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.     

Three-Dimensional Finite Element Analysis of the Stress Distribution in Bi-Adhesive Bonded Joints

A 3-D elastic finite element model was developed to investigate the stresses distribution of bi-adhesive bonded joints (i.e., the bond line of joints filled with two adhesives of dissimilar toughness). The effects of the loading mode on the stress distribution of joints, including the single-lap joints under tensile loading (i.e., single-lap joints) and the butt joints under cleavage loading (i.e., cleavage joints), were also studied in detail. Results showed that higher stress, distributed at the contact position of the dissimilar adhesives placed along the bond line of bi-adhesive bonded joints. Also, the maximum stress of the adhesive layer decreased when the length ratios and bonding sequence along the bond line, filled with two dissimilar adhesives, was appropriately designed. At the same time, stress convergence in the adhesive layer of bi-adhesive joints was also obviously reduced in contrast to the mono-adhesive joints. The numerical investigation shows that it is necessary to take into account the change of loading modes when optimizing the bi-adhesive joint design, because of the uneven and complex loading modes of the adhesive bonding structure in the engineering applications.     

Investigation of the Mechanical Properties of Interphases in Adhesively Bonded Epoxy-Aluminum Joints by Localized Micro Extensometry

In recent papers it has been reported that epoxide-based adhesives form so-called interphases in adhesive joints. The properties of these interphases play an important role concerning the performance and durability of structural adhesive joints under detrimental service conditions. In this paper, the formulation of a basic epoxy adhesive and a methodology for gaining insight into the local mechanical properties of polymer interphases in structural adhesive joints made with ambient temperature curing epoxy adhesive are presented. The localized strain analysis in the cross-section of shear-loaded adhesive joints is accomplished by combining a high precision micro tensile tester with a digital microscope and by developing a method for preparing, marking, and digitally tracking the local deformations in a micro shear specimen.     

The Impact Resistance of Structural Adhesive Joints

The mechanical behaviour of structural adhesives and adhesive joints under impact loading is of growing interest as adhesives are used increasingly in the construction of vehicles ranging from the family motor car to large trucks and buses. The present paper describes some initial work on the development of an instrumented impact test to study the impact behaviour of epoxy adhesives and the use of a linear-elastic fracture-mechanics approach to characterising the fracture properties.     

The Impact Resistance of Structural Adhesive Joints

The mechanical behaviour of structural adhesives and adhesive joints under impact loading is of growing interest as adhesives are used increasingly in the construction of vehicles ranging from the family motor car to large trucks and buses. The present paper describes some initial work on the development of an instrumented impact test to study the impact behaviour of epoxy adhesives and the use of a linear-elastic fracture-mechanics approach to characterising the fracture properties.     

Intrinsic mechanical characterization of structural adhesives

Methods and specimens are described for obtaining the mechanical characteristics of structural adhesives needed by engineers for the design and analysis of adhesive-bonded structural joints. These mechanical characteristics can be obtained on bulk adhesives (in tension and compression loading) or on adhesive joints in shear loading. The first experimental results confirm those of other authors that the mechanical behaviour of the bulk adhesive material is similar to that in bonded joints.     

Assessment and prediction of long-term mechanical properties of adhesives with high plasticity

Long-term life prediction for adhesives based on laboratory tests requiring only days, weeks, or months, is still a demanding challenge. Testing is carried out with the intention of simulating and often accelerating time-dependent aging effects that may occur in a joint during its lifetime. Initial strength values of bonded joints, such as shear or peel properties, can often be obtained from the adhesive manufacturers or retrieved from the literature. They are useful to compare different adhesives and to demonstrate the effect of parameters such as bondline thickness, overlap length or curing conditions and, in some cases, also the surface state. On the other hand, only few data are available relative to the long-term mechanical properties of adhesives with high plasticity. Due to the specific network structure of elastomer adhesives their viscoelastic-plastic deformation behavior is strongly time- and temperature-dependent. Therefore, the main objective of this paper is to illustrate methods for investigating the viscous properties of elastomer-based adhesives and creating basic data for the design and engineering of adhesive joints with enhanced plasticity.     

An experimental method dedicated to the dynamic characterization of structural adhesives under drop weight conditions

Adhesive bonding is a commonly used method in multi-materials assemblies dedicated to the transport fields. In order to ensure structures integrity and users safety, the knowledge of the mechanical behaviour of structural adhesives used in these assemblies under impact conditions appears to be an essential prerequisite. To date, numerous tests combining usual specimens geometry e.g. single lap joint, butt joint, etc. and high velocity testing rigs exist and are used. Among these, most allow comparative studies and a few provide a partial identification of the material properties of the investigated adhesive.In this study, an experimental method dedicated to the dynamic characterization of structural adhesives under drop weight condition is proposed. On the basis of existing works, a modified Arcan specimen and a dynamic tensile testing mean were developed and are presented. The Arcan geometry allows to test the adhesive under various loading directions and so to obtain its mechanical response envelope. Design strategies are also implemented in order to obtain time stable and quasi-homogeneous stress distributions in the adhesive during the tests.At last, the dynamic characterization of a Dow^® Betamate^TM 1496 V adhesive is proposed. Results are repeatable and show a strain rate dependent behaviour validating the appropriateness of the experimental approach.     

Behaviour of environmentally degraded epoxy adhesives as a function of temperature

ABSTRACT

Structural adhesives are increasingly being used in the aerospace and automotive industries. They allow for light weight vehicles, fuel savings, and reduced emissions. However, the environmental degradation of adhesive joints is a major setback in its wide implementation. Moisture degradation of adhesive joints includes plasticization, attacking of the interface, swelling of the adhesive and consequent creation of residual stresses. This may lead to reversible and irreversible damage. The main factors affecting the strength of adhesive joints under high and low temperatures are the degradation of the adhesive mechanical properties and the creation of residual stresses induced by different coefficients of thermal expansion (between the adhesive and the adherends). The effect of the combined effect of moisture and temperature is not yet fully understood. The aim of this study is to shed light on this subject.

In this work bulk water absorption tests were conducted at different moisture conditions in order to assess the diffusion coefficient, maximum water uptake, and glass transition temperature. Aged and unaged small dogbone tensile specimens were tested under different temperature conditions. The glass transition temperature of the adhesives as a function of the water uptake was assessed. The aim is to determine the evolution of the properties of two epoxy adhesives as a function of two variables (environmental temperature and moisture).     

水性环氧树脂包覆有机蒙脱土制备木材胶粘剂

摘要：水性环氧树脂通常含有水溶性分子或分子链，导致在高温和潮湿条件下作为木材胶粘剂时耐水性及力学性能较差。采用有机改性的纳米蒙脱土改性水性环氧树脂增强水性环氧树脂胶粘剂的耐水性及力学性能。并通过乳液包覆蒙脱土的方法与直接共混的方法对比，研究了不同添加量有机蒙脱土（0%，3%，6%，9%）对胶粘剂性能的影响。胶粘剂的耐水性及力学性能通过测量胶粘剂在干燥及潮湿条件下的剪切强度来表示。通过TGA、SEM、TEM、DSC研究了复合胶粘剂的热稳定性和结构。结果表明，在水性环氧树脂中添加有机改性的纳米蒙脱土，可以有效地提高胶粘剂的粘结强度，此外，采用乳液包有机覆蒙脱土的方法比直接共混的方法制备得到胶粘剂，有机蒙脱土在胶粘剂中分布更均匀，具有更优异的力学性能，说明有机蒙脱土在复合材料中的分散质量是影响复合胶粘剂性能的主要原因。     

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.     

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.     

Dynamic strength of single lap joints with similar and dissimilar adherends

The increased use of adhesives for joining structural parts demands a thorough understanding of their load carrying capacity. The strength of the adhesive joints depends on several factors such as the joint geometry, adhesive type, adherend properties and also on the loading conditions. Particularly polymer based adhesives exhibit sensitivity to loading rate and therefore it is important to understand their behavior under impact like situations. The effect of similar versus dissimilar adherends on the dynamic strength of adhesive lap joints is addressed in this study. The dynamic strength is evaluated using the split-cylinder lap joint geometry in a split Hopkinson pressure bar setup. The commercial adhesive Araldite 2014 is used for preparing the joints. The adherend materials considered included steel and aluminum. The results of the study indicated that the dynamic strength of the lap joint is influenced by the adherend material and also by the adherent combination. Even in the case of joints with similar adherends, the strength was affected by the adherend type. The strength of steel–steel joints was higher than that for aluminum–aluminum joints. In the case of dissimilar adherends, the strength was lower than that of the case of similar adherends. The results of this study indicate that the combination of adherend material should also be accounted for while designing lap joints.     

Cohesive and continuum mixed-mode damage models applied to the simulation of the mechanical behaviour of bonded joints

The objective of this work is to discuss the adequacy of cohesive and continuum damage models for the prediction of the mechanical behaviour of bonded joints. A cohesive mixed-mode damage model appropriate for ductile adhesives is presented. The double cantilever beam and the end-notched flexure tests are proposed in order to evaluate the cohesive properties of the adhesive as a thin layer under mode I and mode II, respectively. A new data reduction scheme based on the crack equivalent concept is also proposed to overcome crack-monitoring difficulties during propagation in these fracture characterization tests. An inverse method to determine the cohesive parameters of the trapezoidal softening law is discussed. A continuum mixed-mode damage model is developed in order to better simulate the cases where adhesive thickness plays an important role. The model is applied to evaluate the effect of adhesive thickness on fracture characterization of adhesive joints. Some important conclusions about the advantages and drawbacks of cohesive and continuum damage models are reported.     

Effect of Adhesive Thickness on Joint Strength: A Molecular Dynamics Perspective

Recent studies suggest that adhesion in thin joints depends on several factors including temperature, interface toughness, strain rate, surface roughness of adherends, bondline thickness of adhesives, and many others. Influence of thickness on joint properties is surprising but experimentally well documented without reasonable explanations. In this study, we attempt to address the mechanical behavior of polymer adhesives by molecular dynamics (MD) simulation. We show that interfacial strength of the joints in tensile, shear, or combined loading significantly depends on the coupling strength between adhesives and adherends. Failure of joints is always at the interface when coupling strength is weaker. With stronger interfaces, cohesive failure occurs by cavitation or by bulk shear depending on the loading condition. When joints are loaded in tension, it requires an exceedingly stronger interface to realize pure shear failure, otherwise failure is through interface slip. Under a mixed mode condition, interface slip is difficult to avoid. As long as failure is not at the interface alone, the yield strength of joints improves significantly with the reduction of thickness. Increase in bulk density and change in polymer configurations with the reduction of adhesive thickness are believed to be the two key factors in improving mechanical behavior of adhesives.     

Experimental & numerical study of the Tensile/Compression-Shear Arcan test under dynamic loading

The characterization of the adhesive of bonded assemblies under combined and dynamic loading cases appears to be crucial for the development of the future structures dedicated to the transport industry. To date, most of the tests on adhesive joints are dedicated to comparative studies and only a few ones to characterization. Among these, the stress concentration-free bonded Arcan Tensile/Compression-Shear test specimen (Arcan TCS) developed by Créac’hcadec et al. allows to characterize the adhesive of bonded joints under combined quasi-static loading cases while minimizing the edge effects. This paper deals with an extension of the use of this specimen under dynamic loadings.In a first part, an experimental study of the Arcan TCS device under drop weight conditions is made. The mechanical behaviour of the adhesive appears to be non-linear and clearly dependent of the strain rate. Also, stress-strain curves highlight a significant influence of tests conditions. In particular, the way the kinetic energy is transmitted by the falling mass to the testing device plays a significant role on the vibrational behaviour and the loading rate of the specimen.In a second part, a dedicated finite element model is built under the plane stress and elastic assumptions. Results extracted from this numerical study are in agreement with several experimental observations. Moreover, they allow a better understanding of the loading seen by the adhesive.     

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.     

Influence of Fabrication Residual Thermal Stresses on Rubber-toughened Adhesive Tubular Single Lap Steel-Steel Joints under Tensile Load

The tensile load bearing capability of adhesively-bonded tubular single lap joints which is calculated under the assumption of linear mechanical adhesive properties is usually much less than the experimentally-determined because the majority of the load transfer of adhesively-bonded joints is accomplished by the nonlinear behavior of rubber-toughened epoxy adhesives. Also, as the adhesive thickness increases, the calculated tensile load bearing capability with the linear mechanical adhesive properties increases, while, on the contrary, the experimentally-determined tensile load bearing capability decreases.

In this paper, the stress analysis of adhesively-bonded tubular single lap steel-steel joints under tensile load was performed taking into account the nonlinear mechanical properties and fabrication residual thermal stresses of the adhesive. The nonlinear tensile properties of the adhesive were approximated by an exponential equation which was represented by the initial tensile modulus and ultimate tensile strength of the adhesive.

Using the results of stress analysis, the failure criterion for the adhesively-bonded tubular single lap steel-steel joints under tensile load was developed, which can be used to predict the load-bearing capability of the joint. From the failure criterion, it was found that the fracture of the adhesively-bonded joint was much influenced by the fabrication residual thermal stresses.     

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.     

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.