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.     

Molecular orientation and the tensile strength of polystyrene

The tensile strengths of uniaxially oriented polystyrene films with molecular orientation determined by IR spectroscopy are compared. Two different orientation methods involving different time and temperature regimes are employed. It is found that the tensile strength is a unique function of the degree of orientation of the molecular segments of the polystyrene carbon chain irrespective of the means used to orient the specimens and increases linearly with increase in the degree of molecular orientation of the polymer. The experimental dependence is in satisfactory agreement with the results of a theoretical analysis.A. F. Ioffe Physico Technical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 4, pp. 612–615, July–August, 1969.     

Study of the effect of various factors on the low-temperature strength of polyethylene with graft polyacrylonitrile

Low-density polyethylene film has been subjected to graft polymerization with acrylonitrile. The effect of the graft polymerization conditions and subsequent treatment on the strength of the grafted film has been investigated. The strength was measured at -196°C. The grafted film behaves like a heterogeneous material in which the unoriented polyacrylonitrile macrostructures play the part of filler particles. The strength of the grafted film is additively composed of the strengths of the polyethylene and the polyacrylonitrile and the change in strength after stretching, heating, and other treatments is determined by the changes in the degree of orientation of the polyethylene.V. I. Lenin Belorussian State University, Minsk. Translated from Mekhanika Polimerov, No. 4, pp. 589–593, July–August, 1973.     

Effect of gamma radiation on the cracking and mechanical properties of Kapron (polycaprolactam)

The results of a mechano-optical investigation of irradiated oriented polycaprolactam film are discussed. After irradiation comparatively large cracks running at right angles to the orientation axis appear in the film; these are responsible for the reduced tensile strength and deformability of the polymer.Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad; Lenin Tadzhik State University, Dushanbe. Translated from Mekhanika Polimerov, No. 4, pp. 756–758, July–August, 1971.     

Biaxial orientation of amorphous linear polymers

The mechanical properties of biaxially oriented polymethyl methacrylate, obtained on a broad range of stretch ratios and under a variety of orientation conditions, have been investigated. There is a fundamental difference between the variation of the forced elastic limit with increase in stretch ratio, which is monotone increasing, and the variation of such properties as the brittle strength, brittle temperature, true strength and elongation at break, which have an optimum at a certain stretch ratio. It is shown that the presence of an optimum is associated with the transformation of the supermolecular structures in the process of biaxial high-elastic deformation. A relation is established between the mechanical properties of biaxially oriented polymethyl methacrylate (orientation hardening) and the density of the molecular network.For communication 1 see [3].Moscow. Translated from Mekhanika Polimerov, No. 4, pp. 586–593, July–August, 1971.     

Theoretical and maximum attainable strength of various uniaxially oriented polymers (film,fibers)

The theoretical strength of various ideally oriented synthetic fibers was calculated. The calculation was based on an assumption that the rupture of polymers involves simultaneous breaking of molecular chains and pulling asunder the chain ends. The temperature-time dependence of the strength of fibers was analyzed and the maximum attainable strength of fibers of various kinds was calculated to show that it is 2–5 times higher than that recorded in practice. The main causes of the difference between the attainable and attained levels of strength are associated with imperfections of the supermolecular structure and an insufficient degree of orientation of materials of this kind. The principal means of obtaining high strength levels of synthetic fibers were discussed.Mekhanika Polimerov, Vol. 2, no. 6, pp. 845–856, 1966     

Short-time strength of reinforced plastics in uniaxial compression

The short-time compressive strength of one-way and two-way reinforced plastics compressed in the direction of the axes of elastic symmetry has been theoretically and experimentally investigated with allowance for the mechanical properties of the components and the orientation of the fracture (shear) plane.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 4, pp. 621–628, July–August, 1969.     

Strengthening of Polypropylene Strips by Orientational Stretching in Liquids

Data on the orientational stretching of polypropylene strips in process liquids serving as plasticizers of polypropylene are presented. Such a stretching allows one to avoid many complications characteristic of stretching in air and to increase the maximum stretch ratio significantly (up to 15). The stretching of strips is accompanied by changes in the crystalline structure of polypropylene and by transformation of the amorphous phase into a crystalline one. It is established that the specific strength of the orientationally stretched strips as a function of stretch ratio has a maximum. This is caused by the competing processes of macromolecule orientation in the stretching direction and the destruction of molecular bonds, which leads to the origination of microdamages in the strip surface layer and to a growing content of amorphous phase. It is shown that the extraction of process liquids from the stretched strips increases their specific strength by 20–30%.__________Translated from Mekhanika Kompozitnykh Materialov, Vol. 41, No. 2, pp. 251–264, March–April, 2005.     

Long-time strength of high-elastic polymers

This article examines the question of the inadequacy of the Zhurkov equation for describing the laws of long-time strength of polymers, loading of which is accompanied by the development of high-elastic or induced-elastic deformations. On the basis of general considerations concerning the deformation dependence of the stress concentration coefficient and orientation hardening of polymers, a more general relation between static fatigue life and long-time strength is proposed.Mekhanika Polimerov, Vol. 1, No. 6, pp. 85–91, 1965     

Problem of study of durability properties of carbon fibers based on polyacrylonitrile fibers by ultrasonic methods

Conclusions 1. With increase of the HTT of carbon fibers from 1200°C and above, improvement of the structure of the microfibrils occurs and improves their orientation, which leads to increase of elastic modulus and the speed of propagation of ultrasound.2. At the same time, spread of microcracks at the boundaries of the microfibrils is observed, which leads to decrease of strength during stretching and increase of the damping coefficient of ultrasound.Moscow. Translated from Mekhanika Polimerov, No. 4, 724–728, July–August, 1978.     

Mechanism by which tension on the fibrous filler affects the structure and the strength of wound glass-plastics

Conclusions 1. We have studied how the structure of glass-plastics wound with twisted threads is affected by tension on the fibrous filler. The trend of changes in fiber content and porosity, as well as an increasingly nonuniform distribution of the reinforcing filler over the thickness, has been established.2. The effect of a nonuniform filler distribution over the thickness on the tensile strength has been evaluated. The calculated and the experimental curve of strength as a function of the tension have been compared, the former taking into account variable porosity, fiber content, and nonuniformity of fiber distribution over the thickness.3. It has been demonstrated that the change in the strength characteristics of a wound glass-plastic as a function of the tension on the reinforcing filler during winding is due to a combined complex effect of such factors as the overall fiber content and porosity, a nonuniform fiber distribution over the thickness, the fiber orientation, and the degree of mechanical damage in threads due to their interaction with the active components of the winding machine.Report presented at the Third All-Union Conference on Polymer Mechanics. Riga, November 10–12, 1976.Translated from Mekhanika Polimerov, No. 3, pp. 439–444, May–June, 1977.     

Orientational Design of Anisotropic Materials Using the Hill and Tsai–Wu Strength Criteria

Linearly elastic two- and three-dimensional orthotropic materials are considered. The problems of optimal material orientation are studied in the cases of the Hill and Tsai–Wu strength criteria. The necessary optimality conditions are derived for a 3D orthotropic material. In the case of a 2D orthotropic material, an analytical solution is obtained. An analysis of global and local extrema is presented.     

Effect of heat treatment on the strength and supermolecular structure of oriented polycaproamide

The mechanical properties of stabilized polycaproamide have been studied in relation to changes in supermolecular structure, preliminary orientation and duration of heat treatment. The deformation of the spherulites during the orientation of stabilized polycaproamide has been investigated together with the changes in supermolecular structure in the course of heat treatment. Certain relations between strength, type of deformation, and degree of preliminary orientation have been obtained for stabilized and heat-treated polycaproamide. The cause of the formation of inhomogeneous structural elements during polyamide processing are discussed.Mekhanika Polimerov, Vol. 4, No. 3, pp. 402–405, 1968     

Relation between molecular orientation characteristics determined from birefringence and acoustic data and the effect of orientation on the strength of polycaprolactam fibers

The mean molecular orientation calculated from birefringence and acoustic data has been investigated in relation to the degree of stretching for polycaprolactam fiber. A correlation has been found between the characteristics calculated by these two independent methods. It is shown that the molecular orientation factor calculated from the speed of sound does not depend on crystallinity if the measurements are made below the glass transition temperature. It is confirmed that the strength of the fiber depends importantly on the degree of orientation of the molecular segments in the amorphous zones.Mekhanika Polimerov, Vol. 3, No. 1, pp. 3–7, 1967     

The static strength of thermo-stabilized oriented polyamide

Preliminary orientation and the action of heat, particularly intermittent heat, increase the thermal stability, tensile strength and plasticity of thermo-stabilized polymides. Heat treatment raises the initial modulus of elasticity, and the formation of surface defects during heat treatment can be prevented by thermal stabilization of the polyamide.Mekhanika polimerov, Vol. 1, No. 1, pp. 117–123, 1965     

Elasticity of the intercrystallite zones and the mechanical properties of oriented polymers

As a result of a study of the behavior under load of the elements of the supermolecular structure of oriented amorphous-crystalline polymers it is shown that the stresses on the crystallites and the amorphous zones in series with them in the direction of the chain axes are equal to the mean stress applied to the specimen along the orientation axis. The nature of the elasticity and deformability of the amorphous zones is analyzed. A relation is obtained between the strength properties of oriented polymers and the number of load-carrying chains in the amorphous zones.Ioffe Physico-Technical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 6, pp. 1002–1007, November–December, 1969.     

Age aspects of the torsional strength of compact bone tissue

The variation of the strength of specimens of human tibial compact bone tissue with age has been investigated. It is shown that the torsional strength _i ^* increases sharply in childhood, reaches a maximum at age 25–35, and then gradually falls. There is a correlation between _i ^* and the porosity of the bone tissue. The mode of fracture of the bone tissue has been studied in relation to its orientation with respect to the principal axes of anisotropy.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 3, pp. 493–503, May–June, 1975.     

Biaxial orientation of amorphous linear polymers

The dependence of the birefringence and orientation stress on the biaxial stretch ratio and orientation conditions has been experimentally investigated. The temperature dependence of these characteristics is explained in terms of the network structure of amorphous polymers. It is shown that the transformations of the supermolecular structures in the process of biaxial orientation depend on the orientation temperature — at higher temperatures better organized structures are formed. There is a formal relationship between the effect of orientation temperature on supermolecular structure formation and on the relaxation process responsible for the formation of a more thermally stable molecular network.Moscow. Translated from Mekhanika Polimerov, No. 5, pp. 17–23, January–February, 1971.     

Strength of boron fibers

The tensile and flexural strength and elastic properties of continuous boron fibers 95–105 µ in diameter obtained by depositing boron on a tungsten filament 10–12 µ in diameter have been investigated. The strength properties are primarily determined by the defects present at the boron sheath-core interface and for the fibers investigated have a limit of approximately 400 kgf/mm². Coarse surface or internal defects are capable of reducing the strength of the fibers to 200–250 kfg/mm².All-Union Scientific-Research Institute of Aviation Materials, Moscow. Translated from Mekhanika Polimerov, No. 6, pp. 1126–1127, November–December, 1970.     

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