Complex loading of a polymer material with nonlinear creep

The nonlinear tensor stress, strain, and time relations for a memory-type medium under complex loading are examined using degenerate kernels. The basic expressions for simple loading and the material parameters were determined in [5]. The local strains theory is used to find expressions for the strain components in the presence of stepwise complex variation of the stress components, and these expressions are shown to be in satisfactory agreement with the experimental data for high-density polyethylene.Mekhanika Polimerov, Vol. 3, No. 3, pp. 421–426, 1967     

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     

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.     

Computer approximation of families of curves of physically nonlinear creep of polymer materials

The determination of confidence intervals of deformations was studied for an individual measurement and arithmetical mean common for all points of an experimental family of curves. From the experimental data on the creep of polyamide resin and five different analytical equations of physically nonlinear creep, the problems of the computer approximation of families of creep curves and the determination of the material parameters are discussed. A method is proposed for a quantitative evaluation of the degree of correspondence of the initial data to the similarity of isochronous creep curves and the similarity of the creep curves.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 2, pp. 220–229, March–April, 1976.     

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.     

Viscoelastic creep of polymer materials

The creep of fabrics used in membranes is studied in the region of elastic deformations.     

Multiparametric prediction of the creep of polymer materials

The article discusses phenomenological methods for predicting long-term creep, using stress-, temperature-, and vibro-time analogies. Starting from several previously introduced hypotheses, the article proposes a generalized nonlinear thermo-vibro-rheological relationship in which the complex stress-, temperature-, and vibro-time dependences are reduced separately to a time dependence (with certain basic reference values of , T₀, and , chosen arbitrarily), and to dependences; the latter appears only in the change of the time factorsa (_i),a _T(T), and . The prediction methods have been verified experimentally.Institute for the Mechanics of Polymers, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 3, pp. 420–428, May–June, 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     

Photoelastic determination of polymer creep stresses and strains

A physical basis provided for the relation between the measured optical quantities and the unknown mechanical quantities in the photoelastic determination of stresses and strains. Questions relating to the "freezing in" of strains are considered in connection with creep tests on models made of optically sensitive polymeric materials. The determination of the stress concentration at the edge of an opening in a strip tested in creep is considered as an example of the application of the theory.Moscow. Translated from Mekhanika Polimerov, No. 4, pp. 739–746, July–August, 1970.     

Investigation of the one-dimensional problem of nonlinear creep

A method of analyzing experimental creep curves of a nonlinear viscoelastic material to obtain relaxation curves is examined. It is assumed that the family of creep curves in question cannot be represented as a product of the stress function and a function of time. The investigation is carried out using the memory theory. The sum of the exponentials for curves not having a singularity at the start of the process is taken as the creep kernel [1]. A method of approximation by the sum of the exponentials is given. For processes with an initial singularity it is proposed to use the corresponding kernel, for which the resolvent is given.Mekhanika Polimerov, Vol. 2, No. 5, pp. 678–687, 1966     

Relationship between polymer creep curves and stress-strain diagrams

For polymers whose () diagram has the form of a monotonically increasing curve the creep rate is monotonically damped with time. If, however, the stress-strain diagram has a maximum, the creep curve becomes stepped: after first slowing, the creep sharply accelerates, and then slows down again. The strains corresponding to characteristic points on the stress-strain curves coincide with those corresponding to singular points on the creep curves.A. F. Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, Vol. 4, No. 5, pp. 787–792, September–October, 1968.     

Nonlinear creep of thick-walled polymer tubes under internal pressure

The process of creep in a thick-walled tube of rigid homogeneous epoxide-type polymer under under the action of a steady internal pressure is studied. A theoretical analysis has been made on the basis of a non-linear generalized Maxwell equation, taking account of two terms of the spectrum of polymer relaxation times. A comparison is given with the results of a calculation with a single term of the spectrum and according to linear theory. The theoretical results are compared with those obtained experimentally in a specially constructed apparatus. The possibility is shown of evaluating the long-term strength of a thick-walled polymer tube under the action of a steady internal pressure.Institute of Chemical Physics, Academy of Sciences of the USSR, Moscow. Translated from Mekhanika Polimerov, No. 3, pp. 493–501, May–June, 1970.     

A note on plane nonlinear creep of free boundaries

Résumé On montre que la déformation plane et quasi-statique d'un corps incompressible qui est non-linéairement visqueux du second ordre, et dont les frontières sont soumises aux tractions données, ne peut être distinguée de celle d'un corps fluide, ayant les mêmes frontières et les mêmes tractions prescrites, qui est linéairement visqueux et incompressible. Ainsi, les solutions données préalablement (Berg, 1964) pour le fluage plan des frontières libres de tractions s'appliquent également au fluage plan et nonlinéaire du second ordre des frontières libres de tractions. Cela signifie en particulier que dans le fluage plan les effets visqueux du second ordre ne peuvent pas donner lieu à l'irréversibilité cinématique de la déformation d'une surface libre. Les résultats des effets du second ordre sont quelque peu étendus pour montrer que dans le cas le plus général d'un fluide non-linéaire et incompressible les problèmes de fluage plan peuvent se simplifier si l'on incorpore dans le champ de pression une partie des contraintes non-linéairement visqueuses.     

Application of the theory of creep stability to polymer materials

The buckling and loss of stability of a rod made from a nonlinear viscous-elastic polymer material are studied with due allowance for instantaneous nonlinear strains. The case of a hinge-fixed rod is considered for various initial deflections.Scientific-Research Institute of Mechanics of the M. V. Lomonosov Moscow State University. Translated from Mekhanika Polimerov, No. 1, pp. 98–104, January–February, 1972.     

Influence of temperature and humidity on the creep of polymer materials

Conclusions By analyzing the results of the original creep tests under shear and tensile conditions subject to various temperatures and humidities, and also analyzing some control experiments involving simultaneous shear and tensile strain, we have shown that the displacement function of the relaxation spectrum expressing the maintenance of the temperature-humidity-time analogy is independent of the form of the stressed state in the range of linear viscoelasticity for a Poisson coefficient close to 0.5.For Communication 2 see [2].Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 4, pp. 627–632, July–August, 1976.