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.     

Effect of reinforcements at different scales on mechanical properties of epoxy adhesives and adhesive joints: a review

Improvement of the mechanical properties of adhesives and adhesive joints has been a subject of great interest in recent years. Up to now, several methods have been presented such as modifying substrate shapes, adding microparticles (MPs) and nanoparticles (NPs), and embedding micro and macrofibers in the adhesive layer. This review aims to investigate how these reinforcements of different scales in the adhesive layer influence the mechanical properties of adhesive joints and adhesives. Characteristics and applications of reinforcements are introduced in the first part. In the second part, the effects of several parameters commonly investigated by researchers on the strength, stiffness and fracture toughness improvement of polymeric materials are reviewed for reinforcements of different scales. Finally, damage mechanisms involved in increasing or decreasing the mechanical properties are reviewed and discussed.     

New Developments in Structural Adhesives for the Automotive Industry

Recently developed epoxy paste adhesives, reactive hot melts, adhesive film tape and polyurethane adhesives are presented for structural bonding in the automotive industry. Paste adhesives usually require a precure stage to obtain handling strength of the joints and to guarantee wash-out resistance of the adhesive in the paint baths. This step can be omitted with reactive hot melts and adhesive film tape, which are solid before and after their application. In addition they allow an improved working hygiene. Some mechanical properties of the adhesives are shown such as lap shear strength and peel strength as well as lap shear strength as a function of the bondline thickness. Results of the excellent durability of epoxy one-component pastes, reactive hot melts and adhesive film tape are given from cyclic environmental and salt spray tests.     

A Thermo-Mechanical Study of Some Epoxy Adhesives Subjected to Combined Dynamic and Static Stresses

The object of the present study was to investigate the effect of superimposed dynamic and static stresses on mechanical and thermal properties of some epoxy adhesives. It was found that combinations of shear creep and torsional oscillations, applied simultaneously to adhesive joints at temperatures within the glassy range of the adhesive, led to strengthening of the joints in shear and to an increase in the glass transition temperature of the adhesive. Similar loading stresses applied at temperatures close to the T_g of the adhesive, led to opposite effects on the above mentioned properties of the joints. The width of the glassy-rubbery transition of the adhesives increased, in the whole range of temperatures used in this study and for all epoxy compositions, as a result of subjecting the joints to superimposed dynamic and static stresses. The broadening of the glass transition was interpreted as a result of the stiffening of polymer network during the combined stressing experiments. A linear relationship was found between the area of endothermal peaks in the first DSC scan of specimens subjected prior to test to superimposed dynamic and static stresses at temperatures below T_g, and the shear strength of the joints. In agreement with this observation and with literature data, a linear relationship was revealed also between the glass transition temperature of the resins (measured in the first DSC scan) and the shear strength of the joints. Based on experimental observations and on some literature information, it was suggested that the strengthening of the joint, as well as the changes in thermal properties of the adhesives, are mainly due to physical processes, such as short-range orientation of network chains and an increase in intermolecular interaction between highly polar sites of the network. The possibility that superimposed stressing led to changes in chemical crosslinking was discussed but it seems that no such reactions occurred.     

Review on adhesive joints and their application in hybrid composite structures

Composites have been used extensively in various engineering applications including automotive, aerospace, and building industries. Hybrid composites made from two or more different reinforcements show enhanced mechanical properties required for advanced engineering applications. Several issues in composites were resolved during the last few years through the development of new materials, new methods and models for hybrid joints. Many components in automobile are joined together either by permanent or temporary fastener such as rivets, welding joint and adhesively bonded joints. Increasing use of bonded structures is envisaged for reducing fastener count and riveted joints and there by drastically reducing assembly cost. Adhesive bonding has been applied successfully in many technologies. In this paper, scientific work on adhesively bonded composites and hybrid composites are reviewed and discussed. Several parameters such as surface treatment, joint configuration, material properties, geometric parameters, failure modes, etc. that affect the performance of adhesive bonded joints are discussed. Environmental factors like pre-bond moisture and temperature, method of adhesive application are also cited in detail. A specific case of adhesive joints in hybrid bonded-bolted joints is elaborated. As new applications are expanding in the field of composites joining and adhesive joints, it is imperative to use information on multiple adhesives and their behaviour in different environmental conditions to develop improved adhesive joint structure in mechanical applications.     

Influence of the mechanical behaviour of different adhesives on an interference-fit cylindrical joint

Hybrid adhesive joining techniques are often used in many industrial sectors to design lightweight structures. A hybrid adhesive joint results from the combination of adhesive bonding with other traditional joining methods such as welding and mechanical fastening, with the aim of combining the advantages of the different techniques and overcoming their drawbacks.This study focuses on the interference fitted/adhesive bonded joining technique. In this application, two cylindrical components are coupled together by inserting one into the other, after having placed an adhesive between them. Generally anaerobic acrylic adhesives, also known as “retaining compound” are used for this application. However the effect of the adhesive nature and of its mechanical and adhesive responses on the performance of the hybrid joint is still unclear. The aim of the present research is to improve the understanding of the behaviour of different adhesives, including rigid epoxies and flexible polyurethanes, in the presence of an interference-fit. Static strength of bonded and unbonded interference fit joints have been compared in order to investigate the role of the different adhesives.     

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.     

Analytical modelling of the automated dispensing of adhesive materials

The integrity of joints is a critical issue in structural bonding and the shape of adhesive beads needs to be consistent. The right amount of adhesive must be dispensed and to ensure this, the dispensing process must be automated. To have a fully automated dispensing cell, the use of a knowledge-based expert system for set-up and diagnosis is appropriate. To achieve this, the modelling of dispensing flow of adhesive material is essential to ensure that the correct sized bead is dispensed at a variety of robot speeds. For a given adhesive, it is often necessary to perform many ad hoc adjustments and tests to produce optimal adhesive beads at different operational and environmental conditions. To avoid this, the rheological behaviour of the adhesives is analysed, to find a constitutive equation for representing an adhesive model which best describes the way that different types of adhesives flow under a variety of conditions. Modelling the material properties provides an initial approximation of the flow rate for dispensing the adhesive. The automated dispensing cell can then adapt to the operational conditions where the model also provides for its initial set-up and diagnosis. This modelling method can also be applied to similar adhesives with known rheological data and nozzle flow rate.     

Investigating tensile behaviour of toughened epoxy paste adhesives using circumferentially notched cylindrical bulk specimens

Toughened epoxy adhesives are frequently used to bond metals and polymer-matrix composite materials in many structural applications. The mechanical properties of adhesives are often characterised by testing either bulk adhesive specimens or bonded joints (i.e. in-situ form). In this paper, cylindrical bulk specimens with circumferential notches were manufactured and tested to investigate the tensile behaviour of an epoxy paste adhesive toughened with hollow glass microspheres. Bulk specimens were manufactured from the paste adhesive using injection moulding. Tensile tests were conducted for strain-rate and stress triaxiality effects by varying displacement rates and notch radii, respectively. Fracture surfaces were examined using optical and scanning electron microscopy to identify failure mechanisms. The results obtained from the toughened paste adhesive indicate that strain-rate and stress triaxiality influence its tensile fracture behaviour.     

Assessments for impact of adhesive properties: modeling strength of metallic single lap joints

Adhesively bonded joints are widely used in a variety of industrial and engineering activities. Their overall strength is dependent on the properties of the adhesives. In the present research, assessments of adhesive properties were performed systematically through defining both strength mixity and energy rate mixity and using them to characterize the overall strength of metallic single lap joints. By means of the cohesive zone model, the adhesive strength mixity was defined as the ratio of the shear and tensile separation strength, and the energy rate mixity was defined as the ratio of the area below the shear cohesive curve and the area below the tensile cohesive curve. For each specified group of mixity parameters, corresponding to the properties of a specified adhesive, the overall strengths and the critical displacements of bonded joints were characterized. A series of strength and energy rate mixities were taken into account in the present calculations. A comparison of the present calculations with some existing experiments was carried out for both brittle and ductile adhesives. Finally, in the calculations presented here, damage initiation and evolution of the adhesive layer were also undertaken. The results showed that the overall strength of the joints was significantly depended on the adhesive properties, which were characterized by the strength and energy rate mixities of the adhesive. Furthermore, the shear adhesive stress components played a dominate role in both the damage initiation and evolution in the adhesives, which were also affected by the overlap length of the joints.     

Adhesion of modified polymers to fibres: Maxima on adhesive strength-modifier amount curves and the causes of their appearance

Adhesion of modified polymers to fibres is considered. The dependences of the shear adhesive strength of polymer/fibre joints on the modifier content are analysed. It is shown that an increase in the adhesive strength can often be achieved by adding a component with a lower adhesive ability to the basic adhesive. As a result, a maximum strength is observed for a certain concentration range of the minor component. This paradoxical phenomenon can be explained with the use of simple qualitative reasoning, which is based on the analysis of the concentration dependences of surface and bulk properties. By the example of the ED-20 epoxy resin (analog of DGEBA), competitive processes whose superposition produces such a maximum are shown to be different for one-phase and two-phase adhesives.     

The Effect of Adhesive Thickness on the Mechanical Behavior of a Structural Polyurethane Adhesive

One parameter that influences the adhesively bonded joints performance is the adhesive layer thickness. Hence, its effect has to be investigated experimentally and should be taken into consideration in the design of adhesive joints. Most of the results from literature are for typical structural epoxy adhesives which are generally formulated to perform in thin sections. However, polyurethane adhesives are designed to perform in thicker sections and might have a different behavior as a function of adhesive thickness. In this study, the effect of adhesive thickness on the mechanical behavior of a structural polyurethane adhesive was investigated. The mode I fracture toughness of the adhesive was measured using double-cantilever beam (DCB) tests with various thicknesses of the adhesive layer ranging from 0.2 to 2 mm. In addition, single lap joints (SLJs) were fabricated and tested to assess the influence of adhesive thickness on the lap-shear strength of the adhesive. An increasing fracture toughness with increasing adhesive thickness was found. The lap-shear strength decreases as the adhesive layer gets thicker, but in contrast to joints with brittle adhesives the decrease trend was less pronounced.     

Medical-grade acrylic adhesives for skin contact

Pressure-sensitive acrylic adhesives for application to skin are made from 2-ethylhexyl acrylate, isooctyl acrylate or n-butyl acrylate copolymerized with polar functional monomers such as acrylic acid, methacrylic acid, vinyl acetate, methyl acrylate, N-vinylcaprolactam, or hydroxyethyl methacrylate. Functional comonomers increase cohesive strength, provide surface polarity, and enhance wear performance. Tack, adhesion to skin, adhesive transfer to skin, and wear performance of the adhesive are governed by the molecular weight, glass transition temperature, and the viscoelastic behavior of the adhesive. Viscoelastic properties of the adhesive as measured by the Williams plasticity number (WPN), dynamic storage modulus (G′), dynamic loss modulus (G″), and tan δ are important polymer properties for good wear performance. Sweating skin, a moist environment, and physical activity are the most important factors influencing the failure of an adhesive tape during wear. A medicalgrade adhesive for application to human skin should be hypoallergenic. Medical-grade adhesives are utilized in making surgical tapes for holding dressings in place, adhesive bandages, adhesive dressings to cover wounds, and surgical operating drapes.     

Corrosion properties of adhesives

Advantages and disadvantages of the anodic oxidation method, which is widely used for the preparation of metal and alloy surfaces for adhesive bonding, are considered. A new technology is proposed for the preparation of metal surfaces using a combined electrolyte and an organic corrosion inhibitor. The advanced technology permits increasing the moisture resistance of adhesive joints by three to five times. The results of comparative tests of corrosion activity of a large body of adhesives based on modified epoxy and phenol-formaldehyde oligomers, as well as rubbers, are described. Phenol.-rubber adhesives are shown to possess protective properties and demonstrate corrosion activity, thus increasing the durability of adhesive joints.     

Effect of using sepiolite silicate + fumed silica mixtures as fillers on the characteristics of solvent-based polyurethane adhesives

Abstract —Both fumed silica and sepiolite have been used as a filler of polyurethane (PU) adhesives. Although effective, the small particle size and the relative high cost of fumed silica are limitations in some applications. Sepiolite is cheaper than fumed silica, but its relatively large particle size facilitates its settling from the adhesive solutions. In this study, the usefulness of using sepiolite + fumed silica mixtures as a filler in solvent-based PU adhesives is demonstrated. The rheological and adhesion properties of the PU adhesive solutions and the rheological and mechanical properties of the PU films (without solvent) were studied. SEM micrographs of PU films showed the morphology and compatibility of the fillers with the PU matrix. The use of sepiolite + fumed silica mixtures inhibited the settlement of the filler from the PU adhesive solutions, increased both the storage and the loss moduli, and improved the rheological and mechanical properties of the PU. On the other hand, the green (immediate) T-peel strengths of roughened styrene-butadiene rubber/PU adhesive joints and plasticized PVC/PU adhesive joints were greatly improved in filled PU adhesives. The effects produced by using fumed silica alone or sepiolite + fumed silica mixtures were very similar, although in general, somewhat more marked in fumed silica-filled PU.     

A study on durability of joints bonded with pressure-sensitive adhesives

The use of adhesive bonding as fastener presents very high usage potential, especially for different environments. However, the designers do not have enough data yet to address the changes in the mechanical properties of adhesives in the course of time. In this study, the ageing of film-type adhesives, Structural Bonding Tapes (SBT) 9244 and 9245, which are different in thickness and possess pressure-sensitive and visco-elastic properties, was investigated. First, the bulk adhesive specimens were prepared and exposed to two different environmental conditions until saturated. In the same time, single lap joints with two different adhesive layer thicknesses, consisting of AA2024-T3 as the adherend, were exposed to the same environmental conditions for exposure times of 90 days. The immersion environments were 100% relative humidity (RH) and 3.5% NaCl solution. At the end of exposure time, the failure surfaces were examined by Scanning Electron Microscopy (SEM) after the strength of joints was determined with the lap shear test. Both SBT adhesives absorb more water from 100% RH than from 3.5% NaCl solution and, therefore, they deteriorate rapidly in 100% RH. In addition, as the thickness of adhesive layer increases, the loss in the strength increased.     

Boundary Layers and Physico-Mechanical Properties of Adhesive Joints: Part I Experimental Background

Abstract

Various hypotheses of boundary layer formation in adhesive joints are reviewed. The features of boundary layers in joints obtained by means of thermoplastic and thermoreactive adhesives are studied, and substrate boundary layers are estimated. The examples of influence of density, elasticity modulus and other properties of boundary layers upon adhesive joint properties are presented.

An original method to design adhesive joints is proposed, based on the concept of special boundary layer properties. By means of this method the problem of mechanical performance of an adhesive lap shear joint is solved. The effects of various parameters of the theoretical model are compared with the experimental data.     

Tensile stress–strain behaviour of a structural bonding tape

The tensile properties of a pressure-sensitive adhesive tape which can be cured to impart structural performance have been measured at different temperatures using quasi-static tests. In order to assess the linear viscoelastic behaviour of the tape, results from the vibrating beam technique are also presented. Bulk specimens and non-contacting extensometers were found to be suitable for tensile measurements on such a flexible adhesive. The results show that the strength of the tape is not as high as, and that its behaviour is very different from, traditional structural adhesives, but its high strain-to-failure with an adequate strength offers exciting possibilities for structural 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.     

Shear strength of adhesively bonded polyolefins with minimal surface preparation

Interest in polyethylene and polypropylene bonding has increased in the last years. However, adhesive joints with adherends which are of low surface energy and which are chemically inert present several difficulties. Generally, their high degree of chemical resistance to solvents and dissimilar solubility parameters limit the usefulness of solvent bonding as a viable assembly technique. One successful approach to adhesive bonding of these materials involves proper selection of surface pre-treatment prior to bonding. With the correct pre-treatment it is possible to glue these materials with one or more of several adhesives required by the applications involved. A second approach is the use of adhesives without surface pre-treatment, such as hot melts, high tack pressure-sensitive adhesives, solvent-based specialty adhesives and, more recently, structural acrylic adhesives as such 3M DP-8005^® and Loctite 3030^®.In this paper, the shear strengths of two acrylic adhesives were evaluated using the lap shear test method ASTM D3163 and the block shear test method ASTM D4501. Two different industrial polyolefins (polyethylene and polypropylene) were used for adherends. However, the focus of this study was to measure the shear strength of polyethylene joints with acrylic adhesives. The effect of abrasion was also studied. Some test specimens were manually abraded using 180 and 320 grade abrasive paper. An additional goal of this work was to examine the effect of temperature and moisture on mechanical strength of adhesive joints.