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.     

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.     

A Simple Dilatometric Method for Determining Poisson's Ratio of Nearly Incompressible Elastomers

Abstract

A simple apparatus which was developed for measuring the dilatation of specimens tested in uniaxial tension is described. The dilatometer can be used on an Instron testing machine. In spite of its simplicity, this dilatometer enables an accurate determination of Poisson's ratio of nearly incompressible elastomers. We present typical curves showing the effect of strain on Poisson's ratio of filled and unfilled elastomers. We also describe the dilatometric processes observed during straining of granular filled elastomers.     

Prediction of the stress distribution in an elastic pancake using a neo-hookean strain energy function

The aim of this article is to evaluate the axial stress of a thin cylindrical rubber specimen bonded between two steel cylindrical plates. The thickness of the rubber with respect to its diameter is small. It was assumed that the rubber is described by a Neo-Hookean strain energy function. A theoretical equation was developed to correlate the diametral contraction η with respect to strain ε within the sample. The equilibrium equations of motion and the incompressibility constraint with the free stress condition of R = A were used to determined the axial stress state.     

Characterization of ethylene–propylene rubber and ethylene–propylene–diene rubber networks

The stress–strain (S/S) and the swelling equilibrium behavior in a series of ethylene propylene rubber (EPR) and ethylene propylene diene monomer (EPDM) networks were investigated and the results were employed to evaluate the effects of varying the cure conditions on the crosslinking efficiency in these networks. The S/S curve of completely swollen vulcanizates is in agreement with the predictions of rubber elasticity theory, while that of dry or partially swollen vulcanizates is fully described by the Mooney-Rivlin equation. ? values determined in benzene were found to vary linearly with v_r (v_r = equilibrium volume fraction of rubber in swollen sample). Crosslinking efficiency, moles of crosslinks produced per moles of crosslinking agent used, ranges from 3.7 in peroxide-cured EPDM (55% wt ethylene and 2.6% unsaturation) to 0.15 in similarly cured EPR (43% ethylene). Efficiency in the latter system improves to 0.6 by addition of a coagent (sulfur) to the cure formula. Crosslinking efficiency in EPDM (55% ethylene) was found to increase in the order: peroxide- > resin- > sulfur-cured. In the EPDM sulfur vulcanizates, changing the terpolymer in the cure formula resulted in significant changes in the crosslinking efficiency.     

Slipping of carbon nanotubes in a rubber matrix

The interactions of carbon nanotubes (CNTs) and carbon black (CB) with rubber matrices are of great interest. Although both belong to the carbon filler family, their interactions are different. In this study the adhesion of CNTs, if any, with natural rubber (NR) was examined. Scanning electron microscopy examinations made on cryogenically fractured surfaces of a crosslinked NR sample containing 7% by weight of CNTs showed that the CNT bundles emerged from the side surface (narrowed by Poisson's ratio) and slowly slid back in when the deformation was removed. The protruded lengths were many times larger than the nanotube bundle diameters. This extensive slipping out of CNTs from the rubber matrix suggests that interfacial interactions between CNTs and NR are quite weak. In contrast, relatively strong interactions were found between CB and rubber, indicated by the large amount of bound rubber formation. Reinforcement of rubber by CNTs is therefore attributed to the large aspect ratio of CNT bundles. Physical entanglement with rubber molecules is then able to generate effective load transfer, replacing the strong adhesion found with CB. Copyright © 2010 Society of Chemical Industry     

Introductory behavior of rubber concrete

This article introduces a new type of concrete, the so‐called rubber concrete, and thereupon presents a way of modification of waste rubber to construction articles. The conventional cement concrete is made by mixing cement with sand and pebbles, but the rubber concrete proposed here virtually excludes cement completely. The manufacturing process of rubber concrete can be divided into two methods, which are designated for dry and wet processes, but this article focused just on the dry process. The physical properties of rubber composite increased with the silane treatment of added aggregates, but the volume of the aggregate might not be a critical factor affecting the compressive strength in the range of the aggregate contents used in this study, that is, the interfacial adhesion between the matrix rubber and the aggregates was a key factor to improve the mechanical properties of rubber concrete. The compressive strength of rubber concrete was about 89 MPa and the Poisson's ratio, which is the ratio of compressive‐to‐tensile strength, was 5.5%. From the viewpoint of the compressive strength and the Poisson's ratio, rubber concrete had better properties than those of conventional cement concrete. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 35–40, 1999     

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.     

Mechanical and swelling behavior of green nanocomposites of natural rubber latex and tubular shaped halloysite nano clay

Nanocomposites based on natural rubber latex as the matrix and naturally occurring tubular shaped nanoclay, halloysite nanotubes (HNTs) as the reinforcing phase were prepared through co‐coagulation method. XRD, morphology, mechanical, and solvent transport properties of the nanocomposites with special reference to weight percentage of nanoclay were analyzed and discussed in detail. Matrix–filler interaction was estimated from Kraus, Cunneen–Russell, and Lorentz–Park plots. Theoretical estimation of reinforcement effect revealed a better interaction between rubber and filler at lower concentration of filler. At higher loading properties decreased due to the formation of filler–filler networks than polymer–filler networks resulting in the reduction of aspect ratio of fillers. Properties of nanocomposites depend on the aspect ratio and volume fraction of reinforcing filler. Morphological analyses of the nanocomposites were done in detail from scanning electron micrographs. Theoretical modulus of nanocomposites was computed using different composite theories by varying the aspect ratio of filler and compared with experimental data. A good agreement between experimental and theoretical values was observed at lower concentration of filler. Solvent transport properties of nanocomposites were found to decrease at lower concentration of HNT because of the tortuosity of the path. POLYM. COMPOS., 37:602–611, 2016. © 2014 Society of Plastics Engineers     

Mechanical properties of urea–formaldehyde foam

The mechanical behavior of urea-formaldehyde foam was studied to evaluate its potential for energy absorption applications. The apparent elastic modulus (E_f) as a function of foam density was obtained from force-deformation tests. The values of energy absorption capacity were derived from a numerical integration technique. Poisson's ratio (v) was determined by a method of uniaxial compression of cylindrical samples. An increase in foam density results in an increase in the apparent elastic modulus of the material and therefore in its energy absorption capacity. Poisson's ratio is independent of the foam density. The mechanical properties' values obtained can be incorporated in various analyses for predicting desired characteristics for energy absorption applications.     

Application of an interpenetrating network model to the necking in the microcrystalline region in four annealed isotactic polypropylene films subjected to uniaxial stretching at room temperature

The microscopic infrared dichroism, mesoscale deformation and macroscopic stress measurements are made on the microcrystalline region in four annealed isotactic polypropylene (iPP) thin films subjected to uniaxial stretching at room temperature. Results reveal that volume dilatation might occur during stretching and the necking causes the anisotropic shrinkage in the thickness and the width directions. The average orientation function f_av and the true stress as a function of local draw ratio in the samples showing volume dilatation can be respectively overlapped onto those of the sample undergoing constant volume deformation. The pseudo-affine deformation is applicable for molecular orientation at f_av<0.50 and the true stress-strain relationship on the mesoscale can be well described in the same region by the interpenetrating network model previously proposed for necking in the quenched iPP film. This model becomes invalid for deformations above f_av=0.50 due to that plastic deformations in the crystalline phase, depending on the annealing time, start to play a major role.     

On the complex poisson's ratio of a urethane rubber compound

The complex Poisson's ratio of a urethane rubber compound was determined for frequencies up to 700 cps. It is shown that the assumption made by earlier workers using this material, that Poisson's ratio is a numerical constant slightly less than 1/2, while approximately correct at low (creep) frequencies is definitely invalid in certain more elevated frequency bands.     

An interface model for the prediction of Young's modulus of layered silicate-elastomer nanocomposites

Recent experiments on layered silicate-elastomer nancomposites by Burnside and Giannelis have shown that there is a discrepancy between theoretical modulus predictions and experimental modulus measurements. A theory is proposed to explain this discrepancy. We hypothesize that the discrepancy is due to imperfect bonding between the matrix/inclusion interface which effectively reduces the aspect ratio and the volume fraction of the inclusion. We use a simple interface model to quantify the imperfect interfacial bonding. From this model, we introduce the concept of the effective aspect ratio and effective volume fraction of the inclusions. These effective quantities depends on a single material parameter, namely, the constant interfacial shear stress, τ. The interfacial shear stress for the elastomer-silicate nanocomposites is found by fitting the theory to the experimentally measured modulus of Burnside and Giannelis. The interfacial shear stress is in the range of thousands of Pascals. For the elastomer-silicate nanocomposite systems considered here, the interfacial shear stress can be decomposed into two parts; intrinsic shear stress τ_i and frictional shear stress τ_f. The intrinsic interfacial shear stress τ_i depends only on the volume fraction of inclusions and decreases with increasing volume fraction of inclusions. On the other hand, the frictional shear stress τ_f is found to increase linearly with the applied strain. Since the mean stress is also proportional to the applied strain, this gives rise to an effective coefficient of friction, which is found to be 0.0932 for the nanocomposite system considered here.     

A pressure drop correlation for low Reynolds number Newtonian flows through a rectangular orifice in a similarly shaped micro-channel

Current microfabrication methods mean that rectangular orifices in similarly shaped micro-channels are often found in microfluidic devices. The power required to overcome the pressure drop across such orifices is often of importance. In the contribution reported here, numerical results for low Reynolds number incompressible Newtonian fluid flow through rectangular orifice in similarly shaped micro-channel have been used to develop a correlation for pressure drop arising from the orifice. The correlation, which was motivated by theoretical developments, indicates that the pressure drop is proportional to the average velocity through the orifice, and a function of the orifice contraction ratio, length-to-width ratio and, most particularly, aspect ratio.     

Unidirectional fiber–polymer composites: Swelling and modulus anisotropy

The solvent swelling of unidirectional rubber–fiber composites was studied. The amount of matrix swelling was constrained to the extent that would be predicted from the thermodynamic theories of elasticity and polymer–solvent interaction. The geometry of swelling was found to be orthotropic in nature. A simple trigonometric function was derived to relate linear deformation due to swelling to the angle which the direction of its measurement makes with the fiber direction. The validity of the derivation was demonstrated experimentally. Considering swelling to be the imposition of tensile forces of equal magnitude in all directions, and considering a swelling-induced linear deformation to be analogous to a tensile compliance, a simple set of relationships between elastic parameters and their direction of measurement was derived: where E_θ, G_θ, v_θ, and η_θ are Young's modulus, shear modulus, Poisson's ratio, and the shear coupling ratio measured in a longitudinal transverse plane at an angle with the fiber direction, respectively, and E_L, G_LT, and θ_LT are the longitudinal Young's modulus, the longitudinal transverse shear modulus, and the longitudinal transverse Poisson ratio, respectively. Further simplifying the case of combined transverse isotropy and special orthotropy was the conclusion that 1/G_LT = 1/E_T + (1 + 2v_LT)/E_L. The relationships for G and E were experimentally demonstrated.     

Phenomenological model of interfacial stress transfer in carbon nanotube reinforced composites with van der Waals effects

A pullout model is presented to analyze interfacial stress transfer in the double‐walled carbon nanotube (DWCNT) reinforced composites. The effects of the van der Waals (vdW) interaction between two layers of DWCNT and the Poisson's effects of DWCNT and matrix are taken into account in the model. Based on the equilibrium of the interfacial shear stress and the DWCNT axial stress as well the continuous condition of the displacement and stress on the interface of DWCNT and matrix, normalized interfacial shear stress, DWCNT axial stress and matrix axial stress are derived, respectively. Moreover, the effects of DWCNT aspect ratio, DWCNT volume fraction and relative modulus between the DWCNT and matrix are analyzed in details. Finally, the maximum normalized interfacial shear stress with vdW effect is compared with that without vdW effects. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Interrelation of mechanical and optical properties of plasticized epoxy polymers

A series of cold-setting epoxy polymers, plasticized with different amounts of plasticizer, ranging between 0 and 90% by weight of the amount of the epoxy prepolymer, were studied for their mechanical, optical, and fracture behavior properties. Quantities defining the mechanical properties were considered: the elastic modulus E, Poisson's ratio v, and fracture tensile stress σ_f. These were accurately measured with electric strain gauges in specimens tested in a 5-ton Instron tester. The optical behavior was characterized by the stress optical coefficients of the materials in both principal directions, α and β, as well as by the coefficient of optical anisotropy, ζ. The values of these quantities were measured by a Fizeau interferometric method. Finally, the optical method of caustics was applied to cracked epoxy polymer specimens to provide a new experimental technique for determining the stress optical properties of these polymers in terms of their mechanical properties. This method was used to check the previous results found by established experimental methods.     

Dispersion state and hyperelastic behavior of tire tread compounds reinforced with nanoclay: Uniaxial tensile and compression studies

Tire-tread compounds based on natural rubber, butadiene rubber, and styrene-butadiene rubber (65/20/15) were reinforced with Cloisite 15A. Clay state-of-dispersion in the ternary matrix (clay aspect ratio and clay/matrix interface yield strength) was estimated using Halpin–Tsai, Guth, and Leidner–Woodhams–Pukanszky micro-mechanical models. The aspect ratio suggested by Halpin–Tsai (9.7) and Guth (16) models both propounded partially intercalated microstructure. Transmission electron microscopy micrographs indicated higher reliability of Halpin–Tsai theory. Wetting parameter values indicated the affinity of Cloisite 15A to disperse in butadiene rubber. However, it seems that clay particles were not provided with proper compounding conditions to further stabilize their thermodynamic state. The poor matrix/clay adhesion was responsible for the decrease in matrix/clay interface strength and thickness upon increasing clay content according to Leidner–Woodhams–Pukanszky. Hyperelastic modeling was conducted using Abaqus software (five strain energy potential forms) on the basis of large deformation uniaxial tension/compression measurements. Effect of nanoclay on the crosslink-density of samples was justified by C₁₀ (Mooney–Rivlin) and locking-stretch (Van der Waals) values. The sample containing 1 phr nanoclay presented the best fit to the hyperelastic models among the rest conforming to its small value of global interaction parameter “a”(Van der Waals model) calculated explaining minimum deviations. Overall, Marlow and Ogden provided the best consistency with the experimental stress–strain results.     

Surface crack in tension or in bending – A reassessment of the Newman and Raju formula in respect to fracture toughness measurements in brittle materials

The Newman and Raju formula for the stress intensity factor of a semi-elliptical surface crack loaded in uniaxial tension or in bending has been developed about 30 years ago using an FE-analysis for several geometric parameters and fitting an empirical equation to the data points. The Poisson's ratio analyzed was 0.3.In this paper a reassessment of the Newman and Raju formula is made, where all relevant geometric parameters of crack and specimen and the Poisson's ratio are considered. The deviations of the old formula from the new results are up to 21%, if the full range of Poisson's ratio is taken into account. Furthermore the influence of the crack-surface intersection angle is discussed.The results of this work are important for more precise fracture toughness measurements in brittle materials and give a practical guidance for appropriate specimen preparation for fracture toughness measurements, which is also considered here.