The Effect of Temperature on the Strength of Adhesively-Bonded Composite-Aluminium Joints

Different materials have different coefficients of thermal expansion, which is a measure of the change in length for a given change in temperature. When different materials are combined structurally, as in a bonded joint, a temperature change leads to stresses being set up. These stresses are present even in an unloaded joint which has been cured at say 150°C and cooled to room temperature. Further stresses result from operations at even lower temperatures.

In addition to temperature-induced stresses, account also has to be taken of changes in adhesive properties. Low temperatures cause the adhesive to become more brittle (reduced strain to failure), while high temperatures cause the adhesive to become more ductile, but make it less strong and more liable to creep.

Theoretical predictions are made of the strength of a series of aluminium/CFRP joints using three different adhesives at 20°C and 55°C. Various failure criteria are used to show good correlation with experimental results.     

The Effect of PEEK Fibres and Powder on Joints Made with a High Temperature Adhesive

High temperature adhesives typically exhibit low levels of peel strength since they tend to be more brittle than typical toughened adhesives used for lower temperature applications. It was found that incorporating thermoplastic fibres or powder into the bondline of a joint made with a high temperature epoxy-based adhesive resulted in significant improvements in peel strength. Poly(ether ether ketone) (PEEK) fibres and powder were incorporated into the adhesive resin and used in aluminium joints. These were tested in peel and single lap shear using a range of fibre lengths, orientations and volume fractions. It was seen that large increases in peel strength could be achieved but that lap shear strength was degraded with most types of modification. However, some modifications resulted in significant increases in peel strength with limited decrease in lap shear strength. These improved properties have been achieved using physical modifications rather than chemical alteration of the resin.     

Effect of Temperature on the Ultimate Strength and Modulus of Whisker-Reinforced Ceramics

Whiskers are a very attractive means of reinforcing a ceramic material. It has been shown that the whiskers dramatically improve the ultimate strength and modulus of the materials at room temperature. However, recent studies indicate that at high temperatures the improvement is less pronounced, or there is no improvement at all. In this paper a model is developed to explain why the properties are degraded at high temperature. The parameters which seem most important for high-temerature performance are the coefficients of thermal expansion and Poisson's ratios for the constituents, the heat treatment temperature, and the cofficeint of friction between the whisker and the matrix. The model compares favorably with the experimental data available, so a parametric study was done to show the effect of different parameters on the strength and modulus of the composite.     

Predicting the Fatigue Life of Adhesively-Bonded Joints

The fatigue behaviour of adhesively-bonded joints, which consisted of an epoxy-film adhesive bonding fibre-composite substrates, has been studied. Using a double-cantilever beam specimen, the rate of crack growth per cycle has been measured as a function of the maximum strain-energy release rate, G_max. These data have then been modelled, and used to predict the fatigue lifetime of bonded single-lap joints. The agreement between the theoretical predictions and experimental results for the fatigue behaviour of the single-lap joints was found to be excellent.     

温度变化对复合材料双面搭接接头拉伸强度的影响分析

介绍了复合材料三种连接方式之一——胶接。对双面搭接接头进行了有限元建模,在不同的高温环境下进行拉伸试验,运用相关的失效准则,分析判断复合材料胶接接头的破坏形式,确定温度对破坏形式的影响。通过试验,验证了分析的正确性,对复合材料胶接接头的强度校核及设计改进有重要的指导作用。     

磷化处理对金属胶接接头拉剪强度的影响

以结构钢为研究对象,对常用磷化液的各主要成份对胶接接头拉剪强度的影响进行了研究并进行了优化,结果指出,采用本文的磷化液对被粘件进行表面处理,可使胶接接头的拉剪强度在现有常规处理工艺的基础上得到一定程度的提高。     

Effect of the Engagement Ratio and of Temperature on the Shear Strength of Epoxy Adhesive Bonded Aluminum Alloy Pin-and-Collar Joints

ABSTRACT

Epoxy adhesives are widely used in industrial applications, as they are particularly suitable to bond many types of materials. Conversely, possible drawbacks may arise from the use under high temperature, which is likely to imply a drop of mechanical properties. Previous research indicated that the Engagement Ratio (ER), namely, the ratio between the joint length and its coupling diameter, has an effect on the shear strength of an epoxy adhesive applied to steel adherents. Moreover, the shear strength decreases for increasing temperature, with loss of any ER effect beyond the glass transition temperature. The present research is focused on EN AW 7075-T6 alloy adherents that are widely applied in lightweight constructions. The study has involved LOCTITE 9466 with experimental tests on Pin-And-Collar samples with ER varying from 0.4 to 1.7 over four levels (10 replications). The effect of temperature has also been assessed, by campaigns at room temperature and at 40°C, 60°C and 80°C. The results, also interpreted by an analytical model, indicate that keeping ER around 0.9–1 is advisable to optimise strength. Temperature leads to a shear strength drop, to a loss of ER effectivity and to higher scattering, when exceeding the glass transition temperature of the adhesive.     

The Effect of Sintering Temperature on Porous Silica Composite Strength

The effect of sintering temperature on porous silica composite strength was studied by discussing three factors, namely crystal phase, glassy phase and porosity. The fired products of clay and silica are composed of crystalline phase and glassy phase. The crystalline phases consist of alpha-quartz and mullite and the glassy phase contains a disordered silica network. With the increase of sintering temperature up to 1360°C, the crystalline silica decreased gradually. The disappearing silica dissolved into the glass and became a part of glass network and resulted in the enhancement of glass strength. This change in glass played an important role in the improvement of sample strength. At the same time, the increase of sintering temperature promoted the densification of samples and reduced the porosity of products, which also contribute to the increase of sample strength. When the sintering temperature is up to 1390°C, the silica in glass tended to convert to cristobalite with the expansion of glassy phase. This expansion weakened the connection of atoms in glass network and brought some closed pores into products, which led to the decrease of sample strength.     

The Interaction Parameter and the Strength of Adhesive Joints

A “blister test” technique has been used to determine the fracture surface energy of a range of adhesive joints formed using a polyurethane adhesive and a range of solid substrates. For each adhesive pair examined the work of adhesion was calculated from the contact angles formed by liquids for which the polar and dispersion force components of the surface tension are known. For each adhesive pair, the solubility parameter of adhesive and substrate were determined by swelling measurements in a range of liquids. Although cohesive failure of the joints was observed for some of the pairs for which the solubility parameters were matched, this was not true for all such pairs and an explanation of this behaviour has been sought in a new calculation of the volume interaction component of the molecular interaction parameters.     

The Effect of the Silane Deposition Conditions on the Durability of Aluminium Joints Pretreated using 3-Aminopropyltrimethoxysilane

The interfacial adhesive fracture energies and durabilities of aluminium/polyurethane joints pretreated with 3-aminopropyltrimethoxysilane using a range of deposition conditions has been investigated using the blister test. For substrates that were rinsed in order to remove the excess silane, the initial adhesive fracture energy was found to increase markedly compared with the unpretreated joints, but was not affected by changes in the pretreatment solvent (water and toluene), time (five and twenty minutes), or, for substrates pretreated from aqueous solutions, pH (6.8 and 10.4). However, the durability of these rinsed silane pretreated joints varied considerably with the silane pretreatment conditions. The most durable joints were formed when the aluminium was pretreated from aqueous solutions at pH 6.8 for five minutes. XPS analysis of the failed fracture surfaces revealed that the failure was associated with the very thin silane film, occurring at the metal/silane/polymer interface. When the excess silane was not removed by rinsing, the initial adhesive fracture energy was approximately half that obtained from the rinsed joints and the durability was also very poor. XPS analysis of the fracture surfaces showed that failure had occurred within a thick silane film.     

The Effect of the Silane Deposition Conditions on the Durability of Aluminium Joints Pretreated using 3-Aminopropyltrimethoxysilane

The interfacial adhesive fracture energies and durabilities of aluminium/polyurethane joints pretreated with 3-aminopropyltrimethoxysilane using a range of deposition conditions has been investigated using the blister test. For substrates that were rinsed in order to remove the excess silane, the initial adhesive fracture energy was found to increase markedly compared with the unpretreated joints, but was not affected by changes in the pretreatment solvent (water and toluene), time (five and twenty minutes), or, for substrates pretreated from aqueous solutions, pH (6.8 and 10.4). However, the durability of these rinsed silane pretreated joints varied considerably with the silane pretreatment conditions. The most durable joints were formed when the aluminium was pretreated from aqueous solutions at pH 6.8 for five minutes. XPS analysis of the failed fracture surfaces revealed that the failure was associated with the very thin silane film, occurring at the metal/silane/polymer interface. When the excess silane was not removed by rinsing, the initial adhesive fracture energy was approximately half that obtained from the rinsed joints and the durability was also very poor. XPS analysis of the fracture surfaces showed that failure had occurred within a thick silane film.     

Effect of Cross-Linking on Tack and Peel Strength of Polymers

In this work the influence of cross-linking on the adhesive fracture energy and the peel strength is studied choosing polydimethylsiloxane (PDMS) as a model polymer. A series of samples was prepared by electron-beam irradiation which covers the whole range from a viscoelastic liquid to a cross-linked rubber. The mechanical behaviour of these PDMS samples was characterized by mechanical spectroscopy. Tack measurements with an instrument described elsewhere⁵ and peel measurements show that the adhesive fracture energy after short contact times as a measure of tack and the peel strength have a pronounced maximum in the range above the gel point, where the PDMS consists of a very loose and imperfect network and a high fraction of soluble polymer. In this range debonding is connected with the formation of fibrillar structures within the polymer.     

Effect of Cross-Linking on Tack and Peel Strength of Polymers

In this work the influence of cross-linking on the adhesive fracture energy and the peel strength is studied choosing polydimethylsiloxane (PDMS) as a model polymer. A series of samples was prepared by electron-beam irradiation which covers the whole range from a viscoelastic liquid to a cross-linked rubber. The mechanical behaviour of these PDMS samples was characterized by mechanical spectroscopy. Tack measurements with an instrument described elsewhere⁵ and peel measurements show that the adhesive fracture energy after short contact times as a measure of tack and the peel strength have a pronounced maximum in the range above the gel point, where the PDMS consists of a very loose and imperfect network and a high fraction of soluble polymer. In this range debonding is connected with the formation of fibrillar structures within the polymer.     

A Unified Approach for Predicting the Strength of Cracked and Non-Cracked Adhesive Joints

This paper presents an investigation of suitable failure criteria for predicting the strengths of uncracked and interfacially-cracked adhesively-bonded joints. A detailed experimental study of both bulk adhesive and adhesive joint behaviour has been carried out. The effect of both strain rate and temperature on the response of the adhesive to mechanical loading has been investigated through a series of tensile tests. The resulting data were used to construct an empirical model for the behaviour of the adhesive, A novel test method based on a four-point bend specimen has been used to investigate how the hydrostatic stress affects the response of the adhesive. Extensive tests on adhesive joints, subjected to different modes of loading and different lengths of interfacial cracks, have provided comprehensive joint strength data and insight into the site and locus of joint failure initiation. Following this, various failure criteria have been evaluated by carrying out detailed linear elastic and non-linear elasto-plastic two-dimensional analyses of the joints tested. Three-dimensional analyses provided modified loads for these two-dimensional analyses that more accurately reproduce the conditions on the plane of failure. Criteria based on critical stress or strain components at a distance from the point of singularity were investigated A procedure for accounting for the strain rate effects of the adhesive has been incorporated with the non-linear analyses. Criteria based on critical energy release rates have been evaluated from the linear elastic analyses of the joints with interfacial cracks diminishing to very small sizes. Finally, non-linear springs along a plane of failure have been used to model a line of localised damage, resulting in joint failure criteria based on a critical opening displacement. This last method provides the most physically acceptable way of predicting the strength of cracked and non-cracked joints using the same failure criterion.     

The Fatigue and Durability Behaviour of Automotive Adhesives. Part III: Predicting the Service Life

In Part I [1] a fracture mechanics approach has been successfully used to examine the cyclic fatigue behaviour of adhesively-bonded joints, which consisted of aluminium-alloy or electro-galvanised (EG) steel substrates bonded using toughened-epoxy structural paste-adhesives. The adhesive systems are typical of those being considered for use, or in use, for bonding load-bearing components in the automobile industry. The results were plotted in the form of the rate of crack growth per cycle, da/dN, versus the maximum strain-energy release-rate, G_max, applied in the fatigue cycle, using logarithmic axes. In Part II [2] the mechanisms of failure were considered, particularly the mechanisms of environmental attack. The present paper, Part III, discusses the use of the relationship between da/dN and G_max, which can be obtained in a relatively short timescale, to predict the fatigue lifetime of (uncracked) single-overlap joints cyclically loaded in tension. An analytical and a finite-element model have been derived to predict the number of cycles of failure, N_f, for lap joints and, particularly when the latter model was used to deduce the value of the strain-energy release-rate, G, in the lap joints, the agreement between the theoretical predictions and the experimental results is found to be very good.     

Effect of Temperature on the Quasi-static Strength and Fatigue Resistance of Bonded Composite Double Lap Joints

Fibre reinforced polymer composites (FRP's) are often used to reduce the weight of a structure. Traditionally the composite parts are bolted together; however, increased weight savings can often be achieved by adhesive bonding or co-curing the parts. The reason that these methods are often not used for structural applications is due to the lack of trusted design methods and concerns about long-term performance. The authors have attempted to address these issues by studying the effects of fatigue loading, test environment and pre-conditioning on bonded composite joints. Previous work centered on the lap-strap joint which was representative of the long-overlap joints common in aerospace structures. However, it was recognised that in some applications short-overlap joints will be used and these joints might behave quite differently. In this work, double-lap joints were tested both quasi-statically and in fatigue across the temperature range experienced by a jet aircraft. Two variants on the double-lap joint sample were used for the testing, one with multidirectional (MD) CFRP adherends and the other with unidirectional (UD) CFRP adherends. Finite element analysis was used to analyse stresses in the joints. It was seen that as temperature increased both the quasi-static strength and fatigue resistance decreased. The MD joints were stronger at low temperatures and the UD joints stronger at high temperatures. It was proposed that this was because at low temperature the strength was determined by the peak stresses in the joints, whereas, at high temperatures, strength is controlled by creep of the joints which is determined by the minimum stresses in the joint. This argument was supported by the stress analysis.     

Effect of Temperature on the Quasi-static Strength and Fatigue Resistance of Bonded Composite Double Lap Joints

Fibre reinforced polymer composites (FRP's) are often used to reduce the weight of a structure. Traditionally the composite parts are bolted together; however, increased weight savings can often be achieved by adhesive bonding or co-curing the parts. The reason that these methods are often not used for structural applications is due to the lack of trusted design methods and concerns about long-term performance. The authors have attempted to address these issues by studying the effects of fatigue loading, test environment and pre-conditioning on bonded composite joints. Previous work centered on the lap-strap joint which was representative of the long-overlap joints common in aerospace structures. However, it was recognised that in some applications short-overlap joints will be used and these joints might behave quite differently. In this work, double-lap joints were tested both quasi-statically and in fatigue across the temperature range experienced by a jet aircraft. Two variants on the double-lap joint sample were used for the testing, one with multidirectional (MD) CFRP adherends and the other with unidirectional (UD) CFRP adherends. Finite element analysis was used to analyse stresses in the joints. It was seen that as temperature increased both the quasi-static strength and fatigue resistance decreased. The MD joints were stronger at low temperatures and the UD joints stronger at high temperatures. It was proposed that this was because at low temperature the strength was determined by the peak stresses in the joints, whereas, at high temperatures, strength is controlled by creep of the joints which is determined by the minimum stresses in the joint. This argument was supported by the stress analysis.     

Fatigue Growth of Cohesive Defects in T-Peel Joints

This paper uses 2D and 3D finite element models to predict the stresses within bonded and weld-bonded T-peel joints. Epoxy adhesive is modelled as a homogeneous layer providing a perfect bond between aluminium adherends. Knowledge of the critical tensile stresses enables the likely region of fatigue crack initiation to be predicted. The long term reliability and durability of a joint depend directly on its fatigue strength. This research elucidates the region of cohesive crack initiation, the subsequent direction of crack propagation and the relative duration of the different stages of fatigue crack growth. The various stages of embedded, surface and through-width fatigue growth of cohesive defects within a T-peel joint are compared. This establishes fatigue life from crack initiation to final joint fracture for typical bonded and weld-bonded T-peel joints.     

The Effect of Ageing and Environment on the Static and Fatigue Strength of Adhesive Joints

This paper describes the results of a durability programme designed to test the effects of ageing and environment on the performance of adhesive joints. Specimens were kept under a variety of loading and environmental conditions and the paper reports results of static and fatigue tests after 8-9 years storage. Some adhesive joints showed excellent durability performance, while others were adversely affected by the environment, particularly high humidity and natural exposure. It was found that the effect of ageing on static and fatigue performance is not necessarily the same.