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     

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.     

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.     

Mechanical properties of a hollow-fiber glass-reinforced plastic

The strength and elastic properties of a hollow-fiber glass-reinforced plastic are investigated together with the effect of temperature, and long-time and cyclic loading on its mechanical characteristics. The instantaneous moduli of elasticity and the creep kernel parameters are obtained on the basis of the creep diagrams.Translated from Mekhanika Polimerov, No. 2, pp. 341–345, March–April, 1971.     

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.     

Hereditary theory of creep and low-cycle fatigue of polymeric materials

The results of an investigation of the creep of polymethyl methacrylate under alternating loading and recovery are presented. The experiments ended in the failure of the specimen. The creep data obtained are described on the basis of the nonlinear Boltzmann-Persoz theory and Bryzgalin's theory of hereditary recovery. Korabel'nikov's result, according to which for alternating loading and recovery the total time under load up to failure is less than the static life of the polymer, is confirmed.Institute of Problems of Mechanics, Academy of Sciences of the USSR, Moscow; Scientific-Research Institute of Mechanics, Lomonosov Moscow State University. Translated from Mekhanika Polimerov, No. 4, pp. 615–621, July–August, 1970.     

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.     

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.     

Variants of the local strains theory

By means of the proposed new variants of the local strains theory it is possible to solve a broad class of complex loading problems for initially isotropic materials. The basic equations for a series of variants of incompressible and compressible materials are derived. The author investigates the possibility of using the relations obtained to solve complex loading problems for materials with irreversible plastic strains or with different physical reversible or irreversible creep relations depending on the loading path.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 2, pp. 233–240, March–April, 1970.     

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     

Effect of relaxation parameters on the behavior of rigid polymers in pure shear

The authors have investigated the effect of the relaxation parameters of a rigid polymer corresponding to the individual terms of the relaxation spectrum on the nature of the theoretical curves calculated for creep, strain relaxation and loading at constant strain rate in pure shear on the assumption that the stress-strain relation is described by the generalized Maxwell equation.Institute of Chemical Physics, Academy of Sciences of the USSR, Moscow. Translated from Mekhanika Polimerov, No. 3, pp. 431–436, May–June, 1969.     

Vibrational creep of polymeric materials

An experimental study of the effect of vibration on the creep process has been carried out in the case of the rigid porous polyurethane PPU-3, as a function of the magnitude of the vibrational loading and the level of basic static stresses. It has been shown that with increase in the velocity amplitude of the dynamic stresses, the creep process is accelerated, without being accompanied thereupon by vibrational heating of the material. The possibility has been established of approximating vibrational creep curves by the integral equation of Volterra, using a discrete series of relaxation times transformed by the vibro-time analogy method.For Communication No. 3, see [1].Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 2, pp. 223–232, March–April, 1970.     

Grapho-analytical method of calculating the deformation of a viscoelastic rod in axial tension under nonstationary temperature conditions

The strains in the material are calculated on the basis of a compound mechanical model consisting of a system of individual elastico-viscous models of the Maxwell type. This model makes it possible to allow for the time dependence of the temperatures and stresses in the cross sections of bodies of simple shape—rod, plate, thin-walled shell—and to calculate the strains under nonstationary temperature conditions from the creep curves for reinforced plastics with allowance for the previous loading history both under a constant load and for an arbitrary loading law.Mekhanika Polimerov, Vol. 2, No. 3, pp. 413–420, 1966     

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.     

A study of creep of plastic (polymethyl methacrylate) simultaneously stressed in tension and torsion

It was established that isotropic polymethyl methacrylate in the vitreous states is, in the physical sense, a tensorially nonlinear material in creep. The tensorial nonlinearity is not very substantial, and the hypothesis of the similarity of stress and strain deviators gives sufficiently accurate results. The creep strain under relatively simple programmed loading conditions is dependent on the form of the stress state.Mekhanika Polimerov, Vol. 2, No. 5, pp. 671–677, 1966     

Quasilinear theory of viscoelasticity

By postulating equal contributions the number of kernels in the principal cubic theory of viscoelasticity and in the theory with regular kernels of two arguments is reduced to three. For certain quasilinear relations all the kernels and functions are determined from creep, relaxation, and simple loading and deformation tests. In the case of simple loading and deformation the problems for a viscoelastic incompressible material reduce to problems of the theory of small elastoplastic deformations of an incompressible material. Several problems relating to this case are considered.Moscow M. V. Lomonosov State University. Translated from Mekhanika Polimerov, No. 4, pp. 603–611, July–August, 1969.     

Creep of certain polymers under hydrostatic pressure

The problems of an experimental investigation of polymer creep under omnidirectional hydrostatic pressure are considered. The method of investigation is discussed, and the loading devices are described. The results of tests on low-pressure polyethylene and teflon are reported.Mekhanika Polimerov, Vol. 4, No. 2, pp. 232–236, 1968     

Creep on an initial small interval

The creep processes of hereditary media on a small initial interval are described, using as influence functions the resolvent functions for the generators of the Dirac function, which are employed as relaxation kernels. Equations are constructed for determining the instantaneous modulus and the parameters of these functions from experimental stress-strain diagrams obtained at three different constant loading rates.Moscow Institute of Electronic Machine Building. Translated from Mekhanika Polimerov, No. 5, pp. 945–950, September–October, 1970.     

Multiparameter prediction of polymer creep

An investigation of the thermocreep of low-density polyethylene (LDP) and the vibrocreep of porous polyurethane (PPU) in complex states of stress has shown that multiparameter creep prediction based on the combined application of the time-stress, time-temperature, and time-vibration superposition principles can be used for rapid analysis of the nonlinear viscoelasticity and thermovibrocreep of polymeric materials under complex loading.For communication 1 see [1].Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 3, pp. 416–420, May–June, 1971.     

Multifibre polymer composites: Prediction of deformation and thermophysical properties

Conclusions A theoretical and experimental investigation was carried out to examine the possibilities of a structural approach for prediction of elastic constants, creep functions and thermophysical characteristics of hybrid polymer composites reinforced with anisotropic fibres of several types. The theoretical solutions were obtained by generalizing the self-consistent method for the case of a three phase model. The effects of brittle fibre breakdown under tension in the direction of reinforcement of a unidirectional hybrid composite were studied under conditions of a short-term loading and a long-term creep. It has been shown that a creep of viscoelastic fibres plays a principal role in creep of the hybrid composite. It is just this creep that significantly increases the fibre damage during creep of the composite.A variant of the solution has been proposed for predicting the thermorheologically complex behavior of hybrid composites containing not only elastic but also viscoelastic thermorheologically simple components with different temperature-time shift factors. The peculiarities of thermal expansion of hybrid composites and the possibilities for a purposeful control of thermal expansion coefficients by hybridization were studied. The considered thermal interval included a region of transition of the polymer matrix from a glass state into a viscoelastic one.The control tests were performed for specimens of organic/glass, organic/carbon, glass/carbon and organic/boron polymer composites with different ratios of fibre volume contents. On the whole, the obtained accuracy of predicting the characteristics of the examined hybrid composites may be considered as acceptable for engineering applications.Published in Mekhanika Kompozitnykh Materialov, Vol. 30, No. 3, pp. 299–313, May–June, 1994.