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.     

Amine-terminated poly(ethylene glycol) benzoate (ATPEGB)-modified epoxy: mechanical and thermal properties

Amine-terminated poly(ethylene glycol) benzoate (ATPEGB), synthesized from the esterification reaction of poly(ethylene glycol) (PEG) with 4-amino benzoic acid, was used to modify the toughness of bisphenol-A diglycidyl ether epoxy resin (DGEBA) cured with room temperature curing agent, triethylene tetramine. ATPEGB was characterized by FT-IR and H-NMR spectroscopies, viscosity measurements, solubility parameter calculation and molecular weight determination with gel-permeation chromatography (GPC). The modified epoxy network was evaluated for its impact, adhesive, tensile, flexural and thermal properties. Improvement in mechanical properties depends upon the concentration of the ATPEGB modifier. The optimum properties were obtained at 12.5 phr (parts per hundred parts of resin) concentration of the modifier. The ATPEGB modified cured epoxy was thermally stable up to 315°C. The morphology of cured epoxy was also analyzed by scanning electron microscopy (SEM) investigation of fracture surfaces.     

Viscoelastic and adhesion properties of EVA/tackifier/wax ternary blend systems as hot-melt adhesives

Two types of wax were added to a ethylene vinyl acetate (EVA) copolymer/aromatic hydrocarbon resin (tackifier) blend in the molten state and the miscibility, viscoelastic and adhesion properties of ternary blends as hot-melt adhesives (HMAs) were investigated. Miscibility and viscoelastic properties were studied using differential scanning calorimetry (DSC), Brookfield viscometry and dynamic mechanical thermal analysis (DMTA), and their adhesion strength was determined in terms of single lap shear strength. DSC thermograms of both types of waxes showed their melting peaks in a similar region to that of EVA/tackfier blend. It was difficult to evaluate the miscibility of ternary blends using DSC because the melting peaks of the waxes overlapped with those of the EVA/tackifier blend, although the glass transition temperature (T _g) of the ternary blend systems slightly increased with increasing wax concentration. However, their storage modulus (E′) increased slightly and loss tangent (tan δ) showed different peaks when two types of wax were added to the EVA/tackifier blend. Therefore, the miscibility of EVA/tackifier blend altered with addition of waxes. In addition, their melt viscosity decreased with increasing wax concentration. Furthermore, the adhesion strength of the ternary blends decreased with increasing wax concentration, despite the increment of storage modulus. These results suggested that the ternary blends of EVA/tackifier/wax were heterogeneous.     

Matrix-assisted pulsed-laser evaporation of DOPA-modified poly(ethylene glycol) thin films

3,4-Dihydroxyphenyl-L-alanine-modified poly(ethylene glycol) (mPEG-DOPA₃) is a biologically-inspired material that exhibits unique adhesion properties. In this study, mPEG-DOPA₃ thin films were prepared using a novel laser process known as matrix-assisted pulsed-laser evaporation (MAPLE). The films were examined using Fourier transform infrared spectroscopy, atomic force microscopy, profilometry, antifouling studies and cell adhesion studies. The Fourier transform infrared spectroscopy data demonstrated that the main functional groups in the MAPLE-deposited mPEG-DOPA₃ films remained intact. Profilometry and atomic force microscopy studies confirmed that MAPLE provides excellent control over film morphology, as well as film thickness. High resolution patterns of mPEG-DOPA₃ thin films were obtained by masking. MAPLE-deposited mPEG-DOPA₃ thin films demonstrated an absence of cytotoxicity and acceptable antifouling properties against the marine bacterium Cytophaga lytica. MAPLE-deposited mPEG-DOPA₃ thin films potentially have numerous biomedical and marine applications.     

Sodium montmorillonite intercalated with poly(ethylene glycol): a modifier of reactive hot-melt polyurethane adhesive

We have examined the effects of a sodium montmorillonite (Na-MMT) intercalated with poly(ethylene glycol) (PEG) on the properties of a reactive hot melt adhesive (RHA) based on polyurethane pre-polymer with terminal isocyanate groups. The bond strengths of the RHA, from the initial to final time period after application, were enhanced and the set time was reduced by the addition of 1–3 phr Na-MMT intercalated with PEG (Na-MMT/PEG). The modifier increased the viscosity and a pseudo-solid-like behavior at low shear rates when subjected to a dynamic shear stress. Further, both the tensile modulus and tensile strength of cured RHA films were improved by the reinforcing effect of Na-MMT. The X-ray diffraction pattern of RHA showed that the gallery of Na-MMT was intercalated with PEG segment.     

Determination of Structural Damping and Optimal Vibration Control of an Adhesively-Bonded Double Containment Cantilever Joint

In this study, the loss factors of an adhesively-bonded double containment cantilever joint were determined for different plate and support lengths. The response of the adhesive joint subjected to a transverse excitation force was measured with a contactless eddy-current sensor and the first bending natural frequency was determined using the Fast Fourier Transform method. The loss factor was calculated using the half-power bandwidth method based on the power spectrum of the joint vibration. After an excitation force was applied to the joint, the damped free vibration analysis was carried out using the finite element method and its measured loss factor. The transverse vibration attenuation was actively controlled with different numbers of actuators located on the top surface of the plate. The optimal control of the vibration attenuation was achieved based on a performance index by considering the strain energy, the kinetic energy, the work done on the adhesive joint by the actuators as well as the vibration attenuation time. Genetic Algorithm was implemented to this optimization problem in which the optimal control force histories, the optimal locations and the optimal numbers of the actuators were searched. Eight actuators exhibited the best control force history minimizing the performance index to 3.34 × 10^–2. Thus, the attenuation time was reduced from 16 s to 0.15 s and the absolute displacement was decreased from 13.1 mm to 17.15 × 10^–3 mm for 0.15 s. In addition, the modal strain energy and kinetic energy were found to be at lowest levels. As the actuator number was increased only a minor decrease in the performance index was observed after four actuators.     

The roles of the degree of substitution and the degree of polymerization on the behaviour of cellulose ethers applied as adhesives for artwork conservation

Quite recently cellulose ethers have been introduced into the conservation field as an adhesive agent. Cellulose ethers are available in a wide range of products. Their properties are related not only to the nature of the substituent but also to the degree of polymerization (DP) and the degree of substitution (DS). Thus, the number of OH groups that are bound to C1 and C4 or to C2, C3 and C6 carbon atoms of the cellulose ether molecule affect their properties. Paper traction resistance is increased by using cellulose ethers as an adhesive to paste Tenguyo paper, a paper with a low density of fibres usually used by restorers. This increase is directly related to the molecular structure of the cellulose ethers. Therefore, the nature of substituents, the DP and the DS of cellulose ethers have to be taken into account in conservation practice. DP equals the number of anhydroglucose units in the chain and DS equals the number of hydroxyl groups replaced by the substituent in every anhydroglucose unit in the chain. As this information is not usually provided by suppliers, viscosity measurements are proposed as a means of predicting the behaviour of different products in relation to the conservation of paper artefacts, or to any other field where cellulose ethers solutions are used.     

A study of the assembly process and reliability performance of contactless smart cards fabricated using a non-conductive adhesive film

Contactless smart cards are widely used in various applications, such as access control, transit fare payment systems, etc. In order to improve the performance and at the same time to reduce the production costs, research effort on the application of new materials and the development of new processes continues. In this study, the assembly process of contactless smart cards using a film-type non-conductive adhesive (NCA) was investigated and modified taking into account the limited heat resistance of the poly(ethylene terephthalate) substrate used in the tests. Pressure cooker tests were then performed to further evaluate the reliability performance of the adhesive joints in high temperature and high humidity conditions. The assemblies underwent both mechanical and electrical characterizations after the assembly process, as well as during and after the reliability tests. Examination of surface morphologies and cross-sections by scanning electron microscopy and optical microscopy was conducted to establish the failure mechanism.     

Inspection of composite and metal adhesive bonds with an electrochemical sensor

An in-situ sensor, based on Electrochemical Impedance Spectroscopy (EIS), has been used to monitor the health of adhesive bonds constructed from various combinations of aluminum, graphite/epoxy, glass/epoxy, glass/polyester, and glass/vinylester composites and exposed to high humidity and temperature conditions. Modeling of the EIS data as an electric circuit demonstrated that several circuit parameters of the impedance spectra were sensitive to bond performance, as determined by wedge tests and lap shear tests. Moisture absorption by the adhesive and composite was calculated from the circuit capacitance, which was also a function of bonded area and bondline thickness (bondline + composite thickness for glass composites). Material differences, including saturation level of moisture, rate of absorption, and bondline stability, were readily seen among the various materials sets. The sensor electrodes are attached to opposite sides of a bond after fabrication, i.e. they are not embedded. Thus, they are suitable for monitoring existing bonded structures. They have the potential to identify bondlines that are in the early stages of degradation, prior to significant loss of bond strength. As an input to a condition-based maintenance system, they would identify weakening bondlines and allow preventative action to be scheduled and performed.     

Adhesive wear and frictional characteristics of UHMWPE and HDPE sliding against different counterfaces under dry contact condition

Abstract

The current work evaluates the wear and frictional performance of ultrahigh molecular weight polyethylene (UHMWPE) and high density polyethylene (HDPE) sliding against different metal counterfaces, stainless steel(SS), mild steel (MS) and aluminium (Al), under dry contact condition. The experiments were conducted using pin on disc machine at different sliding distances (0–40·32 km), 15 N applied load and 2·8 m s^–1 sliding velocity. Interface temperatures and frictional forces were measured simultaneously during the sliding, while specific wear rates were determined for every 1·68 km sliding distance. Based on the optical microscopy of the worn surface and wear track, frictional and wear results were analysed and discussed. The experimental results showed that the type of counterface material significantly influences both frictional and wear performances of the selected polymers. This was mainly due to the film transfer characteristics. Higher temperature and friction coefficient for UHMWPE and HDPE were evident when sliding took place against Al counterface. Sliding the polymers against stainless steel showed low friction coefficients compared to other counterfaces.