Effect of temperature on the joint strength of a silyl-modified polymer-based adhesive

Temperature is a very important factor that must be fully considered in the study on the adhesive joint strength. In this paper, a silyl-modified polymer-based adhesive ISR 70-08 which is widely used in engineering was studied. Dog-bone specimens were fabricated and tested at ?40°C, room temperature (RT), and 90°C. Results show a decrease in the main mechanical properties with increasing temperature. Butt joints (BJs), single-lap joints (SLJs), and Scarf joints (SJs) were fabricated and tested at different temperatures. A quadratic polynomial expression was an ideal choice to express the joint strength as a function of temperature which was obtained using the least-squares method. Temperature combinations of ?40°C, 0°C, and 90°C were obtained to study the effect of temperature on the joint strength more easily for this adhesive. A three-dimensional surface, consisting of temperature, adhesive angle, and joint strength was presented to facilitate the application of bonding structures in engineering     

Effect of the temperature on the strength of adhesively bonded single lap and T joints for the automotive industry

Adhesively bonded lap and T joints are used extensively in the manufacture of automotive structures. In order to determine the effect of using a structural adhesive instead of spot-welding, a detailed series of tests, supported by finite element analyses, was conducted using a range of loadings. The adhesive was a toughened epoxy and the adherend was a grade of mild steel typical of that used in the manufacture of car bodyshells. The lap joints were tested in tension (which creates shear across the bondline) and three point bending. Previous studies at room temperature have shown that joint failure is dictated by adherend yielding and adhesive strain to failure. In the present study, to asses the effect of temperature that an automotive joint might experience in service, tests were carried out at ?40 and +90 °C. It is shown that the failure criterion proposed at room temperature is still valid at low and high temperatures, the failure envelope moving up and down as the temperature increases or decreases, respectively.     

An Experimental Technique on the Dynamic Strength of Adhesively Bonded Single Lap Joints

This paper reported an experimental technique on the shear strength of adhesively bonded single lap joints subjected to impact loads by means of a split Hopkinson tensile bar. The experiments were conducted at two velocities (V = 20 m/s, 7 m/s) and testing temperatures ranging from ?40°C to 80°C. The results indicated that the shear strength of the specimen decreased with the increase of temperature and increased with the increase of velocity. The strength degradation from room temperature to high temperature was more severe than that from low temperature to room temperature. The effects of the pins, thermal stress and peel stress were also examined and found to have limited effects on the determination of the shear strength of the joints. It was concluded that the shear strength of the adhesively bonded single lap joints under impact loads can be determined by this experimental technique.     

Modification of high‐performance polymer composite through high‐energy radiation and low‐pressure plasma for aerospace and space applications

In this investigation, attempts are made to modify a high‐performance polymer such as polybenzimidazole (PBI) (service temperature ranges from ?260°C to +400°C) through high‐energy radiation and low‐pressure plasma to prepare composite with the same polymer. The PBI composites are prepared using an ultrahigh temperature resistant epoxy adhesive to join the two polymer sheets. The service temperature of this adhesive ranges from ?260°C to +370°C, and in addition, this adhesive has excellent resistance to most acids, alkalis, solvents, corrosive agents, radiation, and fire, making it extremely useful for aerospace and space applications. Prior to preparing the composite, the surface of the PBI is ultrasonically cleaned by acetone followed by its modification through high‐energy radiation for 6 h in the pool of a SLOWPOKE‐2 (safe low power critical experiment) nuclear reactor, which produces a mixed field of thermal and epithermal neutrons, energetic electrons, and protons, and γ‐rays, with a dose rate of 37 kGy/h and low‐pressure plasma through 13.56 MHz RF glow discharge for 120 s at 100 W of power using nitrogen as process gas, to essentially increase the surface energy of the polymer, leading to substantial improvement of its adhesion characteristics. Prior to joining, the polymer surfaces are characterized by estimating surface energy and electron spectroscopy for chemical analysis (ESCA). To determine the joint strength, tensile lap shear tests are performed according to ASTM D 5868–95 standard. Another set of experiments is carried out by exposing the low‐pressure plasma‐modified polymer joint under the SLOWPOKE‐2 nuclear for 6 h. Considerable increase in the joint strength is observed, when the polymer surface is modified by either high‐energy radiation or low‐pressure plasma. There is further significant increase in joint strength, when the polymer surface is first modified by low‐pressure plasma followed by exposing the joint under high‐energy radiation. To simulate with spatial conditions, the joints are exposed to cryogenic (?196°C) and high temperatures (+300°C) for 100 h. Then, tensile lap shear tests are carried out to determine the effects of these environments on the joint strength. It is observed that when these polymeric joints are exposed to these climatic conditions, the joints could retain their strength of about 95% of that of joints tested under ambient conditions. Finally, to understand the behavior of ultrahigh temperature resistant epoxy adhesive bonding of PBI, the fractured surfaces of the joints are examined by scanning electron microscope. It is observed that there is considerable interfacial failure in the case of unmodified polymer‐to‐polymer joint whereas surface‐modified polymer essentially fails cohesively within the adhesive. Therefore, this high‐performance polymer composite could be highly useful for structural applications in space and aerospace. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1959–1967, 2006     

Geometric Factors Impacting Adhesive Lap Joint Strength and Design

Too often adhesive thickness, adherend thickness and other geometric factors are not explicitly considered in adhesive joint design. This study includes experimental and computational research exploring the means of enhancing the engineering design process for adhesive lap joints to include such effects. It clearly demon-strated that both the cleavage stresses and the shear stresses, near the bond termini, play important roles in lap 'shear' joint failure. Finite Element and Fracture Mechanics analyses were used to examine the energy release rate applied to growth of cracks in adhesive lap joints. Lap joints with similar geometries to those analyzed were designed, fabricated and tested. In a separate set of experiments the bond termini were constrained in the direction normal to the uniaxial loading. If the strength of lap shear joints is dominated by the adhesive shear strength, then constraining the lateral motion of the bond termini should have little or no effect on the overall shear strength of the adhesive joint. This work clearly demonstrates that this is not the case. If cleavage stresses are important in lap joints then constraining the bond termini, in a direction normal to the bond area, should have a commensurate effect on the overall strength of the lap joint. None of the ASTM standardized 'lap shear tests' provide any insight into this premise. This paper also presents analyses and experimental results for lap joints to which several methods of lateral constraint were applied near the bond termini. The analytical and numerical methods described and used for explaining and predicting such effects might be a useful adhesive joint design tool.     

Experimental study of adhesively bonded CFRP joints subjected to tensile loads

The tensile performance of adhesively bonded CFRP joints has been investigated experimentally. In this study, overlap length, adherend thickness, adherend width and scarf angle were chosen as design parameters. All load–displacement curves are linear, except that the thicker single-lap joints behave slightly nonlinearity due to the bending effect caused by eccentric loading. The lap shear strength is not directly proportional to overlap length, adherend thickness, adherend width and scarf angle for the brittle adhesive studied in the paper. The major failure mode includes adhesive shear failure and adherend delamination failure, sometimes accompanying with some fiber pull-out. Finally, the lap shear strength of three different lap types with similar bonding area (W=25 mm, L=10 mm, θ=5.71°) and adherend thickness (0.96 mm) was analyzed. It is found that the double-lap joint has the highest ultimate failure load. However, when considering the lap region weight, the scarf-lap joint is the most efficient.     

The effect of adhesive thickness on tensile and shear strength of polyimide adhesive

The effect of adhesive thickness on tensile and shear strength of a polyimide adhesive has been investigated. Tensile and shear tests were carried out using butt and single lap joints. Commercially available polyimide (Skybond 703) was used as adhesive and aluminum alloy (5052-H34) was used as adherends. The tensile strength of the butt joints decreased with increasing adhesive thickness. In contrast, adhesive thickness did not seem to affect the shear strength of single lap joints. The fabricated joints using the polyimide adhesive failed in an interfacial manner regardless of adhesive thickness. The linear elastic stress analysis using a finite element method (FEM) indicates that the normal stress concentrated at the interface between the adherend and the adhesive. The FEM analysis considering the interfacial stress well explains the effect of adhesive thickness on the joint strength.     

Photoelastic thermal stress measurements in scarf adhesive joints under uniform temperature changes

This study deals with the investigation of thermal stresses and delamination growth in scarf joints under a uniform temperature change by photoelastic measurements and a two-dimensional finite element analysis. The adherends were fabricated from aluminum plates, and an adhesive layer was modeled and fabricated from an epoxide resin plate. The adherends and the epoxide resin plate were bonded using a heat-setting and one-component-type adhesive. The adhesive was cured at 85 °C and cooled down to room temperature. The thermal stress was then generated in the scarf joint under a temperature change and measured by photoelasticity. After the scarf joints were cooled in a stepwise manner, the delamination growth, which initiates from the edge of the interface, was measured. It was found that the delamination initiates from the edge of the interface with the acute angle side and it never initiates from the edge with the obtuse angle side. When the scarf angle is 90°, i.e. in adhesive butt joints, the resistance against the delamination is minimal. The thermal stresses in the scarf joints with a thin adhesive layer were also analyzed. It was found that the thermal strength increases as the adhesive thickness decreases. The stress singularity near the edge of the interface was calculated from the stress distributions in the joints with different scarf angles. As a result, it was found that the stress singularity in the scarf joints under thermal loads is quite different from that under static tensile loads.     

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.     

A Method of Predicting the Stresses in Adhesive Joints after Cyclic Moisture Conditioning

The durability of adhesive joints is of special concern in structural applications and moisture has been identified as one of the major factors affecting joint durability. This is especially important in applications where joints are exposed to varying environmental conditions throughout their life. This paper presents a methodology to predict the stresses in adhesive joints under cyclic moisture conditioning. The single lap joints were manufactured from aluminium alloy 2024 T3 and the FM73®-BR127® adhesive-primer system. Experimental determination of the mechanical properties of the adhesive was carried out to measure the effect of moisture uptake on the strength of the adhesive. The experimental results revealed that the tensile strength of the adhesive decreased with increasing moisture content. The failure strength of the single lap joints also progressively degraded with time when conditioned at 50°C, immersed in water; however, most of the joint strength recovered after drying the joints. A novel finite element based methodology, which incorporated moisture history effects, was adopted to determine the stresses in the single lap joints after curing, conditioning, and tensile testing. A significant amount of thermal residual stress was present in the adhesive layer after curing the joints; however, hygroscopic expansion after the absorption of moisture provided some relief from the curing stresses. The finite element model used moisture history dependent mechanical properties to predict the stresses after application of tensile load on the joints. The maximum stresses were observed in the fillet areas in both the conditioned and the dried joints. Study of the stresses revealed that degradation in the strength of the adhesive was the major contributor in the strength loss of the adhesive joints and adhesive strength recovery also resulted in recovered joint strength. The presented methodology is generic in nature and may be used for various joint configurations as well as for other polymers and polymer matrix composites.     

Glass/epoxy composites and aluminium alloy joint using Kevlar® intermediate layer and nanoclay-reinforced epoxy adhesive

Adhesive lap joint between glass fibre/epoxy composites and aluminium alloy (2014 T4) was prepared by an in situ moulding process using a matched die mould. The surface of aluminium alloy was treated with chromic acid before adhesive bonding. Lap shear strength and fatigue life were evaluated in tensile mode and tension–compression mode (at 40% of lap shear load of adhesive joint), respectively. Knurling on the surface of aluminium alloy improved the lap shear strength of the adhesive joint but did not influence the fatigue life of the same. Lap shear strength and fatigue life of adhesive joint made with neat epoxy adhesive and reinforcement of an intermediate layer of Kevlar^® between glass/epoxy composite and aluminium alloy were observed to be 0.44?kg/mm² and 3.6?×?10⁵ cycles, respectively. In another case, lap shear strength and fatigue life of similar type of adhesive joint made from nanoclay (Cloisite 30B)-reinforced epoxy adhesive and without reinforcement of an intermediate layer of Kevlar^® were observed to be 0.38?kg/mm² and 2.3?×?10⁵ cycles, respectively. Whereas, lap shear strength and fatigue life of adhesive joint made from nanoclay-reinforced epoxy adhesive along with the reinforcement of an intermediate layer of Kevlar^® were 0.48?kg/mm² and 3.9?×?10⁵ cycles, respectively. Therefore, adhesive joint made from nanoclay-reinforced epoxy adhesive along with the reinforcement of an intermediate layer of Kevlar^® was the best.     

Long Term Strength of Structural Adhesive Joints

The long term static strength of adhesive joints is analyzed in terms of a modified Prot method and sustained load tests. Data from the failure times under different loading rates are used to predict the static stress that an adhesive joint will withstand for an infinite time, i.e., the endurance limit. Despite theoretical shortcomings, the method is found to give reasonable estimates of the endurance limit as determined by standard sustained load tests. The ratio of the short term lap shear strength to the endurance limit is found to be independent of adhesive modulus, temperature, and sample geometry. For engineering calculations on lap shear structural adhesive joints under a static load (at 23°C, 50% R.H.), the endurance limit may be assumed to be equal to 0.25 of the short term strength.     

“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.     

Improving the mechanical behavior of the adhesively bonded joints using RGO additive

In this research, Araldite 2011 has been reinforced using different weight fractions of Reduced Graphene Oxide (RGO). Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) analyses were conducted and it has been shown that introduction of the RGO greatly changes the film morphology of the neat adhesive. Uni-axial tests were carried out to obtain the mechanical characteristics of the adhesive-RGO composites. It has been observed that introducing 0.5 wt% RGO enhances the ultimate tensile strength of the composites by 30%. In addition, single lap joints using neat adhesive and adhesive-RGO composites were fabricated to investigate the effect of the added RGO on the lap shear strength of the joints. Results show that the joints with added 0.5 wt RGO exhibited 27% higher lap shear strength compared to the joints bonded with neat adhesive. Finally, Finite Element (FE) numerical solutions using Cohesive Zone Modeling (CZM) have been carried out to simulate the failure behavior of the joints, and it has been shown that the FE models can predict the joint’s failure load.     

Synthesis and properties of allyloxyethyl 2-cyanoacrylate adhesive

Allyloxyethyl 2-cyanoacrylate monomer was synthesized and characterized for the first time. It was found that this monomer retains the typical properties of cyanoacrylate adhesives such as fast setting time at room temperature, adhesion to most materials, and high strength of bonded joints. Because of its long ester group and the reactive allyl group, this cyanoacrylate monomer produces adhesive bonds which have improved elasticity and heat resistance. IR and DSC studies showed crosslinking of the adhesive layer when subjected to elevated temperature, which explains the increased tensile shear strength of steel bonded joints. It was found that allyloxyethyl 2-cyanoacrylate can also be used as a crosslinking component for cyanoacrylate adhesives, based on ethyl 2-cyanoacrylate. Less than 10% of allyloxyethyl 2-cyanoacrylate in the mixture is needed for increasing, over three times, the tensile shear strength of the adhesive joint after ageing at 100°C.     

A Method of Estimating the Strength of Adhesive Bonded Joints of Metals

The strength of adhesive bonded joints is investigated both analytically and experimentally. The deformed states of lap joints under tensile shear loading are analysed by the finite element method on the assumption of elastic deformation. A method of using the adhesive strength law is proposed to estimate the joint strength. The adhesive strength law is experimentally determined by subjecting butt joints of two thin-walled tubes to combined axial load and torsion. The strength of lap joints is determined by adopting the adhesive strength law to the adhering interface as well as the strength law of adherend and adhesive resin. The calculated strain distribution and strength of the joints are compared with the experimental results. The effects of the joint configurations on the deformation and strength are discussed. It is shown that the proposed method is useful to predict the joint strength.     

A Method of Estimating the Strength of Adhesive Bonded Joints of Metals

The strength of adhesive bonded joints is investigated both analytically and experimentally. The deformed states of lap joints under tensile shear loading are analysed by the finite element method on the assumption of elastic deformation. A method of using the adhesive strength law is proposed to estimate the joint strength. The adhesive strength law is experimentally determined by subjecting butt joints of two thin-walled tubes to combined axial load and torsion. The strength of lap joints is determined by adopting the adhesive strength law to the adhering interface as well as the strength law of adherend and adhesive resin. The calculated strain distribution and strength of the joints are compared with the experimental results. The effects of the joint configurations on the deformation and strength are discussed. It is shown that the proposed method is useful to predict the joint strength.     

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.     

Durability of PBI adhesive bonded joints under various environmental conditions

Structural adhesives are finding increasing use in many applications. However, their utilization at elevated temperature has always been a challenge due to their low thermal and mechanical properties. However, in recent years, the development of high performance polymers have overcome the problem of using adhesive bonding at high temperature to some extent. Polybenizimidazole (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 temperature (T_g) of 425 °C. Due to its excellent thermal and mechanical properties, it has the potential to be used as an adhesive under various environmental conditions. In the present work, efforts are devoted to explore the potential of using PBI at high temperature and in hot-wet environmental conditions. M21 and DT120 epoxy based carbon fiber composite bonded joints were prepared and tested. Both M21/carbon composite and DT120/carbon composite have exhibited a reduction in joint strength of about 16% and 25% respectively after 1000 h of conditioning in a hot-wet environment. However, a reduction in lap shear strength of 52% and 56% is observed when composite bonded joints were tested at 80 °C.     

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.