Determination of coupled creep functions from experimental creep curves

A method is given for constructing the coupled creep functions g(t) from the experimental longitudinal creep curves and the known Poisson's ratio. It is shown that for this purpose it is sufficient to substitute for one of the parameters of the singular kernel a quantity containing the Poisson's ratio and the parameter determined from the solution of the elastic problem.I. M. Gubkin Moscow Institute of the Petrochemical and Gas Industry. Translated from Mekhanika Polimerov, No. 2, pp. 216–220, March–April, 1974.     

Characteristics of the creep and repeat creep deformation process for polymers in uniaxial tension. Part 2

Physical relations for approximating the creep deformation and recovery have been worked out for a material in the physically nonlinear state with a nonmonotonic θ (t) dependence. It has been shown experimentally that for filled low-density polyethylene and fluoroplastic there is no time similarity for the processes of longitudinal and transverse deformation, deformation under primary and repeat loading, and creep deformation and recovery.     

Analysis of nonlinear polymer creep

The relaxation properties of polyethylene are analyzed. The nonlinear time-dependent stress-strain relations and the creep and relaxation equations are obtained from the experimental creep data. The analysis is based on an appropriate variant of the nonlinear memory theory with singular functions whose parameters, together with the modulus of elasticity, are determined by the method described in [1].Moscow. Translated from Mekhanika Polimerov, No. 3, pp. 410–414, May–June, 1969.     

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.     

Transient creep of glass-reinforced plastic

Short-time (up to 30 min) creep tests on SVAM glass-fiber orthotropic material at 20°C have shown that it is subject to creep even at room temperature. At the same time, the behavior of this material in creep is observed to be markedly anisotropic.Mekhanika Polimerov, Vol. 1, No. 2, pp. 47–54, 1965     

Acoustic creep of glass-fiber-reinforced plastic

A method of investigating acoustic creep of polymer composites developing under the effect of static tensile forces and small amplitudes of dynamic stresses with a frequency of 20 kHz acting coaxially with the static loads is considered. Results of investigating acoustic creep of glass-fabric-reinforced plastic are presented. It is shown that the presence of a high-frequency stress component regularly accelerates relaxation processes and reduces considerably the deformation resistance of the material. Generalized curves simulating the long-time static creep of reinforced plastic are constructed by the method of temperature-time, stress-time, and vibration-time analogies. The results of prediction are compared with the control experiment.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, Vol. 9, No. 3, pp. 397–404, May–June, 1973.     

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.     

Viscoelastic creep of polymer materials

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

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.     

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.     

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.     

On a stability criterion for creep

Stability in the presence of creep has been studied by different authors; some of these investigations were made as much as ten years ago.

The majority of the published papers are concerned with problems of the stability of longitudinally compressed rods, since this represents the simplest formulation of the problem by which many of the particular characteristics of stability can be elucidated.

It has to be noted that there exists a series of principally different formulations of the problems of stability in the presence of creep. In this paper, consideration will be restricted to stability of rectilinear forms of equilibrium.     

Micromechanical creep model for pure copper

A Unit Cell made of copper is simulated and investigated under creep conditions within the framework of micromechanics. Geometrical 3D model of the copper microstructure is represented as a Unit Cell with grains of random crystallographical orientation and geometry. Such simulation enables algorithm of Voronoi tessellation. The stress-strain behavior of grains in the general case is anisotropic due to the ordered crystalline structure. The anisotropic model for a material with a cubic symmetry is implemented in Abaqus and used to assign behavior of grain interior in elastic and creep regions. Material parameters for elastic model are taken from elastic tests of single crystal copper [1]. Power law material parameters for creep model are taken from creep test performed for single crystal copper [2]. The model parameter ξ is validated numerically. Creep results are presented for the case of proportional loading during the primary and secondary creep. Statistical analysis of creep curves received for 55 different realizations of Unit Cell geometry is carried out. As a result confident interval and mathematical expectation of creep data are calculated. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Theory of plasticity and creep taking account of microstrains

A relationship between the theories of plasticity and creep of the type /1, 2/ and theories based on the concept of slip is set up. A most logical structure is proposed for the constitutive equations of the theory which is convenient for engineering calculations.

It has been shown /3/ that the theory of slip /4/ results from the theories /1, 2/. However, it remains unclear whether a deeper connection exists between these theories. Moreover, the connection between creep theories constructed using the approach in /1, 2/ and creep theories based on the slip concept was not generally examined. A survey of the development of polycrystalline strain theory /5/ yields a complete representation of the state of matters in plasticity and creep theories.     

Fractional Burgers models in creep and stress relaxation tests

Classical and thermodynamically consistent fractional Burgers models are examined in creep and stress relaxation tests. Using the Laplace transform method, the creep compliance and relaxation modulus are obtained in integral form, that yielded, when compared to the thermodynamical requirements, the narrower range of model parameters in which the creep compliance is a Bernstein function while the relaxation modulus is completely monotonic. Moreover, the relaxation modulus may even be oscillatory function with decreasing amplitude. The asymptotic analysis of the creep compliance and relaxation modulus is performed near the initial time-instant and for large time as well.