Metal Adherends Research Articles

A Study of the Effects of Moisture on Composite-to-Metal Double-Lap Shear Joints.

This work investigated the effects of moisture absorption treatment on composite-to-metal double-lap shear joints (DLSJs) bonded with epoxy adhesive film through experiments and simulations. The composite-to-metal DLSJ can be divided into five parts (the interface between the composite and adhesive, the interface between the adhesive and metal, the composite adherend, the metal adherend, and the adhesive layer). First, the wet-dependent properties of the adhesive and interfaces were obtained through adhesive tensile tests and GC tests, which showed that the properties of the adhesive and interfaces were significantly affected by the moist environment. Then, tensile tests of the composite-to-metal double-lap shear joints were carried out in dry and wet environments. Finally, based on the experimental investigations, a finite element (FE) model that considered cohesive damage was established for simulating damage evolution and predicting the failure loads and failure modes of the DLSJs. The results of both the experimental and numerical tests show that the DLSJ failure load decreases significantly after immersion in 95 °C water, and the major failure mode transfers from adhesive failure to interface failure. The research results provide a theoretical basis or basic data for the structural design of adhesively bonded composite-to-metal.

Non-covalent functionalization at Titanium carbide nano-particles for strengthening the interface with polyaryletherketone matrix for developing high-strength adhesives

In this study, a highly effective cationic polymer, Poly (diallyldimethylammonium chloride) (PDDA), was used to bond Titanium Carbide (TiC) nano-particles (NPs) non-covalently with matrix polyaryletherketone (PAEK). Functionalization of PDDA to TiC NPs was confirmed using Fourier-transform infrared spectroscopy (FT-IR), Zeta potential, and Field Emission Scanning Electron Microscopy (FE-SEM). Both TiC NPs (3 wt%) (untreated and PDDA-treated) were added to PAEK, as a filler for reinforcement and elevating friction. The addition of TiC NPs (untreated and treated) in the PAEK led to an improvement in selected performance properties such as thermal stability (by 11 and 13 °C), surface free energy (17 and 29%), and lap shear strength (LSS) (116 and 161%) respectively. Plastic deformation, cup formaion and micro-striation formation were the major failure mechanisms. PDDA treatment to TiC NPs resulted in better and more uniform deagglomeration in the PAEK matrix, strengthened particle-matrix interface, higher surface energy, and hence more adhesion with the adherend. The improved strengthening of both the interfaces (NPs with the matrix and nanocomposite with the metallic adherend) led to an enhanced performance by the treated NPs.

Electronic Interaction of Epoxy Resin with Copper at the Adhered Interface.

A better understanding of the aggregation states of adhesive molecules in the interfacial region with an adherend is crucial for controlling the adhesion strength and is of great inherent academic interest. The adhesion mechanism has been described through four theories: adsorption, mechanical, diffusion, and electronic. While interfacial characterization techniques have been developed to validate the aforementioned theories, that related to the electronic theory has not yet been thoroughly studied. We here directly detected the electronic interaction between a commonly used thermosetting adhesive, cured epoxy of diglycidyl ether of bisphenol A (DGEBA) and 4,4'-diaminodiphenylmethane (DDM), and copper (Cu). This study used a combination of density functional theory (DFT) calculations and femtosecond transient absorption spectroscopic (TAS) measurements as this epoxy adhesive-Cu pairing is extensively used in electronic device packaging. The DFT calculations predicted that π electrons in a DDM molecule adsorbed onto the Cu surface flowed out onto the Cu surface, resulting in a positive charge on the DDM. TAS measurements for the Cu/epoxy multilayer film, a model sample containing many metal/adhesive interfaces, revealed that the electronic states of excited DDM moieties at the Cu interface were different from those in the bulk region. These results were in good accordance with the prediction by DFT calculations. Thus, it can be concluded that TAS is applicable to characterize the electronic interaction of adhesives with metal adherends in a nondestructive manner.

Cavitation and crack nucleation in thin hyperelastic adhesives

The present study investigates in the failure of adhesive bondings with structural silicone sealants. Point connectors of two circular metal adherends bonded with DOWSIL™ TSSA are subjected to tensile loading. We formulate and use a constitutive law that captures volumetric softening owing to the formation of cavities. Therein, cavitation is considered a process of elastic instability which is homogenized with a pseudo-elastic approach. Ultimate failure initiating from the free edges is predicted employing the framework of finite fracture mechanics. The concept requires both a strength-of-materials condition and a fracture mechanics condition to be satisfied simultaneously for crack nucleation. For the former, we use a novel multiaxial equivalent strain criterion. For the latter, we employ literature values of the fracture toughness of DOWSIL™ TSSA . The predicted onset of cavitation and ultimate failure loads are in good agreement with our experiments. The proposed model provides initial crack lengths that allow for the derivation of simple engineering models for both initial designs and proof of structural integrity while simultaneously extending the range of usability of the structural silicone compared to standardized approaches.

Effect of outdoor environmental exposure on the mechanical behavior of steel/GFRP single-lap joints

ABSTRACT Adhesive joints GFRP/Steel bonded with epoxy adhesive were exposed to the external environment for one year, a tropical environment with temperature variations, humidity, radiation, and salt spray, due to its proximity to the sea. The shear strength of the joint was measured over the exposure time, as well as their chemical and physical changes. Tg of the adhesive was evaluated through DMA, and changes in the composite matrix were studied using FTIR. A significant reduction in joint strength was observed within the first 30 days, but this reduction quickly stabilized. High corrosion of the metal adherend was evident, which was also observed in the bonded region, significantly impacting the adhesive strength of the joints. Changes in the color of the composite were also identified due to exposure to solar radiation, which also affected the chemical bonds, as indicated by FTIR. The Tg of the adhesive decreased in the first 90 days and then gradually increased. These changes may have been related to the humidity, initially decreasing the Tg, and the exposure to higher ambient temperatures, which causes post cure effect. Therefore, the proposed external exposure conditions contain factors capable of causing significant changes in the joint and its components.

Metal-metal and metal-composite joints with 3D printed aluminium substrates: effect of surface treatment on the mode I fracture toughness

ABSTRACT Adhesive bonding of composite laminates to metals is an increasingly employed technique to manufacture lightweight structures. However, this type of joints is sensitive to defects and composite delamination near the bonded area. Thus, increasing their damage tolerance is fundamental to design reliable multi-material components. This work is the first step in a project aimed at improving the fracture toughness of Carbon Fibre Reinforced Polymer (CFRP)/metal joints, exploiting Additive Manufacturing (AM) to produce small features on the metal adherend. This first step is devoted to the qualification of the Mode I fracture toughness of the as-built AM surface. Thus, Double Cantilever Beam (DCB) tests are carried out on metal/metal secondary bonded and CFRP/metal co-bonded joints. The metal part is obtained by Selective Laser Melting (SLM) of AlSi10Mg aluminium alloy, without surface pre-treatment to preserve the as-built roughness. The fracture toughness of the AM un-treated joints is also compared to that of joints in which the metal adherend is sandblasted. Optical profilometry is employed to evaluate the surface morphology of both as printed and sandblasted adherends. The results show that the untreated joints have comparable fracture toughness to their treated counterparts, despite manifesting dissimilar failure modes due to the different surface morphologies.

Prediction of the strength of adhesive lap joints. An investigative review

The principal methods of predicting the strength of lap shear bonded joints have been introduced for metallic and composite adherends. The early theories of Volkersen and of Goland and Reissner provided indications of the shear and transverse [peel] stresses. These equations were not easy to use and were increasingly displaced by finite element analysis [FEA]. The importance or otherwise of the high stresses around the embedded corner of the unloaded adherend were examined and shown to be the most likely cause of initial cracking, but not necessarily of joint failure. By combining an extensive experimental programme of testing both adhesives and joints with FEA, knowledge was gained of the reasons why lap shear joints fail. Some simple equations were developed with failure criteria based on yield of the adherend and/or the adhesive which produced predictions in good agreement with experimental results. Finally, the bonding of composites was considered, and it was shown that the key factor is the transverse tensile strength of the composite.

Mode I fracture toughness determination and environmental durability evaluation of adhesive bonds by wedge test

This paper presents a study of a method for determining the fracture toughness of adhesive bonds under Mode I loading using the wedge test. The slope of adhesively bonded specimen arms at loading points was used to provide an effective crack length for the wedge test specimen. A simple expression was derived based on beam theory for the Mode I energy release rate. Finite element analyses were conducted on wedge specimens with a range of material and geometrical properties. Good agreement was found between the energy release rates from the beam theory-based expression and the virtual crack closure technique (VCCT). A comparison of various energy release rate expressions showed that the present expression was more accurate than the other expressions available in the literature. The results of this study suggest that the environmental durability of adhesive bonds with composite adherends as well as metallic adherends can be assessed in terms of their fracture toughness using the present method. Experiments under hot and humid conditions were performed on wedge specimens with adherends made of carbon fiber reinforced polymer (CFRP) composite laminate and titanium alloy, and the fracture toughness results were obtained.

Precision and Dimensional Stability of Bonded Joints of Carbon-Fibre-Reinforced Polymers Parts

This article aims to investigate the accuracy and dimensional stability of bonded metal and CFRP (Carbon Fibre Reinforced Plastic) adherends. The motivation behind this study was to assess the suitability of CFRP for optical devices through the evaluation of precision bonding technology. A binocular was selected as a reference optical device. A technological sample was designed, with required total runout of key dimensions 0.05 mm. The sample underwent testing according to ISO 9022-1. The total runout was evaluated after production and environmental tests. Eight out of 15 samples were turned after gluing due to insufficient accuracy. None of the turned samples exceeded the total runout deviation of 0.01 mm, and the average value of the maximal deviation was 0.0041 mm. The noncalibrated samples performed significantly worse with the average value of the maximal deviation of 0.0164 mm. The measurements during the climatic tests showed that the largest deviation (on average 77.6% of the maximum achieved deviation) occurs at the first temperature loading. Subsequent temperature cycles caused lower deviations. The results highlight the significance of addressing deformations resulting from adhesive volume shrinkage-induced stress as a crucial factor in precision bonding technology.

Evaluation of hot-pressing scarf joints between polyamide 6,6 (PA66) composite and AA5754-H111 under tensile and flexural loading

As a direct bonding method, hot pressing connection offers versatility for manufacturing high quality multi-material structures. This experimental study focuses on the hot-press bonding of short-glass-fiber reinforced PA66 and Al5754 alloy. Laser surface structuring with grid pattern is performed on the metallic adherend to enhance the bonding strength and the scarfing ratio of 1:4 is analyzed for the dissimilar joint. Contact angle measurements and X-ray photoelectron spectroscopy (XPS) are used to evaluate the surface modification. Mechanical performance of the joints is studied in terms of the tensile-shear and flexural properties. Morphology of the fracture surfaces is scrutinized through a scanning electron microscopy (SEM). According to the results, it is found that the laser ablation treatment significantly affects the surface of the metallic adherend and subsequent joint strength. The scarf adhesiveless joint shows a mixed fracture with a negligible elastic deformation under tensile-shear loading. Furthermore, the joint performs better under the bending load from the metallic adherend.

Analysis of the adhesively bonded composite double cantilever beam specimen with emphasis on bondline constraint, adherend through-thickness flexibility and fracture process zone relative size

ABSTRACT The double cantilever beam (DCB) specimen is widely used to characterise the mode I fracture of adhesive joints. This paper analyses some particular characteristics of adhesively bonded composite DCB specimens which could affect test results. Three-dimensional (3D) and two-dimensional (2D) finite element analyses (FEA) were conducted in order to evaluate the effects bondline constraint and adherend through-thickness flexibility on the specimen response. Since beam theory-based data reduction schemes are widespread, beam models were also employed to analyze the effects of adherend through-thickness flexibility and fracture process zone relative size. It is shown that, although composite adherends are usually thinner and have much lower transverse moduli than metal adherends, the level of bondline constraint is similarly high. This may: limit the level of adhesive plastic deformations in the fracture process zone; generate high bondline tractions that increase the likelihood of interface failure and interlaminar damage in the composite adherends. The present analyses also show relevant effects of adherend through-thickness flexibility in the adhesive elastic loading stage. Finally, smaller fracture process zones relative to metal adherend DCB specimens were predicted by a beam cohesive zone model. This may explain lower fracture energy values reported with composite adherends in some studies.

A novel profiling concept leading to a significant increase in the mechanical performance of metal to composite adhesive joints

Traditional adhesive joints with straight edged adherends suffer from a significant stress concentration in the composite coincident with the edge of the metal adherend, which can lead to accelerated translaminar failure of the substrate. In this work, we developed a novel profiling concept which improves the mechanical performance of adhesive joints between metallic adherends and composite substrates. We conducted quasi-static four-point bending (4PB) tests which showed that profiling the edge of the metallic adherend could improve the peak load by at least 27%, and that the stability of failure was simultaneously improved. We investigated varying the profile parameters and were able to conclude that further significant mechanical performance gains could be achieved by increasing any of the profile: amplitude, frequency, or number of fractal length-scales. By analysing in-situ acoustic emission (AE) monitoring data we were able to observe that profiling of the metallic adherend results in failure initiation occurring at higher loads, which suggests that the concept is successful in providing better stress distributions and lowering peak stresses. By analysing the fracture surfaces, it is apparent that the profiling concept is successful in deflecting the translaminar fracture path; and additionally that a debonding mechanism occurs at the profile tips which is thought to be an important additional mechanism for creating damage tolerant joints.

Electrical properties of adhesives designed for smart debonding by a pulsed discharge method

ABSTRACT Facile techniques are required for the debonding of polymeric adhesives from metals for material recycling. A promising solution is a pulsed discharge debonding method. In this approach, a pulsed discharge with a high voltage is applied to the system over a short time. Rapid expansion of gasified materials and shock-wave emissions are produced from the discharge passage, which results in debonding. A drawback of the conventional pulsed discharge method is that it must be conducted in water, otherwise, the pulsed discharge creeps along the material surface without destroying the adhesion. In this study, filler was added to the structure adhesive for the effective pulsed discharge debonding of metal adherend. Epoxy resins with fillers of carbon black, copper and barium titanate particles were fabricated, and their properties and adhesive strength before and after application of the pulsed discharge were investigated. We succeeded in introducing the pulsed discharge inside the adhesive by adding 3.4 vol% of carbon black to the epoxy adhesive; the rapid expansion of gasified adhesion that was caused by the discharge, resulted in debonding. The threshold volume fraction of carbon black for effective pulsed discharge inside the adhesive was discussed based on the percolation model with experimental data.

Effect of Surface Treatment on Stiffness and Damping Behavior of Metal-Metal and Composite-Metal Adhesive Joints.

In aerospace and automotive applications, composite materials are used as a major structural material along with metals. Composite-metal and metal-metal joining are very crucial in such structures. Adhesive bonding is commonly used for this purpose. Since such structures are exposed to varying temperatures and dynamic loads, it is essential to investigate the response of such joints under thermomechanical loading. Though various studies have been reported in the literature to assess the thermomechanical properties of composites, adhesives, and their joints, the effect of the surface treatment of metals and composites on the improvement in the thermomechanical behavior of the joints has not been reported. The metal and composite surfaces were modified using chemical etching techniques. The interaction between adhesives and adherends was studied using the DTMA technique in compression mode. Anodizing treatment on aluminum alloys improved the stiffness properties of metallic joints to 36% and decreased the damping to 23%, while chemical treatment on composite and metal adherends increased the stiffness of composite-metal joints to 34% and reduced the energy dissipation to 20%.

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).

A beam model for the fracture of adhesive joints with dissimilar metal adherends

The natural ability to join different materials is one of the main advantages of adhesive joints. It is thus important to characterize the fracture resistance of adhesive joints with dissimilar materials, as they are increasingly used in relevant industrial applications. There are, however, well-known problems in data analysis of such tests, especially where mode partitioning is concerned. A beam model is here developed for mixed-mode I-II fracture testing on adhesive joints with dissimilar metal adherends. The model predicts quite accurately the strain-energy release rate, the mode-mix and the load-point displacements with fully closed-form equations. Therefore, it can be the basis of future data reduction schemes for fracture tests. Furthermore, it is shown that a wide range of mode-mix combinations can be achieved with simple well-known tests on dissimilar metal adherend joints. They could thus be an interesting alternative to developed mixed-mode I-II tests that require complex test fixtures.

Shear bond strengths of various resin cements between three types of adherends and bovine teeth with and without thermocycling.

This study evaluated the shear bond strengths of various types of resin cements between three types of adherends (composite resin, metal, and ceramic) and bovine teeth with and without thermocycling. A conventional resin cement (Variolink N), two adhesive resin cements (PANAVIA F 2.0, Multilink N), and three self-adhesive resin cements (MAXCEM ELITE, Rely X Unicem 2, Speed CEM) were used. The adherends were cemented on the superficial dentin of bovine incisors using each resin cement. Herein, 10 specimens from each group were thermocycled 5,000 times, and the other 10 were stored without thermocycling. With the resin and ceramic adherends, the shear bond strengths of Rely X Unicem 2 were significantly higher than those of the other resin cements both with and without thermocycling (p<0.05). With the metal adherend, the shear bond strengths were not significantly different among the cement groups, except MAXCEM ELITE, which showed the lowest strength.

Determination of Mechanical Properties of Single Lap Joints Having Sandwich Composite and Metal Adherend by Experimental and Numerical Approach

Bu çalışmada, farklı malzemeler kullanılarak oluşturulan tek bindirmeli yapıştırma bağlantılarının mekanik özellikleri deneysel ve nümerik yaklaşım ile belirlenmiştir. Tek bindirmeli yapıştırma bağlantılarında plaka malzemesi olarak çelik, alüminyum ve alüminyum sandviç kompozit malzeme, yapıştırıcı olarak ise Araldite 2015 epoksi yapıştırıcı kullanılmıştır. Aynı malzemeye sahip plakalar kullanılarak elde edilen yapıştırma bağlantılarının yanı sıra, farklı malzemelere sahip plakalardan oluşan yapıştırma bağlantıları da bu çalışma kapsamında incelenmiştir. Deneysel yaklaşımda çekme testleri 1 mm/dak çekme hızında gerçekleştirilmiştir. Deneysel yaklaşım sonunda her bir yapıştırma bağlantısının gerilme-birim şekil değiştirme grafikleri belirlenmiştir. Ayrıca, plaka malzemesi olarak kullanılan alüminyum sandviç kompozit malzeme de yer alan polietilen polimerinin XRD testi ile karakterizasyonu yapılmıştır. Nümerik yaklaşımda, deneysel yaklaşımda kullanılan tek bindirmeli yapıştırma bağlantı geometrisi Ansys sonlu elamanlar programında oluşturulmuştur. Model oluşturulurken yapıştırıcı ile plakalar arasında non-lineer kontak kurulmuş ve deney şartlarını karşılayan sınır şartları uygulanmıştır. Bindirme bağlantılarının non-lineer modellenmesi, doğru sınır koşullarının uygulanması ve doğru temas tipinin seçilmesi nedeniyle nümerik sonuçların deneysel sonuçlarla benzer bir eğilim gösterdiği görülmüştür.

Thermomechanical characterization of metal-polyurethane bonded joints: effect of manufacturing parameters and working temperature

ABSTRACT Metal-elastomer bonded joints are typical of different industrial applications, specifically pallet truck wheels, where the solid elastomeric layer is bonded to the metal body. This paper focuses on the effect of manufacturing parameters and working temperature, on the shear strength of a metal-elastomer bonded joint. Through a comprehensive experimental test plan, the paper investigates the effect of thermal conditioning of the adhesive, shot peening of the metal adherend, type of solvent-based adhesive, and working temperature of the joint. The test plan uses a TAST-like specimen with a sandwich configuration (metal-elastomer-metal), loaded in order to originate a nearly pure-shear stress state. The results show which parameters influence most the shear strength of the joint helping the designer in maximizing the load-carrying capacity of the joint.

Design of adhesively bonded lap joints with laminated CFRP adherends: Review, challenges and new opportunities for aerospace structures

Adhesive bonding is one of the most suitable joining technologies in terms of weight and mechanical performance for current carbon fiber reinforced polymer aircraft fuselage structures. However, traditional joint topologies such as single overlap joints induce high peel stresses, resulting in sudden failure and low joint strength when compared to metal adherends. This drawback in using carbon fiber reinforced polymer is hindering their performance and efficiency in full-scale structures where joints are essential.The goal of this paper is to review how the joint design can help to improve the lap shear strength of composite bonded joints, to recognize the challenges that still need to be understood and to give insight into new opportunities. The focus is thereby on means to increase the matrix-dominated out-of-plane strength of the adherend in order to postpone delamination failure, as it is known to be the most prone type of failure of composite bonded joints. The paper is divided in two main parts: firstly, a review of topology-related and material-related design parameters is given and secondly, future opportunities to improve out-of-plane strength of CFRP bonded joints yet to be explored are discussed.