Adhesion strength of glass/epoxy composite embedded with heat-treated carbon black on the surface

The surface of a glass/epoxy composite material was embedded with oxidized carbon black by heat treatment to enhance the adhesion strength of the glass/epoxy composite structure. Quantitative chemical bonding analysis with X-ray photoelectron spectroscopy (XPS) was conducted to observe the chemical binding states of the surface of the carbon black particles with heat treatment. The morphological effects of the carbon black on the surface of the composite were observed using SEM and AFM. The surface free energies and lap shear strengths of the glass/epoxy adhesive joints whose adherends were embedded with oxidized carbon black were investigated in terms of the heat-treatment conditions and the amount of embedding. From the experimental results, embedding the heat-treated carbon black particles on the composite surface was found to greatly improve the adhesion strength of the composite due to the increased oxidation radicals on the carbon black surface.     

Using silane-functionalized graphene oxides for enhancing the interfacial bonding strength of carbon/epoxy composites

Silane-functionalized graphene oxides (sGOs) were fabricated with four different self-assembled monolayers (SAMs) to reinforce an epoxy adhesive, with the aim of improving the bonding strength of carbon/epoxy composites. The oxygen-containing groups on the surface of graphene oxide (GO) were converted by the SAMs to amine, epoxy, or alkyl groups. The successful reaction between the silane molecules of the SAMs and functional groups of GO was evidenced by the results of different characterization methods such as Fourier transform infrared spectroscopy. It was found that the average thickness of the sGO flakes was higher than that of GO flakes. The bonding strength of a carbon fiber/epoxy composite, tested with a single lap joint bonded with an epoxy adhesive, was increased by 53% after the addition of a sGO that contained amine groups. These results show that sGOs, especially those containing amine functional groups, can strengthen the interfacial bonding between the carbon fibers and epoxy adhesive.     

Effect of surface pretreatment on adhesive properties of aluminum alloys

The lap-shear strength and durability of adhesive bonded AI alloy joints with different pretreatments were studied by the lap-shear test and wedge test. The results indicate that the maximum lap-shear strength and durability of the bonding joints pretreated by different processes are influenced by the grade of abrasive papers and can be obviously improved by phosphoric acid anodizing. Alkali etching can obviously improve the durability of bonding joints although it slightly influences the maximum lap-shear strength. The process which is composed of grit-finishing, acetone degreasing, alkali etching and phosphoric acid anodizing, provides a better adhesive bonding property of Al alloy.     

超声强化氨基化多壁碳纳米管改性环氧黏接接头性能研究

目的 探究超声处理对氨基化多壁碳纳米管(MWCNTs–NH₂)改性环氧黏接接头黏接性能和热稳定性的影响,为强化MWCNTs–NH₂改性环氧胶黏剂与铝合金的黏接提供参考。方法 通过机械搅拌与声波破碎的方法将质量分数为0.75%的MWCNTs–NH₂添加到环氧胶黏剂基体中,使用MWCNTs–NH₂改性环氧胶黏剂制备铝合金黏接接头,基于超声辅助黏接工艺在铝合金黏接过程中进行超声处理。通过傅里叶变换红外光谱仪(FTIR)分析MWCNTs–NH₂改性环氧胶黏剂基体官能团的变化情况。采用单搭接剪切强度试验测定黏接接头的拉伸剪切强度。通过扫描电子显微镜(Scanning Electron Microscope,SEM)观察黏接接头拉伸失效断面以及铝合金与胶黏剂间的黏接界面。通过热失重分析仪(TGA)测试并记录胶黏剂试样质量随温度变化的曲线。结果 经超声处理后,MWCNTs–NH₂与树脂基体间的化学反应增强。与纯环氧黏接接头相比,超声处理后的MWCNTs–NH_2...     

Innovative Leichtbau‐Fügetechnologien

Using examples from the adhesive bonding and ultrasonic welding technique shows how various methods of vacuum‐assisted analysis can contribute to explain joining mechanisms and derive optimization potentials for strength and durability of composites. Employing Time‐of‐Flight Secondary Ion Mass Spectrometry (ToF‐SIMS) to characterize epoxy‐adhesives, locally resolved and quantitative structural informa‐tions can be achieved. Multivariate data analysis methods prove to be very helpful here. Analytical investigations with high spatial resolution at the interface of ultrasonic welded aluminium joints provide structural and chemical data and allow the characterization of the bonding zone in detail. Thus the physical and chemical processes occurred during the welding process can be reconstruct. The increase of strength and longterm durability of ultrasonic‐ or induction welded hybrid joints (aluminium and carbon‐fibre reinforced thermoplastics) can be attributed to micromechanical interlocking of the CF‐matrixpolymer with Al‐oxide‐layer.     

Waterproof characteristics of nanoclay/epoxy nanocomposite in adhesively bonded joints

When adhesively bonded joints are exposed to a moist environment, the tensile load capability of the joint is significantly decreased because moisture absorption weakens the mechanical properties of epoxy adhesive. In this paper, a nanoclay with excellent penetration resistance properties was used as a filler in epoxy adhesive in order to enhance adhesive strength in moist environments. The water absorption of the epoxy adhesive and the adhesive strength of the adhesively bonded joints were measured in water absorption experiments with respect to the weight fraction of the nanoclay and the moisture exposure time. These results showed that the tensile load capability of the nanoclay-filled adhesively bonded joint was greatly enhanced, even in a moist environment, because the nanoclay reduced water absorption into the epoxy adhesive as well as into the interface between the epoxy adhesive and the steel adherend and increased the strength of the epoxy adhesive itself.     

Oxide morphology and adhesive bonding on titanium surfaces

Titanium metal was subjected to two surface treatments (alkaline peroxide etch and chromic acid anodization) and resulting oxide morphology examined by high-resolution scanning electron microscopy in a Jeol 100-CX STEM. The effects of treatment time in alkaline peroxide upon oxide morphology were followed and parallel mechanical measurements made on the strengths of adhesive bonds between the metal and an epoxy resin. These strengths were measured after a standard environmental exposure, namely 120 h in water at 80° C. As time-of-treatment increases, a micro-porous oxide layer is developed and adhesive strength rises to a maximum. Prolonged treatment with alkaline peroxide produces a drastic fall in adhesive strength accompanied by gross etching of the metal surface without changes in the oxide morphology. The loss of adhesive durability in this case is therefore attributable to surface chemistry effects rather than morphological changes.     

Analysis of adhesively bonded CFRE composite scarf joints modified with MWCNTs

The main objective of the present work is to improve the performance of bonded joints in carbon fiber composite structures through introducing Multi-Walled Carbon Nanotubes (MWCNTs) into Epocast 50-A1/946 epoxy, which was primarily developed for joining and repairing of composite aircraft structures. Results from tension characterizations of structural adhesive joints (SAJs) with different scarf angles (5–45°) showed improvement up to 40% compared to neat epoxy (NE)–SAJs. Special attention was considered to investigate the performance of SAJs with 5° scarf angle under different environments. The tensile strength and stiffness of both NE-SAJs and MWCNT/E-SAJs were dramatically decreased at elevated temperature. Water absorption showed a marginal drop of about 2.0% in the tensile strength of the moist SAJs compared to the dry one. Cracks initiation and propagation were detected effectively using instrumented-SAJs with eight strain gauges. The experimental results agree well with the predicted using three-dimensional finite element analysis model.     

Smart cure cycles for the adhesive joint of composite structures at cryogenic temperatures

Adhesive joints are employed for composite structures used at the cryogenic temperatures such as LNG (liquefied natural gas) insulating tanks and satellite structures. The strength of the adhesive joints at the cryogenic temperatures is influenced by the property variation of adhesive and the thermal residual stress generated due to the large temperature difference (ΔT) from the adhesive bonding process to the operating temperature. Therefore, in this work, the strength and thermal residual stress of the epoxy adhesive at cryogenic temperatures were measured with respect to cure cycle. Also, the cure cycles composed of gradual heating, rapid cooling and reheating steps were applied to the adhesive joints to reduce the thermal residual stress in the adhesive joints with short curing time. Finally, a smart cure method was developed to improve the adhesive joint strength and to reduce the cure time for the composite sandwich structures at cryogenic temperatures.     

Durability of metal joints bonded with aluminium powder filled epoxy adhesive

Abstract

Durability of the metal joints bonded with aluminium powder filled epoxy adhesive was investigated by measuring the joint strength by the single lap shear test before and after exposure to distilled water and the hot and humid Arabian Gulf atmosphere. Fractured specimens were examined by photography. The epoxy adhesive retained its strength with as much as 50 wt-% addition of aluminium filler. Moreover, varying the Al filler content in the adhesive did not have a significant effect on adhesive behaviour in either of the two environments studied. Exposure to atmosphere for as long as 6 months did not cause a deterioration of strength for metal joints bonded with aluminium powder filled epoxy. They failed almost completely within the adhesive, similar to the cohesive mode of unexposed specimens. However, a significant strength decrease was observed in adhesive joints after exposure to distilled water for 6 months. The joints failed in more than a single mode. The interior part of the adhesive lap area failed in cohesive mode while an adhesion failure mode was observed near the edges of the adhesive lap area, which is believed to be a result of moisture diffusion through the edges.     

Adhesion and durability of metal-polymer bonds

A review is presented of those factors responsible for promoting the integrity and longterm durability of metal-polymer bonds used in the fabrication of aircraft and aerospace structures. Using a multidisciplinary approach and a variety of surface analytical techniques such as extended resolution scanning electron microscopy (XSEM), X-ray photo-electron spectroscopy (XPS), ellipsometry, and a new technique called surface behaviour diagrams (SBD), investigators at the author's laboratories have evolved several important concepts. First, it has been determined that the initial integrity of metal-polymer bonds depends critically upon the morphology of the surface oxide on the metal. For aluminium and titanium, the metals studied, it is demonstrated that certain etching or anodization pretreatment processes produce oxide films on the metal surfaces which, because of their porosity and microscopic roughness, mechanically interlock with the polymer forming much stronger bonds than if the surface were smooth. Second, the long-term durability of metal-polymer bonds is shown to depend strongly on the environmental stability (or lack of stability) of the same oxide which is responsible for good initial bond strength. For aluminium moisture intrusion at the bond line causes the oxide to convert to an hydroxide with an accompanying change in morphology and bond strength. For titanium the oxides appear to be much more stable than those on aluminium but under severe environmental conditions the oxide undergoes a polymorphic transformation which may lead to bond degradation. Third, it is observed that significant improvements in durability of adhesive bonds to aluminium can be achieved using an extremely simple treatment in which monolayer films of certain organic acids are applied to the adherend oxide to protect it against the effects of moisture.     

The influence of mode mixity and adhesive system on the fatigue life of adhesive joints

In real applications, adhesive joints are commonly subjected to fatigue and mixed mode loading conditions. The aim of the current work is to experimentally analyse the influence of mode mixity on the fatigue strength of joints with an epoxy‐based adhesive. Different adhesive systems (acrylic and epoxies) were considered for pure mode I fatigue loading conditions. To achieve this, Arcan joints with an epoxy adhesive were manufactured and tested at different mode mixities. Based on stiffness degradation, monitored during the tests, damage evolution was calculated for different loading conditions and for all the tested adhesives. Finally, fatigue envelopes were constructed for different fatigue life regimes. Results show that a shear loading component reduces both the static strength and fatigue life of the joints. A small reduction rate of the stiffness was found throughout the most part of the life until a sudden drop was observed, indicating a smooth damage evolution.     

Effect of crack-growth mechanism on the prediction of fracture load of adhesive joints

This paper presents observations regarding the cracking behavior of tensile-loaded structural adhesive joints. Experiments showed that fracture occurred by the development and propagation of a damage zone, rather than a single, sharp crack, and that the presence of the adhesive spew fillet did not affect the fracture load of the adhesive joints studied. For joints bonded with the mineral-filled epoxy Cybond 4523GB (American Cyanamid), there was approximately 5 mm of subcritical crack propagation prior to final fracture. Fracture-load predictions based on the initial uncracked geometry made in previous papers were unaffected by this small change in geometry. For joints bonded with the rubber-toughened epoxy Permabond ESP 310, approximately 50 mm of subcritical crack propagation was observed. It was again found that predictions made in previous papers on the basis of the initial geometry gave a good estimate of the final fracture load even though this subcritical crack propagation significantly altered the geometry, and thus the applied energy release rates. The effect of shear deformations of the adherends was also investigated, and it was found that shear deformations could be neglected in engineering calculations for joints subject to remote tensile loading.     

Influence of post-curing and coupling agents on polyurethane based copper filled electrically conductive adhesives

Effects of post curing and silane coupling agents with different functional groups such as epoxy, isocyanate and ureide on the electrical and mechanical properties of copper (Cu) filled electrically conductive adhesives (ECAs) were studied. Micron-sized Cu particles were used as conductive fillers and polyurethane resin was applied as the adhesive material. Significant differences could be observed on the as cured electrical resistivity and shear strength of the Cu filled ECAs joints prepared with different silane coupling agents. Silane coupling agents functionalized with epoxy groups yielded the lowest electrical resistivity and highest shear strength among the ECAs in this study. Besides, effect of post-curing at 170 °C for 1 h on the ECAs was also investigated. Results showed that ECAs after post-curing exhibited enhanced electrical conductivity and shear strength compared to the as cured ECAs.     

有机硅改性丙烯酸酯聚合物对环氧树脂胶粘剂性能的影响

采用有机硅改性丙烯酸酯聚合物改性环氧树脂,以聚酰胺为固化剂,研究了有机硅改性丙烯酸酯聚合物对环氧树脂黏度、粘接性能、固化行为、耐热性能和微观形貌的影响。黏度、剪切强度、差示扫描量热（DSC）、动态力学性能（DMA）和透射电镜（TEM）测试表明：有机硅改性丙烯酸酯聚合物与环氧树脂形成互穿网络结构,并且存在较多氢键,剪切强...     

Mechanical performance of statically loaded flat face epoxy bonded concrete joints

One of the main challenges in the offshore renewable energy industry is the reduction in the levelised cost of energy of wind, wave and tidal devices. The use of concrete as the primary construction material in such devices presents a low unit cost, high marine durability alternative to steel, however, to maximise material efficiency factors such as mix constituent design, structural detailing and manufacturing processes have to take into account the specific conditions of the marine environment. Pre-cast segmental construction can be considered as one of the fastest and cheapest construction options. However the challenges regarding performance of epoxy bonded concrete in marine environment should be taken into account. This paper presents the results of an experimental programme on the performance of shear and tensile capacity of flat face concrete joints, focussing on the effect of substrate surface preparation, joint thickness, properties of epoxy resins, exposure to seawater and presence of joint defects on the ultimate failure load. The ultrasonic pulse velocity (UPV) method for detection of defects in the adhesive layer was examined and digital image correlation is used to observe the surface strain flow through the joint. The results indicate that the epoxy joints behave monolithically and remain undamaged under different types of static loading. The joints do not significantly interrupt the flow of strain but can locally affect the distribution of strain (and thus stiffness and stresses) in a structure. An increase in the density of the epoxy (and the filler content) leads to the increase in the joint strength and thicker joints are less affected by small defects in the bonding layer. The majority of tested specimens failed by cracking of concrete rather than by debonding of the joint, whilst compressive stresses acting on the joint can help to augment its shear strength. Sandblasting of bonded surfaces can improve performance of joints, whereas UPV testing may be used for quality control of epoxy-bonded joints.     

Improvement of the fracture toughness of adhesively bonded stainless steel joints with aramid fibers at cryogenic temperatures

Adhesives should be reinforced with reinforcing fibers for the bonding of adherends at cryogenic temperatures because all the adhesives become quite brittle at cryogenic temperatures. In this work, the film-type epoxy adhesive was reinforced with randomly oriented aramid fiber mats to decrease the CTE (Coefficient of Thermal Expansion) of the adhesive and to improve the fracture toughness of adhesive joints composed of stainless steel adherends at the cryogenic temperature of −150 °C. The cleavage tests of the DCB (Double Cantilever Beam) adhesive joints were performed to evaluate the fracture toughness and crack resistance of the adhesive joints. Also, the thermal and mechanical properties of the fiber reinforced adhesive layer were measured to investigate the relationship between the fracture toughness of adhesive joints and fiber volume fraction of aramid fibers. From the experiments, it was found that the crack propagated in the adhesive with the stable mode of significantly increased fracture toughness when the film-type epoxy adhesive was reinforced with aramid fiber mats. The optimum volume fraction of aramid fibers was suggested for the film-type epoxy adhesive in the adhesive joint at the cryogenic temperature of −150 °C.     

Surface modification of carbon fiber/epoxy composites with randomly oriented aramid fiber felt for adhesion strength enhancement

To increase the strength of an adhesive joint whose adherend is composed of a carbon fiber/epoxy composite, the surface of the adherend is reinforced with randomly oriented aramid fiber felt before the full cure of the adherend. With this smart cure cycle, the aramid fibers are exposed from the adherend, promoting a bridging effect between the fibers and the adhesive. The cured carbon fiber/epoxy composite material, on which the aramid fiber felt is placed, is co-cure bonded with a smart cure cycle developed in this work. The improvement of the adhesive bonding strength due to the aramid fiber felt is measured with the single-lap shear test of adhesively bonded joints. Additionally, the flexural strength of the carbon fiber/epoxy composite adherend with the co-cure bonded aramid fiber felt is measured.     

Mechanical characterization of nanofiber-reinforced composite adhesives

Tensile and shear strength tests of metal/metal and polymer/polymer joints featuring a new functionalized nanofiber/epoxy composite adhesive were conducted. Strength increase is not as high as we expected (only up to 30%) although we used GCNF-ODA reactive linkers to improve the interface. The moderate strength increase is due to high interfacial stress developed in nanocomposites because of the high stiffness property mismatch, and inefficient interfacial shear stress transfer through shear-lag mechanism. In order to design strong nanocomposite materials, continuous or at least aligned nanofibers/nanotubes should be employed.     

Effect of carbon nanotube modified epoxy adhesive on CFRP-to-steel interface

The effects of carbon nanotube (CNT) modified epoxy adhesive on CFRP-to-steel interfaces were investigated using double strap joints. The bond behaviours studied were failure modes, bond interface at microlevel, bond strength, effective bond length, CFRP strain distribution and bond-slip relationships.For the first time, a novel type of failure in the CFRP-steel joint was discovered, attributable to weak bonding between woven mesh and CFRP fibres. This failure mode prevented exploitation of the full potential of the carbon fibres and the CNT modified epoxy adhesive. Joints bonded with CNT-epoxy adhesive had an effective bond length of about 60 mm, whereas that of joints bonded with pure epoxy was about 70 mm. The CNT-epoxy adhesive can transfer more load from the host structure to the bonded CFRP laminates, consequently modifying bond behaviour. It is therefore expected that CNT-epoxy nanocomposites will assist in the strengthening and rehabilitation of steel infrastructures using CFRP laminates.