Correlation between the Mechanical Properties and the Amount of Desorbed Water for Composites Based on Low-Density Polyethylene and Linen Yarn Production Waste

The effect of the amount of desorbed water on the mechanical properties of composites based on low-density polyethylene and linen yarn production waste (LW) is analyzed by statistical methods. It is shown that the amount of absorbed water decreases during the desorption process at room temperature both for specimens modified and unmodified with diphenylmethane diisocyanate (DIC.) The most sensitive to the action of water is the elastic modulus, which decreases considerably under the effect of water and is fully restored in the desorption process. The tensile strain also increases with the amount of absorbed water. It is found that the elastic modulus of the unmodified composite correlates linearly with the amount of desorbed water. Between the amount of desorbed water and the tensile strain, as well the specific work of deformation, a negative linear correlation is revealed. After water desorption, all strength and deformation characteristics of both the modified and unmodified composites are fully recovered.__________Translated from Mekhanika Kompozitnykh Materialov, Vol. 41, No. 4, pp. 515–524, July–August, 2005.     

Effect of Water on the Physicomechanical Properties of Composites Containing Secondary Polyethylene and Linen Yarn Production Waste

The effect of the amount of absorbed water on the physicomechanical indices (tensile modulus, tensile strength, and ultimate strain) of composites based on secondary polyethylene (SPE) of two trademarks and linen yarn production waste (LW), both with and without a coupling agent (diphenylmetane diisocyanate - DIC), is evaluated. It is shown that the strength properties considerably depend on the time of water sorption and on the blend composition. The tensile strength decreases with increased amount of absorbed water (with increased time of exposure to distilled water) and with increased content of LW in the composites. This can be explained by the plasticizing effect of water molecules, which is confirmed by the increase in the ultimate strain of specimens after their exposure to water. The slight increase in the strength observed for the systems modified with DIC is probably caused by cross-linking of the free diisocyanate in the system under the action of moisture. The diisocyanate intensifies the interfacial interaction and retards the water-sorption process. Therefore, the resulting strength indices of these systems are higher than those of the unmodified compositions.     

Effect of Water on the Physicomechanical Properties of Composites Containing Low-Density Polyethylene and Linen Yarn Production Waste

The effect of the amount of absorbed water on the physicomechanical indices (tensile modulus and tensile strength) of composites based on low-density polyethylene (LDPE) and linen yarn production waste (LW), both with and without coupling agents (stearic acid - SA and diphenylmetane diisocyanate - DIC), is studied. It is shown that the strength properties depend considerably on the time of water sorption and on the blend composition. The tensile strength decreases with increased amount of absorbed water (with increased time of exposure to distilled water) and with increased content of LW in the composite. Somewhat different results are obtained for systems containing SA and DIC modifiers. The modifiers, intensifying the interfacial interaction, retard the process of water sorption, therefore the drop in the strength indices is not so significant. Of special interest is the behavior of systems with DIC. In some cases, a slight increase in strength (after a two to five day exposure to water) is observed, which is probably caused by cross-linking of the free diisocyanate in the system under the action of moisture.     

The influence of water sorption-desorption cycles on the mechanical properties of composites based on recycled polyolefine and linen yarn production waste

The effect of water on the mechanical properties (tensile modulus, ultimate tensile strength, tensile strain, and specific work at break) of both chemically treated and untreated composites based on a recycled low-density polyethylene and linen yarn production waste is analyzed. It is found that three water sorption-desorption cycles change the tensile properties of both the materials irreversibly. This effect is considered as the result of partial fracture of the fiber-matrix interface. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 43, No. 6, pp. 839–848, November–December, 2007.     

Nanocomposites based on a styrene-acrylate copolymer and organically modified montmorillonite 1. Mechanical properties

The preparation of polymer nanocomposites by mixing a solution of a styrene-acrylate copolymer with a suspension of organically modified montmorillonite in dimethyl formamide is described. Seven different compositions with organomontmorillonite content from 0 to 7 wt.% were prepared and tested. Results of their X-ray diffraction analysis are presented. Data on the influence of organomontmorillonite content on the tensile stress-strain curves, elastic modulus, strength, and ultimate elongation of the nanocomposites are obtained. The concentration dependences of elastic properties of materials with differently oriented platelike nanoparticles is analyzed by using an algorithm elaborated for stepwise calculations of elastic constants with account of the features of structural hierarchy of intercalated and exfoliated nanocomposites. __________ Translated from Mekhanika Kompozitnykh Materialov, Vol. 42, No. 3, pp. 375–388, May–June, 2006.     

Evaluation of Deformation and Strength Characteristics of Composites Based on Low-Density Polyethylene and Linen Yarn Production Waste by the Methods of Mathematical Statistics

The possibilities of using various methods of mathematical statistics for processing and analyzing the results of deformation and strength tests on composites made from a low-density polyethylene and linen yarn production waste are evaluated. The hypothesis that the experimental strength data agree with the Gaussian distribution is examined by the Shapiro–Wilk test (W-test.) It is shown that the Gaussian distribution, both for systems unmodified and modified with diphenylmethane diisocyanate (DIC), is valid only for two parameters: the maximum tensile strength _max and the elastic modulus E _t. For the other parameters (the relative elongation _max corresponding to _max, the specific total work of failure A _b), and the specific work of failure to the tensile strength A _max), a non-Gaussian distribution is observed. An analysis of measurements for different specimens by the Bartlett test shows that the E _t data have equal variances for both systems (with and without DIC), but for the system containing DIC, the _max data have different variances. A two-factor ANOVA analysis reveals that DIC considerably affects the tensile strength and modulus of composites, but the influence of test conditions is a statistically significant factor only for the modulus. The coefficient of variation is considerably lower for _max than for E _t and can be used as a quantitative measure for the degree of heterogeneity of the composites investigated.     

Environmental Effects on the Mechanical Properties of Glass-FRP and Aramid-FRP Rebars

In the paper, the experimental results on the effect of temperature and moisture on the mechanical properties of FRP (Fiber-Reinforced Polymer) reinforcements are presented. FRP rebars made from glass and aramid fibers were subjected to cyclic thermal actions at temperatures ranging between 20 and 70°C, typical of natural hot-climate environments. Tensile tests were also carried out on FRP rebars. The effect of moisture was investigated by cyclic wetting and drying the FRP rebars under laboratory conditions before their testing in tension. Finally, the elastic modulus and tensile strength of the FRP rebars exposed to these cyclic actions were compared with those obtained for unexposed ones, in order to evaluate the mechanical damage caused by environmental conditions.     

Strength and deformation characteristics of polymer blend films obtained from water systems

The relationship between the tensile strength and deformation characteristics, composition, and structural organization of films obtained by casting of two-component water-based system blends — a solution of the rigid partly crystalline polymer polyvinyl alcohol (PVA) and an emulsion of the compliant amorphous polymer polyvinyl acetate (PVAc) — has been investigated. The aim of this investigation was to ascertain the possibility of obtaining film materials with increased deformability based on the biodegradable PVA. The composition dependences of the initial modulus of elasticity, the maximum stress, yield stress, the ultimate strength, the ultimate strain, and of the unit work of fracture and other characteristics of films have been analyzed. An analysis of the tensile true stress–strain curves of systems with volume fractions of PVA less than 0.5 points to their considerable orientation strengthening upon tension.     

Measuring the tensile and bending properties of nanohoneycomb structures

The mechanical properties, including Young’s modulus, the effective bending modulus, and the nominal fracture strength, of nanohoneycomb structures were measured by using an atomic force microscope (AFM) and a nano-universal testing machine (UTM). Anodic alumina films were taken as the nanohoneycomb structures. Bending tests were carried out on cantilever beams by pressing AFM tips, and the results were compared with three-point bending tests and tensile tests conducted by using the nano-UTM. A new and less damaging method for gripping the specimens was elaborated for the tensile tests. The results obtained can serve as design guidelines in applications of nanohoneycomb structures. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 42, No. 2, pp. 247–264, March–April, 2006.     

On the mechanical and thermomechanical properties of low-density polyethylene/ethylene-α-octene copolymer blends

Blends of low-density polyethylene (LDPE) and ethylene-octene copolymer (EOC) were obtained. The effect of EOC content and absorbed radiation dose on the mechanical and thermomechanical properties of LDPE/EOC blends are investigated. Particular attention is given to a tensile stress-strain analysis and the “form-memory” effect of the blends. With growing LDPE content, the elastic modulus, the yield stress, and the thermorelaxation and residual stresses of the blends increase, but the ultimate elongation at break decreases, which is caused by the higher crystallinity of polyethylene. As a result of radiation-induced cross-linking, the elastic modulus, the yield stress (at a 1% strain), the ultimate yield strength, and the thermorelaxation and residual stresses increase, while the ultimate elongation at break and the melt flow-behavior index decrease, which is confirmed by the growing gel fraction in the blend. __________ Translated from Mekhanika Kompozitnykh Materialov, Vol. 44, No. 2, pp. 279–286, March–April, 2008.     

Strength and deformation characteristics of poly(ethylene terephtalate)/elastomer blends

Changes in the toughness of crystalline poly(ethylene terephtalate) upon addition of two elastomers — ethylene-propylene-diene terpolymer and ethylene-α-octene copolymer — are investigated. Blends with increasing elastomer content (up to 30 wt.%) were obtained and modified by γ-radiation up to a 300-kGy absorbed dose. The interrelation between the toughness and certain tensile characteristics (elastic modulus, yield stress, ultimate stress, elongation at break, and specific fracture energy) of the blends, with various structural features of components (crystallinity and the degree of cross-linking) and morphology of the blends, is discussed. __________ Translated from Mekhanika Kompozitnykh Materialov, Vol. 44, No. 4, pp. 521–534, July–August, 2008.     

Water uptake and mechanical characteristics of wood fiber-polypropylene composites

The influence of mixing process (in a two-roll mill, high-speed mixer, or twin-screw extruder) on the strength properties of polypropylene/wood fiber composites was studied. The best results were obtained for composites compounded in a twin-screw extruder. The water uptake and the influence of moisture on the flexural strength (σ_fl) and modulus (E_fl) were studied by immersion of the composites in water at 20, 50, and 90°C. Most strongly the moisture affected the value of E_fl, but the degree of water uptake and the change in σ_fl and E_fl also depended on temperature and the presence of a modificator—maleated polypropylene (MAH). MAH improved the strength properties of the composites both in dry and wet states and also decreased the extent of water uptake and swelling in cyclic (soaking/drying) tests. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 42, No. 1, pp. 101–114, January–February, 2006.     

Reduced strength of polymethyl methacrylate saturated with carbon dioxide at high pressure

The effect of external pressure on the amount of carbon dioxide sorbed by specimens of polymethyl methacrylate and on the strength of the specimens after removal of the pressure during the initial period of desorption has been experimentally investigated. The experiments were conducted at 20°C, the pressure varied from 20 to 56 bar.S. M. Kirov Ural Polytechnic Institute, Sverlovsk. Translated from Mekhanika Polimerov, No. 2, pp. 243–246, March–April, 1971.     

The effect of C<Subscript>60</Subscript> fullerene on the toughening of epoxy compositions

The effect of C₆₀ fullerene on the mechanical properties of epoxy resins has been investigated. It is found that this filler affects the tensile modulus and tensile strength of epoxy compositions only slightly, but their impact strength at a 0.01–0.12 wt.% content of C₆₀ increases by about 100–200%. A molecular mechanism of the toughening effect of C₆₀ on epoxy resins is suggested.     

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.     

Influence of nanofillers on the thermal and mechanical behavior of DGEBA-based adhesives for bonded-in timber connections

Results of an experimental investigation into the thermal behavior and mechanical properties of a room-temperature-cured epoxy adhesive (diglycidyl ether of bisphenol A, DGEBA) cross-linked with polyetheramines and filled with different fillers, namely nanosilica, liquid rubber (CTBN), and clay, are reported. The nanosilica and liquid rubber increased the flexural strength and elastic modulus of the adhesive systems; the addition of clay particles raised the elastic modulus significantly, but embrittled the adhesive. Establishing a correct cure time is very important for bonded-in timber structures, as it will affect the bond strength. A study on the effect of cure time on the flexural strength was carried out, from which it follows that the adhesives should be cured for at least 20 days at room temperature. The damping characteristics and the glass-transition temperature of the adhesives were determined by using a dynamic mechanical thermal analysis. The results showed that the filled adhesives had a higher storage modulus, which was in agreement with the elastic moduli determined from static bending tests. The introduction of the fillers increased its glass-transition temperature considerably. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 42, No. 5, pp. 599–614, September–October, 2006.     

Effect of anisometry of a platelike nanofiller on the elastic constants of a transversely isotropic composite

Analysis results for the elastic properties of a composite with a small amount of coplanarly arranged platelike filler particles are presented. The geometrical form of the particles is described by an oblate ellipsoid of revolution. The calculations are performed by formulas obtained by using the Eshelby approach for media with a low concentration of inclusions. The effect of anisometry of the ellipsoidal particles and of the ratio between the elastic moduli of the filler and matrix on the effective elastic constants of the composite is discussed. Calculation results are compared with experimental data for the elastic moduli of a nanocomposite containing completely exfoliated particles of an unmodified montmorillonite. __________ Translated from Mekhanika Kompozitnykh Materialov, Vol. 44, No. 4, pp. 493–504, July–August, 2008.     

Stability of the physicomechanical properties of carbon filaments (fibers)

The relationship between the tensile strength and cross-sectional area of high-modulus carbon fibers is examined. An equation is given for the strength of these promising fillers of composite materials; it is shown that the stability of their tensile strength is determined by the average defectiveness of the surface and degree and stretch, as well as variations in these quantities over any particular group of fibers. The elastic modulus and strength of the carbon fibers are related to the degree of stretch.     

混凝土轴拉加卸载随机损伤模型的建立与试验验证

为了分析及深入探讨混凝土在受拉加载及卸载情况下的力学特性,基于随机损伤本构关系提出了一种混凝土轴拉加卸载模型,推导出了混凝土加卸载的应力 应变关系表达式.为了印证理论表达式,进行了混凝土轴向拉伸及加卸载的试验研究,测得了混凝土的材料参数及其相应的轴拉加卸载应力-应变曲线.结合模型的计算结果,对混凝土的轴拉加卸载试验结果进行了对比分析,结果表明:混凝土轴拉加卸载模型能够预测混凝土的极限强度,同时能描述混凝土的强度软化、加载过程中的弹模折减及卸载后的塑性变形.