Mechanical properties of bonded elastomer discs subjected to triaxial stress

The aim of this article was to evaluate and analyze the mechanical properties of bonded elastomer discs subjected to triaxial stress on an MTS (machine for testing samples) equipment. Saveral pulling tests were run on an Instron machine using an O-ring type of samples to evaluate the mechanical properties of testing unfilled nitrile rubber subjected to uniaxial tension. It was found from the stress–strain curve of the O-ring samples that a very small stress softening occurred when the maximum strain is less than 200%. It was also found that the stress and strain at break does not drastically vary with respect to strain rate. The initial modulus does not vary with respect to strain rate up to ε = 2 min⁻¹, and only for large values of ε does the modulus depend on the strain rate. The material used for the uniaxial tension experiments were bonded between two rigid cylindrical steel plates and the specimens were subjected to uniaxial tension on an MTS machine. It was found that the initial modulus in tension was smaller than in compression. The theoretical predicted initial modulus from Gent's equation was much larger than experimentally estimated. It was shown that the elastomer in the pancake tests was not incompressible and a value of 0.494 was determined for the effective Poisson's ratio. A mathematical equation was derived for the effective Poisson's ratio as a function of the volume fraction of voids within the testing material. © 1996 John Wiley & Sons, Inc.     

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.     

The Uniaxial Tensile Response of Porous and Microcracked Ceramic Materials

The uniaxial tensile stress–strain behavior of three porous ceramic materials was determined at ambient conditions. Test specimens in the form of thin beams were obtained from the walls of diesel particulate filter honeycombs and tested using a microtesting system. A digital image correlation technique was used to obtain full‐field 2D in‐plane surface displacement maps during tensile loading, and in turn, the 2D strains obtained from displacement fields were used to determine the Secant modulus, Young's modulus, and initial Poisson's ratio of the three porous ceramic materials. Successive unloading–reloading experiments were performed at different levels of stress to decouple the linear elastic, anelastic, and inelastic response in these materials. It was found that the stress–strain response of these materials was nonlinear and that the degree of nonlinearity is related to the initial microcrack density and evolution of damage in the material.     

Time,temperature, and strain effects on viscoelastic Poisson's ratio of epoxy resins

Poisson's ratio of polymeric materials, although generally assumed as a constant, is known to display a viscoelastic dependence on time, temperature, and strain. This article investigates the phenomenology of this dependence on two crosslinked epoxy systems with different glass transition temperatures. Poisson's ratio measurements are performed by contact extensometers simultaneously measuring the axial and transverse deformations under two different tensile testing conditions: (i) constant deformation rate, in which the effects of strain, strain rate, and temperature are highlighted; (ii) stress relaxation (or constant deformation), where the dependence of Poisson's ratio on time is studied at various strain levels. The viscoelastic Poisson's ratio increases as strain, temperature, and time increases, with trends markedly depending on the materials glass transition. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers.     

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.     

Poisson's ratio for rigid plastic foams

Poisson's ratio for several low-density plastic foams has been determined in both tension and compression. For polystyrene bead foams and a polyurethane foam, Poisson's ratio is greater in tension than compression. In compression, Poisson's ratio is not linear, showing a larger value below the yield strain and a value near zero for high strains. For 0.05 and 0.10 g/cc polystyrene bead foam, Poisson's ratios are 1/3 in tension and 1/4 in compression below the yield strain; at higher strains, the value in compression is in the range 0.03–0.07.     

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%.     

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.     

Intrinsic Mechanical Properties of 20 MAX‐Phase Compounds

The intrinsic mechanical properties of 20 MAX‐phase compounds are calculated using an ab initio method based on density functional theory. A stress versus strain approach is used to obtain the elastic coefficients and thereby obtain the bulk modulus, shear modulus, Young's modulus, and Poisson's ratio based on the Voigt–Reuss–Hill (VRH) approximation for polycrystals. The results are in good agreement with available experimental data. It is shown that there is an inverse correlation between Poisson's ratio and the Pugh ratio of shear modulus to bulk modulus in MAX phases. Our calculations also indicate that two MAX compounds, Ti₂AsC and Ti₂PC, show much higher ductility than the other compounds. It is concluded that the MAX‐phase compounds have a wide range of mechanical properties ranging from very ductile to brittle with the “A” in the MAX phase being the most important controlling element. The measured Vickers hardness in MAX compounds has no apparent correlation with any of the calculated mechanical parameters or their combinations.     

A method of determining the elastic properties of diamond-like carbon films

The elastic properties of diamond-like carbon (DLC) films were measured by a simple method using DLC bridges which are free from the mechanical constraints of the substrate. The DLC films were deposited on a Si wafer by radio frequency (RF) glow discharge at a deposition pressure of 1.33 Pa. Because of the high residual compressive stress of the film, the bridge exhibited a sinusoidal displacement on removing the substrate constraint. By measuring the amplitude with a known bridge length, we could determine the strain of the film which occurred by stress relaxation. Combined with independent stress measurement using the laser reflection method, this method allows the calculation of the biaxial elastic modulus, E/(1−ν), where E is the elastic modulus and ν is Poisson's ratio of the DLC film. The biaxial elastic modulus increased from 10 to 150 GPa with increasing negative bias voltage from 100 to 550 V. By comparing the biaxial elastic modulus with the plane–strain modulus, E/(1−ν²), measured by nano-indentation, we could further determine the elastic modulus and Poisson's ratio, independently. The elastic modulus, E, ranged from 16 to 133 GPa in this range of the negative bias voltage. However, large errors were incorporated in the calculation of Poisson's ratio due to the pile up of errors in the measurements of the elastic properties and the residual compressive stress.     

Superior Mechanical Properties of Double-Network Hydrogels Reinforced by Carbon Nanotubes without Organic Modification

A facile method is developed to fabricate nanocomposite double-network (DN) gels with excellent mechanical properties, which do not fracture upon loading up to 78 MPa and a strain above 0.98, by compositing of carbon nanotubes (CNTs) without organic modification. Investigations of swelling behaviors, and compressive and tensile properties indicate that equilibrium swelling ratio, compressive modulus and stress, fracture stress, Young’s modulus, and yield stress are significantly improved in the presence of CNTs. Scanning electron microscopy (SEM) reveals that the pore size of nanocomposite DN gels is decreased and some embedded micro-network structures are observed on the fracture surface in comparison to DN gels without CNTs, which leads to the enhancement of mechanical properties. The compressive loading-unloading behaviors show that the area of hysteresis loop, dissipated energy, for the first compressive cycle, increases with addition of CNTs, which is much higher than that for the successive cycles. Furthermore, the energy dissipation mechanism, similar to the Mullins effect observed in filled rubbers, is demonstrated for better understanding the nanocomposite DN polymer gels with CNTs.     

Toughening mechanism of polymer blends: Influence of voiding ability of dispersed-phase particles

The parameters which effect the cavitation strain of polymer blends toughened with a shear yield mechanism have been studied by analysis of the stress acted on the equatorial plane of dispersed-phase particles. As a result, the cavitation strain of polymer blends depends on the Young's modulus and the Poisson's ratio of the dispersed-phase particles and the matrix and also on the break stress of dispersed-phase particles. We tried to provide a criterion for selecting the materials used as dispersed-phase particles which can effectively enhance the toughness of polymer blends. © 1996 John Wiley & Sons, Inc.     

Effects of voids on the response of a rubber poker chip sample. III

The effects of voids on the response of a rubber poker chip sample are examined. A theoretical estimation of the diametral contraction of the sample was performed, using the linear theory of stress analysis. Experimental measurements of the lateral contraction at the middle plane of the poker chip elastomer specimen have shown that the testing rubber is not incompressible. By comparing the experimental data with the theoretical predicted equation, the value of the Poisson's ratio v_eff was found to be 0.487, for a given aspect ratio a* of the sample. A theoretical equation for the volume dilatation of the poker chip rubber sample was developed. Using the given aspect ratio, the value of v_eff, and the experimental stress/strain curve of the sample, an estimation of the volume dilatation was formed. The effective Poisson's ratio was also found using the linear stress analysis, by comparing the developed mathematical equations for an incompressible rubber with voids with a compressible one.     

Nonlinear finite element analysis of the butt-joint elastomer specimen

The practical strength of a butt-joint specimen is of great importance to many industrial applications such as adhesive joints, elastomer mountings, flexible couplings, etc. A butt-joint specimen could fail either cohesively or interfacially, depending on the strength of the materials and the stress distribution in the specimen. In the past, engineering design has been based either on theoretical linear analysis or on empirical rules of thumb. A more realistic analysis based on the nonlinear finite element (FE) method is presented here. The elastomer layer in the butt-joint specimen is modeled by a modified Ogden-Tschoegl strain energy function. The nonlinear axisymmetric FE program is formulated on the total Lagrangian procedure. The nominal strain, the thickness of the rubber layer, the compressibility (or Poisson's ratio), and the strain-hardening (or softening) parameter are taken as the variables in the analysis. The maximum radial and axial stresses are found along the central axis, while the maximum shear stress is near the corner of the bond plane and the free lateral surface. The stiffness as a function of the apparent strains is obtained for various thicknesses, various Poisson's ratios, and various strain-hardening parameters. The lateral contraction and the volume dilatation of the specimen are also calculated and related to the stress distribution in the specimen. A well-defined peak load occurs at a critical strain for thin specimens made of materials with a low strain-hardening parameter and high Poisson's ratio values.     

Mechanical characterization of agarose micro-particles with a narrow size distribution

A micromanipulation technique was used for the mechanical characterisation of two types of agarose microspheres with different material properties. Narrow-size distributions having a mean diameter in the range of 15–22 µm were prepared using (a) conventional emulsification followed by filtration and (b) membrane emulsification. Single microspheres were compressed to a range of deformations at different speeds up to a maximum of ~ 70 µm/s, and then held at constant deformation to permit relaxation to occur. It was found that the loading data could be satisfactorily described by the Hertz equation up to 30% deformation. The Young's moduli calculated on this basis were found to correlate with the gel strength of the agarose which was used to prepare the microspheres. However, the values of the moduli increased with the compression speed and significant stress relaxation occurred. Consequently, a modified Hertz analysis was employed that accounts for the viscoelastic behaviour. Two relaxation times were sufficient to describe the stress relaxation function. The Young's moduli from the Hertz analysis corresponded to the long-time values of the stress relaxation function, which is reasonable given the relatively slow compression speeds used. The predominant process occurring at short times was ascribed to water transport from the interior of the microspheres and the process occurring at longer times was interpreted as that arising from the residual viscoelasticity of the polymer network. As a result of the stress relaxation during loading, the Tatara model could not be used to describe loading data at large deformations.     

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.     

Characterisation of mechanical properties of thin polymer films using a bi-axial tension based on blow-up test

Abstract

Blow-up tests were carried out to evaluate mechanical properties of the thin Nylon film used as bagging films. A new method for calculating bi-axial stress and strain of the thin film in blow-up tests was developed based on the theory of membrane with large strain solutions. The bi-axial tensile elastic modulus, Poisson's ratio, yield strength, fracture stress and bi-axial stress–strain relationship of the thin Nylon film were obtained. Meanwhile, uni-axial tensile tests were conducted and the results were compared with those from blow-up tests. For the Richmond HS-8171 thin Nylon film studied, the bi-axial tensile elastic modulus was slightly more than 2 times greater than the uni-axial tensile elastic modulus. The yield strength was the same for both bi-axial and uni-axial tension. The bi-axial fracture stress was about one-third greater than the uni-axial one, while the bi-axial failure strain was about two-thirds greater than the uni-axial counterpart.     

Effect of coating thickness on the stress profile at the epoxy/silicone interface subjected to pull-off loading: A finite element analysis

Previous experimental studies of silicone coatings have shown three distinct types of release behavior in the tensile flat punch test, depending on coating thickness. The mechanical response in the punch test is highly dependent upon the Poisson's ratio of the coating and its confinement ratio (punch radius divided by coating thickness). This study developed a high accuracy finite-element model of the punch test using the adaptive p-method with extensive mesh refinement to produce smooth stress profiles up to the punch edge. Stress distributions were found for a wide range of confinement parameters and Poisson's ratios. At a typical Poisson's ratio of 0.49, the highest center stress occurred for the intermediate thickness coatings—not thin or thick. Also, the thickest coatings demonstrated steadily increasing high stress towards the edge, while other thicknesses showed the steep singularity at the edge with a protective stress depression bordering inside it. The results further help explain why the critical pull-off force continues to increase as the thickness decreases, even with different release mechanisms. The stress profiles for thick coatings have almost no sensitivity to Poisson's ratio, unlike other thicknesses which show high sensitivity. Edge peeling is most likely to occur for all thick coatings, while other debonding modes are most likely for thin and intermediate thickness coatings. Together, results show the stress mechanics of the flat punch test follow three distinct types of confinement.     

变内压条件下膨胀水泥与地层机械性能的匹配

以膨胀水泥和地层作为研究对象,以维持井筒完整性为目的,利用弹性力学理论,借助有限元方法模拟分析了套管内压变化条件下膨胀水泥和地层机械性能对井筒完整性的影响,研究了两者机械性能的匹配关系。研究表明:膨胀水泥弹性模量越大,水泥环内最大米塞斯(Mises)应力越大;膨胀水泥泊松比越大,水泥环内最大Mises越小,水泥环内最大周向应力越小。地层弹性模量越大(地层越硬),水泥环内最大Mises应力越大,水泥环内最大周向应力越小;地层泊松比对水泥环内最大Mises应力和最大周向应力的影响较小。建议硬地层(弹性模量大)匹配弹性模量较小、泊松比较大的膨胀水泥,重点预防水泥环的挤压破坏;软地层且内压较小时匹配弹性模量较大、泊松比较大的膨胀水泥,重点预防水泥环的周向拉伸破坏。