Long-time strength of polyethylene in biaxial tension

The long-time strength of high-density polyethylene (HDPE) in biaxial tension has been experimentally investigated at ratios of the principal stress components =₁/₂=0, 0.5, 1, 4. The maximum duration of the experiments was 2500 h. The limit surface for HDPE has been constructed. It is shown that the limit surface for this material varies with time as the mode of fracture changes from quasibrittle at medium stresses to brittle at low stresses. In the case of quasibrittle fracture the condition of equivalence of simple and plane states of stress is satisfactorily described by the Malmeister and Gol'denblat-Kopnov criteria, and in the case of brittle fracture by the maximum normal stress criterion.All-Union Scientific-Research Institute of Hydromechanization, Sanitary Engineering, and Special Construction, Leningrad. Translated from Mekhanika Polimerov, No. 3, pp. 401–408, May–June, 1976.     

Comparison of the long-term strengths of polymers under uniaxial alternating and pulsating loads

The laws of fracture of PMMA and PS under uniaxial alternating and pulsating cyclic loading have been investigated at several loading frequencies and two test temperatures. An analysis of the dependence of the long-term strength on stress, heating kinetics and the form of the specimen fracture surface has shown that transition from cyclic tension to more complex alternating loading in the tension-compression regime does not change the basic nature of the cyclic fatigue effect and is reflected only in an increase in the local stress concentration and heating.A. F. Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 3, pp. 483–488, May–June, 1972.     

Effect of temperature — Time factors on the yield point of crystalline polymers in the flat stressed state (high-density polyethylene)

Conclusions 1. Experiments have been performed on biaxial stretching of HDPE over a wide range in ratios of the principal stress components, strain rates, and temperature. The character of change in the limiting characteristics of HDPE are different in different temperature regions.2. The limiting resistance of HDPE in the flat stressed state for T=const and v_i=const is described satisfactorily by the Malmeister and Gol'denblat-Kopnov criteria.3. Prediction of the limiting resistance of HDPE should be carried out within limited temperature ranges, in which similarity of strain diagrams, a similar character of structural changes in the material (accumulation of damage), and identical character of change in its limiting characteristics are observed.Okhtinsk "Plastpolimer" Scientific and Production Union, Leningrad. Translated from Mekhanika Polimerov, No. 2, pp. 247–254, March–April, 1977,     

Molecular mobility in polymers under cyclic loading

Using the NMR method the authors have investigated thermal molecular movement in polymers under the conditions of fatigue tests. They have found that the increase in molecular mobility under cyclic loading (cycle frequency 1–10 Hz) is due only to general self-heating of the polymer. Heating of the specimens as a result of hysteresis heat release takes place uniformly throughout the volume.A. F. Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 6, pp. 1035–1038, November–December, 1972.     

Effect of cycle asymmetry on the cyclic strength of polymers

Cyclic strength diagrams have been obtained for polymethyl methacrylate and kaprolon at different degrees of loading asymmetry (from a pure compression to a pure tension cycle) on the interval from –100 to +80° C at 2·10³ and 3·10⁵ cycles.M. I. Kalinin Leningrad Polytechnic Institute. Translated from Mekhanika Polimerov, Vol. 4, No. 5, pp. 948–951, September–October, 1968.     

Creep of fluoroplastic under the joint action of tension and hydrostatic pressure

Conclusions 1. The effect of hydrostatic pressure on the creep of Teflon has been investigated under isothermal conditions. The applicability of the principle of reduced variables for reduction of creep curves with respect to pressure has been shown.2. It has been ascertained that in the pressure region from atmospheric to 1600 kgf/cm² there are two subregions, which are separated by a transition at a pressure of 700–800 kgf/cm². It has been shown that in these subregions the occurrence of relaxation processes has different characters.3. A direct measurement of the free volume of Teflon has been made in the temperature range from 40 to 120°C. A sharp increase in free volume in the region of the temperature transition has been shown.Leningrad Technological Institute of the Refrigeration Industry. Okhtinsk Scientific and Industrial Union "Plastpolimer," Leningrad. Translated from Mekhanika Polimerov, No. 3, pp. 434–438, May–June, 1977.     

Investigation of the fatigue properties and susceptibility to damage of thin-walled polymer shells

The results of a study of the damage suffered by thin-walled polymer shells are evaluated on the basis of an analysis of the process of crack formation and the variation of the modulus of elasticity under cyclic deformation. The process of fatigue failure has been studied in relation to shells made of three groups of polymeric materials. It is shown that for the polymers investigated there are at least two different fatigue fracture mechanisms. Data on the crack growth kinetics are presented.Plastopolimer Research and Production Association, Leningrad; Lensovet Leningrad Technological Institute. Translated from Mekhanika Polimerov, No. 6, pp. 1019–1026, November–December, 1971.     

Buildup of failures in polymers on micro- and macrolevels

The buildup of damage in polymers has been studied by mechanical and structural—analytical methods. A method of programmed loading has been developed which makes it possible to make an estimate of damage buildup in macroscopic experiments. A comparison has been performed of kinetic curves of failure buildup on molecular and macroscopic levels. It is shown that the kinetic equations where the rate of failure is basically initiated by true stress qualitatively follow the experimental curves for failure buildup on micro- and macrolevels."Plastpolimer" Scientific-Manufacturing Association, Leningrad. Translated from Mekhanika Polimerov, No. 2, pp. 263–268, March–April, 1976.     

Experimental investigation of the propagation of longitudinal stress waves in optically active polymeric rods

A photoelastic technique has been used to investigate the propagation of longitudinal stress waves in free rods. The experiments have made it possible to establish the mechanism of stress wave formation under impulsive loading, explore the dynamic state of stress of the rod, and estimate the effect of the inelastic resistance of the material.B. E. Vedeneev All-Union Scientific-Research Institute of Hydroengineering, Leningrad. Translated from Mekhanika Polimerov, No. 1, pp. 68–75, January–February, 1970.     

Fatigue strength of glass fiber/epoxy angle-ply laminates with different matrix ductility

Conclusion Angle ply laminates made up of glass fiber/epoxy plies have a good static strength for laminate angles up to about 45 °. This is due to the limitation of transverse strain in the plies due to constraints exerted by neighboring plies. At laminate angles of 50 ° and above the transverse and shear strains in the plies are not sufficiently constrained and cause failure in the matrix material. The constraint effect is present in fatigue but since the polymeric matrix material is sensitive to fatigue loading fatigue failure will occur at much lower strain levels than in static loading. This effect cannot be offset by increasing the ductility of the matrix material. Thus, strain to failure under fatigue loading will be only a small fraction of the static strain to failure for angle ply laminates with ply angles up to around 45 ° where the static strength is due to the constraint effect.Published in Mekhanika Kompozitnykh Materialov, No. 5, pp. 632–638, September–October, 1992.     

Construction of fatigue curves in hard high-cycle loading of reinforced plastics

Conclusions The determination of the instant of failure of specimens in hard loading on the basis of load relaxation, the reduction of the elastic properties, of the length of surface cracks to some constant value is not associated with the process of high-cycle fatigue of the composites. It is recommended that the strength criterion of failure in the hard regime should be represented by the rapid reduction of the load in the last 1000–2000 cycles to 10% of its previous value. Endurance in hard loading depends strongly on the form of the specimen and the method of maintaining the given regime and, consequently, these data must be considered in constructing the fatigue curves. It was shown that the fatigue curves in the -log N coordinates in the soft and hard regimes with the symmetric cycle are positioned equidistantly on the axis. This relation may be used for constructing the -log N curves for hard loading from the fatigue curves for soft loading or, vice versa, at a reduced volume of tests. The error in determining the fatigue limits by this method for N10⁶ cycles does not exceed ±5%.Translated from Mekhanika Kompozitnykh Materialov, No. 3, pp. 457–463, May–June, 1987.     

Study of the role of intermolecular interactions (hydrogen bonds) in the initial stage of orientation of polyvinyl alcohol and vinyl alcohol copolymers

The effect of hydrogen bond rupture on the process of destruction of the starting crystallite structure of polyvinyl alcohol and copolymers of vinyl alcohol and N-vinylpyrrolidone and -vinylcaprolactam has been investigated. The differential spectrum method is used to establish the frequency (3200 cm^–1) of the band corresponding to the vibrations of the bound hydroxyl groups; the temperature dependence of the optical density of the 3200 cm^–1 band has been recorded. The strength of the starting structure is calculated from the stress-strain diagram and its temperature dependence is plotted. It is concluded that the strength of the starting structure is chiefly determined by the presence of hydrogen bonds between the polymer macromolecules.Leningrad Kirov Institute of Textile and Light Industry. Translated from Mekhanika Polimerov, No. 2, pp. 327–329, March–April, 1971.     

The effect of overloads on the residual strength and life of laminated GRP

The effect of a short duration cyclic overload on the residual life and strength of laminated glass-fiber reinforced polyester is studied. A uniaxial tensile fatigue loading with the stress ratio 0.1 is considered. The residual life of the composite decreases due to the overload, while the residual strength is almost unaffected. A reasonable agreement of experimental data with the prediction by a residual strength model and by Miner's rule is observed.Translated from Mekhanika Kompozitnykh Materialov, Vol. 35, No. 5, pp. 701–706, May–June, 1999.     

The strength properties of acicular single crystals of polyoxymethylene

The strength properties of acicular formations of polyoxymethylene, obtained by the directed polymerization of trioxane whiskers, have been studied. It has been found that the acicular single crystals obtained in this process exhibit a high strength.A. F. Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 2, pp. 266–270, March–April, 1970.     

The anisotropy of compact bone material

From the measurements of the main elements of the microstructure of compact bone material, it is concluded that the theoretical model of a transtropic material can be applied to bone tissue. This conclusion is confirmed by the experimental data obtained on compression. The correlation connection between the ultimate strength of compact bone material and the elasticity modulus has been found. It is shown that the anisotropy of the compact material is satisfactorily described by the tensor formula.Scientific-Research Institute of Medical Radiology, Academy of Medical Sciences of the USSR, Obninsk. S. M. Kirov Leningrad Order of Lenin Wood Technology Academy. Translated from Mekhanika Polimerov, No. 4, pp. 711–716, July–August, 1972.     

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.     

Durability of polymers as a function of loading regime during repeated loading with a small number of cycles

Specimens of oriented polycaproamide and unoriented polymethyl methacrylate subjected to uniaxial extension were used to study durability as a function of the number of loading cycles and the intervals between them in different segments of the creep curve. It was shown that the deformation hardening during the initial segment of the creep curve governs the ability of the specimen to withstand subsequent loading cycles. With a relatively low loading rate, deloading-loading cycles during the initial segment of the creep curve lead primarily to an increase in local stresses. During the second stage, repeated loading causes principally an increase in local heating. The interval between loading cycles has only a weak influence on durability.A. F. Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 2, pp. 252–256, March–April, 1972.     

Kinetics of crack growth in polymers during repeated loading with a small number of cycles

We have shown that crack propagation, especially in its initial stage, is greatly inhibited by the deformation process involved in hardening. It was established that loading repeated at intervals leads to a brief increase in crack-propagation rate during each loading period, as a result of the development of excessive stresses and the heating effect at the crack apex caused by the finite rate of the relaxation processes.A. F. Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 2, pp. 247–251, March–April, 1972.     

Creep of fluoroplastic in shear with application of hydrostatic pressure

Conclusions 1. It has been shown that under a deformation regime with Vxy = const, with an increase in shear deformation the effect of the action of hydrostatic pressure rises. A considerable effect of pressure on the character of the shear diagram is observed in the pressure region above 500 kgf/cm².Paper presented at the Third All-Union Conference on Polymer Mechanics in Riga, 1976.Leningrad Technological Institute of the Refrigeration Industry. Okhtinsk Scientific-Manufacturing Union Plastpolimer Leningrad. Translated from Mekhanika Polimerov, No. 5, pp. 812–818, September–October, 1977.     

Changes in the mechanical properties of polyimides induced by electron bombardment

The change in the tensile strength and relative elongation of PM polyimide with absorbed dose has been investigated. It is shown that after irradiation with very large doses (5 · 10¹⁰ rad) the mechanical properties are still satisfactory.Scientific Research Institute of Electrophysical Apparatus, Leningrad. Translated from Mekhanika Polimerov, No. 6, pp. 1111–1112, November–December, 1969.