through all the machine learning models. The maximum prediction accuracy attained was an impressive 99.97 %, while the minimum recorded was 89.74 %. This research aims to identify tailored machine learning model specifically for trimaterial bonded joints where nano layer of resin is utilized as the adhesive.

Effect of microstructural roughness on the performance and fracture mechanism of multi-type single lap joints

Surface roughness of adherends is a crucial factor in determining the performance and failure mechanism of adhesive joints. However, the research dedicated to the examination of fracture mechanism for adhesive joints influenced by surface roughness at microscale is limited. This work conducts systematic experimental and numerical investigations into the effect of microstructural roughness on the performance and fracture mechanism of multi-type adhesive single lap joints (SLJ). The adherend materials are aluminium alloy (Al) and polyphthalamide (PPA), ground into three roughness grades for the fabrication of SLJs using an epoxy adhesive. The mechanical properties of the adhesive, adherends and SLJs derived from experimental studies are utilized to calibrate the microparameters in Discrete Element Method (DEM) models for a numerical analysis. The newly developed DEM models demonstrate efficacy in predicting the performance and capturing the failure modes of multi-type SLJs realistically with distinct microroughness profiles. Finally, the influencing mechanisms of microstructural roughness on the performance of multi-type SLJs are investigated, including the microscale interfacial bonds and mechanical interlockings. The effects of microstructural roughness on the microscale failure mechanisms of multi-type SLJs are also explored and discussed, including the crack initiation, coalescence, and propagation within the adhesive and interface.

Uncertainty Analysis of Tensile Strength of Scarf Adhesive Joints using Numerical Method and Validation through Experimentation

Adhesive joints represent a significant advancement in material bonding technology, offering benefits such as reduced weight and uniform stress distribution, which distinguish them from conventional joining methods. Among the various adhesive joints, the scarf adhesive joint stands out due to its mechanical advantages and potential for high-strength applications. This research investigates the impact of key parameters on the strength of scarf adhesive joints, including scarf angle, surface roughness, adhesive type, and adhesive layer thickness. By systematically varying these parameters and employing a design of experimentation (DOE) methodology, we aim to identify the optimal conditions that maximize joint strength. A series of scarf adhesive joint specimens with diverse parameter combinations will be fabricated and tested to evaluate the resultant joint strength. Statistical analysis of the experimental data will facilitate the understanding of how each parameter influences joint performance and contribute to the development of optimized adhesive joint designs. The findings of this study are expected to advance the application of scarf adhesive joints in various high-performance engineering fields, providing insights into the critical factors that govern their strength and reliability. Key Words: Scarf Adhesive Joint, Adherend Material, SS 304 Stainless Steel, Finite Element Analysis (FEA), Tensile Strength, Max von Mises Stress, Scarf Angle, Design of Experiments (DOE), Adhesive Properties, Surface Roughness.

Sikahyflex-Epoxy Mixed Adhesive’s Effect on the Aluminum-Composite Joint’s Shear Tensile Strength for the Automotive Industry

The automotive industry sector encounters challenges in the construction of connections between different materials. Hence, a breakthrough is needed in the automotive industry in manufacturing connections between different wall panel materials. Mixed adhesive materials for different materials are required and represent an innovation in the manufacturing process for joints of different materials due to the need for stiff and slightly ductile adhesives. This research aims to analyze the effect of using a sikahyflex-epoxy mixture adhesive in aluminum-composite joints on the shear tensile strength of the joints for the automotive industry. This research serves as an innovation in dissimilar material connection systems by developing the use of mixed adhesives and cocofiber aluminum-composite adherend materials with environmentally friendly and corrosion-resistant properties. The research material used aluminum 5083-cocofiber composite. The adherend surface was roughened employing sandpaper of #60, #80, and #150. The adhesive used the addition of sikahyflex adhesive to the epoxy adhesive with additional variations of 10%, 20%, 30%, and 40% sikahyflex. Connections between different materials with the single lap joint type refer to ASTM D1002. The roughness test results yielded the best roughness grade #150 on the surface of the aluminum adherend and coco fiber composite. The shear tensile test results by adding 40% sikahyflex adhesive, 0.4 mm adhesive thickness, and #150 sandpapering resulted in a 20% increase in shear tensile strength in the single lap joint of 2.51 N/mm2. The surface roughness enhanced the adhesive bond strength between mechanical interlocking adhesives and adherend. Meanwhile, the failure modes observed in macro observations included thin-layer cohesive failure, cohesive failure, two-stage failure, and stock-break failure modes. The SEM observation revealed that in the initial propagation of microcracking and voids, which mark the initial onset of adhesive failure, tearing took place, leading to a failure mode in the aluminum-composite coco fiber single overlap joint.

Mechanical response of reduced graphene oxide (rGO) reinforced epoxy adhesive joints

The present work aims at investigating the effect of adhesive modification on the lap shear strength of the adhesively bonded joints. The epoxy-based adhesive is reinforced with reduced graphene oxide (rGO) at different weight fractions. Single lap joints (SLJ) have been prepared by considering the commercially available aerospace-grade aluminum alloy AA8011 as the adherend material and Araldite 2014 A/B as the adhesive. Experiments showed that the addition of 0.5 wt.% graphene to the epoxy adhesive resulted an approximately 27% increase in the lap shear strength of the SLJ. Further increase in the reinforcement loading decreases the lap shear strength due to the agglomeration of graphene particles. A detailed analysis of the effect of reinforcement loading and overlap length on the shear strength has been presented and failure mechanisms are discussed.

Mixed-Mode Cohesive Failure of CFRP to Steel and Fe-SMA to Steel Bonded Joints

The integrity of carbon fibre reinforced polymers (CFRP)-to-steel and iron-based shape memory alloys (Fe-SMA)-to-steel adhesively bonded structural strengthening patch must be maintained to ensure a durable application. The influence of the adhesive thickness and adherend material behavior on the joint fracture has been well documented. The influence of mode-mixity has yet to be investigated and is studied experimentally and analytically in this study. In engineering applications, bonded strengthening patch, located in the shear span, may be subject to mixed-mode loading. Lap-shear tests involving mixed-mode fracture of bonded CFRP and Fe-SMA were conducted. The mode-mixity was introduced via an eccentricity between the loading axis and the adhesive plane. Furthermore, a novel theoretical model has been developed considering (i) adherend material behavior and (ii) mode-mixity. Experiments show that loading eccentricity decreases the joint capacity, up to 70% for CFRP and 32% for Fe-SMA. The theoretical analysis shows this difference is caused by the Fe-SMA material nonlinearity. The local mixity contains more Mode II in the Fe-SMA case than the CFRP case. Fe-SMA yielding mitigates the development of opening forces at the crack tip. These findings hold significant meaning relative to the structural resilience and robustness of joints. A strengthening solution employing Fe-SMA can benefit from material ductility to hinder the influence of loading eccentricity or misalignment of structural surfaces after damage.

Characterization of Bending Strength in Similar and Dissimilar Carbon-Fiber-Reinforced Polymer/Aluminum Single-Lap Adhesive Joints

In recent years, the adhesive bonding method has gained increased attention, especially in the automotive industry, for constructing efficient body structures from dissimilar and lightweight materials such as aluminum and polymeric composites. Adhesively bonded automotive structures endure complicated loading conditions, including tensile and bending loading, during their service lives. To the best of the authors’ knowledge, there is no published work on the assessment of bending strength in single-lap adhesive joints (SLJs) when considering dissimilar adherends under three-point bending. In this study, three-point bend experiments were carried on the bending strength and the failure mechanisms of dissimilar SLJs made of carbon-fiber-reinforced polymer (CFRP) and aluminum substrates bonded with Araldite 2015 adhesive. Additional experiments were conducted individually on similar SLJs, including aluminum/aluminum and CFRP/CFRP, to investigate and compare the effects of adherend material type on the bending strength and failure behavior of SLJs. The results indicate that a CFRP/CFRP single-lap adhesive joint exhibits significantly higher joint strength in comparison to an aluminum/aluminum single-lap adhesive joint under three-point bending. The strength of dissimilar CFRP/aluminum single-lap joints usually falls between that of an aluminum/aluminum and that of a CFRP/CFRP single-lap adhesive joint. When the CFRP adherend is situated at the bottom of the joint in three-point bending, it imparts significantly greater joint strength and deformation compared to situations where the aluminum adherend is placed at the bottom.

Effect of cork powder concentration and service temperature on strength characteristics and failure modes of adhesively bonded lap joints

In this study, the combined effects of cork powder particles addition and service temperature on the strength characteristics of lap joints was experimentally investigated. Two types of lap joints i.e. single lap joint (SLJ) and double strap joint (DSJ) with two different adherend material configurations were manufactured using a two-component epoxy system Araldite©️ LY 556. CFRP and Aluminum alloy 5083 was used as adherends in the fabrication of the lap joints. Cork powder was added in the amounts of 0.25, 0.5, 0.75, and 1 wt% in the host resin and was mechanically mixed at an elevated temperature of 50 °C, below the Tg of epoxy adhesive. The prepared adhesive was then used to make adhesive lap joints. The joints were then tested at four different temperatures of 25, 50, 75, and 100 °C by utilizing a universal testing machine equipped with thermal chamber. The results of the study showed that the joint strength can be increased up to a certain extent with the increase in cork powder addition, however, in general, will be negatively affected by the increase in service temperature. Also, the reinforcing effect of cork powder remained significant even at temperatures close to the glass transition temperature of epoxy adhesive.

Ultrasonic monitoring of adhesive curing process between adherends

An ultrasonic method to monitor the curing of a thin adhesive layer was developed theoretically and verified experimentally. The method was based on a model in which an interface spring represented an adhesive layer sandwiched between adherends of the same material. In this method, the interfacial stiffness in the spring model, which included the ultrasonic longitudinal wave velocity and the adhesive's density and thickness, was obtained from the measured reflection or transmission coefficient for the wave in the adhesive layer. The ranges of interfacial stiffness for valid application of the method were theoretically clarified in terms of the stiffness's sensitivity to the reflection and transmission coefficients and the stiffness error with respect to errors in the measured coefficients. The applicable ranges were verified by experimentally evaluating the curing process of an epoxy resin with adhesive several micrometers thick when joining aluminum alloy adherends. Specifically, the ranges were obtained as 3.1ρC<K/f<62ρC[Pa⋅s/m] for the reflection coefficient and K/f<6.3ρC[Pa⋅s/m] for the transmission coefficient, where ρC, K, and f are the acoustic impedance of the adherends, the interfacial stiffness of the adhesive, and the ultrasonic frequency. The measurable ranges of the adhesive's interfacial stiffness were also determined for typical adherend materials such as alloys, polymers, composites, and ceramics.

Submerged ultrasonic plastic welding: A computational and experimental investigation

Thermoplastics hold utmost importance in the day-to-day life of modern-day society, being widely employed in water transport and storage vessels commonly made from thermoplastics such as PVC, i.e., polyvinyl chloride, and PP, i.e., polypropylene. This urge for a joining technique is applicable even with water in the vessel or the submerged case. The present investigation explores ultrasonic welding as an option to weld thermoplastics in water-submerged conditions. We have performed FEM simulations and experimental validation to prove the viability of the proposed ultrasonic welding process used in welding PVC and PP in water-submerged conditions. The discussed results demonstrate a decrease in melting and degradation of adherend material at the weld interface while welding PVC and PP in water-submerged conditions and attaining 65.78% and 107% of weld strength, unlike their open-air counterparts, respectively.

Investigation on the transitional micromechanical response of hybrid composite adhesive joints by a novel adaptive DEM model

This work developed a discrete element model to adaptively capture the transitional micromechanical response and failure mode of adhesive joints with dissimilar adherend materials and different configurations. Especially, the development of the model only requires one-time calibration. Loctite EA 9497 epoxy adhesive, aluminium (AL) and polyphthalamide (PPA) were selected to make different types of hybrid adhesive joints for the lab tests. The modelling applicability in simulating Mode I, Mode II cohesive failure and adhesive, mixed failure modes was subsequently validated with the experimental data. The validation shows that the proposed model can accurately capture the observed failure modes and joint performances. It is followed by investigating the ability to adaptively obtain the variation of fracture energies with different adhesive thicknesses. The results agree well with the current reports on the growth trend of fracture energies which see a rise till the thickness reaching 0.8 mm and subsequently decline to a plateau. Finally, key factors including the adhesive thickness, lap length were selected to perform a parametric study to investigate their influences on the failure mechanism and micromechanical response of hybrid joints. It is found that a thickness range of 0.1–0.3 mm is adequate to obtain satisfactory joint strength whilst thicker adhesive over 0.6 mm will decrease the joint strength. This is due to that a thinner adhesive layer can facilitate its cohesive fractures and thus fully use the resistance of adhesive. A range of lap length from 6 mm to 12.5 mm was found to have a higher efficiency of improving the joint strength when AL adherend was used.

Strain Rate Effect on Mode I Debonding Characterization of Adhesively Bonded Aluminum Joints

In adhesive bonding, two different substrate materials are joined together, usually by forming chemical bonds. The adhesive can stick things together. The loading rate and deformation mode can easily change the mechanical properties of the adhesive material. Hence, a vital aim of the current study is to evaluate the strain rate effect on the damage response of adhesive joints for Mode I loading scenarios. The adherend material was aluminum AL6061-T6, and Araldite 2015 was the adherent material. This experiment for delamination had a prescribed adherend size of 200 mm × 25 mm × 3 mm and an adhesive thickness of 0.5 mm. In situations where the strain rate affects the failure mechanism, a displacement rate of 5, 50, or 500 mm/min is sufficient to attain the failure mechanism. A double cantilever beam (DCB) specimen was employed to construct the FE model geometry for simulation. A hybrid experimental–FE technique was utilized to extract the properties of the adhesive interface. FE simulation has proven to have an excellent correlation with the experimental findings.

Prediction of bonded asymmetric metallic cross-tension and single lap shear joints using finite element model with material-level adhesive properties and cohesive zone method

Predictive finite element (FE) models of adhesive joints are needed to enable the design and evaluation of adhesively joined structures, particularly large-scale structures that may be costly to assess experimentally. Although models calibrated to coupon-level tests have provided an important first-step, models based on material-level characterization are needed to enable joint assessment in complex modes of loading. In the present study, a structural epoxy adhesive was characterized using rigid double cantilever (Mode I) and bonded shear (Mode II) specimens to provide material-level input properties. Three single-lap shear (SLJ) and seven cross-tension (CT) specimen configurations were fabricated with aluminum and steel sheet materials. The specimens included symmetrical (the same adherend material and thickness) and asymmetrical (dissimilar adherend material or unequal thickness) configurations, with three loading angles (0°, 45°, 90°) for the CT specimens. Finite element models of the SLJ and CT specimens were developed using Cohesive Zone Modeling for the adhesive, with properties determined from the Mode I and Mode II characterization tests. The FE models of the SLJ and CT test specimens predicted the peak load within an average difference of 2%–19%. The joint strength varied between different test configurations, owing to adherend deformation, load eccentricity and mixed-mode loading. Importantly, the model parameters were not calibrated to the SLJ and CT tests. The FE models were able to predict joint response for varying test specimen geometry, adherend thickness, adherend material, and modes of loading.

Effect of using fibre reinforced epoxy adhesive on the strength of the adhesively bonded Single Lap Joints

In this paper, the static strength and elongation of the adhesively bonded Single Lap Joints (SLJs) at failure are enhanced by using glass fibres reinforced with epoxy adhesive. Commercially available Araldite, a standard epoxy adhesive, is utilised as the binding agent, while Aluminium plate and Glass Fibre Reinforced Plastic (GFRP) are used as metal and composite adherend materials, respectively, in the experiment. The performance of glass fibre reinforced epoxy adhesives in terms of strength improvement has been tested for two types of SLJs: MTM (Metal-to-Metal) and MTC (Metal-to-Composite). The strength controlling parameters considered during the experiment are certain variables such as the alignment of the bundle of long glass fibres along the loading direction, the size of glass fibres and the weight ratio of short glass fibres reinforced with epoxy adhesive in the overlap region of the SLJ. The effects of these controlling parameters on the strength and elongation have been discussed. In addition, the bundles of long glass fibres are overlaid at the fillet region along the loading axis, extending beyond the overlap ends to observe the effect of the peel stress at the overlap ends and examine its effect on the failure strength of the SLJ. The experimental results show that there is a substantial increase in failure load and elongation. Fractography analysis has also been carried out to analyse the fracture surfaces using a Scanning Electron Microscope (SEM).

Crack-arresting behavior of adhesively bonded joints with a single rectangular convex/concave shape formed on adherend surfaces

ABSTRACT The effect of a rectangular convex/concave shape formed on adherend surfaces in terms of preventing the crack growth of adhesively bonded joints was analytically investigated. The finite element method was used to evaluate crack growth resistance along the crack path by simulating a double cantilever beam test in which a single convex/concave shape was formed on the adherend material. The obtained crack growth resistance clearly demonstrated that both concave and convex shapes improved the apparent fracture toughness; i.e., the convex/concave shape functioned as a crack arrester. We identified the cause of the improvement as the dispersion of energy allocated to crack propagation due to the accumulation of the strain energy or the reduction of the stress concentration at the crack tip. It is noteworthy that the crack growth was arrested even when the convex shape did not interrupt the crack path.

Mode I tensile fracture behavior of adhesively-bonded metal–metal, metal–CFRP, and CFRP–CFRP bi-material combinations analyzed by size effect method

Understanding the adhesive and interfacial fracture is important for developing better adhesive performance in bi-material joints, i.e. adhesive joint between dissimilar materials. However, to characterize the fracturing behavior of various adhesively-bonded materials, it was shown in this work that the Mode I fracture energies estimated from conventional methods (e.g., work-of-fracture, (modified) compliance calibration method, (modified) beam theory, etc.) can be strongly affected by adherend thickness, adhesive bond length, and adherend material type. Consequently, the proper understanding of fracturing in bi-material joints has been hindered since the estimated fracture energies can exhibit unreasonable difference among various material combinations depending on the method of fracture energy calculation. This has led to confusion in the literature due to the unfair comparison of these non-objective results estimated by leveraging conventional methods on the specimens with different geometries or dissimilar adherend materials.This work introduced the size effect method to adhesively-bonded joints and compared the results with conventional methods of calculating the Mode I fracture energies of metal–metal, metal–CFRP, and CFRP–CFRP material combinations via Double Cantilever Beam (DCB) tests. The results showed that the estimated fracture energies obtained using the size effect method were not dependent on the specimen geometries, whereas conventional methods were strongly dependent on specimen geometry for most material combinations. The size effect method allowed objective comparison particularly on the interfacial fracturing between metal/adhesive and CFRP/adhesive combinations. The difference was further explained by and correlated with the damage morphology on the material surface after failure, which was quantified by three-dimensional profilometer.

Failure load prediction of single-lap bonded joints by damage zone method

The failure load of single-lap bonded joints with dissimilar adherend materials was predicted using the damage zone method. A number of six joint configurations with different thicknesses of the adherends were considered. Three-dimensional finite element models were built to represent the behavior of the bonded joints, and the damage zone method was applied to predict the failure loads of the joints. The predictions of failure loads of the joints were achieved, showing a good agreement between the numerical and the corresponding experimental results.

Effect of different joint angles on the mechanical strength of adhesive-bonded scarf and double butt–lap joints

Abstract The effect of different joint angles and joint types in adhesively bonded joints was investigated. Two joint types were chosen to examine the effect of the joint type. A total of 12 samples were produced at six different angles (30°, 45°, 52°, 60°, 75°, and 90°) to examine the effect of joint angle on the scarf and double butt–lap joint types. St 37 steel was used as the adherend material. The adhesion distance, sample thickness, and adhesion area were kept constant in the samples. There were differences in the sample widths to make the angle change by keeping the adhesive bonding length constant. 3M Scotch-Weld DP810 epoxy adhesive was used as the adhesive. The adhesive thickness was chosen as 0.1 mm. An axial tensile load was applied to the samples, and the results were recorded and evaluated. When scarf and double butt–lap joint samples are compared with each other according to their angles, it was seen that double butt–lap joints were more successful in samples with 30, 45, and 52° joint angles, and scarf lap joints were more successful in samples with 60, 75, and 90° joint angles.

On quasi-static large deflection of single lap joints under transverse loading

This study aims to characterize the mechanical behavior of a carbon fiber reinforced plastics (CFRP)/aluminum (Al) adhesive bonded single lap joint (SLJ) subjected to transversal three-point bending load with different adherend materials and kinematic boundary conditions. The experimental results divulged that the boundary conditions had noticeable influence on the load bearing capacity of the joint; and the stiffness and peak load for the fixed-support condition were much higher than those for the simply-supported condition. The stress distribution in the adhesive layer varies under different boundary conditions, leading to the different morphology of the residual adhesive at the fracture surface. The different cracks in the adhesive layer were further analyzed by SEM under peel stress and shear stress loadings. The finite element (FE) models were developed by using the material properties characterized in-house here. It is found that the load displacement curves and the crack energies predicted by finite element analysis (FEA) was well correlated with those measured by the experiment in the designated four groups of adhesive joints. This study is anticipated to provide systematic understanding of adhesive joints with dissimilar adherend materials under different boundary conditions subject to transverse loading.

Low-speed bending impact behaviour of adhesively bonded dissimilar single-lap joints

This study investigates the low-speed bending impact behaviour of adhesively bonded dissimilar single-lap joints and the effects of both strength and plastic deformation capability of adherend material on adhesive failure. Dissimilar adhesive single-lap joint specimens, such as Al 2024-T3 (top adherend)-Al 5754-0 (bottom) and Al 5754-0 (top)-Al 2024-T3 (bottom), were tested at two impact energy levels (3 and 11 J) for two overlap lengths (25 and 40 mm). The progressive failure analysis of the adhesive layer was also conducted by the non-linear explicit finite element method. The adhesive layer was modelled with a 3D cohesive layer along with the upper and lower adhesive interfaces and a non-linear continuum adhesive region between two cohesive layers. The continuum adhesive region had elasto-plastic adhesive properties whilst the cohesive layers obeyed 3D cohesive rules. The experimental and predicted contact force-time, contact force-displacement diagrams, axial separation lengths of the failed adhesive region, permanent deflection of the bonded region, fracture surfaces were in good agreement. The strength and plastic deformation capability of adherend materials and impact energy levels affected the progressive adhesive failure behaviour. The proposed finite element model was successful reasonably in predicting the initiation and propagation of the adhesive failure.