Simple loading of a polymer material with nonlinear creep

Nonlinear tensor relations between strain, stress, and time are examined for a memory-type medium using degenerate kernels. The material parameters are determined from creep tests in a simple state of stress. Expressions for the strain associated with a complex state of stress and simple loading, found on the basis of the local strains theory, are in satisfactory agreement with the experimental data obtained for specimens of high-density polyethylene.Mekhanika Polimerov, Vol. 3, No. 2, pp. 236–242, 1967     

Endurance limit of polyethylene during cyclic loading under conditions of a plane stressed state

Conclusions The effect of the asymmetry of the loading cycle on the fatigue strength of HDPE subjected to biaxial tension has been studied. An increase in the acuteness of the cycle leads to an increase in the durability of the material. Under conditions of cyclic loading, the material became embrittled as the duration of the experiment was extended. The effect of various different ratios between the principal stresses on the fatigue strength of HDPE has been investigated. It has been shown that the form of the isochronous boundary curves depends on the duration of the experiment. It was established that the mechanism of HDPE fracture changes depended on the duration of the loading. The accumulation of defects, when an experiment was of average or longer duration, built up at a greater rate than would have been predicted by a linear rule for summation.M. I. Kalinin Leningrad Polytechnic Institute. The "Plastpolimer" Okhtinsk Scientific Manufacturing Association, Leningrad. Translated from Mekhanika Polimerov, No. 1, pp. 35–40, January–February, 1977.     

Problems of the theory of deformation of polymeric materials under complex loading

Research on problems of the theory of deformation of polymeric materials under complex loading based on a synthesis of creep theories and the local strains theory is reviewed.Presented at the 2nd All-Union Conference on Polymer Mechanics, Riga, November 10–12, 1971.M. V. Lomonosov Moscow State University. Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 3, pp. 434–443, May–June, 1972.     

Reasons for the lowering of the lifetime of polymers under cyclic loading

A model is proposed which explains the lowering of the lifetime of polymers on transition from static to cyclic loading by the interaction of local stresses from an external load with residual stresses which arose after preceding loading cycles. The results predicted on the basis of the model have been checked experimentally on polymethyl methacrylate, and a good agreement has been shown.M. I. Kalinin Leningrad Polytechnic Institute. Translated from Mekhanika Polimerov, No. 2, pp. 279–283, March–April, 1976.     

Creep and damage accumulation in orthotropic composites under cyclic loading

Experimental results and theoretical prediction of the response of glassfiber-reinforced polyester under quasi-static, static (creep), and cyclic (fatigue) loading are presented. The nonlinear strain component at static loading and the strain amplitude rate at cyclic off-axis loading of an orthotropic composite are shown to follow the associated flow rule with a single-parameter quadratic potential function. The influence of fatigue damage on deformation is considerable due to the reduction in the elastic modulus of the composite and is apparently negligible with respect to its effect on the parameters of the creep kernel.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. 447–460, July–August, 1998.     

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 polymer materials under periodically varying loads

A method is proposed for constructing the creep curves of a material whose nonlinear memory properties are described by Rozovskii's nonlinear integral equation [2] (with allowance for the stress dependence of the relaxation time) under given periodic loading from known creep curves recorded at constant stress. In deriving the theoretical relation certain simplifying assumptions are made (the creep strain accumulated in 1–2 cycles is small, no vibration [4–6]). An experimental check shows that the proposed method can be used to predict the behavior of a material under periodic loading with an accuracy sufficient for practical purposes.Mekhanika Polimerov, Vol. 2, No. 3, pp. 330–336, 1966     

Creep of a multilayer cylinder under nonisothermal loading

A method is proposed for calculating the creep of a multilayer cylinder under nonisothermal loading with allowance for the temperature dependence of the physico-mechanical characteristics of the material. Numerical examples are considered. The states of stress and strain of the viscoelastic and elastic components of a two-layer cylinder are compared.Baranov Central Institute of Aviation Engine Construction, Moscow. Translated from Mekhanika Polimerov, Vol. 5, No. 2, pp. 321–327, March–April, 1969.     

Fatigue strength and structure changes of polyethylene-oxide films under static and cyclic loading

Conclusions The behavior of the PEO films under long-term loadings show certain special features reflected mainly in the unusual form of the fatigue curve and the presence of three types of creep curves. The most significant factors affecting the fatigue characteristics of the PEO films are the molecular mass and the degree of crystallinity — when these factors increase the fatigue resistance also usually increases. To evaluate suitability of PEO films for service it is promising to use the cyclic bending test. This loading mode characteristic of practice makes it possible to carry out tests at low strain levels and cumulate considerable damage. Examination by the x ray diffraction method showed that in cyclically bent PEO films not only the amorphous but also crystalline structures are damaged.Translated from Mekhanika Kompozitnykh Materialov, Vol. 3, No. 3, pp. 404–412, May–June, 1994.     

Forward and reverse creep of a physically nonlinear polymer material

From tension and torsion experiments under constant load, the authors determine the parameters of a material with physically nonlinear creep. They examine the applicability of these parameters to represent the deformation properties of materials under other loading systems. They give a method of drawing the reverse creep curve (in integral form) with parameters different from those of active creep.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 5, pp. 786–795, September–October, 1973.     

Investigation of the integral creep equations (cubic nonlinearity) for different loading paths

Expressions for the creep strain obtained in accordance with the Leaderman-Rozovskii theory and in the form of a multiple-integral Volterra series are compared for different loading paths. The influence functions are assumed to be symmetrical. The strain intensity-time curves are calculated for a complex loading path for which the stress intensity is constant. It is found that these curves are nonmonotonic for both linear and nonlinear creep.Read at Fourth Symposium on Rheology, Moscow, May 27–30, 1969.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 1, pp. 35–42, January–February, 1970.     

Plane-stress creep of a glass-reinforced plastic

The creep behavior of an orthotropic glass-reinforced plastic based on Polimal'-109 resin and 19-S4 glass fabric is described. A special device for testing tubular specimens in plane-stress creep has been designed. The loading conditions include combined torsion and tension, internal pressure, and loading along the principal directions of anisotropy. Constitutive equations describing creep and recovery are proposed for the two-dimensional case.Warsaw. Translated from Mekhanika Polimerov, No. 3, pp. 398–410, May–June, 1971.     

Accumulation of defects during nonstationary loading of Plexiglass

The fatigue resistance is studied in programmed tests on Plexiglass samples. The effect of the nonstationary character of loading and, in partcular, that of the shape of the load spectrum on durability of Plexiglass is estimated.Institute of Mechanics, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Mekhanika Polimerov, No. 1, pp. 169–171, January–February, 1972.     

Calculation of one-dimensional polymer creep under stepped loading

A graphic method is proposed for calculating polymer creep under stepped loading. The method is based on the equations of nonlinear viscoelasticity. It can be used to describe the aftereffects of specimens of polyethylene terephthalate film and the creep of tubular plexiglas specimens at sharply varying stresses. The agreement with experimental data is satisfactory.Mekhanika Polimerov, Vol. 3, No. 4, pp. 671–675, 1967     

Deformation of cylindrical composite shells under combined dynamic loading

Conclusions A procedure has been shown for calculating the stress-strain state of cylindrical multilayer shells made from composite materials under the combined action of dynamic axial compression and dynamic external pressure, as well as with different variants of combined loading with static and dynamic forces. An investigation has been made of the effect on the mode of the buckled shell surface of the ratio of the application rate of dynamic loads; ranges of loading rates have been established in which stresses predominate caused either by axial compression or external pressure. It has been shown that, as a result of preliminary static loading, a marked change occurs in the initial imperfections of the shell mode which affects subsequent dynamic buckling. To calculate the time when the first defect occurs and its location in the shell body, a procedure has been devised for layer-by-layer strength analysis employing a tensor-polynomial criterion. It was demonstrated that the level of preliminary static loading noticeably affects the time until the first failure of the layer, not only a reduction of this time being possible with an increase in the static loads, but also an increase in it.We should also point out the work in [10] where it is shown that it is possible to weaken the susceptibility of the shell to initial imperfections when internal pressure is applied.Translated from Mekhanika Kompozitnykh Materialov, No. 3, pp. 461–473, May–June, 1981.     

Deformation of thermoplastic materials at low-cycle loading

The possibility is discussed of deducing analytical equations for evaluating the deformation of polymer materials at a low-cycle periodic loading on the basis of the linear prehistory theory. Finite expressions are given for the case of loading by rectangular and triangular pulses. Comparison of the results of the experimental study of the deformation of low- and high-pressure polyethylenes (LPP and HPP, respectively) with the conclusions of the theory shows satisfactory agreement for tests of up to 5·10³ loading cycles.Leningrad Institute of Aviation Instrument Engineering. Translated from Mekhanika Polimerov, No. 2, pp. 230–234, March–April, 1976.     

Distribution of the strength deviations of plastics under multiple pulse loading

Logarithmically normal dependences, similar to the corresponding graphs for cyclic loading at low rates, have been obtained for the distribution of the number of cycles to failure.Translated from Mekhanika Polimerov, No. 3, pp. 560–562, May–June, 1971.     

Viscoelastic-viscoplastic-damage modeling of thermoplastics under long-term cyclic loading

Thermoplastics tend to creep under mechanical loads and recover deformation during unloading. A viscoelastic-viscoplastic-damage model is presented to accommodate the behavior of a thermoplastic under cyclic loading-unloading. Finite element simulations may be computationally expensive in case of many loading cycles. A cycle jump method is proposed to mitigate parts of the cost. It can efficiently approximate large parts of the cyclic calculations by extrapolating internal variables over several cycles at once. Hence, only a fraction of the original computational effort remains. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Dependence of the rheological characteristics of filled polypropylene on filler concentration,loading regime and temperature

The rheological characteristics of polypropylene have been investigated in relation to filler concentration, loading rate in the =wt regime, creep stress and time interval and temperature. A study has been made of the relationship between the changes in the rheological characteristics of the material, determined from - curves obtained from tensile tests at different constant loading rates, creep tests and by acoustical methods, with variation in filler concentration and temperature.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, Vol. 4, No. 5, pp. 853–858, September–October, 1968.     

Torsional mode of buckling in nonlinear creep with allowance for the difference in loading and unloading laws

The torsional mode of creep buckling of a thin-walled element of material with nonlinear creep is investigated. The local strains theory is used for the physical relations.Mekhanika Polimerov, Vol. 3, No. 1, pp. 142–145, 1967