Effects of Moisture and Stresses on the Structure and Properties of Polyester Resin

The results of a complex study of structural changes in a cured Norpol 440 polyester resin under the action of damp environment and mechanical loading are presented. A considerable effect of absorbed moisture on the structure and some characteristics of the material is revealed by using thermophysical methods and X-ray diffractometry. The joint effect of moisture and mechanical stress is estimated by investigating the creep in stationary and nonstationary moisture conditions. The anisotropy of the material structure formed during creep is evaluated from the results of dilatometric measurements. It is found that the degree of anisotropy of the material after creep accompanied by moisture sorption is higher than that after creep in the conditions of moisture equilibrium with atmosphere. It is established that the aftercure and relaxation of the residual creep deformation come to an end at heating to 80-85°C. At a further rise in temperature and repeated heating, changes in the material structure are not observed.     

Effects of temperature and moisture on the mechanical properties of polyester resin in tension

The effect of environment on the physical and mechanical properties of composite materials in some cases is determined by the environmental sensitivity of the binder. The results of experimental investigation of the deformability and strength of polyester resin, widely used as a binder in composites, upon the action of stationary and quasi-stationary loads, temperatures, and moisture are presented. The ranges of acceptable values of these services factors are determined. The elastic modulus and tensile strength of the material are obtained from quasi-static tests. The viscoelastic behavior of the resin is investigated in creep tests. From the results of a short-term experiment with stepwise loading up to failure, it is found that the creep of specimens with a moisture content of 0.14% can be described by a linear viscoelastic model for stresses up to 20 MPa (two thirds of the strength). The action of single loading impulses is summarized according to the Boltzmann superposition principle. The temperature and absorbed moisture are considered as factors accelerating the relaxation processes in the material. The creep activation under the action of these factors is described using the methods of time-temperature and time-moisture equivalence. The results of short-term creep tests allow us to determine the relaxation characteristics of the material in stationary conditions. The long-term creep under close-to-service conditions is predicted using these data and quite good agreement with the control test is obtained. The sensitivity of the material characteristics (strength, elastic modulus, and creep strain) to the action of temperature and moisture is estimated. The creep strain is most sensitive to the action of environmental factors. For a fully saturated material (moisture content 1.64 wt.%), after one hour creep, this strain four times exceeds that of a dry one. The same growth in deformability is caused by an 18°C increase in temperature. A quantitative comparison of the characteristics of polyester and epoxy resins allows us to choose the binder for composites and to estimate the expected environmental effect. Presented at the 11th International Conference on Mechanics of Composite Materials (Riga, June 11–15, 2000). Translated from Mekhanika Kompozitnykh Materialov, Vol. 36, No. 3, pp. 395–406, May–June, 2000.     

考虑剪力滞效应的先简支后连续-箱梁徐变效应分析

为了分析混凝土箱梁的徐变效应与剪力滞效应的相互影响,基于箱梁剪力滞效应计算的能量变分方法,推导了混凝土箱梁徐变受剪力滞效应影响的次内力和应力计算公式.结合先简支后连续箱梁算例,计算了受剪力滞效应影响后的徐变弯矩和应力,分析了考虑徐变效应影响的剪力滞系数.结果表明,与不考虑剪力滞效应的结果相比,剪力滞效应增大了箱梁徐变效应.对算例箱梁,考虑剪力滞效应后,对中支点截面处徐变的影响最大,次内力增大42.56%,腹板处应力减小8.5%.     

Time — Moisture superposition

The results of creep tests on PN-3 polyester resin are given for various values of the temperature and moisture content of the material. It is shown that moisture content and time, like temperature and time, have an interchangeable effect on creep; i.e., in addition to a time-temperature (T-t) there is also a time-moisture (w-t) analogy. The (w-t) analogy can be used to predict the long-time creep from accelerated test data.Institute of Polymer Mechanics, Academy of Sciences of the Latian SSR, Riga. Translated from Mekhanika Polimerov, No. 5, pp. 780–786, September–October, 1972.     

Prediction of the creep of polyester resin in the presence of randomly acting temperature and humidity factors

The possibility of using the theory of random processes to predict the temperature and moisture creep of polymeric materials in the presence of randomly acting temperature and humidity factors is considered. A procedure for estimating the long-term deformation under such conditions is described with reference to the uniaxial creep of PN-3 polyester resin using time—temperature and time—moisture superposition. The possible correlation of the ambient temperature and humidity is taken into account. The results of a computer calculation show that the theoretical and experimental data are in satisfactory agreement.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 2, pp. 219–226, March–April, 1973.     

泥质盐岩单轴蠕变寿命研究

岩石流变力学的研究中，蠕变寿命是一个重要问题.由于长期蠕变试验资料的缺乏，难以估计蠕变破坏时间.该文进行了泥质盐岩单轴全应力 应变压缩试验，并采用陈氏加载法进行了单轴蠕变试验.对蠕变曲线进行了处理，获得了不同应力水平下的蠕变曲线簇，进而得到了等时应力-应变曲线簇.通过拟合分析，建立了等时应力-应变曲线割线模量随时间变化关系模型和等时应力 应变曲线的数学模型.对等时应力 应变曲线与全应力-应变曲线之间的关系进行了分析，获得了蠕变破坏强度和破坏应变分别与蠕变寿命之间的数学表达式.该文研究成果可以估计泥质盐岩的蠕变寿命、长期强度、长期模量、蠕变破坏线和蠕变终止线，对相关岩石流变寿命的估计具有借鉴意义.     

Water Resistance of Polyester Polymer Concrete

The results of experimental investigation of polyester resin and polymer concrete at a long-term (four-year) exposure to water and air with 98% RH are presented. The polymer concrete was composed of a polyester resin as a binder, lime flour, quartz sand, and granite chips. The resin content in concrete was 20 wt.%. The features of sorption properties of the materials investigated are discussed. Data on the water effect on the compressive strength in short-term loading are reported. The creep tests of virgin polymer-concrete specimens were carried out for five years at different stress levels from 0.11 to 0.44 of the short-term prismatic strength. The effect of moisture on the creep behavior was also studied.     

Effect of temperature and moisture on the creep of polymeric materials 1. One-dimensional extension under stationary temperature-moisture conditions

Creep tests have been carried out on PN-3 polyester resin for one-dimensional tension in the linear region of the stress-strain relation at various fixed values of the temperature and moisture content of the material. The temperature and moisture dependence of the instantaneous elasticity and creep characteristics of the material have been determined. It is shown that time-temperature-moisture superposition can apply in creep under various fixed temperature-moisture conditions.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 3, pp. 393–399, May–June, 1975.     

Moisture permeability of a polymer nanocomposite containing unmodified clay

Results of an investigation into the kinetics of moisture sorption by an unfilled styrene-acrylate copolymer and a nanocomposite containing unmodified montmorillonite clay are reported. The sorption tests were performed at 25°C and different levels of the relative humidity of air up to 80%. The moisture diffusivity was calculated using Fick's equation for one-dimensional diffusion. Data on the influence of montmorillonite concentration on the coefficients of diffusion, solubility, and permeability are obtained. The experimental values of the permeability coefficient are compared with those calculated by using a model taking into account the increased path of a diffusing water molecule caused by the shielding effect of plate like filler particles. Translated from Mekhanika Kompozitnykh Materialov, Vol. 44, No. 5, pp. 723–736, September–October, 2008.     

Creep equations for orthotropic material based on the theory of local deformations

By introducing a time factor into the local deformation function, the theory of local deformations can be used to unite the theories of plasticity and creep. The local deformation function is taken as a rheological dependence in the form of an integral equation, and it is assumed to vary in a way that depends on the direction relative to the principal isotropic axes. Thus, equations are obtained for an orthotropic material with nonlinear creep. The relations obtained also allow for the variation of Poisson's ratio with time. It is shown that special cases of these expressions were previously used to describe the creep of glass-reinforced plastics at low stresses.Mekhanika polimerov, Vol. 1, No. 1, pp. 44–49, 1965     

Deformability of an expanded polystyrene EPS 200 under long-term compression

The results of a study on the creep of EPS 200 plates under compressive stresses σ_c in the interval 0.22 ≤ σ_c/σ_10% ≤ 0.80 are presented. From experimental data, the domain of linear creep of the material in the interval of compressive stresses is determined. Under variations of the stress on the interval 0.22 ≤ σ_c/σ_10% ≤ 0.36, the prognostic value of creep strain of the EPS 200 plates for a lead time of 25 years is found to be 2.4%, with a coefficient of variation equal to 18%.     

Creep of thermally cycled polyethylene

A method for determining creep strains of polymer tube specimens (in three mutually perpendicular directions) tested at constant and cyclically changing temperatures is described. The equipment for the programmed variation of specimen temperature and for the application of complex loads is described. The application of the method is illustrated by the results of creep tests on tube specimens of low-density polyethylene (LDP). The creep strain _c of thermally cycled specimens is noticeably larger than that of specimens tested isothermally at the median temperature of a given thermal cycle, and in time it becomes larger than the strain of specimens tested isothermally at the maximum cycle temperature.Mekhanika Polimerov, Vol. 3, No. 4, pp. 676–685, 1967     

Time-dependent magnetothermoelastic creep modeling of FGM spheres using method of successive elastic solution

Magnetothermoelastic creep behavior of thick-walled spheres made of functionally graded materials (FGM) placed in uniform magnetic and distributed temperature fields and subjected to an internal pressure is investigated using method of successive elastic solution. The material creep, magnetic and mechanical properties through the radial graded direction are assumed to obey the simple power law variation. Using equations of equilibrium, stress-strain and strain-displacement a differential equation, containing creep strains, for displacement is obtained. A semi-analytical method in conjunction with the Mendelson’s method of successive elastic solution has been developed to obtain history of stresses and strains. History of stresses, strains and effective creep strain rate from their initial elastic distribution at zero time up to 55 years are presented in this paper. Stresses, strains and effective creep strain rate are changing in time with a decreasing rate so that after almost 50 years the time-dependent solution approaches the steady state condition when there is no distinction between stresses and strains at 50 and 55 years.     

A biodegradable polymer nanocomposite: Mechanical and barrier properties

The preparation of an environmentally friendly nanocomposite based on plasticized potato starch and unmodified montmorillonite clay is described. Data on the influence of montmorillonite concentration on the mechanical properties of the materials obtained are reported. The effective elastic constants of the nanocomposites are calculated. The calculation results are compared with experimental data. The influence of montmorillonite content on the moisture permeability is also investigated. __________ Translated from Mekhanika Kompozitnykh Materialov, Vol. 44, No. 1, pp. 61–76, January–February, 2008.     

Coupled heat and moisture transport in paper with application to a warm print surface

In this work we present a mathematical model describing the coupled heat and moisture transport in paper. The model is solved numerically and the numerical solution is used to study the interdependency of the moisture and temperature distribution in paper. The results show that variation with temperature of the saturated water vapor concentration and the sorption isotherm parameters are both important for inducing moisture desorption. It is also found that for steep relative humidity ramps moisture sorption generates temperature increments that slow down the sorption process itself. The model is also used to study the moisture gradients in a paper sheet inside a printer from Océ Technologies, which contains a warm print surface. The results predict changes in moisture content of only 0.2%, which suggests that no deformations are induced on the printed sheet.     

Anisotropic moisture depending viscoelastic material model for wood

A moisture depending anisotropic viscoelastic material model is presented in this paper. The necessity of consideration cylindrical anisotropy is caused by the growing process. Wood exhibits different creep characteristics depending on the state of stress. Therefore, the consideration of anisotropic viscoelasticity is required. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Creep of Heat-Resistant Composites of an Oxide-Fiber/Ni-Matrix Family

A creep model of a composite with a creeping matrix and initially continuous elastic brittle fibers is developed. The model accounts for the fiber fragmentation in the stage of unsteady creep of the composite, which ends with a steady-state creep, where a minimum possible average length of the fiber is achieved. The model makes it possible to analyze the creep rate of the composite in relation to such parameters of its structure as the statistic characteristics of the fiber strength, the creep characteristics of the matrix, and the strength of the fiber-matrix interface, the latter being of fundamental importance. A comparison between the calculation results and the experimental ones obtained on composites with a Ni-matrix and monocrystalline and eutectic oxide fibers as well as on sapphire fiber/TiAl-matrix composites shows that the model is applicable to the computer simulation of the creep behavior of heat-resistant composites and to the optimization of the structure of such composites. By combining the experimental data with calculation results, it is possible to evaluate the heat resistance of composites and the potential of oxide-fiber/Ni-matrix composites. The composite specimens obtained and tested to date reveal their high creep resistance up to a temperature of 1150°C. The maximum operating temperature of the composites can be considerably raised by strengthening the fiber-matrix interface.     

Prediction of creep of polymeric materials during random processes of change in loading and temperature-humidity conditions of surrounding media

Conclusions 1. On the basis of the exponentialsmoothing method, a method is suggested for estimating the probability characteristics (statistical moments) of nonstationary random processes of change in temperature conditions of the surrounding medium.2. The possibility is shown of the application of the methods of theory of nonstationary random processes to the prediction of creep of polymeric materials which belong to the class of thermal—moisture rheologically simple solids during nonstationary random change in temperature and humidity of the material. The proposed method of prediction has been experimentally confirmed for the example of prolonged (27,000 h) creep of PN-3 polyester resin during simple shear under the climatic conditions of Riga.For Communication 1 see [1].Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 4, pp. 616–621, July–August, 1976.