Strength of epoxy adhesive-bonded stainless-steel joints

The strength of stainless-steel joints bonded with two epoxy adhesives was investigated. The experimental programme included tests on single-lap and butt joints, as well as thick-adherend and napkin ring shear tests. Results suggested that the tensile and shear strengths of the epoxy adhesives were quite similar. However, finite element (FE) analyses raised doubts on the true adhesive strengths, due to the complex stress state in joint tests and pressure-dependent adhesive behaviour. In spite of some uncertainties, FE analyses showed that failure could be fairly well predicted by a maximum shear strain criterion.     

Methanol permeability and proton conductivity of polybenzimidazole and sulfonated polybenzimidazole

The preparation of sulfonated polybenzimidazole (sPBI) by the grafting of (4‐bromomethyl) benzenesulfonate onto polybenzimidazole (PBI) has been investigated. The methanol permeability and proton conductivity of PBI and sPBI have been studied, and the effects of methanol concentration and temperature on the methanol permeability of PBI and sPBI membranes are discussed. The results showed that the PBI membrane is a good methanol barrier. Methanol permeability in this membrane decreases with increasing methanol concentration and increases with increasing temperature. The temperature‐dependence of methanol permeability of PBI and sPBI membranes is of the ‘Arrhenius type’. Methanol permeation of sPBI is less sensitive to temperature than that of PBI. However, sPBI is a poorer methanol barrier when compared to PBI. Methanol permeability in sPBI membranes increases with increasing methanol concentration and temperature. The proton conductivity of sPBI is 4.69 × 10^?4 S cm^?1 at room temperature in the hydrated state. The DC conductivity of sPBI–H₃PO₄ increases with increasing temperature. Proton transport in sPBI–H₃PO₄ is less sensitive to temperature than that in PBI–H₃PO₄. Copyright © 2004 Society of Chemical Industry     

Strength and interface failure mechanism of adhesive joints

Adhesive joints have a wide range of applications in the civil engineering, automotive and aircraft industries. In the present research, we use the finite element method to systematically study the overall strength and interface failure mechanism of single lap joints, which are subjected to tensile loading, focusing on the effects of various system parameters including fracture energy of the adhesive layer, overlap length and adhesive layer thickness on the load-bearing capability of the joints. The results show that the overlap length and the adhesive fracture energy have combined influences on the load-bearing capability. On the other hand, a preliminary damage analysis of the adhesive layer is carried out, considering the situations when the loads arrive to the peak values. Furthermore, the interface behavior is investigated, including the interface stress analysis and interface slip. The rotation of the joint during loading and its influence factors are studied as well. Obtained results suggest that the interface stress distributions are related to the slip and the rotation angle.     

聚苯并咪唑的改性及其应用

为了提高聚苯并咪唑(PBI)的溶解性、化学稳定性和导电性等,需对PBI进行改性。介绍了通过单体改性、在聚合物主链上引入柔性链(基团或原子)、在—N—H上引入脂肪族和芳香族磺酸盐侧基等方法提高PBI的溶解性;通过调节PBI的相对分子质量或者用甲基取代咪唑环上的氢,提高PBI的稳定性能;通过物理改性和化学改性(单体改性、掺杂改性、磺化改性)提高PBI的导电性能。综述了PBI在纤维、膜材料、基体树脂、粘结剂等方面的应用情况,并对其进行了展望;指出了PBI的改性大部分属于化学改性,通过单体改性和聚合物主链改性对PBI性能影响很大,应进一步加大PBI物理改性的开发,如低温等离子体改性,拓宽PBI应用范围。     

Strength and failure of bulky adhesive joints with adhesively-bonded columns

Strength and failure properties of bulky (i.e. with thick substrate) adhesive joints with adhesively-bonded columns (ABCs) subjected to external loads were investigated experimentally and analytically. From the experimental results, it was found that the strengths of the bulky adhesive joints with ABCs increased considerably when they were subjected to external tensile loads or lateral bending loads. And the joint strengths increased with increasing depths of the blind holes. The failure process of the joints with ABCs was simulated by the technique of element birth and death developed in the finite element method (FEM). The conclusions obtained from FEM coincide with those obtained from the experiments.     

Strength model of adhesive-bonded double-lap joints under cantilevered bending

The objective of this study was to develop mathematical relations for predicting the strength of adhesive-bonded double-lap joints under cantilevered bending. Based on the strength of composite materials theory, two models were proposed to predict the stress-strain distribution and vertical deflection of the laminates and the adhesive under this loading condition. The first model was based on the basic beam theory with the assumption that every cross section in a plane before bending remains plane after the bending load is applied. In the other model, a strain gap between each bonded surface is assumed. Based on the second model and the predicted peel failure mode, the effects of shear modulus of the adhesive, joint length, and adhesive thickness on the joint strength were evaluated. Scotchply composite laminates were used as the adherends of the double-lap joints in the experimental investigation. An Instron machine fitted with a special apparatus was used for conducting the experiments. By the attachment of strain gages to the adherends and through the use of a dial indicator, the theoretical models were verified experimentally.     

Strength and bonding mechanisms in vibration-welded polycarbonate to polyetherimide joints

Under the right conditions, high strengths are shown to be achievable in vibration welded polycarbonate to polyetherimide Joints. While welding of thermoplastic interfaces of the same material can be understood in terms of interchain diffusion at elevated temperatures, this mechanism is severely limited in the case of dissimilar materials. Scanning electron microscopy is used to show that part of the bond strength in such dissimilar materials results from mechanical interlocking of the two polymers, which is caused by viscous mixing. The effects of the weld parameters on the weld morphology are considered in detail.     

Strength of glass-filled modified polyphenylene oxide vibration-welded butt joints

The strengths of glass-filled modified polyphenylene oxide (GF-MPPO) welds relative to the strengths of GF-MPPO are shown to depend on specimen thickness. (Modified polyphenylene oxide is a blend of poly (2,6-dimethyl-1,4-phenylene ether) and high-impact polystyrene.) Relative strengths on the order of 70 and 87 percent can be achieved in 6.1 and 3.18-mm-thick specimens, respectively. Welds of GF-MPPO to modified polyphenylene oxide (MPPO) can easily attain the strength of MPPO, the weaker of the two materials. In contrast to MPPO, in which weld strength decreases with increased weld pressure, the strengths of GF-MPPO to GF-MPPO welds and GF-MPPO to MPPO welds, are not affected by weld pressure.     

Strength prediction of adhesively bonded carbon/epoxy joints

A finite element approach has been used to obtain the stress distribution in some adhesive joints. In the past, a strength prediction method has not been established. Therefore in this study, a strength prediction method for adhesive joints has been examined. First, the critical stress distribution of single-lap adhesive joints, with six different adherend thicknesses, was examined to obtain the failure criteria. It was thought that the point stress criterion, which has been previously used for an FRP tensile specimen with a hole, was effective. The proposed method using the point stress criterion was applied to adhesive joints, such as single-lap joints with short non-lap lengths and bending specimens of single-lap joints. Good agreement was obtained between the predicted and experimental joint strengths.     

Strength of adhesively bonded joints under mixed axial and shear loading

This study aims at optimising adhesive properties in an aluminium/structural epoxy assembly for different conditions of surface pre-treatment. We consider the mechanical behaviour and failure under proportional, multi-axial loading using an instrumented, Arcan-type test. Values of fracture strength were found to be dispersed (even for a given surface treatment). Typically dispersion was of the order of 15%. This statistical behaviour, also observed with a simple tensile test, seems to be related to the heterogeneous nature of the microstructure of the adhesive bond, which contains voids, as well as mineral particles for reinforcement. A statistical analysis is suggested for use in conjunction with a strength envelope in practical design, for cases when the stress distribution is significantly heterogeneous. It is believed that this approach may be developed in order to understand the well-known scatter of adhesion strength results, and thus contribute to better reliability assessment.     

Strength and bonding characteristics of adhesive joints with surface-treated titanium-alloy substrates

This paper presents a study on the effect of surface treatments on the mechanical behavior of adhesively bonded titanium alloy joints. Several different treatments were selected for the preparation of Ti-6Al-4V alloy faying surfaces, and bonded joints were fabricated using surface-treated titanium alloy substrates and a film adhesive. Tensile tests were performed on single-lap specimens to evaluate the joint strength and to assess the failure mode, i.e. cohesive failure, adhesive (interfacial) failure or a mix of both. Contact angle measurements were also carried out, and the surface free energies of titanium alloys and the thermodynamic works of adhesion for the adhesive/titanium alloy interfaces were obtained. A three-dimensional finite element analysis was used to predict the strength of the specimens exhibiting cohesive failure. In addition, an expression of the relationship between the joint strength corresponding to interfacial failure and the thermodynamic work of adhesion was introduced based on the cohesive zone model (CZM) concept. It is shown that two surface treatments, Itro treatment and Laseridge, lead to cohesive failure and a significant increase in the joint strength, and the numerically predicted strength values are fairly close to the experimental values. These surface treatments are possible replacements for the traditional surface treatment processes. For degreasing, emery paper abrasion, atmospheric plasma treatment, sulfuric acid anodizing, nano adhesion technology and high-power lasershot, the specimens fail at the adhesive/substrate interface and the joint strength increases linearly with the thermodynamic work of adhesion as expected from our CZM-based expression.     

Strength and finite element analyses of single-lap joints with adhesively-bonded columns

This paper introduces a novel approach to increase the loading ability of adhesive joints by incorporating adhesively-bonded columns. Strengths of single-lap adhesive joints with adhesively-bonded columns were measured experimentally. Stress and strain distributions at selective positions in the adhesive layer were analyzed using the Finite Element Method (FEM). Failure mechanisms of the joints were analyzed. It was found that the metal-adhesive columns increased the joint strength and also the joint strength increased with increasing length of the metal-adhesive columns. Therefore, using metal-adhesive columns in adhesive joints is an effective approach for enhancing the strength of bulk adhesive joints.     

Photodegradation of polybenzimidazole/polyvinyl chloride composites and polybenzimidazole: Density functional theory and experimental study

In this study, density functional theory (DFT) and experimental study were conducted to investigate the photodegradation of PVC composites. The results indicate that chain crosslinking, chain scission, dehydrochlorination, and photooxidation are suppressed, while the tensile strength retention of PVC composite reaches a maximum with the incorporation of polybenzimidazole (PBI) and polyacrylic acid modified carbon black. The allyl group can obviously reduce Gibb's free energy (∆G) of PVC in attack by triplet state oxygen (³O₂), and photooxidation of PVC is accelerated in the presence of PBI due to the photogeneration of singlet state oxygen (¹O₂). Photodegradation of PBI was also studied by UV–vis spectroscopy and matrix-assisted laser desorption ionization time-of-flight mass spectroscopy, while the possible photodegradation pathways of PBI were proposed. The disappearance of UV-absorption of PBI in PVC matrix after 600 h UV-weathering is probably related to the byproducts and molecular properties.     

Studies on proton conductivity of acid doped polybenzimidazole/polyimide and polybenzimidazole/polyvinylpyrrolidone blends

Polybenzimidazole (PBI)/polyimide (PI) and PBI/polyvinylpyrrolidone (PVP) blends were prepared. All of these polymers are suitable for acid doping as a matrix because of the existence of the N‐heterocycle in the polymers, which can eventually be used as proton conductors. The miscibility of PBI/PVP blend is confirmed by Fourier transform infrared spectroscopy (FTIR) and atomic force microscopy (AFM), while the miscibility of PBI/PI is already reported in references. Temperature dependence of proton conductivity of acid doped PBI/PI and PBI/PVP blends are studied. As the temperature increases, proton conductivity of H₃PO₄ doped PBI/PVP blends increases. Temperature dependence of proton conductivity of the blends can still be fitted by a simple Arrhenius relation. The proton transport of the blend is similar to that of PBI. The methanol permeability of PBI/PI blends is also studied. Methanol permeability in PBI/PI, PI, and PBI membranes decreases with increasing methanol concentration. POLYM. ENG. SCI., 45:1395–1400, 2005. © 2005 Society of Plastics Engineers     

Strength model of adhesive bonded composite pipe joints under tension

A theoretical model is developed to predict the strain of the pipe, coupling, and adhesive under tensile loading of an adhesive bonded joint. The model is found to be within 10 percent of the experimental pipe and coupling strain. Based on the model, several failure modes and their locations are defined and related to the measured data. In this investigation, delamination is the dominating mode of failure. The delamination stress for each test sample is within 7 percent of the average theoretical delamination stress. In addition, the effect of the coupling length, coupling Young's modulus, adhesive shear modulus, and adhesive thickness on the delamination failure are investigated. The model shows that decreasing the modulus of the coupling improves the delamination failure load; however, the coupling strain at the middle of the joint is increased by this variation. Increasing the shear modulus of the adhesive provides the most significant improvement of the joint delamination failure load. Two geometric factors, the joint length and the adhesive thickness also affect the joint failure load. The joint delamination failure load can only be significantly improved by increasing the bonding length up to a certain limit. Increasing the adhesive thickness increases the delamination failure load, however, a large gap between the pipe and coupling may contribute to misalignment during installation which may result in imposed moments under tensile loading. This study can supply the manufacturers with the appropriate design parameters to improve the joint performance significantly under tensile loading.     

Strength of resin-coated-adhesive-bonded double lap-shear pultrusion joints at ambient temperature

The inherited adhesion limitation of polyester and vinyl ester resin-based pultruded GFRP makes pultrusions difficult to bond, especially when a thixotropic adhesive is used. While such an adhesive is necessary for gap filling, it has a limited wettability. Therefore, coating the adherend with low-viscosity epoxy resin, prior to bonding, improves wetting and hence increases joint strength. The paper describes the experimental methodology to achieve this, using double lap-shear (DLS) joints with various materials combinations. A significant strength improvement was reached as a result of coating the inner adherend in conjunction with using a “high adhesion” outer adherend. To further understand the effect of coating, numerical stress analysis was undertaken, including preliminary micro-models representing the composite/adhesive interface as well as overall DLS models.     

Strength, deformation and relaxation of joints bonded with modified cyanoacrylate adhesives

The introduction of high molecular weight poly(methyl methacrylate) or poly(butadiene-co-acrylonitrile) into ethyl 2-cyanoacrylate produced viscous adhesives with a homogeneous or heterogeneous structure after cure. Steel joints bonded with these adhesives are shown to have improved tensile shear strength, deformability and stress relaxation of bonds compared with pure cyanoacrylate adhesive. Poly(methyl methacrylate)-modified adhesive is recommended for static load-bearing joints while poly(butadiene-co-acrylonitrile)-modified adhesive is more suited to cyclic or vibrating loads.     

High-modulus polyaramide and polybenzimidazole blend fibers

Polymer blends from polybenzimidazoles (PBI) and high-performance polyaramides were spun to give a PBI variant with better tensile properties than PBI itself. Infrared absorption spectra of a hot-drawn fiber showed 10-wavelength shift at the carbonyl band, C?O, of high-modulus polyaramide (HMA) due to an interaction of the C?O with N? H. Dynamic mechanical analysis (DMA) also confirmed the interaction is at a molecular level. The mechanical properties of hot-drawn fibers from PBI/HMA polymer blends obey Halpin and-Tsai's theory. Hot-drawing “immediately after” dry-spinning yielded fibers with impressive properties. Aging as-spun PBI/HMA fibers was very harmful for fiber tensile properties. The effects of sulfonation and heat setting on fiber thermal and mechanical properties are also reported.     

Electrical resistance of polybenzimidazole membranes

Measurements of electrical resistance were made on several polybenzimidazole membranes prepared for use in reverse osmosis. In contact with 1 M KCl, different samples had resistances which ranged from 0.087 to 22 cm². Smaller ranges of resistance were seen with membranes in contact with 36% H₂SO₄ (0.022 to 0.030 cm²) and 40% KOH (0.0063 to 0.096 cm²). With all three electrolytes, however, increasing resistance followed increasing salt rejection measured under reverse osmosis conditions. This offers a rapid and convenient diagnostic test for reverse osmosis performance of the membranes.     

Strength prediction of epoxy adhesively bonded scarf joints of dissimilar adherends

In this study, strength of epoxy adhesively bonded scarf joints of dissimilar adherends, namely SUS304 stainless steel and YH75 aluminum alloy is examined on several scarf angles and various bond thicknesses under uniaxial tensile loading. Scarf angle, θ=45°, 60° and 75° are employed. The bond thickness, t between the dissimilar adherends is controlled to be ranged between 0.1 and 1.2 mm. Finite element (FE) analysis is also executed to investigate the stress distributions in the adhesive layer of scarf joints by ANSYS 11 code. As a result, the apparent Young's modulus of adhesive layer in scarf joints is found to be 1.5-5 times higher than those of bulk epoxy adhesive, which has been obtained from tensile tests. For scarf joint strength prediction, the existing failure criteria (i.e. maximum principal stress and Mises equivalent stress) cannot satisfactorily estimate the present experimental results. Though the measured stress multiaxiality of scarf joints proportionally increases as the scarf angle increases, the experimental results do not agree with the theoretical values. From analytical solutions, stress singularity exists most pronouncedly at the steel/adhesive interface corner of joint having 45-75° scarf angle. The failure surface observations confirm that the failure has always initiated at this apex. This is also in agreement with stress-y distribution obtained within FE analysis. Finally, the strength of scarf joints bonded with brittle adhesive can be best predicted by interface corner toughness, Hc parameter.