The effect of primers on adhesive properties and strength of adhesive joints made with polyurethane adhesives

The paper presents selected aspects of the effect of primers on adhesive properties and strength of aluminium sheet adhesive joints, made using polyurethane adhesives. The strength of adhesive joints was determined based on two cure time variants: 15 and 64 h. It was found that the longer cure time at a humidity of 33% is more desired, as it leads to a substantial increase in strength of the tested adhesive joints. In addition, two variants of surface preparation were applied: degreasing and degreasing followed by the application of a primer (a pro-adhesive agent). It was observed that the primer application prior to the application of an adhesive leads to a significant increase in strength compared to the variant where the adhesive application is preceded only by degreasing. Moreover, the aluminium sheet surface that was subjected to cataphoretic painting and priming exhibits better adhesive properties. It has a higher value of both surface free energy and its dispersion and polar components compared to the surface that was only subjected to degreasing.     

Processing and property of carbon-fiber aluminum-foam sandwich with aramid-fiber composite adhesive joints

Thin tissues (or called webs) of short aramid fibers were added at the adhesive joints between carbon-fiber face sheets and aluminum-foam core to form aramid-fiber ‘composite adhesive joints’ for improving the interfacial bonding strength of sandwich structures and therefore other structural properties. In-plane critical compression loads and failure modes of carbon-fiber aluminum-foam sandwich beams with short aramid-fiber composite adhesive joints are investigated and discussed in this study. Improvements in critical compression loads were achieved for all specimens with aramid-fiber composite adhesive joints of different densities. It is suggested that the composite adhesive joints using low-density short aramid fibers is effective in promoting reinforcements against cracking and delamination at the interface between carbon-fiber face sheets and aluminum-foam core. The underlying interfacial strengthening and toughening mechanisms were discussed and analyzed based on observations from optical image and scanning electron microscopy.     

Effect of geometry and hybrid adhesive on strength of finger joints of Pinus elliottii subject to humidity and temperature

The optimized bonding of glued finger joints is required for structural and nonstructural applications. The use of nonspecific adhesives, combined with the joint geometry and exposure of joints to humidity and temperature, are factors that can compromise the durability of glued joints. The main objective of this study is the development of cross-linking poly(vinyl acetate) (PVAc) hybrid adhesive to produce nonstructural finger joints of Pinus elliottii with finger lengths of 6.5 and 4.5 mm. The adhesives were produced by emulsion copolymerization of vinyl acetate with n-butyl acrylate with different amounts of N-methylol acrylamide and blended with resorcinol-formaldehyde resin (RF) and aluminum chloride (AlCl₃). The rheological behavior of adhesives was investigated. We found that the joint configuration and the exposition time employed influenced joint strength. The PVAc/RF adhesive showed a thicker bond line and consequent deeper penetration into the pores of the wood as verified by microscopy analysis. Statistically differences in bond strength of the adhesive joints were found with respect to different conditioning times and finger length. The highest values were exhibited by the joints produced with a finger length of 6.5 mm and glued with the hybrid adhesive (AD-4) than that joints produced with a finger length of 4.5 mm.     

Characterization of the mode I fracture energy of adhesive joints

This investigation was aimed at improving the calculation of the Mode I fracture energy, G_IC, of adhesive joints by incorporating the elasticity of the adhesive layer. It was also aimed at proposing ways to improve the calculation of G_IC over the existing standard for the measurement of that material property.

Our experiments were performed with a variety of aluminum specimens bonded with a large number of different adhesives in various thicknesses. All specimens were tested under mode I loading. In calculating G_IC, it is common to neglect the properties of the adhesive layer, especially when the compliance of the system is considered. The validity of this attitude was tested in the present study, and it was found to be in accord with the experimental results but only for joints bonded with a relatively thin layer of adhesive. A method for improving the calculation of the fracture energy of standard specimens, using a theoretical model for the compliance is proposed. This method comprises all the adhesive parameters and is appropriate for linear elastic joints.     

环氧树脂胶黏剂在石墨粘接接头的热分解动力学

引言 与传统的铆接、螺栓连接工艺相比,粘接技术因具有操作灵活、简便、生产效率高、生产成本低等优点,而广泛用于化工、汽车、航空、航天等现代结构工程,在工程设计和制造领域的应用持续增长[1-3].     

Determination of the strain distribution in adhesive joints using Fiber Bragg Grating (FBG)

This work focuses on the development and testing of a technique used to measure strain levels inside an adhesive joint. As more industries adopt high performance structural adhesives, the need for structural monitoring and quality control of adhesive joints rises. The method presented in this work, based on optic fibers, is proposed as a possible means for real-time health monitoring of adhesive connections. In the first part of this work a procedure for embedding optical fibers etched with Bragg sensors is explained. Instrumented, single lap joints were fabricated and subjected to tensile test. The results were compared with finite element models to ensure the accuracy and provide a better understanding of the measurement process.     

XFEM modelling of single-lap Kenaf fibre composite hybrid joints under quasi-static loading

ABSTRACT

The present paper aims to predict the bearing stress at failures of woven fabric kenaf fibre reinforced polymer with single-lap hybrid joints under quasi-static testing. Testing series investigated includes a variation of joint types, normalised W/d, lay-up types, plate thickness and bolt loads. Initially, cohesive failures appeared within adhesive layer, followed by net-tension failure mode in all testing series investigated associated with stress concentration. Strength prediction was carried out subsequently by implementing XFEM framework with an embedded traction-separation relationship within ABAQUS CAE. The experimental datasets and XFEM results were evaluated where good agreements were found in the combination of cross ply and thicker coupon with a discrepancy of less than 5%.     

A hybrid bondline concept for bonded composite joints

Based on the experience in the past and the occurrence of in-service damages, the authorities restrict today the application of adhesive bonding of composite structures for aircraft applications. However, certification limitations can be overcome if occurring disbonds within a bond are stopped by implemented design features, so-called disbond stopping features. Consequently, a novel bondline architecture for bonded composite joints is proposed. By implementing a distinct rather ductile thermoplastic phase, a physical barrier for growing disbonds is obtained and thus a fail-safe design, respectively. Moreover, the joint is established by using two different joining technologies, namely adhesive bonding and thermoset composite welding. A sophisticated manufacturing technique is developed for the hybrid bondline concept to achieve a high strength joint. The joint׳s quality is examined by means of several analytical methods like microsections, scanning electron microscopy (SEM), and energy-dispersive X-Ray (EDX) analysis. Additionally, the mechanical performance is evaluated by static Double Cantilever Beam (DCB) and Single Lap Shear (SLS) tests.     

Fatigue crack initiation and damage characterization in Brazilian test specimens for adhesive joints

The present study is focused on the fatigue failure initiation at bimaterial corners by means of a configuration based on the Brazilian disc specimens. These specimens were previously used for the generalized fracture toughness determination and prediction of failure in adhesive joints, carried out under static compressive loading. Under static loading, local yielding effects might affect the asymptotic two-dimensional linear elastic stress representation under consideration. Fatigue loading avoids this fact due to the lower load levels used. The present tests were performed using load control; video microscopy and still cameras were used for monitoring initiation and crack growth. The fatigue tests were halted periodically and images of the corner were taken where fatigue damage was anticipated. Damage initiation and subsequent crack growth were observed in some specimens, especially in those which presented brittle failure under static and fatigue tests. These analyses allowed the characterization of damage initiation for a typical bimaterial corner that can be found in composite to aluminium adhesive lap joints.     

Geometrical study of mixed adhesive joints for high-temperature applications

The use of adhesives for high-performance structural applications has significantly increased in the last decades. However, the use of adhesive joints in adverse environmental conditions is still limited due to the reduced capability of adhesives to withstand large thermal gradients. Dual adhesive joints, which contain two adhesives with remarkably different mechanical behaviours, are a technique suitable for being used in extreme temperatures. The object of this study is a ceramic–metal joint, representative of the thermal protection systems of some aerospace vehicles. In this paper, several joint-mixed joint geometries are presented, studied with recourse to finite element analysis. In a first phase, the three-dimensional finite element models and the material properties are validated against experimental data. In a second phase, the model geometry is modified, with the aim of understanding the effect of several changes in the joints’ mechanical behaviour and comparing the merits of each geometry. The models’ presented good agreement was found between experimental and numerical data and the alternative geometries allowed the introduction of additional flexibility on the joint but at the cost of lower failure load.     

胶粘剂特性和厚度对劈裂载荷作用下胶接接头应力分布的影响

运用三维弹塑性有限元法对劈裂栽荷作用下的胶接接头(即劈裂接头)承载后的应力分布特征进行了分析，重点研究了胶粘剂层厚度对劈裂接头应力分布的影响。结果表明，胶粘剂的性能对应力分布有较大影响，提高胶粘剂强度和减小胶层厚度，均导致胶层应力集中加剧，各向正应力峰值呈上升趋势，各向剪切应力则正好相反；并且劈裂接头中应力分布以三向主应力为主，剪切应力的存在亦不可忽略。故在不引起过大应力集中和较大胶层缺陷条件下采用高强度的胶粘荆和较厚胶层对提高劈裂接头强度有利，实验结果与有限元分析相吻合。     

Characterization and prediction of fracture properties in hygrothermally degraded adhesive joints: an open-faced approach

One of the challenges in the application of structural adhesive joints is the prediction of their long-term durability. During the service life, moisture diffuses into the adhesive layer and eventually degrades its fracture properties. Environmental degradation should thus be taken into consideration in the design and analysis of adhesive joints. This work first provides an overview, summarizing the recent efforts regarding the hygrothermal exposure of adhesive joints, accelerated aging methods, water diffusion modeling, and characterization of fracture properties in adhesively bonded joints. The second part presents a recent degradation methodology by which the fracture toughness evolution of adhesive joints can be predicted using fracture test data obtained using the accelerated open-faced degradation method.     

Crack detection in adhesive joints: use of strain gages in aggressive environments

A large three-dimensional finite element model was used to predict the response of miniature resistance strain gages to the initiation and growth of center and corner cracks in the fillet of an asymmetric single lap-shear joint. Experiments were conducted to assess the feasibility of the technique under conditions of accelerated durability testing. Drift in the strain-gage output complicated the interpretation of the data and placed restrictions on the slowest rate of crack growth which could be detected. The finite element model was also used to predict the strain-gage response to two-dimensional cracking under conditions of cyclic loading. This was seen to eliminate the experimental problem of signal drift and to create a more robust technique for the detection of crack initiation and growth. Using this method, it is predicted that cracks as small as 0.2 mm can be detected by comparing the best-fit slopes of the strain-gage output vs. the applied load in subsequent loading cycles.     

Ultrasonic testing for bond efficiency of carbon steel to stainless steel adhesive joints

Structural adhesive joints between carbon steel and stainless steel were made and evaluated. Various combinations of a two-component epoxy adhesive were used and diverse polymerization cycles were employed to obtain joints of different mechanical strengths. Two ultrasonic procedures were used: ultrasonic scanning with a focused beam to ascertain the quality and the uniformity of the joints; and ultrasonic spectrum analysis to evaluate bond strength. The influence on adhesion of surface calamine of the carbon steel was also investigated: bond strengths were comparable with those attained with ground carbon steel adherends. Adding lubricating oil to the adhesive was found to increase the mechanical properties of the joints.     

胶层局部缺陷对胶接接头承载能力的影响研究

针对汽车车身中应用日益广泛的钢板胶接结构,通过实验得到了不同胶层局部缺陷对接头承载能力的影响规律,进而采用接头的三维弹塑性有限元模型,解释了实验结果,并提出了接头承载危险系数(K)的概念。研究结果表明,胶层为四角缺陷时,接头处的危险系数随缺陷面积的增加而增大(从0.66增加到0.91),接头的断裂强度由8640N降低到8121N;胶层为中央缺陷时,接头处的危险系数随中央缺陷面积的增加增幅较小,接头承载能力基本不变。     

Evaluation of interfacial properties in adhesive joints of aluminum alloys using angle-beam ultrasonic spectroscopy

This paper reports an ultrasonic angle-beam technique for the evaluation of interfacial properties of adhesive joints. The technique is based on measurement of the frequency response of obliquely incident ultrasonic signals from an adherend-adhesive interface. A theoretical model was developed for analysis of the interaction between the obliquely incident ultrasonic waves and multilayered adhesive joints. A special ultrasonic goniometer using only one ultrasonic transducer was built to measure the reflected signals. Samples with controlled interfacial properties were prepared for experimental determination of these properties. The experimental results were in good agreement with those predicted from the model. It was found that some of the minima loci in the reflected frequency spectra are independent of either the thickness or the elastic properties of the interfacial layer. This enables us to develop a simple and stable reconstruction procedure to determine the thickness and interfacial properties from experimental data.     

Quality analysis of hybrid adhesive-spot welded joints

In the paper, practical aspects of hybrid adhesive-welded joints are described. For the joints made of low-alloy car-body sheet steel HS340LA 1.2 mm thick at selected variable process parameters, a wide range of destructive and non-destructive tests was carried-out. The joints were made using three constructional adhesives, one of those dedicated for hybrid joints. For this adhesive, the most favourable hardening conditions were established. Quality analysis of hybrid joints was based on ultrasonic testing with use of an RSWA (Resistance Spot Weld Analyser) device, metallographic examinations, basic mechanical testing and tests at complex stress state, visual testing with a high-speed camera. Results were compared with adhesive bonding and spot welding. The whole research is summarized with conclusions drawn on the grounds of experimental and analytical results.     

Functionally graded adhesive joints by graded mixing of nanoparticles

The main objective of the present study was to use a functionally modified adhesive in order to have an adhesive with properties that vary gradually along the overlap, allowing a uniform stress distribution along the overlap. This allows for a stronger and more efficient adhesive joint. It is possible to work with much smaller areas, reducing considerably the weight of the structure and obtaining more reliable joints. The adhesive stiffness would vary along the overlap, being maximum in the middle and minimum at the ends of the overlap. The processes tested in this work were dielectric heating using a domestic microwave oven and conventional oven heating. Different amounts of carbon black were used and functionally dispersed along the bondline length in order to obtain a functionally graded joint. The functionally graded joints were found to have a higher joint strength compared to the cases where the carbon black was dispersed uniformly along the overlap or where the adhesive was used without carbon black. An analytical model was used to assist with the prediction and the assessment of the possible effectiveness of a graded joint concept.     

Study of the singular term in mixed adhesive joints

In the numerical modelling of mixed adhesive joints, where different materials are disposed along the bond-lines, trimaterial singularities may arise at the adhesive bands transition points, resulting in mesh-dependent models. This work is focused on the study, characterization and treatment of these type of singularities. Firstly, the selection of an intermediate adhesive material for generating a tailored transition between bands is addressed, using analytical solutions and then corroborating by means of a novel technique for determining the influence distance of the singularity with the finite element method as calculation tool. Furthermore, it is proved how a continuum material field transition between bands produces a convergent finite element solution applying the considerations of this framework (analytical asymptotic solutions). Finally, in order to achieve the best numerical convergence rate in the models, the optimum material transition function is obtained and checked through FEM.     

Experimental study of the influence of adhesive reinforcement in lap joints for composite structures subjected to mechanical loads

The paper deals with experimental investigations on reinforcing the adhesive in single lap joints subjected to mechanical loads such as tensile, bending, impact and fatigue. The adhesive used for bonding was an epoxy reinforced with unidirectional and chopped glass fibres as well as micro-glass powder. The adherends were glass reinforced composite laminates. The bonding surfaces were prepared before joining. In the case of unidirectional fibres in the adhesive region, the fibre orientations considered were 0°, 45° and 90°. The volume fraction of fibres in the adhesive layer in all the cases was 30%. The volume fractions of micro-glass powder were 20%, 30% and 40%. The tensile, bending, impact and fatigue tests on the prepared specimens were conducted according to ASTM standards. The results show that except the 90° unidirectional orientation, reinforcing the adhesive with glass fibres or powder increases the joint strength. The use of volume fraction of 30% of micro-glass powder gave the best performance in the above loading conditions. The fatigue life increased by 125%, the ultimate joint strength in tension increased by 72%, the bending ultimate joint strength increased by 112% and the impact joint strength increased by 63%. The microstructure of the debonded area was examined and three modes of failure could be observed namely cohesive failure, light fibre-tear failure and thin layer cohesive failure.