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.     

Relation between the strength of polyethylene-steel adhesion bonds and the thickness of the ultrathin layer of polymer remaining on the surface of the metal

On the basis of the principle of ellipsometry and, moreover, the method of pyrolitic decomposition of the polymer with subsequent chromatographic analysis of the products of pyrolysis, a procedure has been developed for experimentally determining the thickness of the ultrathin layer of polymer that remains on the metal after failure of a polyethylene-steel adhesion bond. The nature of the relationship between the thickness of the residual polymer layer (200–800 Å) and the strength of the adhesion bond is established. It is concluded that the cohesion strength and the deformation at failure of the layer of adhesive next to the more rigid and stronger boundary layer have a decisive influence on the strength of the adhesion bond.Riga Polytechnic Institute. Translated from Mekhanika Polimerov, No. 3, pp. 420–424, May–June, 1976.     

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 adhesion of epoxy resin ED-5 to metals

The strength and thermostability of metals bonded together by epoxy resin ED-5 pass through a maximum with increase in the concentration of curing agent (tetraethylene pentamine). Cohesion type fracture changes to adhesion type at a certain concentration of the hardener. It has been suggested that a change in the character of the polymermetal bond is the cause of the observed dependences.The appearance of breaks or maxima at 80°–120° C on the curves of the temperature dependence of the strength of the glued joints is explained by changes in the physical state of the polymer.Mekhanika Polimerov, Vol. 1, No. 6, pp. 108–113, 1965     

Current state and prospects for development of the physical theory of brittle strength of polymers

Various mechanisms of brittle fracture and theories of the time dependence of the strength of rigid polymers are examined. The effect of various types of mechanical losses (deformation, dynamical, surface) on polymer strength and life is analyzed. The principal shortcomings of Griffith's theory of strength are pointed out. From an examination of the two basic mechanisms of brittle fracture of polymers-nonthermal and thermal (thermal fluctuation)-it is concluded that the latter is the more important. A comparison is made of the fluctuation theory of polymer strength with new experimental data on polymethyl methacrylate, and the causes of the transition from brittle to nonbrittle fracture are discussed. The effect of molecular orientation on the strength of polymers is examined on the basis of the fluctuation theory of strength.Mekhanika Polimerov, Vol. 2, No. 5, pp. 700–721, 1966     

On the fracture mechanism in filled rubbers

Possible fracture mechanisms in filled rubbers are examined in the light of the adhesion theory of elastomer reinforcement. The cases of rubbers containing inactive fillers, when the crack grows along the elastomer-filler boundary, and rubbers containing active fillers, when the crack propagates through the polymer, are considered. The effect of the presence of chains of filler particles on the fracture mechanism is examined.Lomonosov Moscow Institute of Fine Chemical Technology. Translated from Mekhanika Polimerov, Vol. 5, No. 1, pp. 127–133, January–February, 1969.     

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.     

Flexible composites, strength, deformation, and fracture processes. 1. Reinforcement structures and tensile strength

Fiber-reinforced flexible composites are extensively used for different kinds of applications, for example, tubes, drive belts, tires, and coated fabrics. Typical for these materials are matrix materials allowing large strain deformation and reinforcement structures allowing bending. Apart from the tensile strength and limited bending stiffness, damage resistance and ductile-brittle transition characteristics are discussed. The tensile strength usually follows the rule of mixture. The mode of fracture and damage resistance, however, strongly depend on penetration of the matrix into the fiber bundles, textile structure, and internal friction. Models for the work of fracture and the ductile-to-brittle fracture transition are discussed.Presented at the 10th International Conference on the Mechanics of Composite Materials (Riga, April 20–23, 1998).Published in Mekhanika Kompozitnykh Materialov, Vol. 34, No. 6, pp. 747–760, November–December, 1998.     

Micromechanics of polymer fracture

Small-angle x-ray scattering has been used to investigate the formation of embryonic submicroscopic cracks in polymers under a load. The main characteristics of crack formation in various loading regimes are analyzed. It is shown that there is a relation between the submicrocrack concentration and the deformation of the loaded polymer. The principal parameters of crack formation determining the strength properties of the polymer are found to be the transverse dimension of the initial submicrocracks relative to the loading axis, which is determined by the structural heterogeneity of the material, and the submicrocrack concentration in the prefracture state. The principles of the micromechanics of polymer fracture are formulated on the basis of the results of an analysis of the quantitative relationship between these parameters. The dominant role of the surface in the fracture process is demonstrated by comparing the parameters of crack formation in the interior and at the surface of the loaded polymer.A. F. Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 5, pp. 792–801, September–October, 1974.     

Fracture propagation in a boron-aluminum composite

Conclusions 1. Application of the linear mechanics of fracturing to composites of the boron-aluminum type is justified, since it has proven possible to determine in an experiment the value of the fracture strength which characterizes the resistance of the material to fracture.2. The fracture strength of boron-aluminum turns out to be higher than the same characteristic of the matrix material. Boron-aluminum is a material with a high resistance to fracture, whose surface is normal to the direction of the fibers. The fracture work of boron-aluminum with a fiber content of 50% is approximately three times higher than the fracture work of the unreinforced matrix.3. At present there is no computational model of a composite which would permit reliably estimating the value of the fracture strength and optimizing a composite for this characteristic. Such a model should intrinsically take account of the statistical characteristics of a fiber.4. The data obtained can also be interpreted as confirmation of the existence of a scaling dependence of the strength of a composite in the case of supercritical reinforcement.Institute of Solid-Body Physics, Academy of Sciences of the USSR, Moscow. Translated from Mekhanika Polimerov, No. 6, pp. 1010–1017, November–December, 1976.     

Effect of hydrostatic pressure on the strength properties of polymer materials

On the assumption that the strength characteristics of homogeneous polymer materials depend on the specific volume, equations are derived for the dependence of the strength or high-elastic limit on the hydrostatic component of the stress tensor and temperature. The ultimate strengths in simple tension, compression and shear are considered in relation to brittle and plastic fracture.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 3, pp. 449–454, May–June, 1969.     

Effect of structural and processing parameters of the reinforcement on the strength of unidirectional metalfiber composites under different types of loading

Conclusion The above studies show that the strength of unidirectional FCP's in uniaxial and biaxial tension is largely determined by the strength of the bond between the fibers and matrix and the internal geometry of the reinforcement. This dependence is particularly significant when the FCP product is loaded across as well as along the fibers. Impulsive loading as a method of making FCP's makes it possible to broadly vary the strength of the fiber-matrix interfaces. The unidirectional FCP's and tubular products of FCP's obtained by this method have a high strength under different types of loading, and this strength can be predicted by calculation.Presented at the Sixth All-Union Conference on the Mechanics of Polymer and Composite Materials (Riga, November, 1986).Translated from Mekhanika Kompozitnykh Materiaiov, No. 6, pp. 1033–1038, November–December, 1986.     

Time dependence of the strength of polyvinylene carbonate and its low-molecular-weight analog

The temperature-time dependences of the strength of polyvinylene carbonate (PVCa) and its solid hydrogenated monomeric analog, ethylene carbonate (EC), have been investigated. The parameters of these relations have been determined graphically and by calculation. The fracture energy barriers of the polymer and the monomer are compared and the sublimation energy of the monomer is determined. It is concluded that the polymer fracture process is directly related to the rupture of both chemical and intermolecular bonds.Mekhanika Polimerov, Vol. 3, No. 6, pp. 1054–1059, 1967     

“Fundamental” and “practical” adhesion in polymer-fiber systems

The relationship between adhesion and bond strength in thin fiber-polymer matrix systems was studied. Adhesive interaction in composite materials was analyzed within the scope of thermodynamic and molecular-kinetic theories of adhesion. Methods based on wetting are shown to give poor estimation of the work of adhesion in fiber-polymer systems, which is due to their low sensibility to donor-acceptor interactions taking place at the interface. Important information about the acidity and basicity of contacting surfaces can be obtained by using inverse gas chromatography to investigate the thermodynamics of adsorption. The calculation of the work of adhesion including acid-base interactions shows the best agreement with the bond strength in the same systems. The local (ultimate) interfacial shear strength is proposed to characterize the quality of fiber-matrix bonding. Analysis of the relationship between the work of adhesion and adhesive pressure for various systems allowed us to differentiate the dispersive and acid-base components of the local bond strength as well as to estimate distances characteristic of these twoTypes of interaction. For dispersive forces, our estimation gives 7–8Å, i.e., of an order of magnitude of the center-to-center distance for van der Waals interactions. At the same time, the acid-baseInteractions have a characteristic range of 4–5Å and can be attributed to hydrogen bonding. The agreement between the calculated distances and literature data is evidence for the applicability of the proposed method to the analysis of the adhesive interaction in fibrous polymer composites.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. 4, pp. 431–446, July–August, 1998.     

The effect of glass fiber inelasticity on the long-term strength of reinforced plastics subject to uniaxial tension along the fibers

Relationships are presented for determining the long-term strength of reinforced plastics subject to uniaxial tension by a constant load directed along the fibers; these relationships are based on the rheological characteristics of the components of the plastics. The proposed expressions agree quite well with experiment. Treatment of the experimental data by mathematical statistics showed that the relative strain at fracture remains constant; it does not depend either on the relative reinforcement content or on the applied load (i. e., on the time to fracture).Mekhanika Polimerov, Vol. 3, No. 4, pp. 719–725, 1967     

Investigation of the acoustic emission associated with the fracture of cylindrical and flat specimens

The problem of the brittle tensile fracture of an infinite cylinder is examined theoretically. Relations are derived between the parameters of the nonstationary ultrasonic pulse generated at fracture and the fracture characteristics. The conclusions obtained are extended to the tensile fracture of an infinite plate and of two face-bonded plates with a bond zone in the form of an infinite strip. Recommendations are given concerning the choice of specimen geometry. The theoretical conclusions have been checked experimentally. The agreement between the theoretical and experimental data is good.Scientific-Research Institute of Standard and Experimental Design of Residential and Communal Buildings, Leningrad Zone. Translated from Mekhanika Polimerov, No. 1, pp. 141–148, Janaury–February, 1976.     

Strength of adhesion of epoxy polymer to carbon,glass, and steel fibers

A study has been made of the strength of adhesion of epoxy polymer to fibers of various chemical natures, and it has been shown that the magnitude of the adhesion depends not only on the nature of the fiber but also on its surface geometry. Questions are discussed on the effect of residual (internal) stresses arising during the process of heat-treating or cooling samples of bonded articles on the strength of the adhesive bond.Institute of Chemical Physics, Academy of Sciences of the USSR, Moscow. Translated from Mekhanika Polimerov, No. 1, pp. 37–42, January–February, 1974.     

Theoretical and experimental investigation of anisotropic damage in textile-reinforced composite structures

In the present work, a phenomenological plane-stress damage-mechanics-based model for textile-reinforced composites is presented and its predictive capability is evaluated by carrying out a series of experimental tests. Damage variables are introduced to describe the evolution of the damage state and, as a subsequence, the degradation of material stiffness. For calculating the nonlinear stress and strain distribution of complexly loaded composites with a textile reinforcement, a special emphasis has to be placed on the interaction between the fiber failure due to the stress in the fiber direction and the matrix failure due to the transverse and shear stresses. This demands the formulation of realistic failure criteria taking into account the microstructural material behavior and different fracture modes. The new failure criteria, like the fracture mode concepts, consider these fracture modes, as well as further fracture types, in the reinforcement plane. The failure criteria are based on equations for failure surfaces in the stress space and damage thresholds in determining the stiffness degradation of the composite. The model proposed was used to characterize the strength and the failure behavior of carbon-fiber-reinforced composites. For this purpose, several unidirectional and bidirectional tests were performed to determine the specific properties of the material. The specimens were investigated by using acoustic emission techniques and strain-controlled tension and torsion tests.Russian translated published in Mekhanika Kompozitnykh Materialov, Vol. 40, No. 6, pp. 791–810, November–December, 2004.     

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.     

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.