Effect of interlaminar stiffness and strength of fiber-reinforced materials in plane stress

The distortion of the strains associated with a difference in the influence coefficients of the reinforcing layers is considered. The effect of the compliance of the intermediate resin layers on the average stiffness of composites is estimated. An attempt is made to explain in these terms the experimentally established deformation characteristics of three-dimensionally reinforced materials [12, 13] and laminated plastics loaded at an angle to the direction of reinforcement [11].Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 5, pp. 876–883, September–October, 1970.     

Creep and long-time strength of unidirectional reinforced plastics in compression

An attempt is made to predict the creep and long-time strength of unidirectional reinforced plastics in compression in the direction of the reinforcement from the properties of the components. The reinforcement is assumed to be elastic, while the resin is described by a Boltzmann-Volterraintegral equation with fractional-exponential Rabotnov kernel. Experimentally obtained creep and long-time strength curves are presented for unidirectional reinforced plastics.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 5, pp. 830–835, September–October, 1970.     

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.     

Three-dimensionally reinforced woven materials

The article presents experimental data on the deformation and strength characteristics of a number of materials reinforced with three-dimensionally woven multilayer glass fabric, which are compared with the corresponding data for traditional glass-reinforced plastics. The accuracy of the formulas proposed [1] is evaluated. The effect of prestressing the warp and weft fibers on the mechanical properties of the material was investigated. The effectiveness of simultaneous prestressing of the warp and weft was evaluated; the magnitude of the optimal force was established.See [1] for report 1.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 3, pp. 471–476, May–June, 1970.     

Creep in unidirectionally reinforced polymer composites

Conclusions We have explored here the possibilities of predicting the permanent creep in unidirectionally reinforced polymer composite materials from the results of accelerated testing of their components. The mutually independent components of the viscoelastic compliance tensor under conditions of creep have been calculated with the aid of the Laplace transformation and an earlier verified variant of determining the mean elastic characteristics of a composite material, whereupon the originals of the sought functions have been obtained by a numerical inverse Laplace transformation. Experiments were performed with unidirectionally reinforced materials, a glass-plastic, an organoplastic, a carbon-plastic, and a boroplastic, all tested in tension in the direction of fiber reinforcement and in the transverse direction, also in longitudinal shear. The possibilities of predicting the creep in composite materials with the aid of functions characterizing the viscoelastic compliance of the binder and obtained by the method of temperature-time analogy have been confirmed twofold: by long-duration (up to 3 yr) control tests performed on the given materials and by control calculation of the creep in these composite materials from results of direct long-duration (up to 5 yr) tests performed on specimens of the binder.Translated from Mekhanika Kompozitnykh Materialov, No. 2, pp. 215–223, March–April, 1984.     

Modern trends in the development of fibrous composites

The two main trends in the development of high-modulus composites are considered. Improved methods of calculation sensitive to the effects associated with the weak shear and transverse characteristics are reviewed. It is shown that the disadvantages of composites with a traditional arrangement of the reinforcement can be overcome. The properties of boron- and carbon-reinforced plastics with a traditional reinforcement structure are described and compared with those of three-dimensionally structured materials with two- or three-strand reinforcing. Whiskerized fiber reinforcement is also considered. A program of further research on high-modulus composites is 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. 541–552, May–June, 1972.     

Deformative characteristics of plastics reinforced with high-modulus anisotropic fibers

The independent elastic constants of plastics unidirectionally reinforced with transversely isotropic fibers have been determined. It has been assumed that the distribution of reinforcement in a transverse section of the plastic is regularly rectangular or hexagonal. To determine the transverse elastic modulus and the shear modulus in the plane of reinforcement, a constancy-of-plane-sections hypothesis was used. Values of deformative characteristics determined by the assumed calculational dependences have been compared with the experimental ones for plastics reinforced with graphite fibers.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 4, pp. 631–639, July–August, 1972.     

Tensile strength of unidirectionally reinforced plates stretched at an angle to the direction of reinforcement

Conditions for the fracture of a unidirectionally reinforced plate under uniaxial tension at an arbitrary angle to the direction of reinforcement are proposed. The fracture conditions are applicable to the case where the adhesion strength between the bond and reinforcement is greater than the strength of the polymer bond. The strength of the polymer bond in the volume stressed state is determined by an energy criterion.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, Vol. 9, No. 3, pp. 482–486 May–June, 1973.     

Effect of the sign of the shear stresses on the shear strength of reinforced plastics

The effect of the sign of the shear stresses on the shear strength has been investigated for a glass-reinforced plastic of the SVAM type with various ratios of the longitudinal and transverse reinforcement in shear in the plane of reinforcement in axes turned through 45° relative to the direction of reinforcement. More than 80 tubularspecimens were tested. The shear strengths T ₄₅ ⁺ and T ₄₅ ^– corresponding to shear stresses of different signs can be found from uniaxial tests in tension and compression in the direction of the reinforcement.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 2, pp. 262–268, March–April, 1971.     

Structural characteristics of materials reinforced with high-modulus fibers

Research on the mechanics of boron and carbon-reinforced plastics is briefly reviewed. The design and testing characteristics of these materials associated with the high degree of anisotropy of their elastic properties, as compared with those of glass-reinforced plastics, are discussed. Problems relating to testing at an angle to the direction of the reinforcement, the effect of misorientation and distortion of the fibers, and the consequences of the low shear strength are considered. Experimental confirmation has been obtained by testing unidirectional (1 : 0), orthogonally reinforced (1 : 1 and 2 : 1), and tridirectional (1 : 1 : 1 in the 0°, +60°, and –60° directions) boron and carbon-reinforced plastics.DeceasedInstitute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 4, pp. 676–685, July–August, 1971.     

Relation between the structural and strength characteristics of textolite glass laminates

The authors derive approximate relations between the microstructure parameters of densely reinforced textolite glass laminates and the static strength and fatigue life of the material. The relations obtained are compared with the results of experiments performed on materials with varying degrees of crimping of the filaments and varying densities of the cloth reinforcement.Mekhanika Polimerov, Vol. 3, No. 2, pp. 302–308, 1967     

Structural theory of the tensile and compressive strengths of reinforced plastics

The strength conditions of unidirectionally and orthogonally reinforced plastics subjected to uniaxial tension and compression in the direction of, and at an angle to, the reinforcement are developed, with consideration given to possible failure of the fibers, binder, or the bond between the fibers and binder. Proposed strength conditions take into account the structure of the material, the properties of its components, and stress concentration.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 6, pp. 988–995, November–December, 1975.     

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 deformability of reinforced polymers in tension normal to the fibers

The strength and deformability of reinforced polymers in tension across the fibers is investigated. It is assumed that the polymer deforms as an ideal elastoplastic body. Relations are obtained for the nature of the deformation of the polymer between the fibers and the strength and deformability of the composite with allowance for the structural distribution of the components. Theoretical stress-strain diagrams are presented for composites with different reinforcement densities and resin elasticities. The theoretical values of the strength and deformation of reinforced polymers with the load applied across the fibers are compared with the results of experiments on model specimens of epoxy-Thiokol polymers.Leningrad Mechanical Institute. Translated from Mekhanika Polimerov, No. 4, pp. 682–687, July–August, 1970.     

Investigation of the mechanical properties and internal stress field of directionally reinforced materials in transverse tension

The dependence of the transverse Young's modulus and Poisson's ration on the reinforcement ratio, type of structure, and internal fiber radius has been investigated. The microstress field in the structure of a composite in transverse tension is analyzed. Simple approximate expressions, which can be used to determine the corresponding mechanical characteristics without analyzing the state of stress, are obtained from the exact equations. A medium reinforced with doubly periodic groups is taken as a model. The exact solutions of the problem are obtained by means of elliptic functions.Institute of Mechanics, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Mekhanika Polimerov, No. 5, pp. 812–817, September–October, 1970.     

Technological anisotropy of chaotically reinforced glass-fiber-resin materials

The authors investigate the strength properties of chaotically reinforced glass-fiber-resin materials in compression-molded and injection-molded artifacts of complex shape and also the density, water absorption, and binder content in various zones in the artifacts. They find anisotropy to be present in the strength indices, owing to orientation of the reinforcing elements during flow of material in the mold. They determine the coefficients of anisotropy of the strength (these are defined as the ratios of the strength in the flow direction to the strength in a perpendicular direction). They suggest a method of estimating the degree of orientation of the reinforcing elements in artifacts made of nonoriented glass-fiber-resin materials.All-Union Scientific-Research Institute of Glass-Fiber-Reinforced Plastics and Glass Fiber, Moscow Region. Translated from Mekhanika Polimerov, Vol. 9, No. 3, pp. 507–515, May–June, 1973.     

Experimental evaluation of the effect of prestressing the fibers in two directions on certain elastic characteristic of woven-glass reinforced plastics

The effect of prestressing the reinforcing fibers in either of two mutually perpendicular directions on the elastic characteristics of woven-glass reinforced plastics has been experimentally investigated. It is shown that an increase in the elastic characteristics in the prestress direction is accompanied by a decrease in the direction at right angles. Applying the same prestress in both directions considerably improves the moduli of elasticity (E_x, E_y). The possibility of using Bolotin's theory [1] and the relations proposed in [4, 6] to estimate the effect of regular distortions of the fibers on the elastic moduli of woven-glass reinforced materials is examined.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, Vol. 4, No. 5, pp. 859–863, September–October, 1968.     

Tensile testing of high-strength unidirectional composites

Conclusions 1. In order to obtain stable values of the tensile strength of high-strength unidirectional composites it is necessary to use bar specimens with pressboard end reinforcement 90–110 mm in length.2. In determining the strength on specimens without end reinforcement it is necessary to select their size and shape with allowance for the transverse compressive strength, the shear strength and the taper of the grip jaws.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. All-Union Scientific-Research Institute of Aviation Materials, Moscow. Translated from Mekhanika Polimerov, No. 4, pp. 717–723, July–August, 1978.     

Strength of orthogonally reinforced plastics in uniaxial tension

Failure conditions are proposed for an orthogonally reinforced plastic in uniaxial tension at an arbitrary angle to the directions of reinforcement. The failure conditions are formulated for the case when the strength of the bond between the resin and the reinforcement is greater than the strength of the resin. The strength of the resin, which is in a volume state of stress, is determined by an energy criterion.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 4, pp. 629–633, July–August, 1973.     

Deformation and stress distribution in a layered plastic

The stress distribution over the unidirectionally reinforced layers is investigated in relation to the layer thickness ratio and the direction of loading of a two-way reinforced plastic. An expression is obtained for the modulus of elasticity of the layered plastic in an arbitrary direction relative to the directions of reinforcement. The effect of the geometry of the structure of the layered material on its deformation properties is experimentally illustrated.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 3, pp. 563–570, May–June, 1972.