Temperature dependence of the strength of a woven composite in a plane state of stress

Conclusions On the basis of an analysis of test results obtained by typical patterns of simple momentary quasistatic loading at various temperatures within the 20–150°C range, we have found a family of strength surfaces for a woven organic plastic material in a plane state of stress in the reinforcement plane. These experimentally found strength surfaces can be approximated by the equation of a second-degree surface. It has been established that the range of safe states of stress in the stress space narrows nonuniformly with rising temperature; namely, this narrowing is accompanied by a shift of the center and a reorientation of the axes of the strength ellipsoid. We have revealed and described the temperature dependence of the components of the strength surface tensors involved in the strength criterion. The data can be used for predicting the strength of a composite material under consideration when the latter is subject to simple quasistatic loading patterns in the three-dimensional (₁₁, ₂₂, ₁₂) stress space in the reinforcement plane within a given test range of temperatures.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 3, pp. 452–457, May–June, 1978.     

Influence of edge effects on the state of stress and strength of carbon composites in tension

Conclusions A difference has been found between the theoretical and experimental strength values. The theoretical strength of flat strips with straight edges, obtained on the basis of the two-dimensional solution within the elastic range for a composite with fiber angle < 30°, is higher than the actual strength. The difference is due to the failure to take account of interlaminar shear. The strength tensor is applied to a monolayer with respect to which the internal state of stress and the nature of the interrelationship between the layers are not taken into account. In this connection, it is desirable to supplement the given criterion with a condition which would take account of the difference between the stresses _xy in adjacent layers. If the directions of reinforcement > 30°, then the agreement between the theoretical strength of flat strips of variable width, obtained on the basis of the two-dimensional solution within the elastic range using the tensor strength criterion, is in good agreement with the actual strength obtained experimentally. The big differences between the actual and theoretical strengths for the three-dimensional solution are attributable to the physical and geometric nonlinearity in the boundary zone between the elementary layers of the composite.Translated from Mekhanika Kompozitnykh Materialov, No. 2, pp. 263–270, March–April, 1985.     

Evaluating the effect of whiskerization of the fibers of composites on the fundamental frequency of a laminated cylindrical shell

Conclusion The calculations showed that whiskerization of the reinforcement of the structural material of multilaminate shells makes it possible in some cases to increase the fundamental vibration frequency of the structure up to 15–20%. In combination with the well-known [1] effect of improved strength characteristics for a whiskerized composite in the transverse and shear directions, this finding allows us to conclude that whiskerized structural materials are more efficient than ordinary laminated composites in shell-type load-bearing structures. Here, the greatest benefit can be expected in the case of whiskers which have higher elastic moduli than the main reinforcement. Since considerably higher reinforcement intensities can be achieved in whiskerized laminated composites than in composites with a reinforcement characterized by an arbitrary three-dimensional structure, it can be concluded on the basis of the results obtained here that, at least for shells of moderate thickness (10 < R/h 50), whiskerized composites are the optimum structural material for load-bearing shells.Translated from Mekhanika Kompozitnykh Materialov, No. 6, pp. 1022–1027, November–December, 1987.     

Strength and deformation properties of fiberglass-plastic in the flat stressed state as a function of reinforcement structure

Conclusion Limiting strength values have been ascertained in the flat stressed state as a function of reinforcement structure. The change in each strength surface tensor component as a function of reinforcement intensity has been approximated by the piecewise-linear approximation method. A strength condition has been derived which can be used in optimization problems. The problem of the optimum reinforcement structure of a composite at various ratios of the stresses ₁₁, ₂₂, and ₁₂ has been examined. By using the strength condition, one can predict strength values for structures which appear in the class of materials in question with various reinforcement intensities. The procedure developed can be used in designing composite materials.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 5, pp. 848–859, September–October, 1978.     

Correlation between static and fatigue strength of reinforced plastics

The conventional estimation of the fatigue strength of glass-fiber-reinforced plastics from the static strength in tension is shown to be inefficient by analysis of experimental data. It was found that a more reliable correlation is observed between their static and fatigue strengths as a result of referring _–1 to *, where * is the smaller of the stresses in tension or compression, determined from the inflection points on the static deformation curves. Views are expressed on the use of the correlation found for reducing the amount of fatigue tests.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, Vol. 9, No. 3, pp. 468–474, May–June, 1973.     

Strength of fibrous composites

Research on the effect of various factors on the strength of fibrous composites in tension, compression, shear, bending and plane stress, published in the journal Polymer Mechanics, is reviewed. The more important future trends are outlined.Presented at the 2nd All-Union Conference on Polymer Mechanics, Riga, November 10–12, 1971.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 3, pp. 553–562, May–June, 1972.     

Experimental investigation of the anisotropy of strength of bolted joints of laminated composites upon variation of geometric and design parameters

Conclusion In the present work we investigated the anisotropy of the characteristics of tensile (buckling) strength and elasticity of plates made of KMU-4L in a bolted joint in a wide range of parameters of the material (stacking, number of layers), geometric parameters of the joint, parameters of the fastening element and of design parameters. An analysis of the results showed that the strength of the bolted joint under static loading can be increased for thin plates with h = 1–2 mm by local reinforcement of the zone of the hole by layers with = ±45 °, and also by increasing the moment of preliminary tightening; for plates with h = 3–4 mm the only feasible measure is increasing the moment of preliminary tightening to 20–40 Nm. All the results of the experiment were graphically generalized in the form of surfaces of maximal stress; this made it possible to predict the load bearing capacity of a joint for intermediate values of the parameters by calculating the safety factors.Translated from Mekhanika Kompozitnykh Materialov, No. 2, pp. 268–272, March–April, 1990.     

Creep and long-time strength of oriented glass-reinforced plastics in interlaminar shear

The creep and long-time strength in interlaminar shear and the creep in compression in the direction of the reinforcement have been experimentally investigated for certain types of oriented glass-reinforced plastics. The specimens in the interlaminar creep tests took the form of short beams loaded in bending. The experimental creep data for shear and compression are well described by the hereditary theory with a kernel of the Abel type (shear) or in the form of a Rabotnov function (compression). If the stresses are constant in time, good agreement with experiment is also given by Findley's form of the aging theory. A deformation criterion of interlaminar shear strength is also obtained. The experimental curves and values of the creep and long-time strength constants are presented.Translated from Mekhanika Polimerov, No. 6, pp. 1003–1012, November–December, 1971.     

Bending of prestressed glass-reinforced plastic beams

Initial waviness of the fibers and prestressing are investigated in relation to their effect on flexural stiffness and strength for beams with low shear strength. It is shown that prestressing the reinforcement increases the flexural stiffness but at the same time adds to the shear correction as a result of an increase in the modulus of elasticity E_x in the direction of reinforcement and the insensitivity of the shear modulus G_xz to prestress. It is established that prestressing increases the shear strength and the degree of anisotropy . Materials of two types are investigated: unidirectional (AG-4S) and cloth-reinforced (SKT-11).Mekhanika Polimerov, Vol. 3, No. 5, pp. 888–893, 1967     

Effect of the geometry of the structure on the strength distribution of multilaminate tridirectional reinforced plastics

Conclusion The proposed analytical method makes it possible to predict the strength distribution of an LRP of the type [0/±]_s from its structure (the geometry of the packet, the number of layers, and the strength properties of the layers) in a plane stress state. Allowance is made for the random character of the strength properties of the layers, which makes it possible to evaluate the reliability of the LRP for both determinate and random loading. A criterion was formulated for the optimum design of the structure of an LRP with respect to ensuring maximum reliability for specific loading conditions. We also evaluated the effect of the parameters of the structure and the characteristics of the plane stress state on reliability. According to the results of a numerical analysis performed with the above-developed structural model of the failure of an LRP — with allowance for the random character of the strength properties of the layers — the imbalance of the laminated packet which occurs during failure can be ignored. The method used to predict the strength distribution of the LRP, involving determination of the strength distribution law of an RSE and subsequent examination of the loading of parallel-connected RSEs, is promising for other reinforcement schemes as well.Translated from Mekhanika Kompozitnykh Materialov, No. 5, pp. 805–812, September–October, 1988.     

Elasticity and strength of polyurethane foams with cylindrical cavities

The strength and elasticity of layers of polyurethane foam with cylindrical cavities have been investigated on the interval of variation of the specific weight of the foam from ₀ = 0.2 to ₀ = 0.7 gf/cm³. A correlation has been established between the proportionality limit in compression along the channel axes and the modulus of elasticity of a cylindrical tube of foam with a radially restrained outer surface taken as a model of a layer with cylindrical cavities. The experimental results are compared with the theory of elastic compression of tubes with free and restrained ends.Translated from Mekhanika Polimerov, No. 5, pp. 813–819, September–October, 1971.     

Deformation and strength properties of carbon-reinforced plastics in compression

The deformation and strength properties of unidirectionally reinforced carbon plastics have been experimentally investigated for uniaxial compression in the longitudinal and transverse directions and at 45° to the direction of reinforcement in both short-time and long-time tests. On the basis of the deformation properties of the components an attempt is made to describe the creep curve of the plastic in all three loading directions. The Mohr theory is used to predict the compressive strengths in the direction of reinforcement and at right angles to the reinforcement.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 1, pp. 29–35, January–February, 1973.     

Elastic behavior and onset of cracking in cement composite plates reinforced by perforated thin steel sheets

Thin cement mortar plates reinforced by perforated thin steel sheets have been tested in four-point flexure loading. Six kinds of sheet reinforcement and to additional ones (for control) were used. Perforated sheets of the Daugavpils Factory of Machinery Chains differed by their thickness (0.6–1.8 mm), shape (round, rectangular, oval, dumbbell), and mark of steel (St. 08, 50, 70). Dimensions of plantes were 100×20×2 cm. Cements-sand mortar with a 12 ratio of cement PZ35 and river sand of 3 mm grains was used as a matrix. Control specimens of similar dimensions and matrix were reinforced by wire cages and meshes (ferrocement). The testing was performed using an UMM-5 testing machine. Maximum deflection (at the midspan), tension, and shear strains were recorded. The expeimental data are presented in tables and graphs. The testing results showed that the elasticity modulus of material was in good agreement with the admixture rule; an onset of cracking for all types (excluding one) practically did not differ from reference samples; the mode of fracture in typical cases included an adhesion failure and significant shear strains. In one case the limit of the tension strength of the reinforcement was achieved.Presented at the Ninth International Conference on the Mechanics of Composite Materials. Riga. October, 1995.Latvian Building Research Institute. Riga, LV-1012 Latvia. Published in Mekhanika Kompozitnykh Materialov, Vol. 32, No. 2, pp. 195–201, March–April. 1995.     

Experimental investigation of the strength of glass-reinforced plastics in biaxial compression in three planes of symmetry

Glass-reinforced plastics (GRP) with fiber (wound) and fabric (hot-molded textolites) reinforcement have been investigated in biaxial compression in the three planes of symmetry. The high strength in biaxial compression in the transverse plane, observed for all the GRP investigated, is explained. A method of investigating the tensile strength of the fiber components of the GRP from the results of testing a unidirectional composite in biaxial compression in the transverse plane is proposed. The possibility of a nondestructive estimation of the strength of GRP in biaxial compression at an arbitrary stress ratio is demonstrated.Translated from Mekhanika Polimerov, No. 1, pp. 63–72, January–February, 1976.     

Effect of chemical nature of matrix on the strength of bonds with Armos aramide fibers

This investigation deals with adhesion between high-strength and high-modulus Armos aramide fibers (polyheteroarylene-co-p-phenyleneterephthalamide) and a series of different thermosetting matrices. The effect of the chemical nature of the matrix, time-temperature conditions of bond formation, and test temperature on the strength of the fiber-matrix interface was studied. Modified epoxy and heat-resistant matrices were used as adhesives. As a measure of adhesion, the shear adhesive strength ₀ determined by the fiber pull-out technique was used. It was found that both the adhesive strength and the fracture location in adhesive bonds depended on the nature of the matrix. At room temperature, chlorine-containing epoxy matrices provide the highest values of ₀, while the smallest strength of the interface is observed for bonds with heat-resistant (bismaleimide, oligomethacrylate) matrices. Fracture of adhesive bonds does not always occur at the fiber-matrix interface. A number of the specimens failed near the interface of the fiber. With temperature increase, the values of ₀ decrease. The adhesive strength falls especially drastically in the region of matrix softening. An advantage of heat-resistant matrices is that they retain 60–67% of ₀ value even at 250°C. The strength of unidirectional composites based on the investigated fibers and matrices was also estimated under different loading conditions such as tension, shear, compression, and bending. It was found that the strength in shear and compression did not correlate with the interface strength. The values of _c in bending and tension increased linearly with increase of ₀. The obtained dependences _c–₀ were compared with those of composites based on the SVM polyheteroarylene fibers determined by us earlier.Presented at the 10th International Conference on the Mechanics of Composite Materials (Riga, April 20–23, 1998).Translated from Mekhanika Kompozitnykh Materialov, Vol. 34, No. 3, pp. 391–406, May–June, 1998.     

The effect of stabilizers on the fatigue strength of polycaproamide under tension - compression

The authors give the results of an investigation of the influence of various stabilizers (quin-hydrone, iodine, potassium iodide, DNPDA, and MoS₂) on the fatigue strength and endurance of polycaproamide (PCA) under cyclic compression/tension, both with _a = const and with _a = const. The effectiveness of the stabilizers is mainly governed by the structural changes in the polymer during the stabilization process.Kaunas Polytechnic Institute. Translated from Mekhanika Polimerov, Vol. 9, No. 3, pp. 562–564, May–June, 1973.     

Adhesion of polysulfone—Liquid crystalline polymer blends to fibers

The effect of modifying a thermostable thermoplastic matrix with a liquid-crystalline (LC) polymer on its adhesion to fibers was investigated. Poly(arylene sulfone), LC polyether, and blends of them of varying composition were used as the matrices, and high-strength steel wire 150 m in diameter and alkali-free glass fibers 150–300 m in diameter were the substrates. The adhesive shear strength , measured by the pull-out method, was used as the measure of adhesion. The samples were formed at a temperature of T_f=300°C, and the heat treatment time t_f varied from 15 min to 2 h. It was found that when the values of t_f increased, the dependence –t_f was described by a curve with a maximum. The maximum was observed for t_f=30 min. The adhesive strength at the bonds of the poly(arylene sulfone) with the steel wire was sufficiently high (comparable to the adhesive strength of epoxy—4,4-isopropylidenediphenol matrices). The adhesive strength of the LC polyether with the steel wire was much lower. The strength of the interface of poly(arylene sulfone), LC polyether, and blends in bonds with the steel wire was much higher than in bonds with the glass fiber. The adhesive strength of the blends as a function of the concentration of LC polyether was described by a curve with a maximum. The maximum adhesive strength with fibers was provided by matrices containing 2–7% LC polyether.Translated from Mekhanika Kompozitnykh Materialov, Vol. 33, No. 4, pp. 433–442, July–August, 1997.     

Impregnation of fibrous fillers with polymeric binders 6. Effect of parameters of ultrasound treatment on strength properties of wound fibrous composites

Conclusion Tests of annular PCM specimens — fiberglasses and organoplastics formed by the method of wet winding using low-frequency ultrasound — were conducted. Three alternate US-impregnation schemes were investigated. Optimal alternate schemes and insonification parameters were established experimentally: frequency, amplitude, intensity, and time. Optimization was carried out on the basis of investigation of the relationships between the values of the tensile, compressive, bending, and interlayer-shear strengths, and also the relative content of filler as a function of treatment parameters. It was shown that deviation of insonification parameters from the optimal leads to a reduction in strength characteristics, as well as to a different character of specimen failure in tension and interlayer shear.The effect of US modification in producing PCM is provided by an increase in structural uniformity, and improvement of the strength and production characteristics of the polymer matrix, as well as by a reduction in the composite's effectiveness as a result of a reduction in the content of air inclusions and improvement of the binder's distribution across the section of the filler.For previous communication, see [9].Translated from Mekhanika Kompozitnykh Materialov, No. 4, pp. 724–731, July–August, 1989.     

Experimental investigation of the strength of anisotropic materials in biaxial and triaxial compression

It is proposed that the strengths in uniaxial and uniform biaxial compression in the direction of the axes of symmetry be used as parameters. The applicability of a strength criterion in the form of a fourth-degree polynomial to glass-reinforced plastics in biaxial compression in the plane of reinforcement and the transversal plane and to wood in triaxial compression has been experimentally confirmed.S. M. Kirov Leningrad Forest Engineering Academy. Translated from Mekhanika Polimerov, No. 6, pp. 991–996, November–December, 1973.     

Mechanical properties of a composite with a carbonized matrix

The effect of reinforcement schemes that differ in the magnitude of the disorientation angle of neighboring filler layers on the mechanical properties of a "carbon-carbon" composite under extension, compression, or bending is shown.Translated from Mekhanika Polimerov, No. 2, pp. 235–240, March–April, 1976.