Adhesion Characteristics of Carbon Black Embedded Glass/Epoxy Composite

The surface of glass/epoxy composite material was embedded with carbon black which was dispersed in methyl ethyl ketone (MEK) during the curing process to enhance the adhesion strength of the glass/epoxy composite structure. The morphological effect of the carbon black on the surface of composite was observed using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Quantitative chemical bonding analysis with X-ray photoelectron spectroscopy (XPS) was also performed to observe chemical bonding states on the surface. The lap shear strength of the glass/epoxy composite adhesive joints where composite adherends were embedded with carbon black was investigated with respect to the type and amount of embedment. Also, the tensile properties of the carbon black embedded glass/epoxy composites were measured to observe the mechanical degradation of the composite due to the MEK. The surface free energies of carbon black embedded composites were determined from the van Oss–Chaudhury–Good equation to correlate the lap shear strength of the adhesive joints with the surface free energies of composite adherends. From the experimental results, it was found that the carbon black embedment of the composite adherend improved much the bond strength due to the increased surface roughness on nano-scale as well as increased surface free energy.     

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.     

Effect of Adhesive Thickness on Transverse Low-Speed Impact Behavior of Adhesively Bonded Similar and Dissimilar Clamped Plates

This study investigates the effect of adhesive thickness on the transverse low-speed impact behavior of adhesively bonded similar (Al–Al, St–St) and dissimilar (Al–St, St–Al) clamped plates using the three-dimensional explicit finite element method. The contact force and plastic dissipation histories are studied for various impact energies and adhesive thicknesses. The residual plastic strains in both adhesive layer and the two plates increase with increasing impact energy. The central transverse deflections become maximal in Al–Al, moderate in Al–St, St–Al and minimal in St–St bonded plates. The back plates of all configurations deform noticeably. The stiff steel plate results in a shorter contact time, a higher contact force, a lower plastic dissipation energy and the impact energy is absorbed by the adhesive layer rather than by the front and back plates, whereas Al–Al plates dissipate it as much as the adhesive layer. The total contact time gets longer with increasing impact energy. St–St bonded plates experience larger damaged regions in both plates and adhesive layer than those in Al–Al bonded plates. The adhesive thickness has only a minor effect on the magnitude of the contact force and contact time, whereas a stiffer (St) front or back plate affects considerably the contact force and total contact time. Increasing the adhesive thickness decreases apparently residual plastic strains in plates and the adhesive layer, the central transverse deflection. A thick adhesive layer results in a minor increase in the kinetic energy of impactor, a shorter total contact time, a lower plastic dissipation energy and smaller damaged areas on the back faces of the lower plate, along the adhesive–plate interfaces.     

Transverse Low-Speed Impact Behavior of Adhesively Bonded Similar and Dissimilar Clamped Plates

This study concentrates on the transverse low-speed impact behavior of adhesively bonded similar and dissimilar clamped plates using the three-dimensional explicit finite element method. The contact force and plastic dissipation histories of the adhesively bonded dissimilar plates, such as aluminum–aluminum (Al–Al), aluminum–steel (Al–St), steel–aluminum (St–Al) and steel–steel (St–St) layered structures, were studied for different values of the impactor mass, radius and velocity (impact energies). The residual plastic strains in both adhesive layer and plates increased with increasing impact energies. The impactor radius had only a minor effect on the contact force histories for all configurations. The peak transverse deflection in the impact region was maximal in Al–Al, decreased in Al–St, St–Al plates and became minimal in St–St bonded plates. Impact effect was evident in the back plates of all four configurations. Al–Al plates dissipated impact energy as much as the adhesive layer, whereas the adhesive layer rather than plates absorbed the impact energy in Al–St, St–Al and St–St bonded plates and this state became evident in the St–St bonded plates. The number and locations of the steel plates considerably affected impact force history, impact time as well as the plastic dissipation level; thus, the contact force increased, the contact time shortened and the dissipated energy decreased. As the impact energy was increased the impact period got longer. Damage areas in the adhesive layer were minimal in Al–Al bonded plates but maximal in St–St bonded plates.     

Strength Improvement of Adhesively-Bonded Joints Using a Reverse-Bent Geometry

Adhesive bonding of components has become more efficient in recent years due to the developments in adhesive technology, which has resulted in higher peel and shear strengths, and also in allowable ductility up to failure. As a result, fastening and riveting methods are being progressively replaced by adhesive bonding, allowing a big step towards stronger and lighter unions. However, single-lap bonded joints still generate substantial peel and shear stress concentrations at the overlap edges that can be harmful to the structure, especially when using brittle adhesives that do not allow plasticization in these regions. In this work, a numerical and experimental study is performed to evaluate the feasibility of bending the adherends at the ends of the overlap for the strength improvement of single-lap aluminium joints bonded with a brittle and a ductile adhesive. Different combinations of joint eccentricity were tested, including absence of eccentricity, allowing the optimization of the joint. A Finite Element stress and failure analysis in ABAQUS^® was also carried out to provide a better understanding of the bent configuration. Results showed a major advantage of using the proposed modification for the brittle adhesive, but the joints with the ductile adhesive were not much affected by the bending technique.     

Effect of Surface Roughness on the Adhesive Strength of the Heat-Resistant Adhesive RTV88

Heat-resistant adhesive RTV88 is a hyper-elastic material and so far there have been little research on using RTV88 in adhesive joints. In this study, the effect of surface roughness on the adhesive strength of RTV88 was examined. Aluminum adherends were first sandblasted in order to generate rough surfaces, and then tensile–shear tests on Al/RTV88 single lap joints were performed. The shear strength was shown to be influenced by surface roughness. Peel failure was dominant when the surface roughness was at a low level. However, cohesive failure was the major type of failure when the surface roughness was at a high level. Effective area, peel failure area, and cohesive failure area were introduced to explain the effects of surface roughness on the adhesive strength. An empirical relation for the failure force was proposed, based on these parameters. Tensile tests of the RTV88 bonding was performed in order to obtain the necessary data. Finally, the empirical relation for the failure force was verified by tensile–shear test results.     

A Rate-Dependent Cohesive Zone Model for Adhesively Bonded Joints Loaded in Mode I

The present work investigates the rate-dependent failure behaviour of structural adhesive joints loaded in mode I. Butt joint and tapered double cantilever beam (TDCB) specimens were tested at velocities ranging over more than six orders of magnitude. A rate-dependent extension of the bi-linear cohesive zone model is proposed and implemented into the finite element code LS-DYNA via an user-defined subroutine. The parameters for the implemented cohesive zone model are found directly by evaluation of experimental data. The comparison of simulations with experimental results for different specimen types and test velocities validates the proposed model. The critical energy release rate of adhesively bonded joints is usually measured in (tapered) double cantilever beam tests, and evaluated using the Irwin–Kies equation. In this paper a different evaluation method is proposed, which provides additional information on the energy dissipated during crack initiation. The results of this method agree with the results obtained using the Irwin–Kies equation. The investigations have focussed on thin adhesive layers. Parameter identification and validation have been performed using the crash-optimized adhesive Terokal 5077 from Henkel.     

Tensile load-bearing capacity of co-cured double lap joints

The co-cured joining method has several advantages over the adhesively bonded joining method because both the curing and the joining processes for the composite structures are achieved simultaneously. In this study, the tensile load-bearing capacities of co-cured double lap joints were investigated experimentally and compared with the analytical results calculated by finite element analysis. Co-cured double lap joint specimens with several bond parameters such as bond length, surface roughness, and stacking sequence of the composite laminate were fabricated and tested. From the experimental results, it was found that the failure mechanism of the co-cured double lap joint was cohesive failure by delamination at the first ply of the composite laminate in the co-cured double lap joint. Finally, optimum values of several bond parameters were determined. Analytical tensile load-bearing capacities of the co-cured double lap joints were calculated by the three-dimensional Tsai-Wu failure criterion using stress distributions obtained from finite element analysis.     

Single Lap Joints with Rounded Adherend Corners: Experimental Results and Strength Prediction

The objective of the present study was to better understand the effect of the change in the geometry of the adherend corners on the stress distribution in single lap joints and, therefore, on the joint strength. Various degrees of rounding were studied and two different types of adhesives were used: one very brittle and another which had a large plastic deformation. Experimental results on the strength of joints with different degrees of rounding are presented. For joints bonded with brittle adhesives, the effect of the rounded adherend corners is larger than that with ductile adhesives. The strength of joints with brittle adhesives with a large radius adherend corner increases by about 40% compared to that with a sharp adherend corner. It is shown that for joints bonded with brittle adhesives, crack propagation occurs for a short period before it grows into catastrophic failure. However, for ductile adhesives, there is large adhesive yielding and small crack propagation before final failure. Another important feature of joints bonded with ductile adhesives is that there may be more than one crack in the adhesive layer before failure. This makes strength predictions more difficult. The second part of the paper presents an approximate method for predicting the strength of joints bonded with brittle and ductile adhesives, with and without adherend corner rounding. The predictions, based on an average value around the singularity, compare well with the experimental results, especially for joints bonded with ductile adhesives.     

Application of a Genetic Algorithm Associated With Adhesive Joint Analysis to the IC Chip Pick-up Process

In the condition investigated here, a concentrated force is applied to both IC chip and blue tape bonded by an adhesive under pin–pin boundary conditions. The experimental results show that even though IC chips of 0.1 mm thickness are subjected to a concentrated force of 4.8 N, they cannot be fully separated from the blue tape and fail easily during the pick-up process. However, when IC chips of 0.34 mm thickness are subjected to a concentrated force of only 3.5 N, they can be fully separated from the blue tape without breakage. These two experimental findings are then explored analytically by applying the C++ program of the genetic algorithm associated with adhesively bonded joint analysis to the IC chip pick-up process. In accordance with the experimental results, the results for the 0.1 mm thick IC chips reveal no solutions for the material properties or adhesive thickness to satisfy the conditions of the IC chip successful pick-up process, although those for the 0.34 mm thick IC chips show solutions for the values of both the elastic modulus and the adhesive layer's thickness. As regards the easy failure of IC chips with 0.1 mm thickness, if the blue tape's mechanical properties are appropriately chosen and then used in this process and its elastic modulus is greater than one-tenth that of the IC chips, the probability of the IC chips being fully separated from the blue tape can be expected to increase.     

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.     

Influence of Laser Surface Modification on Bonding Strength of Al/Mg Adhesive Joints

Key properties of magnesium alloys, such as the high strength-to-density ratio, are driving the production of lightweight structural components in the automotive and aeronautical industries. Many efforts have been carried out on various aspects of processing and fabrication, but the joining of Mg alloys to dissimilar materials is a subject which attracted much research interest in the last decades. In the present work a preliminary investigation on the strength of Al/Mg (AA6082/AZ31B) single-lap epoxy bonded joints was carried out. To this aim, Mg and Al substrates were laser irradiated using a pulsed ytterbium fiber laser. For comparison, and in order to estimate the beneficial action of the laser surface treatment, single lap joints with grit-blasted substrates were prepared and tested. The interaction between laser treated surfaces and two different epoxy adhesives was also analyzed. Finally, the results and discussion were supported by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) executed on treated and post-failure sample surfaces.     

Effect of overlap length on durability of joints bonded with a pressure-sensitive adhesive

In this study, the effect of overlap length on durability of a film type adhesive, Structural Bonding Tape (SBT) 9244, which possesses pressure-sensitive and visco-elastic properties, was investigated. Single-lap joints with 1.62 and 3.2 mm adherend thicknesses and at 12.5, 25 and 50 mm overlap lengths consisting of AA2024-T3 alloy as the adherend were exposed to two environmental conditions for exposure times of up to 90 days. The exposure environments were 100% relative humidity (RH) and 3.5% NaCl solution. At the end of exposure times, the failure surfaces were examined by Scanning Electron Microscopy (SEM) after the strength of joints was determined with the lap shear test. It was observed that with increasing overlap length, not only the failure load increased, but also the degradation rate decreased. In addition, as the metal adherends do not absorb any water and moisture from the environments, the metal adherend thickness had no effect on durability of the adhesively bonded joints.     

Damage Parameters of Adhesive Joints with General Triaxiality Part I: Finite Element Analysis

In part 1 of this paper, the triaxiality function of several adhesive joints was investigated using the finite element method. The main aim of this investigation was to decide on the most suitable joint type for fatigue experimental tests, which can be used to control triaxiality in adhesive layers. These fatigue experimental tests are presented in this paper (part 2). In part 1 of this paper, three types of adhesive joints were considered. These were: butt joint, cleavage joint and scarf joint. The geometry of the butt joint cannot be manipulated in order to control triaxiality, and therefore triaxiality is controlled through the tension/torsion loading ratios. It was found that the type of load had an effect on the map of triaxiality in the adhesive layer. For the cleavage joint, it was found that triaxiality can be manipulated by changing the adhesive bondline angle. However, the relationship between triaxiality and bondline angle was found to be complex. For the scarf joint, the triaxiality can also be manipulated in a similar way to that used for the cleavage joint by changing the bondline angle. The scarf joints has many advantages over cleavage joints including: (a) simpler relationship between triaxiality and bondline angle and (b) easier manufacturing process related to the geometry of the joint. It was shown that the scarf joint is the best practical choice to control triaxiality in fatigue experimental tests.     

Effect of Substrate Material on Fatigue Crack Propagation Rate of Adhesively Bonded DCB Joints

The effect of substrate material on the fatigue crack propagation rate was investigated using adhesively bonded DCB specimens with CFRP and aluminum substrates. The experimental results show that the increase in thickness of the adherend lowers the fatigue threshold, ΔG _th, and raises the crack growth parameter, n, irrespective of the substrate material, and that the crack growth parameter, n, for the aluminum joints is less than that for the CFRP joints. To elucidate the fatigue crack propagation behavior, fracture surface observation and finite element analysis have been conducted. Besides, Gurson's model is applied to the adhesive layer. SEM images show that numerous voids are formed in the fracture surface for the joints with aluminum substrate, but the growth of voids is suppressed for the joints with CFRP substrate. FEM results also show that the void area fraction for the joint with aluminum substrate is greater than that with CFRP substrate. Thus, the above experimental and numerical trends of voids correspond to the trends of the fatigue crack propagation behavior.     

Improvement of the Adhesive Fracture Toughness of Bonded Aluminum Joints Using E-Glass Fibers at Cryogenic Temperature

Fracture toughness and crack resistance of aluminum adhesive joints were measured at the cryogenic temperature of ?150°C, with respect to the orientation and volume fraction of the E-glass fibers in the epoxy adhesive. Cleavage tests on the DCB (Double Cantilever Beam) adhesive joints were performed using two different test rates of 1.67 × 10^?2 and 8.33 × 10^?4 mm/s to observe the crack propagation trends. From the experiments, it was found that the DCB joints bonded with the epoxy adhesive reinforced with E-glass fibers not only showed a stable crack propagation with a low crack propagation speed, but also higher fracture toughness and crack resistance than those of the DCB joints bonded with the unreinforced epoxy adhesive at a cryogenic temperature of ?150°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.     

Fatigue Behaviour of Adhesive Bonded Joints

This paper presents experimental results of the fatigue behaviour of adhesive bonded plastic-to-plastic joints and metal-to-plastic joints under both dynamic and static loading. The fatigue life of the joints was found to be independent of the test frequencies and humidity for the range of values tested, but dependent on the mean stress level and test temperature with greater reduction in fatigue life observed in metal-to-plastic joints at higher temperature. Empirical equations from which the fatigue life of joints could be predicted were obtained by regression analysis.     

胶接接头剪切和正拉强度试验方法的分析

分析了胶接接头现行剪切强度和正拉强度测试方法中存在的问题，指出由于应力集中及多种应力的综合作用导致测试值出现较大偏差，提出了胶接接头剪切强度和正拉强度测试方法的改进意见。