Properties of Adhesives and their Applications

The stress analysis of an adhesively bonded lap joint requires more information on the mechanical properties of adhesives than it is normally furnished by the manufacturers. For this reason the tests were performed on the three types of adhesives covering a large range of properties. In order to get the true stress-strain curves in tension and compression the change in the Poisson's Ratio with strain was investigated. It was found that the Poisson's Ratio increases almost to the constant volume deformation value until the nonrecoverable deformation sets in. From that point the Poisson's Ratio begins to decrease. Considering only the range of the recoverable deformation, the computer programs developed for the stress analysis of metallic materials can be used for an adhesively bonded lap joint. The recoverable viscoelastic deformation was considered non linear elastic and by applying an effective stress-effective strain relationship the analysis was performed.     

Study on the Viscoelastic Poisson‘s Ratio of Solid Propellants Using Digital Image Correlation Method

Digital image correlation methods were used for further studies of the viscoelastic Poisson's ratio of solid propellants. The Poisson's ratio and the Young's relaxation modulus of solid propellants were separately determined in a single stress relaxation test. In addition, the effects of temperature, longitudinal strain, preload and storage time on the Poisson's ratio of solid propellants were discussed. The Poisson's ratio master curve and the Young's relaxation modulus master curve were constructed based on the time‐temperature equivalence principle. The obtained results showed that the Poisson's ratio of solid propellants is a monotone non‐decreasing function of time, the instantaneous Poisson's ratio increased from 0.3899 to 0.4858 and the time of the equilibrium Poisson's ratio occurred late when the temperature was varied from −30 °C to 70 °C. The Poisson's ratio increased with temperature and longitudinal strain, decreased with preload and storage time, while the amplitude Poisson's ratio increased with preload, decreases with longitudinal strain and storage time. The time of the equilibrium Poisson's ratio occurred in advance with the increase of longitudinal strain, preload and storage time.     

Relaxation of the axial strain induced stresses in double–coated optical fibers

The axial strain induced stresses in double‐coated optical fibers are analyzed by the viscoelastic theory. A closed form solution of the axial strain induced viscoelastic stresses is obtained. The viscoelastic stresses are a function of the radii, Young's moduli, relaxation times and Poisson's ratios of the polymeric coatings. If the applied axial strain linearly increases, the induced stresses increase with the time. On the other hand, if the axial strain is fixed, besides the axial stress in the glass fiber, the stresses exponentially decrease with the time. The relaxation of stresses is strongly dependent on the relaxation times of the polymeric coatings. If the relaxation time of the polymeric coating is very long, the viscous behavior of the polymeric coatings will not appear, and the axial strain induced stresses solved by the viscoelastic theory are the same as those solved by the elastic theory. On the other hand, if the relaxation time of the polymeric coating is very short, the relaxation of stresses is very apparent. A compressive radial stress at the interface of the glass fiber and primary coating will result in an increase of the transmission losses, and a tensile interfacial radial stress will possibly produce debonding at the interface of the glass fiber and primary coating. To minimize this interfacial radial stress, the radius, Young's modulus and Poisson's ratio of the polymeric coatings should be appropriately selected, and the relaxation time of the primary coating should be shortened. Finally, the stresses in single‐coated and double‐coated optical fibers are discussed.     

Time evolutions of non-aging viscoelastic Poisson's ratio of concrete and implications for creep of C-S-H

The viscoelastic Poisson's ratio of concrete is an essential parameter to study creep and loss of prestress in biaxially prestressed structures. Here we first aim to scrutinize the various existing definitions of this ratio. We then analyze all creep data of concrete available in literature that make it possible to compute the evolutions of this viscoelastic Poisson's ratio, which, for mature concrete, is found to remain roughly constant or slightly decrease over time, such as to reach a long-term value always comprised between 0.15 and 0.2. Then, the long-term viscoelastic Poisson's ratio of concrete is downscaled to the level of calcium silicate hydrates (noted C-S-H) with micromechanics. The long-term viscoelastic Poisson's ratio of the C-S-H gel is found to range between 0 and 0.2. Finally, the identification of this range is used to discuss various potential creep mechanisms at the level of the C-S-H particles.     

Moisture in Epoxy Resin Composites

The effect of external loading on the rate of water uptake by unidirectional composites is examined as a function of the loading angle (0) with respect to the fibre direction in glass and graphite-reinforced epoxies. The moisture uptake under the effect of the external load increases with 0, suggesting a dependence on the matrix volume increase, itself a function of the strain and of the Poisson's ratio.     

Determination of poisson's ratio for polyimide films

The Poisson's ratios of polyamic acid and polyimide films were determined using a high pressure gas dilatometer. In this technique, a sample is held at constant length and a hydrostatic pressure is applied to the sample. The resulting change in stress on the sample with applied pressure provides a measure of Poisson's ratio. For fully cured polyimide films based on pyromellitic dianhydride and oxydianiline, Poisson's ratio was measured to be 0.34 at approximately 1% strain. This value increases to 0.48 as the strain is increased to 5%.     

The dependence of the elastic properties of silica/alumina materials on the conditions used for firing

The dependence of the elastic properties of a range of powder compact samples has been measured as a function of firing variables. It was found that both Young's modulus and Poisson's ratio are particularly sensitive to the peak temperature and the time for which the peak temperature is maintained, over a range of these variables for which density is not significantly affected. The material investigated is used industrially for the manufacture of wall tiles. Firing trials conducted in an industrially operated tunnel kiln have indicated that sufficient variation in firing conditions exists, in the cross-section of the tunnel kiln, to cause significant variation in the values of Young's modulus and Poisson's ratio of bodies fired in different positions in the kiln. Microstructural examination of bodies produced to have very similar densities but vastly different values of Young's modulus and Poisson's ratio has indicated that the dependence of Young's modulus and Poisson's ratio on firing conditions can be explained by the extent of sintering within the ceramic matrix.     

Temperature dependence of the tensile properties of lignin/paper composites

The mechanical properties of dioxane lignin (DL)/paper and kraft lignin (KL)/paper composites were investigated as functions of temperature and strain rate. The tensile properties of the lignin/paper composites were governed by the viscoelastic properties of lignins used as a matrix. In the temperature dependence of the tensile properties, the strength of DL/paper composite decreased at 70° and 130°C at which the elongation had maxima. This behaviour was caused by the viscoelastic properties of DL having two relaxations in the primary dispersion region at 120° and 160°C. In the case of KL/paper composite, a drastic decrease in the strength and maximum of elongation were observed at the glass transition of KL (140°C). The strain rate dependence of the strength of DL/paper composite showed behaviour typical of viscoelastic materials. The strength increased with increase of the strain rate and then decreased after reaching a maximum, which showed a transition from a brittle to a ductile type of fracture.     

An approximate method for the determination of poisson's complex ratio in harmonic viscoelastic behavior

To improve the simulation of viscoelastic behavior of composites, an approximate and incremental method for the determination of Poisson's complex ratio of the polymer matrix is proposed. This method is based on the fact that many polymers exhibit a slight variation of their bulk modulus throughout, at their main relaxation temperature. We examine different parameters that affect the proposed method. It is shown that (i) the Poisson's complex ratio of the matrix and the complex modulus of the composite depend significantly on the initial values of the incremental method and (ii) the Poisson's complex ratio of the matrix allows for the effects of filler content on the magnitude of the mechanical relaxation.     

Influence of rheological properties of adhesive polymer on strain energy release rate of mode II and adhesive shear strength

It is well known that adhesive strength shows temperature and rate dependencies reflecting viscoelastic properties of an adhesive used. Specifically, a mechanical relaxation mechanism around the glass transition temperature (T_g) has a strong effect on the adhesive strength, which involves deformation of the adhesive layer. In addition, it is very interesting to know how viscoelastic properties of the adhesive affect the value of strain energy release rate since deformation and failure of the adhesive occur at the measurement of strain energy release rate for adhesive joints. In this study, adhesive tensile strength and strain energy release rate (G_IIC) in plain-shearing mode were measured under a constant experimental condition using adhesives consisting of two types of epoxy resins; the influence of viscoelastic properties on these two values was investigated, and we discuss the relationship between the adhesive shear strength and G_IIC. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 525–536, 1997     

The Energy Balance of a Viscoelastic Material

Abstract

It is well known that the work done by external forces on a viscoelastic material is converted into a conserved part (potential energy) and a dissipated part, each of which may be divided into two other parts: the isotropic one which is connected with volume changes and the deviatoric one which is associated with distortions.

For strain and strain-rate-independent Poisson's ratio (which is reported to be the case for most viscoelastic materials) the time-dependent isotropic and deviatoric moduli differ only by a constant factor. Expressing the relaxation moduli by Prony-Dirichlet series enables the evaluation of the isotropic and deviatoric parts of the stress-power. These calculations are carried out for the case of constant strain-rate uniaxial tension. The positive definite terms of the resulting expression stand for the dissipated stress-power and the remaining terms—for the conserved stress-power. By integrating over time, the different parts of the stress-energy are obtained.

The ratio of deviatoric part to isotropic part of energy is found to be independent of time and equal for both conserved and dissipated energies.

Results of experiments carried out on Perspex (polymethyl methacrylate) and epoxy-resin were used to calculate the different parts of stress-energy. It is found that the ratio of dissipated energy to conserved energy is always smaller than unity decreasing for smaller strains and strain-rates. The energy computations are practically not affected by the choice of the parameters representing the viscoelastic behaviour of material.

The proposed method can be easily applied to other experimental conditions such as relaxation, creep, constant rate of stress or any other loading history.     

Prediction of the creep behavior of reinforced plastics

The time-dependent deformation of orthotropic and transversely isotropic viscoelastic materials under biaxial constant load is given in the range of linear and reversible stress-strain behavior for isothermal processes. This allows one to calculate the deformation of plastics on the basis of isochronous stress-strain diagrams. In addition, a method is presented which allows a calculation of the creep moduli of mat-reinforced unsaturated polyesters and their dependence on glass content, temperature, and time. This calculation requires only specific creep data of matrix material and the elastic modulus of the reinforcement.     

Failure mechanisms in polypropylene with glass beads

The effect of glass beads on the stress-strain behavior of isotactic polypropylene has been examined. Poisson's ratio and secant compliance as a function of strain have been measured. Both sets of data are consistent with interfacial debonding as the initial damage mechanism. Interfacial debonding is then followed by extensive plastic yielding of the matrix at the debond sites. The maximum stress and strain to failure decrease with glass bead content and glass bead diameter. Impact properties correlate with the ability of the composites to reach high strain to failure. The proposed failure mechanisms are supported by fractography and in-situ deformation studies by scanning electron microscopy.     

Poisson's ratio and volume change accompanying deformation of randomly oriented electrospun nanofibrous membranes

ABSTRACT

The present work focuses on the determination of volume change accompanying deformation and Poisson's ratio for electrospun nanofibrous membranes. For this purpose, polyurethane (PU) is considered for the fabrication of electrospun nanofibrous membranes. Three different sample thicknesses are fabricated. Following this, surface morphology analysis and fibre orientation analysis are conducted to investigate the variation of properties between electrospun PU membranes of different thicknesses. Subsequently, PU specimens are subjected to uniaxial extension test where the changes in sample width and thickness are recorded as a function of applied strain. Volume changes are computed while further analysis on the relationship between transverse strains and axial strain provided the values of Poisson's ratio. For all three electrospun PU samples investigated, significant volume changes are observed while the in-plane Poisson's ratio is found to be around 0.55. However, the out-of-plane Poisson's ratio of electrospun PU membranes are not classical and remains undetermined.     

Hybrid-particulate composites based on an epoxy matrix,a reactive rubber,and glass beads: Morphology,viscoelastic, and mechanical properties

The deformation and fracture behaviors of hybrid-particulate epoxy composites have been examined. These materials were based on a DGEBA/DDA matrix with various volume fractions of glass beads and different rubber contents. Young's modulus, yield stress, dynamic mechanical spectra, and fracture energy have been determined at room temperature. The Kerner model fits well the Young's modulus for the hybrid complexes with various glass bead contents. The analysis of the relaxation peak recorded from viscoelastic measurements allow us to discuss the influence of the introduction of the glass beads on the mobility of macromolecular chains and the characteristics of the rubber-separated phase. The fracture energy displays a strong improvement and a synergism effect due to the presence of both kinds of particules. The toughening mechanisms were discussed. © 1993 John Wiley & Sons, Inc.     

Relaxation properties of particle filled coatings: Experimental study and modelling of a screw joint

The present study describes the mechanical behaviour of powder coatings used under very high compressive loads in clamping force joints. Carboxyl functional polyester powder coatings cured with hydroxyl functional β-hydroxyalkylamides with variations in amount of filler have been studied. The coatings were subjected to relaxation tests in tension and in compression. The tests in compression were performed in specially designed tests developed to study the behaviour of powder coatings under compressive loads in clamping force joints. The relaxation results for the matrix were used in a unit cell in micromechanical finite element (FE) model to predict the homogenised viscoelastic properties of the particle composite. These constitutive properties were subsequently used to evaluate the behaviour on a macromechanical scale in a screw joint. The model corresponds well with experimental data at ambient temperature. When increasing the temperature above the glass transition of the coating, however, the model predictions and experimental data differ. Experiments in compression show a much lower relaxation as compared to the FE model. The relaxation simulations of the coating under compressive loads from screw joints showed a significant sensitivity to the Poisson's ratio of the polymer matrix. As the Poisson's ratio approaches 0.5, the matrix becomes hydrostatically incompressible, which resulted in a negligible relaxation of the coating at the screw joint.     

Tensile yield in phenolphthalein polyether ketone

Uniaxial tension tests to the yield point were performed on phenolphthalein polyether ketone (PEK-C) from room temperature to near the glass transition temperature (T_g) at a constant rate of 0.02 min^?1. At room temperature, some measurements were also made at strain rates from 0.002 min^?1 to 2 min^?1. Yield stress was a linear function of temperature and log strain rate. The temperature and the strain rate dependence of yield stress could be modeled using Eyring theory. Yield energy was found to be a linear function of temperature. Young's modulus, yield strain, elastic strain, and plastic strain all decreased with temperature. © 1994 John Wiley & Sons, Inc.     

Computing thermomechanical properties of crosslinked epoxy by molecular dynamic simulations

This paper reports the use of molecular dynamics simulations to study the thermomechanical properties of an epoxy molding compound formed by curing tri/tetra-functionalized EPN1180 with Bisphenol-A. An interactive crosslinking-relaxation methodology is developed to construct the simulation cell. This crosslinking-relaxation methodology allows the construction of highly crosslinked polymer network from a given set of monomers. Based on this computational algorithm, three-dimensional simulation cells can be constructed. By using an existing polymer consistent force-field, several thermomechanical properties of the model epoxy are computed such as the curing induced shrinkage, gelation point, coefficient of thermal expansion, glass transition temperature, Young's modulus and Poisson's ratio. The dependence of these properties on crosslink density and temperature is also investigated. Simulated results are compared with existing theoretical or experimentally measured values when available. Good agreements are observed.     

Effect of viscoelasticity on the spherical and flat adhesion characteristics of photopolymerizable acrylate polymer networks

This research is the first of its kind to study the comparison between spherical and flat probe adhesion behavior as a function of viscoelasticity. Viscoelastic properties were tailored through the use of acrylate networks synthesized from tert-butyl acrylate and poly(ethylene glycol) dimethacrylate (PEGDMA) solutions. The molecular weight and the weight fraction of PEGDMA crosslinker was altered to maintain a constant glass transition temperature of approximately 57 °C, but systematically vary the viscoelastic properties and the rubbery moduli (1–62 MPa). Dynamic mechanical analysis was performed to characterize the low-strain thermo-mechanical behavior of the materials. Viscoelastic behavior of the materials was characterized by creep testing and was observed to inversely correlate with crosslinking density. The samples tested with the spherical probe exhibited low pull-off forces at temperatures well above and well below the glass transition temperature of the material. A maximum in pull-off force was observed in the vicinity of the glass transition temperature owing to the viscoelastic energy dissipative processes. The peak in pull-off force was observed to decrease with an increase in crosslinking density and modulus. Adhesion measurements using the flat probe demonstrated a strong dependence of pull-off force on the modulus of the material above the glass transition temperature. It is concluded that viscoelasticity is a dominating factor in increasing the pull-off force values in the vicinity of the glass transition, while it plays a little or no role for temperatures +/−20 °C away from transition region , opening the possibility of thermally switchable adhesives.     

Effective volume changes during fatigue and fracture of polyacetal

Conventional tensile dilatometry techniques are extended to cyclic fatigue applications to study volume changes that occur during controlled-load cyclic fatigue of polyacetal. During fatigue, in-situ measures of the irreversible and elastic volume change are monitored together with dynamic viscoelastic parameters (E′, E″, and Tan δ), and changes in the energy densities (strain energy, potential energy, and irreversible work). The results show that the effective irreversible volume of the polyacetal gradually increases over a wide range of applied cyclic stress. However, at high stress levels and/or frequencies (i.e., low-cycle, thermally dominated regime), the effective Poisson's ratio of the polyacetal increases as it softens (evidenced by the dynamic viscoelastic data). Conversely, at lower stress levels, the Poisson's ratio continually decreases coincident with decreases in the loss modulus (E″) and the irreversible work density. These results are indicative of entirely different mechanisms governing the low-cycle (high stress level) and high-cycle (low stress) regimes. Also, comparisons between tensile and fatigue dilatometry studies show that the dilational-strain response of samples fatigued at high stress levels are similar to data obtained from monotonic tensile dilatometry. However, the dilationstrain response of samples fatigued at lower stress levels are distinctly different from low-cycle fatigue and tensile dilatometry.