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.     

Effect of anisotropy on the cyclic deformation and heating of SVAM-type glass-reinforced plastics

The effect of anisotropy of the mechanical properties on the cyclic deformation and heating of 1:1 and 5:1 SVAM-type glass laminates has been investigated for symmetrical tension-compression and pulsating tension and compression. It is shown that on the main part of the fatigue curve, depending on the anisotropy and the cyclic stress level, the temperature may be stabilized. At the same time, fracture is invariably accompanied, under fixed experimental conditions, by a rise in temperature to a certain value that depends only on the anisotropy property of the glass-reinforced plastic, the strain also reaching critical values that are determined by the cycle asymmetry and the anisotropy property of the material.Institute of Mathematics and Mechanics, Academy of Sciences of the Armenian SSR, Erevan. Translated from Mekhanika Polimerov, No. 5, pp. 898–903, September–October, 1971.     

Effect of aging on creep of SVAM glass-reinforced plastic in tension with allowance for the orientation of the fibers

The author presents the results of an experimental study of the effect of aging under room conditions on the creep deformation of SVAM glass-reinforced plastic. It is established that with prolongation of the aging period up to two years the resistance of the material to creep deformation increases. The effect of aging is investigated in relation to anisotropy.Mekhanika Polimerov, Vol. 1, No. 6, pp. 20–29, 1965     

Vibrocreep of polymer materials

The vibrocreep of low-density polyethylene (LDP) in uniaxial tension has been investigated in the presence of vibration in the direction of action of the constant load. The material was deformed under nonisothermal conditions owing to heating caused by the dissipation of vibrational energy. Superimposing vibrations leads to a considerable increase in creep rate. It is shown that this increase can not be explained solely in terms of the rise in temperature due to heating of the material; there is also a dynamic creep acceleration effect. Avariant of the vibrocreep approximation with allowance for the dynamic and temperature creep acceleration effects is proposed.Mekhanika Polimerov, Vol. 4, No. 3, pp. 413–420, 1968     

Effect of moisture on the viscoelastic properties of an epoxy-clay nanocomposite

The results of a complex study on the viscoelastic behavior of an epoxy-clay nanocomposite after a long-term exposure to moisture are presented. The main laws of variation in the glass-transition temperature of the nanocomposite in relation to the different content of filler and absorbed moisture were determined by using a thermomechanical analysis. The loading levels in creep experiments were chosen according to the results of quasi-static tensile tests. The sets of creep and creep recovery curves obtained were approximated by the Boltzmann–Volterra linear integral equation with account of the principle of moisture-time analogy. The variation in the spectrum of retardation time of the epoxy resin with introduction of the nanofiller was estimated. It is shown that the moisture-time reduction function correlates with changes in the forced rubber-like elasticity and the volume of nanocomposite specimens upon their moistening.     

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.     

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.     

Representation of the linear viscoelastic behaviour of wood with a constitutive theory based on fractional time derivatives

Like many other materials used in mechanical and civil engineering, wood shows a pronounced history-dependent mechanical material behaviour. Due to its anisotropy its rheological behaviour is strongly dependent on the direction. In this research project, the material behaviour is represented with a phenomenological theory of anisotropic fractional viscoelasticity. In order to identify the material functions and parameters, the time-dependent creep compliances are measured in three orthogonal directions under tension and shear. As a result of the developed constitutive approach, the experimentally observed creep data is described by several power functions. In the second part of the presentation, some differences between classical models of viscoelasticity which are based on Kelvin-Voigt or Maxwell elements and the fractional approach are presented. The assets and drawbacks with respect to wood are discussed. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Comparative Studies on the Mechanical Properties of a Thermoset Polymer in Tension and Compression

Results of an experimental investigation into the mechanical properties of a polyester resin in tension and compression are reported. Features of the stress-strain curves obtained are discussed. Data on the elastic modulus, Poisson ratio, and volume strains are obtained. The results of creep behavior of the material in tension and compression are also presented. It is found that the time-dependent creep obeys a power law, but the nonlinear stress dependence can be described by using the hyperbolic sine function. The effect of load type (tension or compression) on the nonlinearity of the creep is analyzed. __________ Translated from Mekhanika Kompozitnykh Materialov, Vol. 41, No. 5, pp. 633–650, September–October, 2005.     

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)     

Computational mechanical analysis of composite bondlines under environmental moisture loads

A combined moisture diffusion and mechanical computational analysis methodology was developed as part of an independent research program at Lockheed to enhance our understanding of nonlinear and coupled physical processes in materials. The physical behavior of interest is the mechanical response of composite material systems to environmental conditions which can change the materials' moisture content. In particular, it is desirable to know what set of environmental conditions will affect the integrity of a partial adhesive bondline when a bondline gap (no adhesive) is present. The Lockheed proprietary finite element analysis code DIAL was modified to analyze this phenomena in two stages. First, the temporal moisture partial pressures in the materials are calculated from initial and environmental boundary conditions and second, these partial pressures are then passed to a quasi-static mechanical analysis where the moisture data at each time step is treated as a dilatation (swelling) load. The resulting stress and strain state is then calculated, as well as gap closure, if any.Demonstration of this methodology was on a relatively simple 2-dimensional axisymmetric example problem. The results show primarily compressive stresses caused by the positive dilatation (swelling) induced by moisture intrusion of the structure. The stresses concentrate at adhesive bondlines, and bondline gap behavior (i.e., the absence of a contiguous adhesive bond at material interfaces) is correctly simulated. It should be noted that the behavior of the gap and any resultant stress/strains due to a change in moisture is problem dependent, and this example problem is meant as a demonstration of methodology.     

On a model for anisotropic creep of materials

We apply the concepts of proper moduli and states, revealing the structure of the generalized Hooke’s law, to a model of anisotropic steady-state creep of materials. The steady-state creep equations for incompressible materials are expressed in invariant form. The matrix of anisotropy coefficients of these materials reduces to block form with the nine independent components. We consider the special case of an orthotropic incompressible material for which the matrix of anisotropy coefficients corresponds to a nonor.     

Multiaxial constitutive modeling of aircraft engine materials

Based on a newly developed theory (Lu and Weng, Acta Mech., in press) the high temperature behavior of an aircraft engine material is studied under combined stress state. Both monotonic and cyclic deformations are examined to uncover its stress-strain response, as well as its cyclic hardening and strain-ratchetting characteristics. Under a biaxial loading it is disclosed that tensile cyclic hardening is greatly magnified with a superimposed lateral tension, whereas the strain-ratchetting process is led to an enhanced, unsettling state with a superimposed lateral compression. The biaxial transient and steady-state creep strains have also been calculated. The results suggest that while a superimposed lateral tension will inhibit the creep deformation, a lateral compression can greatly promote the inelastic flow. To reflect the practical service conditions of an aircraft engine, the theory is further applied to examine the effect of loading frequency on the development of inelastic strains under concurrent thermal and mechanical loading. It is found that a more frequently flying aircraft will have a greater accumulation of creep strains and, consequently, a greater possibility of material damage in its engine components over the same total flying time.     

Theoretical model of the anisotropy of low-cycle deformability of glass-fiber reinforced plastics

Conclusion A theoretical model is suggested for the anisotropy of low-cycle deformability of glass plastics determined by the anisotropy of the deformational properties of the material in short-term static tension (compression). The effect of the anisotropy of cyclic loss of strength of the deformational properties is taken into account by the introduction of the characterizing function which is taken to be independent of the mechanical properties of the material and of the conditions of low-cycle loading.Translated from Mekhanika Kompozitnykh Materialov, No. 3, pp. 437–442, May–June, 1985.     

Physico-mechanical properties of polymer materials and the friction problem

Aspects of the friction behavior of polymer materials associated with their molecular structure and responsible for the relatively small value of the modulus of elasticity as compared with the ultimate strength are examined. It is shown that, in view of the important influence of hydrostatic pressure on the mechanical properties of polymer materials, the mechanical characteristics obtained from uniaxial testing cannot be used in calculating the contact area and the forces of friction. Formulas are presented for calculating the mechanical characteristics under these conditions. The results of indentation experiments designed to simulate contact processes are discussed. It is shown that the resistance to deformation of the asperities on the surface of polymer materials is of the order of the yield stress, and not two or three times greater, as with metals. The results of contact creep studies are described and evaluated. The results of investigations of the mechanical and antifriction properties of filled polymer materials show that the forces of friction are inversely proportional to the modulus of elasticity, while the thermophysical characteristics are a function not only of the thermophysical characteristics of the filler and the base, but also depend to a great extent on the shape of the filler particles; thus, when a fibrous filler with a low coefficient of linear expansion is used, the thermal stability of the friction material can be considerably improved. The results of a study of the adhesion interaction of polymer materials under conditions of omnidirectional nonuniform compression and simultaneous deformation are presented. It is shown that the adhesion interaction is strong even at room temperature. Aspects of the mechanical properties of lubricants that determine their effectiveness in polymer friction are considered.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, Vol. 5, No. 2, pp. 303–314, March–April, 1969.     

Mechanical properties of blends of liquid crystalline copolyesters with polyprophylene

Conclusions A significant effect of the addition of LCP on the mechanical properties and their anisotropy has been established. Already, if one considers the shape of curves of the stress-strain relationship it can be seen that curves typical for semicrystalline polymers (pure polypropylene) with clearly visible yield point and significant cold drawing leading to an anisotropic stiffening are changing into curves without yielding and with a brittle failure (LC-rich blends). Generally, the tensile elasticity modulus increases with increasing LCP content for both MD and TD. The maximum value of anisotropy of elastic properties was noted for a rather low content of LCP (c = 5%). On the contrary, the stress at yield decreases with increasing LCP content. The same was observed for the strain at yield but in both cases an important increase of anisotropy has taken place. Consequently, the total elongation during drawing (strain at break) showed a drastic decrease for blends with higher LCP content (about 60–80 times). The addition of the LCP to polypropylene has led to a stiffness increase (higher elasticity modulus) but simultaneously to a considerable plasticity decrease. As a confirmation of these observations, there served also the creep test where a decrease of the creep compliance (by two times) for LC-rich blends as compared with pure PP was noted.It also should be emphasized that, generally, a smaller effect of LCP content on the elastic deformation was noted than that on the time dependent effects (nonelastic creep deformation).Published in Mekhanika Kompozitnykh Materialov, Vol. 30, No. 4, pp. 442–450, July–August, 1994.     

Vibrational creep of polymeric materials

An experimental study of the effect of vibration on the creep process has been carried out in the case of the rigid porous polyurethane PPU-3, as a function of the magnitude of the vibrational loading and the level of basic static stresses. It has been shown that with increase in the velocity amplitude of the dynamic stresses, the creep process is accelerated, without being accompanied thereupon by vibrational heating of the material. The possibility has been established of approximating vibrational creep curves by the integral equation of Volterra, using a discrete series of relaxation times transformed by the vibro-time analogy method.For Communication No. 3, see [1].Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 2, pp. 223–232, March–April, 1970.     

Creep damage modeling of a polycrystalline material

A polycrystalline material is investigated under creep conditions within the framework of continuum micromechanics. Geometrical 3D model of a polycrystalline microstructure is represented as a unit cell with grains of random crystallographical orientation and shape. Thickness of the plains, separating neighboring grains in the unit cell, can have non-zero value. Obtained geometry assigns a special zone in the vicinity of grain boundaries, possessing unordered crystalline structure. A mechanical behavior of this zone should allow sliding of the adjacent grains. Within the grain interior crystalline structure is ordered, what prescribes cubic symmetry of a material. The anisotropic material model with the orthotropic symmetry is implemented in ABAQUS and used to assign elastic and creep behavior of both the grain interior and grain boundary material. Appropriate parameters set allows transition from the orthotropy to the cubic symmetry for the grain interior. Material parameters for the grain interior are identified from creep tests for single crystal copper. Model parameters for the grain boundary are set from the physical considerations and numerical model validation according to the experimental data of the grain boundary sliding in a polycrystalline copper [2]. As the result of analysis representative number of grains and grain boundary thickness in the unit cell are recommended. (© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Low-cycle fatigue of a composite substance containing a dispersed filler in cyclic compression

Conclusions 1. The lifetime of a composite material having a dispersed filler has been examined under conditions of low-cycle compression.2. It has been shown that, in distinction from materials of fibrous structure, the lifetime of this material is determined, not by a breakdown in the continuity of structure, but the deformability of the polymer matrix.3. The absence of a correlation between the static and fatigue strengths of composite mateials from cold and hot curing is caused by the different effect of the structure of the cured binder as a component of the material on the ultimate static characteristics and creep, whose relaxation character is more clearly expressed.4. Creep of the binder and the strength of the microspheres are the main factors which determine the lifetime of the material in question in cyclic compression.All-Union Scientific-Research Institute of Synthetic Resins, Vladimir. Translated from Mekhanika Polimerov, No. 4, pp. 653–657, July–August, 1978.     

由压痕蠕变试验确定材料的蠕变性能参数

研究由平头压痕蠕变试验来确定受压材料蠕变性能参数的可行性。利用有限元蠕变分析确定在定压痕应力下的压痕蠕变率,重点放在稳定压痕蠕变率和受压材料蠕变性能参数的关系上。详细地研究了压头形状、大小和宏观约束对压痕蠕变响应的影响:当压头的尺寸和受压材料为同一数量级时,宏观约束将有十分明显的影响。提出两种方法来由压痕蠕变试验来确定受压材料蠕变性能参数,并给出了算例,结果有利于准确认识平头压痕蠕变试验,从而拓宽其应用范围。