A Method of Predicting the Stresses in Adhesive Joints after Cyclic Moisture Conditioning

The durability of adhesive joints is of special concern in structural applications and moisture has been identified as one of the major factors affecting joint durability. This is especially important in applications where joints are exposed to varying environmental conditions throughout their life. This paper presents a methodology to predict the stresses in adhesive joints under cyclic moisture conditioning. The single lap joints were manufactured from aluminium alloy 2024 T3 and the FM73®-BR127® adhesive-primer system. Experimental determination of the mechanical properties of the adhesive was carried out to measure the effect of moisture uptake on the strength of the adhesive. The experimental results revealed that the tensile strength of the adhesive decreased with increasing moisture content. The failure strength of the single lap joints also progressively degraded with time when conditioned at 50°C, immersed in water; however, most of the joint strength recovered after drying the joints. A novel finite element based methodology, which incorporated moisture history effects, was adopted to determine the stresses in the single lap joints after curing, conditioning, and tensile testing. A significant amount of thermal residual stress was present in the adhesive layer after curing the joints; however, hygroscopic expansion after the absorption of moisture provided some relief from the curing stresses. The finite element model used moisture history dependent mechanical properties to predict the stresses after application of tensile load on the joints. The maximum stresses were observed in the fillet areas in both the conditioned and the dried joints. Study of the stresses revealed that degradation in the strength of the adhesive was the major contributor in the strength loss of the adhesive joints and adhesive strength recovery also resulted in recovered joint strength. The presented methodology is generic in nature and may be used for various joint configurations as well as for other polymers and polymer matrix composites.     

Stability of shear modulus of glass-reinforced plastics based on adhesive prepreg in a humid medium

Strength properties and moisture absorption of glass-and carbon-reinforced plastics prepared using solvent-free adhesive prepregs are studied in a humid atmosphere. The main advantage of the studied materials is a more than tenfold-higher sealing of honeycombed constructions. The ultimate moisture saturation, diffusion coefficient, shear modulus, and swelling are studied. A cyclic action of moisture and temperature is shown to cause a reduction in the shear modulus along the sheet plane upon moistening and an incomplete recovery of the modulus after drying. It is established that, after the second, third, and fourth cycles, irreversible changes become insignificant, thus indicating the stabilization of the shear modulus along the sheet plane during the subsequent cycles. The dependences of the shear modulus on the preparation process of the glass-reinforced plastics are determined under humid conditions.     

The Effect of Stress Whitening on Moisture Diffusion in Thermosetting Polymers

The effects of stress whitening on the moisture diffusion rate and concentration in a polymer adhesive containing a secondary phase were investigated. This was accomplished by performing an absorption test on both stress whitened and virgin samples of the bulk adhesive and comparing the rate and amount of moisture diffusion in each. The presence of stress whitening in samples was not only observed visually, but also confirmed analytically using the “Bilinear RAMOD-2” equation. Experimental results reveal that visibly-present stress whitening resulting from fracture does indeed affect the rate and amount of moisture absorption in a polymer adhesive. Consequently, a diffusion model representing two different regions, stress-whitened and non-stress-whitened, is proposed for path of diffusion in polymer adhesives.     

The Effect of Stress Whitening on Moisture Diffusion in Thermosetting Polymers

The effects of stress whitening on the moisture diffusion rate and concentration in a polymer adhesive containing a secondary phase were investigated. This was accomplished by performing an absorption test on both stress whitened and virgin samples of the bulk adhesive and comparing the rate and amount of moisture diffusion in each. The presence of stress whitening in samples was not only observed visually, but also confirmed analytically using the “Bilinear RAMOD-2” equation. Experimental results reveal that visibly-present stress whitening resulting from fracture does indeed affect the rate and amount of moisture absorption in a polymer adhesive. Consequently, a diffusion model representing two different regions, stress-whitened and non-stress-whitened, is proposed for path of diffusion in polymer adhesives.     

Concentration Dependence of Moisture Diffusivity in Aluminum Particle Filled Epoxy Adhesive in Salt Solutions

Moisture and aggressive ion ingress into bonded joints are primary causes of adhesive degradation. In this study, moisture diffusion behavior of aluminum-powder-filled epoxy adhesive was investigated through utilizing fluid immersion tests under complete immersion in salt solutions with varying NaCl concentrations. Aluminum powder is used in the adhesive for the purpose of improvement of its thermal properties, as demanded in a variety of industrial applications. Mass diffusivity for each specimen was determined by two methods, one using the diffusion data at early times (away from the saturation point) and the other using the data at large times (close to the saturation point). The results of the two methods were quite different, indicating that diffusivity is concentration dependent and a constant diffusivity assumption might lead to error in determining moisture diffusivity values in epoxy systems. Qualitatively, however, both methods indicated similar diffusion behavior. According to the results of both methods, the aluminum filler content did not affect the moisture diffusivity in the epoxy adhesive significantly but the effect of salt concentration was significant; the higher the salt content in the test solution, the higher the moisture diffusivity in the adhesive.     

The effect of cyclic loading on residual stresses and strains in a bonded joint

The mechanical performance of a structural bonded joint is mainly dependent on the interlaminar stresses and strains, the high concentration of which is close to the free edges of the adhesive. Under cyclic loading these stresses and strains are expected to be intensified and to accumulate. The present study deals with the distribution of the residual stresses and strains along the interlaminar adhesive layer under cyclic loading and how it is affected by geometrical edge conditions. A numerical non-linear finite element method was applied, the adhesive layer being regarded as an elasto-plastic bi-linear material with kinematic hardening (Bauschinger effect) which accounts for cyclic plasticity. Findings indicate that lateral normal residual strains in the edge of the adhesive layer are the major component which increases significantly with cycling and are probably responsible for failure initiation and propagation. It was also found that a significant reduction of stress concentration at the adhesive edge may be achieved by a modification of the free-edge geometry of both the adhesive and the adherend phases.     

The effect of hygrothermal fatigue on physical/mechanical properties and morphology of neat epoxy resin and graphite/epoxy composite

The effect of moisture absorption, desorption, and thermal spiking on the physical/mechanical properties of TGDDM/DDS epoxy resin was investigated and compared to the Gr/Ep composite. The mechanism of moisture diffusion in the neat resin was described on the morphological level. The diffusion rate of moisture in epoxy resin was found to depend on the mobility of molecular chains within an inhomogeneous epoxy network. Two well-known concepts of plasticization of amorphous polymers, the lubricity theory and the gel theory, were invoked to describe the interactions between the absorbed moisture and the resin network. Slight permanent changes in properties of the neat resin were observed after one absorption-desorption cycle. In the thermal spiking experiment, only the spiking temperature above the glass transition of the moisture saturated epoxy resins changed their internal structure and produced very small (thin) microcracks. By comparison with the neat epoxy resin, the Gr/Ep composites contain the reinforcement-matrix boundary region, characterized by the highest restrictions to molecular mobility. The absorbed moisture during the static hygrothermal fatigue cannot effectively plasticize this region. But during thermal spiking, the formation of microcracks is observed within the reinforcement-matrix boundary region as well as an increase in the moisture content.     

Characterisation of moisture diffusion and strength degradation in an epoxy-based structural adhesive considering a post-curing process

Adhesive bonding techniques have been widely used in many industrial fields, such as aerospace, vehicle and civil engineering, etc., while the bonded structures are designed to perform satisfactorily under complex service conditions. This paper presented an experimental insight into the moisture diffusion process in Araldite 2015 adhesive considering the effect of post-curing process under elevated temperature. Polytetrafluoroethylene mould was adopted for the curing process of dumbbell adhesive specimens and both distilled and salt water were introduced to simulate a severe immersion environment. Quasi-static testing was conducted on both unaged and aged bulk adhesive after certain period of immersion to reveal the mechanical properties variation. Measurements on specimen weight and thickness were performed with different time intervals during the immersion process. It is shown that the strengths of bulk adhesive were significantly degraded due to the presence of water in both immersion conditions, where higher moisture content led to greater strength degradation regardless of the curing and ageing conditions applied. In addition, distilled water environment led to notably higher saturation content and hygroscopic swelling compared to salt water. However, the rates of water diffusion and hygroscopic swelling processes in salt water were detected to be far above the ones in distilled water. It is worth noting that slight increase was observed in moisture saturation content and swelling expansion for post-cured adhesive, while the coefficients of hygroscopic swelling showed no difference for all sample groups studied. Scanning Electron Microscopy observation further revealed the transition in microscopic topography from unaged to saturated samples.     

The effect of residual strains on the progressive damage modelling of environmentally degraded adhesive joints

This work is concerned with developing numerical modelling techniques for predicting the environmental degradation of adhesively-bonded joints. Associated experimental data are also reported. The moisture-dependent mechanical properties of the adhesive were obtained by testing bulk specimens also exposed to various moisture contents. The diffusion parameters for moisture in the adhesive were determined by carrying out gravimetric experiments on bulk adhesive samples. The moisture-dependent interfacial bond strength of the adhesive system investigated has been determined by testing a mixed mode flexure (MMF) specimen, exposed to obtain various levels of moisture content at the interface. Progressive damage in the joints was modelled with a two-parameter cohesive zone model (CZM). The CZM parameters were determined by correlating the experimental data obtained from the MMF test with results from the numerical simulation. The parameters were then used to predict the response of another configuration, the notched coating adhesion (NCA) specimen. When the residual stresses were neglected in the modelling, the predicted NCA response was seen to be in good agreement with the experimental data. However, initial simulations that included the residual stresses resulted in poor predictions of the NCA response. Creep tests on the saturated adhesive, at the ageing temperature, showed large viscoplastic deformations at low loads. Coupled diffusion-stress modelling, including viscoplastic material properties for the adhesive continuum, showed that the residual stresses for the aged specimens decreased significantly and thus did not contribute strongly to the environmental weakening. Good predictions were then obtained for the NCA tests.     

Dielectric Response of Adhesive Joints to Water Absorption

Dielectric measurements have been used to infer the average moisture content of aluminum adhesive joints bonded with three commercial epoxy adhesives. Measurements were made on both aluminum-adhesive-aluminum joints and aluminum-adhesive specimens in order to assess the influence of moisture diffusion gradients. Similar experiments were also performed with foil electrodes embedded in the bondline. For average water concentrations less than approximately 2%, there existed a linear relationship between the permittivity and the average moisture level which was independent of the spatial nonuniformity produced by water diffusion. At higher moisture levels, the dielectric response varied considerably from one adhesive to another, but, in general, could be approximated as a linear dependence on the average moisture concentration. The dielectric response was recoverable upon drying.     

Diffusion of Moisture in Adhesively Bonded Joints

In this article, diffusion of moisture in adhesively bonded composite joints is discussed and analysed experimentally, analytically and numerically. The experimental studies concentrate on moisture diffusion in adhesive films and in unidirectional and multidirectional composite substrates exposed to two different conditioning environments, namely 45°C/85% RH and 90°C/97% RH for the absorption studies and 90°C/ambient for the desorption studies. The coefficients of diffusion are determined from the water uptake plots. The analytical solutions for diffusion in joints with impermeable adherends are based on the classical theory of diffusion and are used to derive equations in two-dimensions for different adhesive fillet shapes, namely radiused fillet, triangular fillet and rectangular fillet. In the finite element analysis, the diffusion of moisture from the composite substrates into lap-strap joints is also taken into account. Both unidirectional and multidirectional composites are considered, as well as two different fillet shapes, i.e., rectangular and triangular fillet. A comparison between the results obtained using FEA and those obtained using the analytical solution is made. Finally, fatigue test data for lap-strap joints aged and tested in different environments is presented and a tentative link between fatigue threshold and water concentration at the site of failure initiation is made, indicating a semi-empirical method of predicting the strength of joints subjected to moisture-induced degradation.     

Characterising moisture ingress in adhesively bonded joints using nuclear reaction analysis

The moisture ingress in bonded structures is usually characterised by gravimetric experiments on bulk adhesive samples. This is a relatively economic and convenient method. However, this approach poses a problem as only the total amount of moisture in the specimen can be determined and not the moisture concentration distribution throughout the adhesive layer. In this study, the moisture profile at two different ageing times (43 and 96 h) in a bonded joint has been determined by use of nuclear reaction analysis (NRA). The moisture ingress profile, which was found to be Fickian in the bulk adhesive sample, was dominated by apparent Case II kinetics in the bonded joint specimens. The ingress in the laminates was seen to be much faster than if the moisture transport had been governed by Fickian diffusion.     

吸湿行为对PMI泡沫性能及应用的影响

研究了在不同条件处理下，聚甲基丙烯酰亚胺（PMI）泡沫的密度、孔径和厚度对其吸湿行为的影响，并实验验证PMI泡沫吸湿后对其力学性能、烘干可逆性及粘接性能的影响。结果表明，PMI泡沫密度越大，吸湿速率越小；孔径越大，吸湿速率越大；厚度越大，吸湿速率越小，饱和吸湿率越低；吸湿后，PMI泡沫刚性下降，韧性增强，粘接时易出现缺陷；在短期的高湿环境或浸水下，PMI泡沫性能烘干可逆，长期在高湿环境或浸水条件下处理后，PMI泡沫性能出现不可逆下降。     

Water absorption of water-based anticorrosive coatings and its effect on mechanical property and adhesive performance

We focused on the water absorption of newly developed water-based anticorrosive coatings, which contain low volatile organic compounds. We investigated (1) the water absorption behavior of the coatings, (2) the effect of water absorption on the mechanical property of the coatings, and (3) the relationship between the water-absorbing properties and adhesive performance under accelerated aging conditions. It was found that the water absorption of the coatings indicated Fickian diffusion, and the saturated water content M _∞ and diffusion coefficient D of the coatings were quantitatively determined. The storage modulus E′ of the coatings decreased due to water absorption. However, in the coating that exhibited higher crosslink density, the effect of water absorption on the decrease in E′ was relatively small in spite of its higher M _∞ . The lower M _∞ and higher D coating resulted in better adhesive performance than the higher M _∞ and lower D coating. Thus, M _∞ value affected adhesive performance under accelerated aging conditions much more than D value.     

A Model for the Diffusion of Moisture in Adhesive Joints. Part III: Numerical Simulations

The coupling mechanisms between the diffusion process and the viscoelastic response of an adhesive are explained. A numerical scheme for fully-coupled solutions is proposed and implemented in a two-dimensional finite element code. A number of numerical simulations are presented in order to illustrate the importance of the following features: (1) the bulk viscoelastic behavior, (2) penetrant size, (3) physical aging, (4) the strain dependence of the diffusion coefficient, (5) the concentration dependence of the diffusion coefficient and (6) differential, swelling. The effect of moisture intrusion on the stress (strain) distribution across a butt joint is also presented.     

Electrochemical Sensors for Nondestructive Evaluation of Adhesive Bonds

An in-situ corrosion sensor based on electrochemical impedance spectroscopy (EIS) has been used to detect moisture ingress into aluminum-aluminum and aluminum-composite adhesive bonds. Both wedge tests and tensile button tests (aluminum-aluminum bonds only) were performed. Upon moisture absorption, the impedance spectra change shape with the low-frequency region becoming resistive. The low-frequency impedance decreases by several orders of magnitude, depending on the adhesive and the experimental conditions. For bonds with stable interfaces, such as phosphoric acid anodized (PAA) aluminum, the absorbed moisture causes an initial weakening of the adhesive resulting in reduced strength or small crack propagation. A substantial incubation time prior to substrate hydration and bond degradation allows warning of potential joint deterioration and enables condition-based maintenance. For bonds with smooth interfaces with little or no physical bonding (mechanical interlocking), crack propagation can proceed interfacially with minimal moisture absorption. A comparison of the incubation times for Forest Products Laboratory (FPL, or sulfuric acid-sodium dichromate) etched surfaces, both bonded to epoxy adhesives and freely exposed to water or humidity at different temperatures, shows that hydration occurs with the same activation energy and, hence, the same mechanism, independent of whether or not the surface is covered with adhesive. However, the pre-exponential factor in the rate constant is dependent on the concentration of free moisture at the interface so that the hydration rate varies by several orders of magnitude.     

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).     

Hygrothermal aging of an adhesive reinforced with microparticles of cork

In the present study, natural microparticles of cork are used with the objective to increase the toughness of a brittle epoxy adhesive. The cork particles act as a crack stopper, leading to more energy absorption. This fact occurs because cork presents a remarkable combination of properties (low density, low cost and sustainability of the raw material). Adhesives are susceptible to the presence of moisture in the environment. There are several studies that refer that moisture can degrade the molecular structure of the adhesive, and, therefore, its mechanical properties. The main objective of this research is to investigate the effect of moisture on the degradation of an adhesive reinforced with micro cork particles, knowing that cork presents a great capability to absorb water. The water absorption and desorption characteristics have been studied, for specimens without cork and with 1% cork, 125–250 μm. The moisture uptake behaviour in the adhesive was studied to obtain the coefficient of moisture diffusion. The effect of water exposure on the mechanical properties and glass transition temperature was also investigated. It was observed that the presence of water alters the mechanical properties of the adhesive (with and without cork), but these changes are not permanent.     

Carbon foam matrices saturated with PCM for thermal protection purposes

In the present work, numerical and experimental studies are proposed to predict and investigate the thermal characteristics of a thermal protection system consists of carbon foam matrix saturated with phase change material, PCM. Several types of carbon foam matrices with different porosities and thermal properties were introduced for the sake of a parametric study. The composite (carbon foam matrix saturated with PCM) was introduced into a cylindrical enclosure while it experiences its heat from a heat source setting on the top of the enclosure. The numerical simulation was performed using the volume averaging technique and a finite volume technique was used to discretize the heat diffusion equation while the phase change process was modeled using the enthalpy porosity method. The results are portrayed in terms of temperature and heat absorption time history and the numerical and experimental results showed good agreement. The results illustrated that the higher the porosity the more stability of the thermal performance of the matrix composite. On the other hand, the thermal conductivity of the composite matrix acts sharply to increase or decrease its heat absorption rate.     

Moisture sorption-desorption-resorption characteristics and its effect on the mechanical behavior of the epoxy system

Thermosetting epoxy resins are attractive materials for many engineering applications, as they are low in density, with excellent mechanical properties and easily fabricated by processes such as injection molding, extrusion and vacuum forming. However, the hostile hygrothermal environment can degrade the epoxy system. In this study, moisture sorption-desorption-resorption characteristics of the DGEBA/DDA epoxy system have been investigated by the hygrothermal aging and molecular dynamic (MD) simulation. Also, the effects of moisture on the mechanical behavior of the epoxy system have been studied by the uniaxial tensile test and a scanning electron microscopy (SEM), for the unaged, moisture saturated, completely desorbed and moisture re-saturated specimens, respectively. Results show that the moisture diffusion in epoxy system is not only dependent on the hygrothermal conditions, but also on the specimen thickness and hygrothermal history. Due to the effect of the hygrothermal aging, both the tensile elastic module and tensile strength of the studied epoxy system have been reduced, that is, the absorbed moisture has deleterious effects on the physical properties of epoxies and can, therefore, greatly compromise the performance of an epoxy-based component.