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.     

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.     

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.     

Mixed-Mode Strength of Thin Adhesive Films: Experimental Characterization Through a Tubular Specimen with Reduced Edge Effect

The present work deals with the characterization of adhesives in thin film under uniform and multi-axial loading conditions. The tests are carried out with a tubular butt bonded specimen, previously developed by the authors, which ensures both shear and normal uniform stress fields inside the adhesive layer. Stress analysis is performed analytically and shows that, in addition to the axial stress, both radial and circumferential stress components are present in the adhesive layer due to Poisson's effect. This leads to a high level of stress triaxiality especially when only axial loading is considered. The experimental tests performed on eight different loading modes show that the adhesive behaves better under shear stress rather than under normal tensile stress, and its strength increases under compressive mixed mode loading. Among literature criteria for equivalent stress, the Stassi D'Alia criterion provides a clear equivalent failure stress value for the adhesive here examined, regardless of the stress triaxiality.     

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.     

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.     

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%.     

Effect of matrix ductility and interface treatment on mechanical properties of glass fiber mat composites

The influence of matrix properties on randomly oriented glass fiber epoxy composites has been studied. It is shown that an increased ductility (flexibility) of the matrix does not result in greater elongation to failure of the composite under tensile and flexural loads. The tensile (and flexural) strength and the modulus of elasticity are decreased as the ductility of the resin is increased. It is concluded that since the matrix material is subjected to a triaxial state of stress when the composite specimen is subjected to uniaxial loads, the effect of matrix modulus, Poisson's ratio, and yield strength are more important than the matrix ductility measured under uniaxial stress. The effect on mechanical properties of various surface treatments applied to the fibers is also investigated. Finally, scanning electron micrographs are presented showing matrix cracks, fiber debonding, and fiber pull-out.     

Influence of the interfacial resin layer on delamination initiation in broken ply composite laminates

The present study has investigated the influence of a resin layer on the delamination initiation at the interface of broken and continuous plies in the case of GR/E (graphite/epoxy) laminates with broken central plies. A full three-dimensional (3D) finite element (FE) analysis was performed with each layer of the laminate modelled as homogeneous and orthotropic. The interface between the broken and the continuous plies was modelled with a thin resin-rich layer. Eight-noded isoparametric layered elements were used to model the laminate specimen. Also, 3D contact elements were used to prevent inter-penetration of the delaminated faces at the interface. Based on the results of the 3D FE analysis, strain energy release rates were calculated at the delamination front using Irwin's 'crack closure integral'. Using the concepts of linear elastic fracture mechanics (LEFM), the strain energy release rate was used as a parameter for assessing delamination initiation. The effects of various factors such as resin layer stiffness, resin layer thickness, and fibre orientation at the interface on the three components of the strain energy release rates, namely G_I, G_II and G_III, were studied for laminates with various crack sizes of the broken ply, and the influence of the resin layer in the delamination initiation was established. It was observed that delamination initiation is a mixed-mode phenomenon even in the case of uniaxial loading and the dominance of the mode of delamination is governed by the resin layer stiffness, thickness, and lamina orientation at the interface. The present work also concludes that an increase in the resin layer modulus leads to an increase in the probability of mode I delamination while the probability of mode II delamination decreases. A 0/90 interface exhibits a higher chance of delamination in modes I and II, while mode III delamination is maximum for 0/30 and 0/60 fibre orientation interfaces. It was also observed that the larger the crack width, the greater the probability of delamination initiation at the interface.     

Application of digital image correlation method to biogel

This study adopts the digital image correlation (DIC) method to measure the mechanical properties under tension in agarose gels. A second polynomial stress–strain equation based on a pore model is proposed in this work. It shows excellent agreement with experimental data and was verified by finite element simulation. Evaluation of the planer strain field by DIC allows measurement of strain localization and Poisson's ratio. At high stresses, Poisson's ratio is found to exceed the standard assumption of 0.5 which is shown to be a result of pore water leakage. Local failure strains are found to be approximately twice those determined by crosshead displacements. Viscous properties of agarose gels are investigated by performing the tensile tests at various loading rates. Increases in loading rate do not cause much difference in the shape of stress–strain curves, but result in increases in ultimate stress and strain. POLYM. ENG. SCI., 50:1585–1593, 2010. © 2010 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.     

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.     

Mechanical Response of Thermoset Polymers under High Compressive Loads, 2

Summary: A nonlinear viscoelastic material model was used to describe the experimental behaviour of thin vinyl ester specimens subjected to compression in thickness direction. The stress‐dependent material functions in the model were found in creep and strain recovery tests on thick cylindrical specimens. The elastic and creep response of thin thermoset polymer specimens subjected to compressive loads was simulated while varying the geometry of the test set samples. The calculated increase in the apparent elastic modulus and decrease of the creep‐strain rate due to reduced thickness‐to‐width ratio is in a good qualitative correlation with experimental results for corresponding geometries. The constraint due to friction and interaction with the material outside the loaded surface area were identified as the cause for high apparent stiffness, which converges with decreasing thickness to an asymptotic value dependent on the modulus and Poisson's ratio of the material.

The shape of a 2 mm‐thick specimen under compression.     

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.     

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.     

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.     

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.     

Stress concentration coefficient in a composite double lap adhesively bonded joint

The main target of this paper is to investigate the effect of peak stress at the extremities of the adhesive layer of a bonded assembly subjected to dynamic shear impact. It is known, that under both static and dynamic loadings such joints endure at their extremities high level of stresses, an aspect known as edge effects. Double lap joint assembly was considered with unidirectional carbon–epoxy substrates and Araldite 2031 adhesive. To quantify this edge effect, a specific coefficient, named coefficient of stress concentration was defined: it is the ratio of the maximum shear stress to the average shear stress. This coefficient helps to calculate maximum strength of the joint since experimentally, only average shear stress could be measured. A numerical analysis at the midplane of the joint was carried out to investigate the effect of geometrical and material parameters on this stress concentration factor. It was found that this factor is constant with the time once the equilibrium is established. Moreover, this stress concentration coefficient decreases with higher Young's modulus of the adherents, lower Young's modulus of the adhesive, thicker and shorter adhesive layer. A unified parameter involving geometrical and mechanical parameters of the specimen was established to quantify this stress concentration factor.