Moisture and temperature degradation of double cantilever beam adhesive joints

In this work, the double cantilever beam (DCB) test is analysed in order to evaluate the combined effect of temperature and moisture on the mode I fracture toughness of adhesives used in the automotive industry. Very few studies focus on the combined effect of temperature and moisture on the mechanical behaviour of adhesive joints. To the authors’ knowledge, the simultaneous effect of these conditions on the fracture toughness of adhesive joints has never been determined. Specimens using two different adhesives for the automotive industry were subjected to two different ageing environments (immersion in distilled water and under 75% of relative humidity). Once they were fully degraded, they were tested at three different temperatures (?40, 23 and 80 °C), which covers the range of temperature an adhesive for the automotive industry is required to withstand. The aim is to improve the long term mechanical behaviour prediction of adhesive joints. The DCB substrates were made of a high strength aluminium alloy to avoid plastic deformation during test. The substrates received a phosphoric acid anodisation to improve their long term adhesion to the adhesive. Results show that even though a phosphoric acid anodization was applied to the adherends, when the aged specimens were tested at room temperature and at 80 °C, they suffered interfacial rupture. At ?40 °C, however, cohesive rupture was observed and the fracture toughness of the aged specimens was higher.     

Fracture of adhesive joints and laminated composites

The mechanisms of resin controlled failure in adhesive joints and composites (delamination and transverse cracking) are examined. An in-situ failure model based on the fracture mechanics principles is applied here to describe the failure processes involved. The model centers on the crack tip plastic zone developed in the thin resin layer between the fibers or the adherends. The plastic zone in the resin layer is heavily influenced by a dominant slow varying stress distribution, approximated to be r^?m/2 dependent with m ? 1 (r is the distance from the crack tip). The adhesive or composite fracture toughness G*_IC can then be expressed as a function of several resin properties of comparable importance: modulus E, yield stress σ_y, resin G_IC and residual stress. The relative significance of the resin properties on the adhesive or composite fracture is discussed. The effects of temperature, loading rate, and resin toughening on such failure as a result of the corresponding variations in resin properties are also addressed.     

Relationship between Phenolic Adhesive Chemistry,Cure and Joint Performance. Part I. Effects of Base Resin Constitution and Hardener on Fracture Energy and Thermal Effects During Cure

In this study the relationships between the composition of phenol resorcinol-formaldehyde resins and paraformaldehyde concentration in the adhesive were explored, using DSC, IR, GPC, and solubility measurements. Differences of chemical composition between base resins and adhesives were compared to the fracture toughness of adhesive bonds.

The cure temperature and cure time effects upon fracture toughness were also investigated. Fracture toughness tests were performed with bonded hard maple tapered double-cantilever beam cleavage specimens.     

Relationship between Phenolic Adhesive Chemistry, Cure and Joint Performance. Part I. Effects of Base Resin Constitution and Hardener on Fracture Energy and Thermal Effects During Cure

In this study the relationships between the composition of phenol resorcinol-formaldehyde resins and paraformaldehyde concentration in the adhesive were explored, using DSC, IR, GPC, and solubility measurements. Differences of chemical composition between base resins and adhesives were compared to the fracture toughness of adhesive bonds.

The cure temperature and cure time effects upon fracture toughness were also investigated. Fracture toughness tests were performed with bonded hard maple tapered double-cantilever beam cleavage specimens.     

Predicting environmental degradation of adhesive joints using a cohesive zone finite element model based on accelerated fracture tests

A framework was developed to predict the fracture toughness of degraded adhesive joints by incorporating a cohesive zone finite element (FE) model with fracture data of accelerated aging tests. The developed framework addresses two major issues in the fracture toughness prediction of degraded joints by significant reduction of exposure time using open-faced technique and by the ability to incorporate the spatial variation of degradation with the aid of a 3D FE model. A cohesive zone model with triangular traction-separation law was adapted to model the adhesive layer. The degraded cohesive parameters were determined using the relationship between the fracture toughness, from open-faced DCB (ODCB) specimens, and an exposure index (EI), the time integration of the water concentration. Degraded fracture toughness predictions were done by calculating the EI values and thereby the degraded cohesive parameters across the width of the closed joints. The framework was validated by comparing the FE predictions against the fracture experiment results of degraded closed DCB (CDCB) joints. Good agreement was observed between the FE predictions and the experimental fracture toughness values, when both ODCB and CDBC were aged in the same temperature and humidity conditions. It was also shown that at a given temperature, predictions can be made with reasonable accuracy by extending the knowledge of degradation behavior from one humidity level to another.     

The influence of the core–shell structured meta-aramid/epoxy nanofiber mats on interfacial bonding strength and the mechanical properties of epoxy adhesives at cryogenic environment

The adhesives for adhesively bonded joints at cryogenic environment should be enhanced by reinforcement with low coefficient of thermal expansion (CTE) and high fracture toughness because the materials become quite brittle at cryogenic temperature. Aramid fibers are noted for their low CTE and have been used to control the CTE of thermosetting resins. However, aramid composites exhibit poor adhesion between the fibers and the resin because the aramid fibers are chemically inert and contain insufficient functional groups. In this work, core–shell structured meta-aramid/epoxy nanofiber mats were fabricated by electrospinning with polymer blending method to improve the interfacial bonding between the adhesive and the fibers under cryogenic temperature. The CTE of the epoxy adhesives reinforced with modified nanofiber mats was measured, and the effect on the adhesion strength was investigated at single lap joints at cryogenic temperature. The fracture toughness of the adhesive joints was measured using a double cantilever beam (DCB) test.     

On the effect of pulsed laser ablation on shear strength and mode I fracture toughness of Al/epoxy adhesive joints

The present work describes an experimental study about the shear strength and the mode I fracture toughness of adhesive joints with substrates pre-treated by pulsed laser ablation. An ytterbium-doped pulsed fiber laser was employed to perform laser irradiation on AA6082-T4 alloy. Morphological and chemical modifications were evaluated by means of surface profilometry, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Thick adherend shear tests were carried out in order to assess the shear strength while the mode I fracture toughness was determined using the double cantilever beam. For comparison, control samples were prepared using classical surface degreasing. The results indicated that laser ablation has a favorable effect on the mechanical behavior of epoxy bonded joints; however, while a + 20% increase was recorded for shear strength, a remarkable threefold enhancement of fracture toughness was observed with respect to control samples. XPS analyses of treated substrates and SEM observations of the fracture surfaces indicated that laser pre-treatment promoted chemical and morphological modifications able to sustain energy dissipation through mechanical interlocking. As a result cohesive failure within the adhesive bond-line was enabled under predominant peel loading.     

Mode-I adhesive fracture energy measurement with modified single-lap joint test geometry

The adhesive fracture energy or fracture toughness of adhesively-bonded joints comprising carbon-fiber-reinforced polymer composite substrates and three different types of adhesives was detemined using a modified single-lap joint (MSLJ). This joint was made by implanting end pre-cracks in the adhesive layer at the center of the bondline of a conventional single-lap joint (SLJ). This modification ensured that the crack propagated from a sharp starter crack from both ends of the overlap during testing, reducing the effect of spew fillets on the measured adhesive fracture toughness scatter band. The MSLJ specimens were tested to failure and the adhesive fracture energy was calculated using the Kinloch–Osiyemi model. The values of the adhesive fracture energy obtained from the MSLJ tests were compared with those from SLJ and the double-cantilever beam (DCB) test geometries. The fracture energy values obtained from the MSLJ specimens were significantly lower than those from SLJ specimens and agreed well with those from DCB specimens. The three differenent types of aerospace grade film adhesives tested were Redux 322, Redux 335K and Redux 319A.     

CNT-reinforced adhesive joint between grit-blasted steel substrates fabricated by simple resin pre-coating method

A simple method of applying and distributing multi-walled carbon nanotubes (MWCNTs) onto grit-blasted steel substrates has been investigated in this study to overcome the difficulty of mixing MWCNTs in epoxy adhesives forming the MWCNT-reinforced adhesive joints. MWCNTs were dispersed in an acetone and resin (no hardener) solution with the weight ratio of 1:3:100 for MWCNT/resin/acetone, which was then applied onto the grit-blasted steel substrates. After evaporation of acetone, an ultra-thin layer of resin pre-coating kept well-distributed MWCNTs within the micro-cavities created by grit blasting. Epoxy adhesives (with hardener) were then applied to bond the steel substrates to create MWCNTs-reinforced adhesive joints. The results show that the MWCNT pre-coating (PC) method is beneficial to strong adhesive bonding. Most importantly, the MWCNT-PC method can be easily applied for structural applications on site. In the current study, MWCNTs were simply dispersed in the acetone and resin (no hardener) solution by simple rod stirring for around 1 minute, which can be adopted for large-scale applications. Scanning electron microscopy (SEM) observations on fracture surfaces and cross sections of the MWCNT-reinforced adhesive joints showed MWCNT micro-bundles were well dispersed within the epoxy adhesive joints taking the contour of microscopically uneven substrate surfaces.     

Selected aspects of epoxy adhesive compositions curing process

The paper focuses on selected parameters of curing process – temperature and time. The tests aimed at evaluating the impact of short-term thermal recuring on 1050A and 2017A aluminium alloy sheet adhesive joints strength. Joints were formed with two different adhesives, the main component of which was in both cases epoxy resin Epidian 53 and two different cure agents – poliamineamide C (PAC) and triethylenetetraamine (PF) curing agents. Curing conditions – first curing time, recuring time and recuring temperature – were modified for each of the four tests conducted. For the sake of comparative analysis, adhesive joints were subjected to a single-stage cure cycle at ambient temperature. A two-stage cure cycle of both Epidian 53 compositions at 80?°C for 1 and 2?h produces a material of different mechanical properties than the same material which submits a single-stage cure cycle at ambient temperature, as well as at 60?°C for 30?min. Simultaneously, Epidian 53/PF/100:50 composition proves to produce higher joint strength after recuring than Epidian 53/PAC/100:80; the strength of a joint formed with the former composition increases up to 50% when compared with joints subjected to a single-stage cure cycle. Moreover, tests show that recuring of the adhesive joint formed with both compositions at 60?°C for 30?min does not have a considerable influence on either 1050A or 2017A aluminium adhesive joint strength.     

Enhancement of phenolic polymer properties by use of ethylene glycol as diluent

One application of phenolic resins is for the inner lining of multilayered composites in fire critical applications. Typically such resins contain water as a diluent to facilitate injection and mold filling. Although water is effective in controlling the viscosity, its evaporation from the resin during cure has been found to cause microvoids in the cured resin that are 8–10 μm in size. These voids are believed to affect the properties of the final product. In addition to the initial water content, evolution of water also takes place as a result of cure. In this study, we investigated the effects of processing parameters such as cure temperature, postcure temperature, catalyst concentration, and the use of ethylene glycol as a replacement diluent on water loss, microvoid distribution, and consequently, the mechanical properties. Weight loss during cure was followed by using a thermogravimetric analyzer (TGA). Scanning electron microscopy (SEM) was used to obtain images of cured resin showing the microvoids. The properties that have been obtained for comparison are density, flexural modulus and strength, and fracture toughness. It has been shown that modification of the resin by removing the initial water of a commercial resin system and adding ethylene glycol as a replacement has the most significant effect on the microvoids as well as the properties of the polymer. A decrease in void content and increase in density along with a significant improvement in flexural modulus and fracture toughness have been observed upon replacement of water with ethylene glycol. This is significant because of the importance of the phenolic layer to the overall mechanical performance of a hybrid composite. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 3096–3106, 2004     

Microstructural effects and the toughening of thermoplastic modified epoxy resins

We have studied an epoxy resin formulation consisting of the diglycidyl ether of bisphenol-A (DGEBA), modified with phenolic hydroxyl-terminated polysulfone (PSF) and cured with an aromatic amine curing agent, diaminodiphenyl sulfone (DDS). A range of microstructures and fracture properties have been obtained by controlling the formulation cure conditions (cure temperature and cure cycle in an isothermal mode). The chemical conversion of the cured resins has been monitored by near-infrared spectroscopy (NIR). Although only a single material formulation was used, three distinct types of microstructure were identified by scanning electron microscope (SEM) observations on samples prepared at different cure temperatures. Surprisingly, the thermal and fracture properties of the cured samples did not vary noticeably, in spite of the significant microstructure variations. The consistency of these fracture toughness results with cure temperature changes was an unexpected result in the light of our earlier observations of a strong dependence of fracture toughness on cure temperature in neat resin systems. The difference in behavior between neat and modified resins reveals that the fracture toughness of the latter is dependent on a combination of the microstructure and the matrix resin properties. This hypothesis was also supported by an observation of high fracture thoughness in a sample cured in a two-step process, which we believe is due to the optimum microstructure and matrix resin properties, being achieved separately during precure and postcure, respectively. The increase in fracture toughness values caused by the modification (ΔG_IC) was calculated from the fracture toughness values of neat and modified resins, prepared under the same cure conditions, using a proposed theoretical equation. © 1993 John Wiley & Sons, Inc.     

Effects of hygrothermal aging on anisotropic conductive adhesive joints: experiments and theoretical analysis

Epoxy-based anisotropic conductive adhesive film (ACF) joints have been used in a number of interconnect applications, including direct chip attachment, i.e., chip on glass, chip on ceramics, etc. The ACF joints can be subjected to high relative humidity environment and are susceptible to moisture sorption, especially at elevated temperatures. The long-term hygrothermal aging will induce irreversible changes to epoxy resin systems due to susceptibility of the polymer resin to hydrolysis, oxidation, etc. In this study, the hygrothermal environment was used as an accelerator for the degradation of ACF joints in chip-on-glass (COG) assemblies, which were fabricated in the form of single-lap joints. The effects of aging on the epoxy-based ACF joints were characterized by shear tests and scanning electron microscopy (SEM) at accelerated aging times of 125, 250, 375 and 550 h. The results show that the strength of ACF joints decreases and the fracture mechanism gradually changes with hygrothermal aging. In order to further interpret the hygrothermally-induced degradation to the ACF joints, an ACF joint aging model with hygrothermal environment has been developed, introducing a dimensionless parameter A, which was obtained from the interfacial fracture energy.     

低温固化铅酸蓄电池用防腐耐酸无溶剂环氧胶粘剂的研制

选用环氧树脂、T -3 1固化剂、低分子聚酰胺类固化剂、促进剂、活性稀释剂及各种填料等原料制备了低温固化防腐耐酸无溶剂胶粘剂 ,测试了胶粘剂的粘接强度及耐酸性能。     

Effects of applied pressure and temperature during curing operation on the strength of tubular single-lap adhesive joints

Adhesive joints have been widely used for fastening thin adherends because they can distribute the load over a larger area than the mechanical joint, require no holes, add very little weight to the structure and have superior fatigue resistance. However, the load capabilities of adhesive joints are affected by both applied pressure and temperature during cure, as well as by service environments because the adhesion characteristics of adhesives are very sensitive to manufacturing and environmental conditions. In this study, the tensile load capabilities of tubular single-lap adhesive joints with an epoxy adhesive were experimentally investigated with respect to service temperature and the applied pressure and temperature during curing operation. The effects of the applied pressure on the tensile load capabilities of tubular single-lap adhesive joints were studied by measuring the actual cure finish temperature using thermocouples and dielectrometry. From the experiments, it was found that the actual cure finish temperature of tubular single-lap adhesive joints increased as applied pressure increased, which increased residual thermal stress in the adhesive layer to decrease the load capabilities of adhesive joints. From finite element analysis and experimental results of tubular singlelap adhesive joints, the optimal geometry condition for adhesive joints was also investigated.     

一种用于PVC粘接的环氧树脂胶粘剂

为解决PVC的粘接问题,采用自制对软质PVC具有良好亲合作用的改性环氧树脂增韧树脂制备的环氧树脂胶粘剂,室温固化24h或70℃固化1h可以达到软质PVC材料破坏,使用温度80～100℃。     

Fracture toughness of adhesive joints. II. Temperature and rate dependencies of mode I fracture toughness and adhesive tensile strength

It has been known that adhesive strength shows temperature and rate dependencies reflecting visoelastic properties of an adhesive. Similarly, a critical strain energy release rate is expected to show temperature and time dependencies deformation and fracture of the adhesive occurs at the time of measurement of the critical strain energy release rate, which is a kind of fracture mechanical parameter for adhesive joints. The term “critical strain energy release rate” has usually been called “fracture toughness.” In this study, the critical strain energy release rate (G_IC) of the opening mode was called mode I fracture toughness. G_IC was measured over a wide range of temperatures and rates, and then a master curve was obtained by applying the temperature–rate superposition principle to the obtained data. Also, on the relation between G_IC and adhesive tensile strength is discussed. © 1995 John Wiley & Sons, Inc.     

Preparation and performance of ceramizable heat-resistant organic adhesive for joining Al2O3 ceramics

Ceramizable heat-resistant organic adhesive (CHA) was prepared by using preceramic polymer polysiloxane as matrix, TiB₂ ceramic powder and low melting point glass powder as additives. The curing mechanism, thermal stability properties, phase composition after pyrolysis, structural evolution of bonding layer and bonding mechanism of the adhesive were investigated by FTIR, TGA-DSC, XRD, SEM, FESEM and bonding strength tests. Results of bonding tests showed the maximum shear strength of the joints was 21 MPa when heat treatment at 1200 °C for 2 h in air. Polysiloxane resin acted as crosslinking adhesive at low temperatures and tended to convert to ceramic bonding layer at high temperatures, resulting from ceramization reaction with active fillers. The formation and growth of ceramic phase after heat treatment enhanced the thermal stability and bonding performance of the adhesive at high temperature.     

Fracture toughness of some adhesive cement systems measured in the temperature range 20–700°C

The adhesive fracture energy (G_IC) of several adhesive cement systems has been measured at temperatures extending to 700°C using alumina adherends in the double torsion test geometry. Two commercially available low-temperature curing systems were evaluated together with three laboratory-formulated cements, two of which were cured at 1000°C. The room temperature values of G_IC rangee from 0.6 to 4.9 J m⁻² for the former and from 6.7 to 11.4 J m⁻² for the latter. Despite the adoption of a linear compliance test geometry it was essential to precrack the specimens in order to obtain ‘valid’ G_IC data. Generally the fracture energy of the adhesive cement joints increased with temperature up to a peak between 400 and 600°C, and then decreased. The maximum G_IC value recorded was 40.2 J m⁻². The increase in toughness can be associated with viscous effects in the glassy phases present in the adhesive cements.     

Environmental effect on the fatigue degradation of adhesive joints: A review

ABSTRACT

Environmental factors, such as temperature and moisture, are known to have a degrading effect on the mechanical properties and performance of adhesive joints, which may be perceived as a non-problem because various works have shown that the static response of an adhesive is normally unaffected by slight moisture and temperature variations that occur in real-world applications. While this may be true, performance under purely static conditions is rarely found in commercial uses and most adhesive joints are subjected to cyclic loadings throughout their life. Interestingly, not much work has been done on the effects of the environment on cyclically loaded adhesive joints, but the consensus is that the fatigue response is much more affected by environmental changes than the static response, which is arguably the most important analysis. The general trend is that hygrothermal ageing decreases the number of cycles the joint can withstand and also decreases the threshold fracture toughness value, which translates to cracks initiating sooner, but exceptions to these behaviours also exist.