The use of the exponential Drucker-Prager material model for defining the failure loads of the mono and bi-adhesive joints

In this study, our previous experimental study was extended applying the exponential Drucker-Prager (EDP) yield criterion to define the numerical failure loads for mono and bi-adhesive single lap joints (SLJs) [Öz and Özer, 2016]. Bi-adhesive (or hybrid adhesive) joint is an alternative stress-reduction technique for adhesively bonded lap joints. The joints have two adhesives with different moduli in the overlap region. Non-linear finite element analyses were carried out for mono and bi-adhesive joints implementing the EDP material model. Distributions of EDP maximum principal stress, equivalent stress and shear stress were obtained along the middle of the adhesive thickness. Numerical failure loads were compared with our previous experimental failure loads. In addition, hydrostatic stress and equivalent plastic strain distributions for these joints under the failure loading were obtained. The general results show that experimental and numerical failure loads were in a good agreement. As a result, when bond-length ratios are selected properly and appropriate adhesives are used along the overlap length, the strength of bi-adhesive joints, compared to mono-adhesive joints, was found to increase considerably.     

A numerical and experimental study of preformed angle in the lap zone on adhesively bonded steel single lap joint

The effect of a preformed angle in the lap zone of steel adherends on the stress distribution in adhesively bonded single lap joints was investigated using an elasto-plastic finite element analysis (FEA). The ultimate loads of the joints were determined by shear tests and the failure surfaces of the lap zone were analyzed using scanning electron microscopy (SEM). The results from the numerical simulation showed that all the peak stresses in the mid-bondline of the adhesively bonded single lap joint were reduced as the preformed deflection angle was increased from 0° to 15°. The highest value of the average ultimate load of the experimental joints occurred when the preformed angle was equal to 7° (about 64% higher than the standard one). The SEM images also indicated that the failure mode in the overlap zone of the joint changed from that of a mainly adhesion one to a mixed one as the preformed angle increased.     

Effect of Aging Time at High Temperature on the Shear Strength of Adhesively Bonded Aluminum Composite Foam Joints

In this study, the shear strength behavior of adhesively bonded joints, made of aluminum composite foams subjected to high-temperature processes, has been investigated. Aluminum composite foam and solid aluminum blocks were used to form single lap joints and as the binder, a methacrylate-based structural adhesive has been selected. Foam-foam and solid-foam joints were formed and cured at room temperature for 24 hours. After curing process, aging at 200 ^oC was performed on the samples for 15, 30, 45, 60 minutes. The aged samples were subjected to lap shear testing for adhesively bonded metals and the influences of aging duration on joint strength and failure type were investigated.

As a result, lower strengths were obtained in all samples that aged under high temperature compared to non-aged samples. After the application of short-term (15-30 min) aging processes on samples, it is observed that they have joint strength values about 50% of the joint strength of non-aged samples. However, strength values of short-term aged joints (15, 30 min) remain higher than the strength values of the foam materials used in the tests. These results show that methacrylate-based adhesives subjected to short-term thermal loads up to 200 °C can be used in constructions.     

Strength of polybenzimidazole and phenolic laminate-to-metal joints

Stress concentration effects and strengths of bonded and bolted butt joints were investigated for a glass fabric polybenzimidazole lalminate at room temperatuer and 700°F for a gloass fabric phenolic laminate at room temperature and 500°F. Specimen configurations included: (1) standard tensile specimen, (2) stress concentration specimen, (3) bolted double shear butt joint, (4) bolted single shear butt joint, (5) bonded double shear butt joint and (6) bonded single shear butt joint. Both polybenzimidazole and phenolic laminates exhibited high room temperature tensile strengths and little degradation of that strength occured as a result of elevated temperature exposure. However, low joint effencies (22 to 32%) were obtained for bolted butt joint specimens. Although bonded joints exhibited higher efficiencies, they suffered from a thermal expansion mismatch between the plastic laminate and the Inconel butt plates.     

Effect of Temperature on the Joints Strength of an Automotive Polyurethane Adhesive

In this paper, the performance of an automotive polyurethane adhesive was studied through adhesive joints tests. Butt joints and single lap joints were fabricated and tested at seven temperature measuring points (TMPs). It is shown that both the tensile strength and lap shear strength decrease with the increasing of temperature. Quadratic polynomial expression obtained by the least square method can represent the tensile and lap shear strength as a function of temperature very well. ?40°C, 0°C, and 90°C were selected as the most ideal TMPs for this adhesive through the comparison of the residual sums of squares of 35 fitting curves with different combination of TMPs. Scarf joints with adhesive angles of 60° and 30° were fabricated and tested at ?40°C, 0°C, and 90°C. It also showed a decrease in joint strength with the increasing temperature. Joint strength as a function of adhesive angle is presented. It was found to closely follow a linear behaviour. A three-dimensional surface, consisting of temperature, adhesive angle, and joint strength, is presented finally to facilitate the design of automotive bonding structures.     

Process optimization of solvent based polybenzimidazole adhesive for aerospace applications

The use of adhesive bonding for high temperature applications is becoming more challenging because of low thermal and mechanical properties of commercially available adhesives. However, the development of high performance polymers can overcome the problem of using adhesive bonding at high temperature. Polybenzimidazole (PBI) is one such recently emerged high performance polymer with excellent thermal and mechanical properties. It has a tensile strength of 160 MPa and a glass transition of 425 °C. Currently, PBI is available in solution form with only 26% concentration in Dimethyl-acetamide solvent. Due to high solvent contents, the process optimization required lot of efforts to form PBI adhesive bonded joints with considerable lap shear strength. Therefore, in present work, efforts are devoted to optimize the adhesive bonding process of PBI in order to make its application possible as an adhesive for high temperature applications. Bonding process was optimized using different curing time and temperatures. Epoxy based carbon fiber composite bonded joints were successfully formed with single lap shear strength of 21 Mpa. PBI adhesive bonded joints were also formed after performing the atmospheric pressure plasma treatment of composite substrate. Plasma treatment has further improved the lap shear strength of bonded joints from 21 MPa to 30 MPa. Atmospheric pressure plasma treatment has also changed the mode of failure of composite bonded joints.     

Degradation of single lap adhesively bonded composite joints due to hot water ageing

Joints, which are the most critical part of fibre-reinforced epoxy plastic structures, can be exposed to continuous hydrothermal action. In order to estimate their long-term performance, an accelerated ageing process was performed on adhesively bonded joints of glass-fibre-reinforced epoxy plastics with [0/90/45/?45]_s fibre orientations. Changes in the static tensile properties of single lap shear samples due to hot-wet exposure were investigated for one- and two-week immersion periods and at three different water temperatures (50°C, 70°C, and 90°C). Both the ageing temperature and immersion time were found to be influential on load–displacement characteristics, maximum failure loads, and apparent failure modes of joints bonded with Loctite Hysol-9466 epoxy type adhesive. Due to the hydrothermal exposure, maximum failure loads, distance to failure values, and stiffness of joints decreased by a certain amount in proportion to the immersion time and temperature. While unaged samples and those aged at 50°C and 70°C exhibited mainly light fibre-tear (LFT) failures, the samples treated at 90°C ruptured through the material cross section in stock-break (SB) failure mode.     

Adhesive bonding of aluminium with structural acrylic adhesives: durability in wet environments

The durability of EN AW 6082-T651 aluminium alloy joints bonded with a toughened acrylic adhesive was investigated upon exposure to wet environments (humidity, water immersion and salt water immersion). Environmentally-friendly surface treatments were used to avoid hexavalent chromium. Single lap shear tests were used to determine the durability of the adhesively bonded joints. Specimens were exposed to 31% and 95% relative humidity and submerged in deionized water and 3 wt% sodium chloride solution at 25°C and 50°C, for 10, 30 and 90 days. The data collected in the experiments showed that the durability was higher for surfaces treated with γ-methacryloxypropyltrimethoxysilane (γ-MPS) and sulfo-ferric etchant (P2 etch) than other surface treatments. Both these treatments improved considerably the durability in all environments tested. The results indicate that specimens even without surface treatment maintained a significant residual strength after exposure to low humidity environment (room temperature at 31% RH). The joints exposed to a high humidity environment showed a higher reduction in adhesive strength than those immersed in deionized water and saline solution.     

Effect of temperature on the shear strength of aluminium single lap bonded joints for high temperature applications

An experimental and numerical investigation into the shear strength behaviour of adhesive single lap joints (SLJs) was carried out in order to understand the effect of temperature on the joint strength. The adherend material used for the experimental tests was an aluminium alloy in the form of thin sheets, and the adhesive used was a high-strength high temperature epoxy. Tensile tests as a function of temperature were performed and numerical predictions based on the use of a bilinear cohesive damage model were obtained. It is shown that at temperatures below T_g, the lap shear strength of SLJs increased, while at temperatures above T_g, a drastic drop in the lap shear strength was observed. Comparison between the experimental and numerical maximum loads representing the strength of the joints shows a reasonably good agreement.     

Strain rate dependence of adhesive joints for the automotive industry at low and high temperatures

In this study the impact and quasi-static mechanical behaviour of single lap joints (SLJ) using a new crash resistant epoxy adhesive has been characterized as a function of temperature. Single lap adhesive joints were tested using a drop weight impact machine (impact tests) and using a universal test machine. Induction heating and nitrogen gas cooling was used in order to achieve a homogeneous distribution of temperature along the overlap of + 80 °C and ?20 °C, respectively. Adherends made of mild steel, similar to the steel used in automobile construction, were chosen in order to study the yielding effect on the strength of the SLJ. Results showed that at room temperature (RT) and low temperature (LT), failure was dictated by the adherends due to the high strength of the adhesive. At high temperature (HT), a decrease was found in the maximum load and energy absorbed by the joint due to the reduced strength of the adhesive at this temperature. The results were successfully modelled using the commercially available finite element software Abaqus^®. Good correlation was found between experimental and numerical results, which allows the reduction of experimental testing.     

“Refractory Metal,RM – Wrap”: A tailorable,pressure-less joining technology

The RM-wrap (RM = Refractory Metal) is a pressure-less, versatile and tailorable joining process: it consists of wrapping Si foils inside a refractory metal wrap (i.e., Mo, Nb, Ta) in order to prevent molten silicon from leaking outside the joined region and infiltrating the facing materials during the joining process.RM-wrap (RM = Mo, Nb, Ta) has been successfully applied to join C/SiC composites in this work: optimized joining treatment consisted of heating to 1450?°C with a heating rate of 1000?°C/h followed by a dwell time of 5?min in a non-reactive environment of Argon flow.The joints were characterized by morphological analysis and lap shear tests at room temperature and 1000?°C.Microscopical analysis revealed an in-situ formed composite joint consisting of a silicon matrix reinforced with silicides of the refractory metals. Joining material exhibited continuous and cracked free bonding with C/SiC irrespective of composite fibre orientation.Joints lap shear strength values at 1000?°C were higher than at room temperature, probably due to the brittle to ductile transition (BTDT) of silicon and silicides.Vickers microhardness on refractory metal disilicides measured inside the joints showed a trend similar to their mechanical strength, with higher lap shear strength and hardness for Mo-Wrap and lower for Ta-wrap joints.     

Epoxy-based composite adhesives with improved lap shear strengths at high temperatures for steel-steel bonded joints

High-performance room temperature-cure epoxy structural adhesives utilizing simplified formulation are developed. The developed structural adhesive consists of diglycidyl ether of bisphenol A (DGEBA) and novolac epoxy blend as a base resin, micrometer-sized silica particles as a reinforcing filler, and triethylenetetramine as a curing agent. The developed ambient temperature-cure epoxy structural adhesive with optimized formulation exhibits outstanding properties including high glass transition temperature of 95°C, high thermal stability with degradation temperature at 5% weight loss of 364°C, exceptionally high rubbery plateau modulus of 320 MPa, good flame-retardant characteristics with limiting oxygen index of 40, and high single lap shear strength for single lap steel-steel bonded joint of 548 MPa at the temperature of 80°C. The silica-filled DGEBA/novolac epoxy composite adhesive is a potential candidate for applying as a structural adhesive for construction with long-term durability.     

Joining and mechanical testing of oxide/oxide (Nextel ™610/alumina-zirconia) ceramic composites

Efficient joining materials and techniques are of critical importance for the integration of CMCs in high performance structures. Continuous oxide fiber Nextel^? 610/alumina-zirconia composites were successfully joined to themselves by using a novel glass-ceramic based on the SiO₂-CaO-Al₂O₃-MgO-Y₂O₃-ZrO₂ system.Crystallization kinetic of the novel glass-ceramic was studied using Matusita, Sakka and Ozawa equations. Single lap off-set shear tests and four-point bending tests were performed at room temperature and at 850 °C to investigate the mechanical strength of the joints. Thermal ageing was performed at 850 °C for 100 h in air to check the thermal stability of the joined components. The results showed that the joints were oxidation resistant and the joined interfaces were well bonded. Single lap off-set shear tests on joined samples resulted in delamination of the composites. The average flexural strengths of the joined samples were 71 MPa and 81 MPa, at room temperature and at 850 °C, respectively.     

预偏角对单搭接接头强度的影响

研究了在被粘物搭接区顸偏转角度对钢单搭接拉伸接头剪切强度的影响,并用弹性有限元法分析了预偏角变化时单搭接接头上胶层中的应力分布情况。数值分析的结果表明：当预偏角从0°增加到12°时,结构钢单搭接接头胶层中的所有应力峰值分量均显著下降。而在所采取实验条件下,接头的剪切强度最高值出现在预偏角为6°时。因而在进一步研究预偏角对单搭接接头承载能力的作用时,应将外载作用下接头的本征偏转情况考虑在内。     

A Method to Evaluate Water Resistance of Acrylic Adhesives

A number of two-part acrylic adhesives were examined for the effect of immersion time in water at 23°C, 80°C and 100°C on the tensile lap shear strength. The mechanism of acrylic bonded joint failure in water was discussed based on the observed results. The relative water resistance between these adhesives was compared from shear strength retention values. The dependence of the water resistance of these adhesives at 23°C and at elevated temperatures was discussed. It has been found that for acrylic adhesives, the water resistance at room temperature can be evaluated on the basis of the shear strength retention values after aging in water at 80°C.     

Creep behaviour of steel bonded joints under hygrothermal conditions

The aim of this research is to study the influence of moisture absorption at low moisture contents on the creep behaviour of an epoxy adhesive in steel bonded joints. Single lap joints were manufactured using high strength steel adherends and a two-component epoxy adhesive. The single lap joints were tested at load levels corresponding to average lap shear stresses of ± 5%, 15%, 30% and 45% of the dry lap shear strength in both 40 °C air and 40 °C distilled water. Specimens were not pre-aged to be able to analyse the coupled effect of moisture and loading. The test results show that an increase in the load level resulted in an increase in the instantaneous strain and in the creep strain rate. The creep strain of single lap joints loaded in water was generally larger than for the ones loaded in air. For joints loaded in water the creep behaviour was found to be dependent on the moisture concentration in the adhesive. At low moisture percentages creep was suppressed, resulting in a lower instantaneous strain. At higher moisture percentages creep was promoted, resulting in a larger strain rate. The suppression of creep at low moisture percentages is attributed to water molecules bonding to the epoxy macromolecules, resulting in a reduction in molecular mobility and a smaller creep strain. At higher moisture percentages the plasticizing effect of the water dominates, resulting in a larger creep strain. The Maxwell three-element solid model and Kelvin-Voigt three-element solid model were used to simulate the creep behaviour of the single lap joints loaded in air and water. The models gave good representations of the creep response across the different load levels in both water and air, they were however unable to give a correct representation of the instantaneous strain of the single lap joints loaded in water. This is attributed to the models being unable to account for the present short-term relaxation process that is dependent on the moisture concentration.     

Evaluation of a thermoplastic polyimide (422) for bonding GR/PI composite

A hot-melt processable copolyimide designated 422 previously synthesized and characterized as an adhesive at NASA Langley Research Center for bonding Ti-6A1-4V has been used to bond Celion 6000/LARC-160 composite. Comparisons are made for the two adherend systems. A bonding cycle was determined for the composite bonding and lap shear specimens were prepared which were thermally exposed in a forced-air oven for up to 5000 h at 204°C. Lap shear strengths (LSSs) were determined at room temperature, 177°C, and 204°C. After thermal exposure to 5000 h at 204°C, room temperature and 177°C LSSs decreased significantly; however, a slight increase was noted for the 204°C test. Initially the LSS values were higher for the bonded Ti-6AI-4V than for the bonded composite; however, the LSS decreased dramatically between 5000 and 10 000 h of 204°C thermal exposure. Longer periods of thermal exposure up to 20 000 h resulted in further decreases in LSSs. Although the bonded composite retained useful strengths ( > 11.1 MPa) for exposures up to 5000 h, based on the poor results of the bonded Ti-6A1-4V beyond 5000 h, the 422 adhesive bonded composites would most likely also produce poor strengths beyond 5000 h exposure. Adhesive bonded composite lap shear specimens exposed to boiling water for 72 h exhibited greatly reduced strengths at all test temperatures. The percent retained after water boil for each test temperature was essentially the same for both systems.     

Internal stress analysis of epoxy adhesively-boned joints based on their thermomechanical properties at cryogenic temperature

Internal stress analysis is essential to structural design of materials applied in cryogenic engineering. In this contribution, thermomechanical properties including dynamic thermomechanical properties and thermal expansion behavior of four epoxy resins, namely the polyurethane modified epoxy resin (PUE), diglycidyl ether of bisphenol A (DGEBA), tetraglycidyl-4,4′-diaminodiphenylmethane (TGDDM) and triglycidyl-p-aminophenol (TGPAP) were studied by dynamic thermomechanical analysis. Internal stress of the epoxy layer in the bonded joint was calculated based on the thermomechanical properties. Meanwhile, the structure-cryogenic property relationship of epoxy resins were investigated. Results demonstrate that internal stress in the four epoxies bonded joints is 6 ~ 21 MPa at −150°C, and is positively correlated with the average thermal expansion coefficient (CTE) of epoxy resins. TGDDM and TGPAP showed higher retention of lap shear strength both at −196°C and after temperature cycling due to their lower CTE. Morphology of the fractured surface of bonded joints demonstrated that internal stress is responsible for the severe interface failure at −196°C. It reveals that selection of epoxy resins with low CTE is beneficial for designing high-performance epoxy adhesive systems served at cryogenic temperature.     

Epoxy–imide resins from N‐(4‐ and 3‐carboxyphenyl)trimellitimides. I. Adhesive and thermal properties

Epoxy–imide resins were obtained by curing Araldite GY 250 (diglycidyl ether of bisphenol‐A and epichlorohydrin; difunctional) and Araldite EPN 1138 (Novolac–epoxy resin; polyfunctional) with N‐(4‐ and 3‐carboxyphenyl)trimellitimides derived from 4‐ and 3‐aminobenzoic acids and trimellitic anhydride. The adhesive lap shear strength of epoxy–imide systems at room temperature and at 100, 125, and 150°C was determined on stainless‐steel substrates. Araldite GY 250‐based systems give a room‐temperature adhesive lap shear strength of about 23 MPa and 49–56% of the room‐temperature adhesive strength is retained at 150°C. Araldite EPN 1138‐based systems give a room‐temperature adhesive lap shear strength of 16–19 MPa and 100% retention of room‐temperature adhesive strength is observed at 150°C. Glass transition temperatures of the Araldite GY 250‐based systems are in the range of 132–139°C and those of the Araldite EPN 1138‐based systems are in the range of 158–170°C. All these systems are thermally stable up to 360°C. The char residues of Araldite GY 250‐ and Araldite EPN 1138‐based systems are in the range of 22–26% and 41–42% at 900°C, respectively. Araldite EPN 1138‐based systems show a higher retention of adhesive strength at 150°C and have higher thermal stability and T_g when compared to Araldite GY 250‐based systems. This has been attributed to the high crosslinking possible with Araldite EPN 1138‐based systems arising due to the polyfunctional nature of Araldite EPN 1138. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1729–1736, 2000     

Epoxy–imide resins based on bis (carboxyphthalimide)s

Epoxy–imide resins have been obtained through the reaction of Araldite GY 250 (diglycidylether of bisphenol-A and epichlorohydrin; difunctional) and Araldite EPN 1138 (Novolac-epoxy resin; polyfunctional) with bis(carboxyphthalimide)s derived from 4,4′-diaminodiphenylsulfone, 3,3′-diaminodiphenylsulfone, 4,4′-diaminodiphenylmethane and 4,4′-diaminodiphenylmethane and trimellitic anhydride. For each epoxy-imide resin system, epoxy equivalent to carboxy equivalent ratio has been optimised to obtain the maximum tensile lap shear adhesive strength on stainless steel substrates at room temperature. The lap shear strength at 100, 150, and 175°C has been determined for the optimum ratio. Araldite EPN-1138-based systems give the lap shear strength of 141–182 kg/cm² at room temperature for the optimum compositions and retain about 84–100% of the lap shear strength at 150°C. Araldite GY-250-based systems have lap shear strength of 183–193 kg/cm² and retain 76–84% of the lap shear strength at 150°C except for the one cured with bis (carboxyphthalimide) prepared from 4,4′-diaminodiphenylmethane, which retains only 17% of the lap shear strength. Among the systems studied, Araldite GY 250 cured with bis (carboxyphalimide) synthesized from 3,3′-diaminodiphenylsulfone appears to be the best, retaining 75% (138 kg/cm²) of the lap shear strength at 175°C.