Mode I characterization of toughened epoxy adhesive joints under shock-wave loading

With the advances in adhesive technology, the use of structural adhesive joints has extended to the broader engineering field as an alternative to traditional joining methods such as bolting, riveting, and welding. Therefore, characterization the adhesive joints under different loading and environmental conditions has been becoming more significant in designing adhesives joints for an engineering application. Since most of the polymer-based adhesives have non-linear mechanical behavior and loading rate sensitivity caused by their viscoelastic properties, testing adhesive joints under quasi-static loading cannot give adequate information to predict the response of adhesive joints to high loading conditions. It is therefore imperative to characterize the adhesive joints under high loading rates in order to integrate them into the applications that require high impact resistance. This study focused on the bending (mode I) characterization of adhesive joints under shock-wave loading generated by a large-scale shock tube. A specially designed adhesive joint that transfers the shock-wave loading to the bond in the mode-I form was designed, fabricated and tested. A series of shock-wave loading experiments were carried out with two different adhesive joints: aluminum-epoxy and steel-epoxy and their performances were compared. In addition to the experimental work, an FEM parametric study by an inverse problem-solving technique was used to estimate the mechanical properties of adhesive in both adhesive joints under different shock wave loading conditions. This technique also allowed to estimate the energy absorption capabilities of aluminum-epoxy and steel-epoxy joints.     

Shear properties of isotropic conductive adhesive joints under different loading rates

Epoxy-based conductive adhesives have been widely used in the electronic field given the lead-free development of electronic packaging. The conductive adhesive joints must be subjected to shear loads during the service of electronic products considering the mismatch in mechanical properties between packaged chip and substrate. In this study, INSTRON 5544 universal material testing machine was used for tensile–shear tests of isotropic conductive adhesive joint specimens, which were prepared using pure copper plate adherend in the form of single-lap joints. Four loading rates, that is, 0.05, 0.5, 5, and 10 mm/min, were adopted. The relationship between shear load and displacement of two overlapping copper plates is deduced from a mechanical perspective. A mechanical model of the conductive adhesive shear specimen was developed by introducing dimensionless parameters, which are obtained from interfacial fracture energy and shear strength, to interpret the effect of loading rate on the shear properties of the conductive adhesive specimen considering the loading rate. Results show that this model can effectively reflect the relationship between shear load and displacement in the range of 0.05–10 mm/min.     

Strength of adhesively bonded joints under mixed axial and shear loading

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

Determination of traction-separation laws on an acrylic adhesive under shear and tensile loading

Structural acrylic adhesives are of special interest because those adhesives are cured at room temperature and can be bonded to oily substrates. To use those adhesives widely for structural bonding, it is necessary to clarify the methodology for predicting strengths of bonding structures with those adhesives. Recently, cohesive zone models (CZMs) have been receiving intensive attentions for simulation of fracture strengths of adhesive joints, especially when bonded with ductile adhesives. The traction-separation laws under mode I and mode II loadings require to estimate fracture toughness of adhesively bonded joints. In this paper, the traction-separation laws of an acrylic adhesive in mode I and mode II were directly obtained from experiments using Arcan type adhesively bonded specimens. The traction-separation laws were determined by simultaneously recording the J-integral and the opening displacements in the directions normal and tangential to the adhesive layer, respectively.     

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。     

Thermal peel,warpage and interfacial shear stresses in adhesive joints

Thermal stresses are determined in a single lap joint with identical adherends, which are due solely to temperature changes. The simple bending model used here includes bending and extension of the adherends and extensional and shear strains in the adhesive. The analytical solution shows 'sinusoidal' deformation consistent with warpage (bending) of the adherends due to thermal mismatch. While a modified shear lag model (MSLM) with no adherend bending leads to peak bondline shear stresses which occur only at the ends of the overlap, the bending model shows that such stresses occur not only near the ends, but also at interior points of the overlap region. Results for aluminum adherends and an epoxy adhesive show how the peel, warpage and interfacial shear stresses are distributed over the overlap region.     

Fracture analysis in adhesive composite material/aluminum joints under mode-I loading; experimental and numerical approaches

Assessment and evaluation of fracture characteristics are very important in adhesive joint for achieving a safety mode. In this paper, fracture was investigated in mode-I in adhesive composite material/aluminum alloy joints. To achieve this aim, Double Cantilever Beam (DCB) was used to evaluate fracture in mode-I loading (opening). Bonding was realized by epoxy adhesive as one of the most important and widely used adhesives in aerospace and automotive industries. Modified Beam Theory (MBT) and Compliance Calibration Method (CCM) were formulated to calculate Strain Energy Release Rate (SERR). The obtained experimental results were verified by comparison with Finite Element (FE) analysis. FE results were derived from using Virtual Crack Closure Technique (VCCT) and J-integral approaches in two and three dimension (2-D & 3-D) simulation. Experiment tests and numerical analyses showed good agreement and demonstrated the effectiveness of the proposed experiment and numerical methods.     

Evaluation of a structural epoxy adhesive for timber-glass bonds under shear loading and different environmental conditions

This article presents a study of timber-glass adhesive joints. It examines the shear specimen and shear tools preparation process and the evaluation of the results backed up with an overview of existing similar studies. The chosen adhesive was a cold-curing two-component structural bonding epoxy resin (Mapei Adesilex PG1). The shear tests were performed under different temperatures and the timber samples had different moisture contents. A simple shear test tool was designed and was clamped into a universal testing machine for the shear test. The force and crosshead displacement values from the universal testing machine were used for evaluating the results. The environmental conditions of 20 °C and 5% timber moisture content resulted in the highest average shear strength obtained from the shear tests of the analysed joints (9.89 MPa), whereas the environmental conditions of 50 °C and 20% timber moisture content resulted in the lowest average shear strength (3.42 MPa). It was found that the joint strength is dependent on the environmental temperature and timber moisture content. Moreover, the shear specimen load-displacement behaviour at the environmental temperature of 50 °C was linear and nonlinear – depending on the timber moisture content. The most frequent failure type was timber failure. Additionally, a nonlinear contact finite element analysis was performed to demonstrate the additional shear specimen rotation due to the clearance between the shear specimen and shear tools. This impact was evaluated regarding the stress distribution in the bond line. The evaluated epoxy resin adhesive was proved to be suitable for timber-glass bonds.     

Analysis of aircraft adhesive joints under combined thermal and mechanical cyclic loadings

The effect of thermal aging on the static and fatigue behavior of adhesively bonded aircraft joints has been investigated. The aging cycle consisted of high and low temperatures at different levels of humidity in accordance with ASTM D1183-70 test procedure C for exterior land and air conditions. Single lap joints of aluminum 7075-T6 and 3M structural adhesive prepreg were used. The static loading behavior showed no effect of thermal cycling on the load-carrying capacity of the joints. However, the joints' static deformation increased with increasing number of aging cycles. The specific energy of damage concept was used to extract parameters characteristic of the joint resistance to fatigue loading. These parameters are the specific energy of damage, y', and the energy dissipation coefficient, β'. It was found that the specific energy of damage, γ', was reduced from 35.40 kJ/m³ for joints without thermal cycling to 28.9, 27.1, and 25.9 kJ/m³ for joints subjected to 2, 4, and 6 thermal aging cycles, respectively. However, the energy dissipation coefficient, β', increased from 23 x 10^-5 for joints without thermal aging to 26.5 x 10^-5, 27.6 x 10^-5, and 28.8 x 10^-5 for joints with 2, 4, and 6 aging cycles, respectively. The decrease in the value of y' and the increase in β' indicate a loss of resistance to fatigue crack propagation. The greatest loss of fatigue resistance was encountered after the first two thermal aging cycles. While the static results showed an enhancement in the joints' fracture toughness, based on the area under the load-displacement curve, the fatigue crack propagation data showed a considerable loss in resistance due to aging. Hence this paper emphasizes that it is important to evaluate the resistance of adhesive joints to combined thermal and mechanical cycling if the joints are expected to serve in such conditions.     

Behavior of epoxy adhesive joints under service conditions

The results of testing of epoxy adhesives joints for long-term strength, chatter stability, and water and heat resistance are reported.     

Failure behavior of adhesively bonded joints with a rubber modified epoxy adhesive under tensile-shear combined cyclic loading

Rubber-modified epoxy adhesives are used widely as structural adhesive owing to their properties of high fracture toughness. In many cases, these adhesively bonded joints are exposed to cyclic loading. Generally, the rubber modification decreases the static and fatigue strength of bulk adhesive without flaw. Hence, it is necessary to investigate the effect of rubber-modification on the fatigue strength of adhesively bonded joints, where industrial adhesively bonded joints usually have combined stress condition of normal and shear stresses in the adhesive layer. Therefore, it is necessary to investigate the effect of rubber-modification on the fatigue strength under combined cyclic stress conditions. Adhesively bonded butt and scarf joints provide considerably uniform normal and shear stresses in the adhesive layer except in the vicinity of the free end, where normal to shear stress ratio of these joints can cover the stress combination ratio in the adhesive layers of most adhesively bonded joints in industrial applications.

In this study, to investigate the effect of rubber modification on fatigue strength with various combined stress conditions in the adhesive layers, fatigue tests were conducted for adhesively bonded butt and scarf joints bonded with rubber modified and unmodified epoxy adhesives, wherein damage evolution in the adhesive layer was evaluated by monitoring strain the adhesive layer and the stress triaxiality parameter was used for evaluating combined stress conditions in the adhesive layer. The main experimental results are as follows: S–N characteristics of these joints showed that the maximum principal stress at the endurance limit indicated nearly constant values independent of combined stress conditions, furthermore the maximum principal stress at the endurance limit for the unmodified adhesive were nearly equal to that for the rubber modified adhesive. From the damage evolution behavior, it was observed that the initiation of the damage evolution shifted to early stage of the fatigue life with decreasing stress triaxiality in the adhesive layer, and the rubber modification accelerated the damage evolution under low stress triaxiality conditions in the adhesive layer.     

Electrically conductive adhesive and soldered joints under compression

Compression of soldered and conductive adhesive joints resulted in both reversible and irreversible changes in the contact electrical resistivity. At a low stress (as low as 0.02 and 0.005 MPa for soldered and adhesive joints, respectively), the resistivity increased with almost complete reversibility as the compressive stress increased. At an intermediate stress (as low as 0.03 MPa) for both soldered and adhesive joints, the resistivity decreased with partial or complete reversibility as the stress increased. At a high stress (as high as 0.21 MPa) for the soldered joint only, the resistivity increased slightly with increasing stress. The resistivity of the soldered joint at no load increased irreversibly and gradually as stress cycling progressed, even at the lowest stress amplitude of 0.12 MPa. However, the resistivity of the adhesive joint at no load decreased irreversibly and gradually as stress cycling progressed, even at the lowest stress amplitude of 0.009 MPa.     

Fracture initiation in bi-material joints subject to combined tension and shear loading

Abstract

Linear elastic solution of the stress field near an interface corner of bi-material joints is of the form Hr^λ?1, where r is the radial distance from the corner, H is the stress intensity factor and λ–1 is the order of the singularity. Finite element analysis is used to determine the magnitude of H for a butt joint subject to remote shear; the obtained solution complements existing solution for remote tension and uniform change in temperature. The theoretical solution of the singular shear stress is shown to be in good agreement with the corresponding finite element solution. The effect of combined remote tension, remote shear and uniform change in temperature on the failure loads and failure mechanisms is experimentally determined for brass/araldite/brass butt joint. It is shown that the failure envelope in tensile stress – shear stress space is elliptical and the failure loads decrease with increasing cure temperature due to thermal residual stress associated with the curing process. The application of the results to the assessment of onset of failure in composite patch repair is discussed.     

Effect of adherend's rigidity on the shear strength of single lap adhesive joints

The present paper compares the tensile shear strength of single-lap joints with different adherends. Three materials were combined in the single lap joints: a carbon/epoxy laminated composite, a high elastic limit steel and the 6082-T6 aluminium alloy. The shear strength of joints was influenced by the adherend stiffness and the highest shear strengths were obtained using high stiffness adherend materials. The overlap length influenced the shear strength in different ways depending on the adherend materials. Numerical analysis concluded that the increase in the rigidity of the adherends decreases the rotation of the specimen and promotes a more uniform distribution of stresses in the glue. In joints with distinct materials, the less stiff material was found to determine the strength of the appropriate joint.     

胶接接头在静态负载条件下的蠕变和松弛行为

Modern high performance adhesives are designed to offer an optimized balance of elasticity,toughness and plastic deformation capacity for the individual fields of application in e.g. the building and construction or transportation and vehicle industry. The long-term life prediction for adhesive joints based on laboratory tests requiring only days,weeks,or months is still a demanding challenge. Testing in practice is carried out with the intention of accelerating time dependent aging effects that may occur in a bonded joint during its service time. Initial strength values of bonded joints,such as shear or peel properties can often be obtained from the adhesive manufacturers or retrieved from literature. They are useful to compare different adhesives and to demonstrate the effect of parameters such as bond line thickness,overlap length or curing conditions,and,in some cases,the surface state. On the other hand only few data are available describing the mechanical long-term properties of adhesives related to creep and relaxation under static load conditions. Due to the nature of the polymer network of organic adhesives their viscoelastic-plastic deformation behavior is strongly time-and temperature dependent. The objective of this paper is to illustrate effective methods for investigating and predicting the creep and relaxation properties of adhesively bonded joints in the long-term region and for creating basic data for the design and engineering with adhesives.     

Composite bonded joints under mode I fatigue loading

Experimental investigation on fatigue behavior of carbon-epoxy composite bonded joints under mode I loading was performed in this work. The objective is to evaluate the performance of different data reduction schemes to obtain the energy release rate (G_I) in the fatigue crack growth (FCG) rate using double cantilever beam (DCB) specimens. This law relates the evolution of the crack along time as a function of the energy release rate (G_I) and is generally composed of three different regions: damage nucleation, stable propagation and abrupt final failure. The second phase corresponding to stable propagation leads to a linear trend on the Paris law representation (log-log scale) and must be well characterized to define the fatigue behavior of the structure. During fatigue tests the classical methods require rigorous monitoring of the crack length during its propagation, which is cumbersome and not easy to perform in some materials. In this work, an alternative data reduction scheme based on specimen compliance and crack equivalent concept is proposed to overcome this difficulty. The results provided by the proposed method, namely Compliance Based Beam Method (CBBM), are compared to the ones obtained from the polynomial and Beam on Elastic Foundation Method (BEFM), both of which require crack monitoring. The first is a compliance calibration method that fits a third-order polynomial curve to the experimental results (compliance (C) versus crack length (a)). The second one uses the beam theory to establish the C=f(a) relationship taking into account the properties of the adhesive. One additional advantage can be pointed to the proposed CBBM relative to the other classical methods. In fact, the equivalent crack is related to the specimen compliance, thus taking into account the influence of fracture process zone on specimen behavior. This issue is particularly important when adhesives with some ductility are being characterized in fatigue tests.     

Strength model of adhesive bonded composite pipe joints under tension

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

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.     

The behaviour of single lap joints under bending loading

Four-point bend tests were performed on single lap joints with hard steel adherends and a structural epoxy adhesive. The effect of the overlap, the adherend thickness and the adhesive thickness was studied. It was found that the length of the overlap has no significant effect on the strength of the joints. This is because the load transfer is occurring in a very localised area around the edges of the overlap, being the failure governed by peel mechanisms. The thickness of the adherends strongly affects the strength of the joints. The thicker the adherend, the stronger is the joint. The experimental results are compared with a finite element model and reinforce the fact that the failure takes place due to local strains at the ends of the overlap in tension. An analytical model is also given to predict in a simple but effective way the joint strength and its dependence on the adherend thickness.     

Fracture of adhesive joints

This paper presents some of the important results obtained from a series of studies on cohesive fracture in adhesively bonded joints. Due to the complex nature of the adhesive joint fracture, an accurate and efficient numerical method particularly suitable for the present problem has been developed, based on a hybrid-stress finite element formulation. Fracture characteristics in adhesively bonded joints are described in terms of local crack-tip deformation and stress fields in the polymeric adhesive layer. Effects of material properties, joint geometry, and bond-line thickness on the crack behavior are studied for classical lap-shear and currently used double-cantilever-beam joints. Of particular interest are the crack-tip stress intensities in the adhesive layer; their values are obtained for several cases of practical importance.