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.     

Auxetic polypropylene fibres:Part 1 - Manufacture and characterisation

Abstract

The term 'auxetic' is applied to materials that possess a negative Poisson's ratio ν. The use of auxetic polymers has been limited because of problems with deploying them in their fabricated forms, i.e. as 10 mm diameter cylinders. This paper reports the successful development of a processing route to produce a more useful and usable form of auxetic polymeric material, namely fibres. A conventional polymer processing technique (melt spinning) is the basis of this technique, with novel modifications. Video extensometry was used to measure the Poisson's ratio and a value of ν = -0.60±0.05 was obtained.     

Auxetic polypropylene films

Auxetic materials are those exhibiting negative Poisson's ratio (ν) behavior. Polymeric auxetic extruded products in the form of cylinders and fibers have previously been reported. This article reports the successful production of auxetic polypropylene films (～0.15‐mm thick) using a melt extrusion process. Video extensometry and tensile testing techniques have been used to measure the in‐plane Poisson's ratios and Young's moduli of the auxetic film, both on an Instron tensile testing machine and a Deben microtensile testing machine. The film is elastically anisotropic with the Poisson's ratio and Young's modulus along the extrusion (x) direction being ν_xy = ?1.12 ± 0.06 and E_x = 0.34 ± 0.01GPa, respectively, while the Poisson's ratio and Young's modulus in the transverse (y) direction to the extrusion direction are ν_yx = ?0.77 ± 0.01 and E_y = 0.20 ± 0.01GPa, respectively. POLYM. ENG. SCI., 45:517–528, 2005. © 2005 Society of Plastics Engineers     

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.     

Strengthening the thermal Negative Poisson's ratio structures by SiC chemical vapor infiltration

Negative Poisson's ratio structures exhibit adjustable thermal expansion behavior as the thermal stress can be dispersed or offset by torsion, bending, and tension of the struts. However, the structural stability under cyclic thermal stress significantly determines the long-term durability. Strengthening the Negative Poisson's ratio structure can ensure high thermal and mechanical reliability. The work designed a heat-induced torsional Negative Poisson's ratio structures and fabricated it by 3D printing. For efficient strengthening, the preforms were further densified by chemical vapor infiltration (CVI) of SiC to enhance the reliability. Pores and gaps in the preforms were homogeneously covered and filled by the SiC, enhancing the surface finish and mechanical performance. The heat induced torsion of the structures dispersed the heat flow in one single direction, reducing the thermal stress concentration. The independent thermal expansion change of the structural unit can offset or consume the heat dissipation stress, and further improve the reliability and thermal stability through the densification process. As a result, the 120° twisted structure exhibited an average coefficient of thermal expansion (CTE) of 6. 12 × 10^?6/K from room temperature (RT) to 500 °C, and the instantaneous CTE reached the minimum value of 4.01 × 10^?6/K at 125 °C. Meanwhile, the load-bearing capacity strengthened significantly, exhibiting the optimized strength of 11.31 MPa and Young's Modulus of 36.44 GPa, revealing a significant improvement than those of preforms, promising for high load-bearing and low expansion application of structure-function integrated low expansion material.     

Permeation of several gases through elastomers,with emphasis on the deuterium/hydrogen pair

The diffusion and permeation coefficients for He, H₂, D₂, O₂, and N₂ in a variety of elastomers were measured by simple manometric methods. The elastomers studied were butyl rubber; Hypalon® 40 and 45; Viton® E60 and GF; Hydrin® 100 and filled Hydrin® 100; Kraton® G, FG, and KG VTEOS; EPDM; epoxidized natural rubber; and neoprene. Consistent with earlier studies, elastomers with higher glass transition temperatures exhibited lower diffusion coefficients. The ratio of diffusion coefficients of the hydrogen isotope pair differed from the purely molecular-weight-based prediction. Deuterium's slightly smaller size relative to hydrogen is consistent with observed deviations from the molecular-weight-based diffusion coefficient ratio. © 1993 John Wiley & Sons, Inc.     

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.     

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.     

A new thickness sensor based on the Hall effect

A highly sensitive thickness sensor has been developed which may be used as a continuously recording extensometer for rubbery materials. The application of the sensor for measuring volume changes in dewetting filled rubbers is also discussed. Further possible applications include the determination of Poisson's ratio on plastics.     

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.     

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.     

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.     

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.     

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.     

Impact of titanium and silica/titanium fumed oxide nanofillers on the elastic properties and thermal decomposition of a polyester resin

The impact of nanoparticles of titanium (rutile) and silica–titanium fumed oxide (STO) on both the acoustic properties and thermal decomposition of a styrene‐crosslinked unsaturated polyester resin were studied with the methods of ultrasonic probing and thermal decomposition mass spectrometry at filler loadings ranging from 0.5 to 5.0%. It was shown that the elastic modulus, Poisson's ratio, and thermal resistivity in the titanium‐filled nanocomposites increased at small loadings of about 0.5%, whereas in the STO‐filled nanoparticles, the decreases in the parameters at loadings of up to 1.5% was replaced by some increases at higher loadings of up to 5.0%. Distinctions in the concentration dependences of the elastic parameters and the thermal decomposition intensity for both fillers could be explained by the features of the polymer–particle interactions because of the differences in both the number of active sites located on the particle's surface and the polymer structure within interface regions. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42010.     

Elastic moduli of glassy polymers at low strains

The short time moduli of polystyrene, poly(methyl methyacrylate), and polycarbonate have been measured in the glassy state. The main methods used were as follows: (1) The Young's modulus of a strip was derived by extrapolating to infinite length. (2) A bidirectional strain gauge was used for Young's modulus and Poisson's ratio. (3) A unidirectional bulk modulus was measured by the method of Warfield. The results obtained made it possible to determine all the isotropic moduli including the bulk modulus, and these are compared with those reported in the literature. Poisson's ratio (v) was found to increase with temperature in all cases. For poly(methyl methacrylate), where results reported in the literature vary widely, our values agreed with the lower reported figures (v < 0.36). The Young's modulus of poly(methyl methacrylate) is found to be more dependent on temperature and frequency than with the other two polymers.     

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.     

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.