Adhesive Joints Research Articles

Surface roughness influence on the behaviour of biocomposite adhesive joints

While research has explored surface treatment effects on adhesive joints made by synthetic composites or metals, there is a lack of research on biocomposite adhesive joints. This work aims to explore the effects of the surface roughness, induced by a sandblasting treatment, on the Mode-I-behaviour of adhesive joints made by flax/epoxy composites. This work focuses on the roughness induced by a variation of the sandlasting pressure. Roughness measurements were carried out on biocomposite adherents for three pressures (5, 6 and 7 bar). A finite element model (FEM) of a double cantilever beam (DCB) with a cohesive description (cohesive zone model, CZM) of the adhesive was proposed. In a macroscopic way, the roughness effect is represented by the change in the CZM parameters, accordingly, an identification procedure for different roughness level, was proposed. Comparisons between experimental Mode-I tests and FE simulations reveal a significant correlation between surface roughness and joint behaviour. A roughness mean depth R≈3μm, induced by a sandblasting pressure of 5 bar, appear to maximize the bonding strength (+16.6% with respect to 7 bar pressure-induced roughness R≈4μm). The Mode-I fracture energy decreases with the increasing of the roughness mean depth R (−33% between 5 and 7 bar conditions). These results provide critical insights for optimizing manufacturing processes of flax/epoxy composite adhesive joints in industries striving for sustainable solutions.

Assessment of Shear Fracture Energy in Hybrid Composites with Natural and Synthetic Fibers.

Composite materials made with synthetic fibers are extensively employed across a diverse array of engineering structures. However, from an environmental point of view, synthetic fibers do not represent the best choice, since they are not renewable and are not biodegradable as natural fibers. This study investigates the application of adhesive joints with hybrid composites, which combine natural and synthetic fibers, as potential replacements for traditional composites made solely from synthetic fibers. The main focus is on assessing the mechanical performance of these hybrid composites through end-notched flexure (ENF) tests on adhesive joints. Four different configurations of substrates were used, two with only one type of fiber (natural or synthetic) and two hybrids. Digital image correlation (DIC) analysis was conducted to provide detailed insights into the changes in displacement fields for the different configurations tested. The results indicate that adhesive joints with hybrid composites exhibit superior shear fracture energy (GIIC) compared with the joints with purely synthetic fibers. This enhancement in fracture toughness, attributed to the synergistic effects of the natural and synthetic fibers, suggests that hybrid composites could be a viable alternative, offering potential benefits in terms of sustainability and cost without compromising mechanical performance.

An Analytical Adhesion Model for Elastic Contact Electrification

Abstract Contact electrification is a universal phenomenon that commonly occurs in almost every solid–solid contact pair. The tribo-charges deposited on two surfaces by contact electrification can significantly affect adhesion; however, contact electrification is often overlooked in the study of adhesive contact. Here, we develop an analytical model to investigate electroadhesion during the contact phase between two initially uncharged dielectric surfaces, namely, an elastic parabolic surface and a rigid flat. A system of nonlinear equations is derived to describe the relationship between the indentation, normal load, radius of contact area, and radius of the charged zone using the Barthel–Maugis–Dugdale model (Barthel, 1999, “Modelling the Adhesion of Spheres: When the Form of the Interaction Is Complex, Colloids. Surf., A., 149, pp. 99105.). The analytical results show good agreement with the numerical results of the full self-consistent contact model. When contact electrification leads to a higher tribo-charge density and a larger charged zone, it has a greater impact on the normal traction, interfacial gap, force-approach curves, jump-out, and dissipated energy. The analytical model developed in this study serves as the foundation for advances in rough surface electroadhesive contact and electroadhesion testing, and it sheds light on the usage of adhesive joints in ultra-high vacuum environments and outer space, where contact electrification has a significant impact.

Effects of laminate stacking sequence on the strength properties of aluminum alloy–carbon fiber-reinforced plastic dissimilar single-lap adhesive joints

Here, two single-lap adhesive joints (SLJs) of dissimilar materials were subjected to static and cyclic loading tests. A2017 aluminum alloy was used as an adherend for one, and carbon fiber reinforced plastic (CFRP) was used as the adherend for the other. Four types of orthogonal laminated CFRPs with different laminate stacking sequences were used as adherends to investigate the effect of adherend stiffness on the strength properties of the joints. Furthermore, the results of the finite element analysis of the dissimilar SLJs revealed that when the tensile load was applied to them, the out-of-plane deformation asymmetry increased with increasing difference in stiffness between both adherends. This asymmetry affected the peel and shear stress distributions. Furthermore, the experiments revealed that the static tensile strength of the SLJs increased with increasing stiffness of the CFRP adherend. Additionally, fracture simulation using cohesive-zone modeling (CZM) revealed that the SLJs with higher CFRP stiffness exhibited higher strength, qualitatively agreeing with the experimental results. CZM analysis and adhesion strain measurements during the tests indicated that failure occurred at the A2017 adherend–adhesive interface. In contrast, no differences were observed between the fatigue strengths of the different types of adherends in the short-life region, with a number of cycles to failure (Nf) being ≤ 2 × 105. However, in the long-life region, beyond Nf = 2 × 105, the SLJ bearing the unidirectional CFRP adherend exhibited lower fatigue strength than the others. The anodizing process on A2017 was found to improve fatigue strength by a factor of two or more.

A developed fatigue analysis approach for composite wind turbine blade adhesive joints using finite-element submodeling technique

The intricate shapes and complex structures of composite wind turbine blades pose significant challenges in conducting local-fatigue simulation analyses. Moreover, adhesive joints, commonly found in composite wind turbine blades, are often neglected in current finite-element methods, leading to a dearth of empirical evidence to substantiate design enhancements for these blades. To tackle these issues, this study introduces a fatigue analysis methodology based on a finite-element submodeling approach to investigate the local fatigue behavior of composite wind turbine blades, with particular emphasis on the adhesive joints. The research begins with a comprehensive fatigue simulation analysis of composite wind turbine blades to establish boundary conditions for subsequent submodeling. A specialized submodel focusing on the trailing edge, a common adhesive joint, is constructed with the inclusion of extra adhesive regions for a more accurate depiction of their actual configuration. This submodel is subsequently utilized to investigate the localized fatigue characteristics unique to the trailing edge. Moreover, an exploration into the influence of the submodeling technique and its parameters on the predicted outcomes is carried out to enhance the applicability of the proposed methodology. These results significantly advance the understanding of local static and fatigue properties in large-scale structures through finite element analysis.

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.

Pattern recognition in electromechanical impedance spectroscopy damage detection of adhesive joints using multidimensional scaling

Adhesive joints are prone to various types of damage sources, which may not be identifiable with current non-destructive tests (NDTs). Structural health monitoring techniques, such as those based on electromechanical impedance spectroscopy (EMIS), aim to outperform NDTs in damage detection, by continuously monitoring structures. Although the EMIS-based algorithmic performance of damage detection has been evaluated on metallic and composite components, integrity monitoring of adhesive joints is yet to be fully determined. Therefore, this article investigates the use of multidimensional scaling (MDS) to cluster and visualize experimental impedance measurements of bonded joints in a three dimensional space. With these results, an Euclidean distance damage metric is used to try and classify the type of damage. The results show that damage detection is easily performed with the MDS algorithm, but effectiveness is dependent on the spectral measurement conditions. Furthermore, reduced dimensional spaces can yield information regarding the size and location of the damage in the adhesive layer, yielding increased knowledge on the integrity of structural adhesive joints.

Effects of surface roughness parameters on the shear strength of AA 6061-T6 aluminium alloy in structural adhesive bonding applications

This study aims to investigate the effects of the surface roughness of an aluminium substrate on the adhesive joint strength of aluminium–aluminium specimens prepared using epoxy and methyl methacrylate adhesives for shear strength tests. Aluminium surfaces were mechanically abraded using silicon carbide (SiC) papers with different grit sizes in two different modes. The evaluated bonding performance characteristics included the maximum bonding strength and residual strength of the adhesive joints after their exposure to various environmental conditions. The results demonstrated that excellent adhesion characteristics were obtained at the optimum SiC grit size (grit-80) regardless of the adhesive type. In addition, the topographical and morphological properties of the aluminium surfaces were studied before and after mechanical abrasion via surface profilometry and scanning electron microscopy, respectively. A possible mechanism was proposed to explain the observed shear strength changes and respective modes of fracture after sample exposure to air, de-ionised water, and aqueous salt solutions.

Dealing with multiaxial non-proportional fatigue with varying mean stress: Invariant and critical plane approaches for component-like structural adhesive joints

A comparative investigation between an invariant-based approach using the signed Beltrami-stress and critical-plane approach using the Findley-stress is carried out for the fatigue lifetime prediction of an epoxy structural adhesive under multiaxial non-proportional loading with varying mean stress. Fatigue experiments with uniaxial and multiaxial loading (proportional and non-proportional) are done at stress ratios of −1 and 0.1. Two adhesive joints are evaluated: a hollow-cylinder butt-joint (HCBJ, sample with homogeneous stresses distribution), and a flange-rod-joint (FRJ, component-like specimen with complex stress state). For both geometries, multiaxial loading and an increasing mean stress lead to fatigue strength reduction, whereas the phase-shift effect is less pronounced. A systematic procedure for parameter determination is implemented for both approaches. For predictions of the HCBJ-sample, stresses are analytically obtained. For validation with the FRJ-sample, FEA-based stress calculation is used. For both joints, the critical plane approach results in more accurate predictions than the invariant-based approach. In comparative terms, the invariant-based approach is experimentally more complex, by requiring a mean stress correction, but its numerically simpler with lower calculation time. The critical-plane approach requires fewer experiments for parameter determination and provides more accurate results for non-proportional loading. However, it is more numerically demanding with larger computing time.

The tensile properties of adhesively bonded single lap joints with short kevlar fiber

In this paper, short Kevlar fiber was added to epoxy resin adhesive to form a short fiber toughened adhesive, and its effect on the static strength and elongation at break of Carbon fiber laminate adhesively bonded single lap joints was investigated. 1 wt%, 2 wt%, 3 wt% and 4 wt% short Kevlar fibers were added to make short fiber toughened adhesive joints to investigate the effects of different contents of short Kevlar fibers on the connecting properties of the test pieces of Carbon fiber laminate single lap adhesively bonded joints; In addition, microscopic toughening mechanism of short Kevlar fiber in adhesives revealed by combining DIC technique and SEM technique. The test results showed that the addition of appropriate amount of short Kevlar fiber could slow down the crack extension compared with the without toughening specimens, but the toughening effect was weakened by excessive content. 2 wt% short Kevlar fiber toughened specimens achieved the best toughening effect, with 30.52 % increase in peak load and 25.78 % increase in elongation in tensile test. The combination of DIC and SEM further showed that short Kevlar fiber toughened joints absorb crack strain energy through fiber-bridging thus increasing adhesive properties.

Inverse estimation of tensile shear strength from fracture surface images using deep learning

To improve the long-term durability of adhesive joints, it is important to analyse the fracture surface and identify the fracture factors. Although conventional optical observation methods are simple and widely used, they were limited to qualitative discussions. In this study, an inverse estimation method utilising deep learning was investigated to clarify the quantitative relationship between the tensile shear strength and the fracture surface of single lap joints immersed in water. The deep learning analysis revealed that the tensile shear strength could be estimated from the fracture surface images with very high accuracy, indicating a strong quantitative correlation between the fracture surface images and the residual tensile shear strength. Grad-CAM indicated that the deep learning model could estimate the residual tensile shear strength by observing from the topology and colour of the remaining adhesive.

Moisture sorption behavior of an epoxy-based structural adhesive and its impact on the mode I interlaminar fracture toughness of bonded joints in the oil and gas industry

Structural adhesives are widely used across industry sectors and, specifically in oil and gas, they appear as an alternative for structural joining instead of welding, to mitigate the risks arising from sparks in an environment with highly flammable products. In this sense, this article aims to evaluate the mechanical behavior of adhesive joints using the Double Cantilever Beam (DCB - to determine the GIC, mode I interlaminar fracture toughness) test before and after exposing these joints to severe environmental conditions, such as high humidity and high temperature. Also, three different surface treatments (manual sanding with abrasive sponge, solvent cleaning, and peel-ply application) of the adherend (carbon fiber/epoxy composite) are examined, as well as two different thicknesses of the adhesive layer (0.5 and 1.0 mm) to verify possible influences on the GIC behavior before and after conditioning. The results show that reducing manual surface treatment processes is a better option, to reduce the factors that cause greater dispersion in GIC values. In particular, cleaning the adherend with just isopropyl alcohol (solvent) before applying the adhesive yielded better GIC results among the tested techniques. Regarding thickness, the findings suggest that a greater thickness of the adhesive layer can lead to an increase in the plastic regions, enhancing the dissipation energy and improving the mode I interlaminar fracture toughness. Hygrothermal conditioning revealed the adverse effects of humidity on the adhesive, resulting in a 20–30 % reduction in GIC. Differential Scanning Calorimetry (DSC) analyses demonstrate that conditioning caused a notable reduction in the glass transition temperature (Tg) of the adhesive.

Assessment of Digital Image Correlation Effectiveness and Quality in Determination of Surface Strains of Hybrid Steel/Composite Structures.

The application of the digital image correlation (DIC) contactless method has extended the possibilities of reliable assessment of structure strain fields and deformations throughout the last years. However, certain weak points in the analyses using the DIC method still exist. The fluctuations of the results caused by different factors as well as certain deficiencies in the evaluation of DIC accuracy in applications for hybrid steel/composite structures with adhesive joints are one of them. In the proposed paper, the assessment of DIC accuracy based on the range of strain fluctuation is proposed. This relies on the use of a polynomial approximation imposed on the results obtained from the DIC method. Such a proposal has been used for a certain correction of the DIC solution and has been verified by the introduction of different error measures. The evaluation of DIC possibilities and accuracy are presented on the examples of the static tensile tests of adhesively bonded steel/composite joints with three different adhesives applied. The obtained results clearly show that in a non-disturbed area, very good agreement between approximated DIC and FEM results is achieved. The relative average errors in an area, determined by comparison of DIC and FEM strains, are below 15%. It is also observed that the use of approximated strains by polynomial function leads to a more accurate solution with respect to FEM results. It is concluded that DIC can be successfully applied for the analyses of hybrid steel/adhesive/composite samples, such as determination of strain fields, non-contact visual detection of faults of manufacturing and their development and influence on the whole structure behavior during the strength tests, including the elastic response of materials.

The Effects of Combined Treatments of Laser Engraving, Plasma Spraying and Resin Pre-Coating on Improving the Bonding Strength of Titanium Alloy and Carbon Fiber-Reinforced Polymer.

This study focused on effective methods of laser engraving treatment (LET), plasma spraying, and resin pre-coating (RPC) to manufacture the reinforced adhesive joints of titanium alloy and carbon fiber-reinforced polymer (TA-CFRP) composites. The combined treatments contributed to the creation of a better adhesive bonding condition and offer a vertical gap between circular protrusions to form epoxy pins and carbon nanotube (CNT)-reinforced epoxy pins. The bonding strength of the TA-CFRP composite was reinforced by 130.6% via treatments with a twice-engraving unit of 0.8 mm, plasma spraying, and RPC. The original debonding failure on the TA surface was changed into the cohesive failure of the epoxy adhesive and delamination-dominated failure of the CFRP panel. Overall, laser engraving has been confirmed as an effective and controllable treatment method to reinforce the bonding strength of the TA-CFRP joint combined with plasma spraying and RPC. It may be considered as an alternative in industry for manufacturing high-performance metal-CFRP composites.

Durability of anticorrosive-coated steel–carbon fiber reinforced polymer structural adhesive joint at high temperature and humidity

Limited information on the long-term reliability of structural adhesive joints hinders their practical application. Moreover, performance improvement in transportation equipment and the solution of concrete problems, such as high strengthening, vibration reduction, and weight saving, are expected by increasing the availability of materials in the shipbuilding industry using highly functional materials that are not welded. This study evaluated the durability of second-generation acrylic (SGA) structural adhesives. The strength of SGA was higher than that of epoxy-based adhesives in the case of thick adhesive layers, and few fluctuations were observed in the strength owing to variations in the adhesive-layer thickness. The static strength following environmental degradation was evaluated using a tensile lap shear structural adhesive joint coated with an anticorrosive coating, assuming application to the exposed part. The acceptable design adhesive strength in shipbuilding was evaluated using the strength retention rate ηd (mean strength after deterioration/initial mean strength) and coefficient of variation CV (standard deviation/mean strength) after deterioration. ηd for each test was greater than 72 %. The CV value for each test was <0.16; this is equivalent to a coefficient of dispersion of deterioration Dy ≥ 0.41 on percent defects allowed = 1/10,000 (upper limit of the defect rate assumed at the design stage). This result indicates that the adhesive joint is available even for the exposure section, if the adhesive joint is protected similar to adherend steel with anticorrosive paint, which is common in the field of shipbuilding.

Backface strain as an index to detect damage initiation in composite single-lap bonded joints: Effects of adhesive type and joint dimensions

In this study, the backface strain (BFS) method applied by digital image correlation (DIC) is used to detect crack initiation and propagation in adhesively bonded single-lap joints (SLJ). By comparing the positive strain, due to the tensile load, and negative strain related to the bending moment, a point, called zero strain point (ZSP), can be detected on the substrate surface of the SLJ. Using the Bigwood and Crocombe analytical model, the presence of the ZSP on the backface is explained and the experimental results are used to detect it. The monitoring of the ZSP reveals useful information about the health condition of the joint. The main aim of this research is to investigate how the ZSP position varies by changing adhesive type (epoxy and polyurethane) and bonding area dimensions both in elastic conditions and damage progression. The results illustrate that the position of the ZSP in polyurethane SLJs is closer to the middle of the joint compared to epoxy SLJs. Additionally, the ZSP is more easily recognizable in epoxy adhesive SLJs when substrates are thicker. Finally, the ZSP showed negligible sensitivity to joint width for both types of adhesive joints regardless of the adhesive type. In conclusion, it is shown that the ZSP can be used as a monitoring index to detect damage initiation and propagation in SLJ specimens.

Mechanical and failure analysis of “outer single lap” adhesive joints of carbon fiber reinforced plastics under hygrothermal conditions

Carbon fiber reinforced plastics (CFRP)laminates are widely used in aircrafts. The laminates are often assembled into components by adhesive joints, which will be affected by elevated temperature and humidity during use, resulting in performance degradation. In this study, a compression test of “Outer Single Lap” adhesive joints of CFRP on the rear fuselage of a certain type of navigable aircraft was carried out to investigate the influence of hygrothermal conditions on their performance. The experimental environment includes the elevated temperature and wet condition (ETW) (71 °C, 85%RH) and Room temperature drying (RTD) (23 %, 30%RH). The test results show that RTD specimens have different stages, and the initial stage has good plasticity and no damage. However, when the load reached 50 %, the initial opening pattern damage occurred, which eventually led to the failure of the 20° opening. Compared with RTD samples, the compression properties of ETW samples decreased by 47.7 %, and the opening damage disappeared. Fracture features and stain analysis were performed. The findings suggest the fracture characteristic can change from a brittle fracture to a ductile fracture in different testing conditions, and the influence of the humid and thermal environment on the resin structural adhesive layer itself, the compression state of the carbon fiber laminates, the interface between the carbon fiber laminates and the resin structural adhesive layer should be fully considered in the design.

Surface pre-treatment of aluminum alloy for mechanical improvement of adhesive bonding by maple-assisted pulsed laser evaporation technique.

Adhesive joints are widely used for structural bonding in various industrial sectors. The performance of bonded joints is commonly attributed to the cleanliness of the substrate and the pre-treatment of the surfaces to be bonded. In this study, the Matrix Assisted Pulsed Laser Evaporation (MAPLE) deposition technique was used for surface modification of aluminum (Al) plates by the deposition of poly(propylene glycol) bis(2-aminopropyl ether) (PPG-NH2) of different number average molecular weights (Mn) of 400 g mol-1, 2000 g mol-1, and 4000 g mol-1, respectively. Fourier-transformed infrared spectroscopy (FT-IR) analysis indicated the characteristic peaks for the deposited layers of PPG-NH2 of different molecular weights in all cases while scanning electron microscopy (SEM) revealed continuous layers on the surface of Al plates. In order to demonstrate alterations in the wettability of Al substrates, a crucial aspect in surface treatment and adhesive bonding, measurements of contact angles, surface free energies (SFE), and adhesion work (W a) were conducted. The tensile strength measurements were performed using the lap-joint test after applying the commercial silyl-based polymer adhesive Bison Max Repair Extreme Adhesive®. It was evidenced that at higher values of the SFE and W a, the tensile strength was almost 3 times higher for PPG-NH2 with Mn = 4000 g mol-1 compared with the untreated Al sample. This study provides valuable insights into the successful application of the MAPLE technique as a pre-treatment method for reinforcing adhesive bonding of Al plates, which can lead to improved mechanical performance in various industrial applications.

Parametric analysis of composite tubular adhesive joints bonded by the bi-adhesive technique

Parametric analysis of composite tubular adhesive joints bonded by the bi-adhesive technique

Effects of cyclic ageing frequencies on the ageing and mechanical behaviour of adhesive materials: Experimental analysis and numerical study

In marine structures adhesive joint structures are often exposed to cyclic conditioning where the ambient humidity changes cyclically during their service. Though some comprehensive studies on the aging of adhesives exist, these researches mainly focus on monotonic aging conditions where the adhesive joints are exposed to a wet condition continuously for a long time. However, the few investigations performed on the cyclic moisture absorption of adhesive materials show that the parameters obtained in monotonic aging conditions are not suitable for estimation of the aging behaviour of adhesive exposed to alternating humidity. An important question that can arise is whether or not this frequency affects the mechanical behaviour of adhesive joints under cyclic aging condition. In this investigation bulk dogbone and square samples were manufactured, subjected to cyclic aging conditions with four different aging frequencies and tested. The results show that the moisture diffusion constant of adhesives exposed to higher aging frequencies increase more than those exposed to lower aging frequency conditions. In addition, the moisture content only affects the degradation of strength and stiffness of the tested adhesives in different aging frequencies.