Investigation of the dynamic behavior of optically active polymer materials

In view of the importance of the dynamic behavior of optically active plastics in connection with their use in model studies of the earthquake resistance of hydraulic engineering structures by the photoelastic method, the static and dynamic characteristics of the commonest types of photoelastic plastics are presented using published data and measurements made in the Photoelasticity Laboratory of the Vedeneev All-Union Scientific Research Institute of Hydroengineering, Leningrad.Mekhanika Polimerov, Vol. 3, No. 6, pp. 1117–1124, 1967     

Acoustic fatigue of polymer materials

An experimental investigation of the fatigue properties of filled rubber under acoustic loading is briefly discussed. It is shown that two processes take place in the material—hardening and softening. A corresponding mathematical model is proposed. The form of the Wöhler fatigue curve in logarithmic coordinates is almost linear for high-frequency cyclic loading and may be assumed linear in the calculations with a risk of not more than 1%.Mekhanika Polimerov, Vol. 3, No. 3, pp. 467–475, 1967     

Acoustic fatigue of polymer materials

Methods are proposed for experimentally estimating the temperature-time aging of a filled rubber and the degree of damage accumulation in high-frequency fatigue tests. A procedure for experimentally determining the energy dissipation function of a material from the specimen temperature kinetics is described. The results of an investigation of the fatigue properties of two series of filled rubbers at a vibration frequency of 20 kHz are presented. It is shown that the fatigue failure of the materials tested is thermal in character. No accumulation of mechanical damage in the material in the course of intense vibration could be detected.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 4, pp. 662–668, July–August, 1969.     

Acoustic fatigue of polymer materials

Work on the development of experimental techniques and methods of testing materials under high-frequency cyclic loading are briefly reviewed. Certain aspects of the acoustic fatigue of polymer materials are examined. Future research trends are discussed.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, Vol. 5, No. 1, pp. 108–126, January–February, 1969.     

A dynamic theory for composite materials

A continuum model is developed for a composite medium consisting of inclusions of arbitrary geometry embedded in a matrix material. The equations are valid up to wave lengths of the order of a unit cell dimension and are found to reduce under special assumptions to Mindlin's equations in the long wave-length approximation. In particular, a pair of characteristic time scales and length scales encountered in stress-gradient theories are evaluated in terms of known constituent material properties. As a basis of the theory, it is shown that adefinite relation exists between the dimensions of a local representative volume and those of a unit cell. The relation is found to depend on the ratio of the maximum and minimum effective elastic impedances. As a consequence, the theory hinges on the evaluation of several material constants that reflect the effect of inclusion arrangement in a local representative volume of definite dimensions.

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Zusammenfassung Ein Kontinuumsmodell für ein Verbundmaterial mit Einschliessungen willkürlicher Geometrie wird vorgeschlagen. Die Gleichungen sind bis zu Wellenlängen von der Grössenordnung der Dimension einer einheitszelle gültig. Sie reduzieren sich unter speziellen Annahmen zu den Mindlinschen Gleichungen in der Approximation für grosse Wellenlängen. Insbesondere können die in Theorien mit Spannungsgradienten auftretenden charakteristischen Zeit- und Längenmasstäbe in Funktion von bekannten Materialeigenschaften berechnet werden Als Grundlage der Theorie ergibt sich, dass zwischen den Dimensionen eines lokalen, representativen Volumens und jenen einer Einheitszelle eine wohldefinierte Beziehung besteht. Diese hängt vom Verhältnis der maximalen und minimalen elastischen Ersatzimpedanzen ab. Als Konsequenz daraus führt die Theorie zur Berechnung verschiedener Materialkonstanten, welche den Einfluss der Verteilung der Einschliessungen in einem lokalen representativen Volumen bestimmter Dimensionen darstellen.

     

Anisotropy of torsional rigidity of sheet polymer composite materials

Wide application of polymer composite materials (PCM) in modern technology calls for detailed evaluation of their stress-strain properties in a broad temperature range. To obtain such information, we use the dynamic mechanical analysis and with the help of a reverse torsion pendulum measure the dynamic torsional rigidity of PCM bars of rectangular cross section in the temperature range up to 600 K. It is found that the temperature dependences of the dynamic rigidity of the calculated values of dynamic shear moduli are governed by the percentage and properties of the binder and fibers, the layout of fibers, the phase interaction along interfaces, etc. The principles of dynamic mechanical spectrometry are used to substantiate and analyze the parameters of anisotropy by which the behavior of a composite can be described in the temperature range including the transition of the binder from the glassy into a highly elastic state. For this purpose, the values of dynamic rigidity are measured under low-amplitude vibrations of the PCM specimens with a fiber orientation angle from 0 to 90°. It is shown that for unidirectional composites the dependence between the dynamic rigidity and the fiber orientation angle is of extreme character. The value and position of the peak depend on the type of the binder and fibers and change with temperature. It is found that the anisotropy degree of PCM is dictated by the molecular mobility and significantly changes in the temperature range of transition of the binder and reinforcement from the glassy into a highly elastic state (in the case of SVM fibers). The possibility of evaluating the anisotropy of composites with other reinforcement schemes, in particular, of orthogonally reinforced PCMs, is shown.Translated from Mekhanika Kompozitnykh Materialov, Vol. 35, No. 3, pp. 291–308, May–June, 1999.     

Aging of polymer composite materials exposed to the conditions in outer space

A comprehensive investigation is made of glass, carbon, organic fiber-reinforced plastics, and epoxy-based hybrid composite materials employed in Salyut-type spacecraft which remained in space for up to 1501 1501 days. In particular, the properties, aging mechanism, and strain-strength variations in these materials due to exposure to the conditions in outer space were studied. After a series of tests were performed in space the standard strain and strength parameters as well as the mass, density, and thickness changes in the composite materials were estimated. Electron-microscopic and dynamic-mechanical analyses were performed, and the thermal expansion was estimated for a wide range of temperatures. The principal, dominant process occurring due to the continuous presence in outer space was found to be post-curing of the resin materials, which in turn affected the mechanical characteristics of the composite materials. After 456–1501 days in space the room-temperature strength of the composite materials (except for organic plastics) did not decrease, while at high temperatures it even increased. The post-curing and restructuring of some composite materials lowered their dynamic shear moduli in the glassy state of the resin. Due to consolidation of the surface layer of hybrid composite materials irradiated and subjected to thermal cycles, failure during bending varied from transverse fracture to delamination. The negative effect of the post-curing process was expressed as higher internal tension in the hybrid composite materials with different linear thermal expansion coefficients. The magnitude of this effect depended on the amplitude of the thermal cycles. The unprotected surface of the composites bombarded by atomic oxygen, microparticles, and space garbage were subjected to pickling and microerosion, the maximum effect occurring at the initial stage of exposure. Desorption of moisture and low-molecular products during the first 100–200 days of thermal cycling in the vacuum of near-earth orbit must be considered when estimating the total mass loss of composite materials. Data from microscopic, dynamic-mechanical, and other types of analyses revealed that the outer-space factors improved the supermolecular order of the resin volume, while the subsurface layer structure of the composite materials had loosened. Microcracks formed in the plastic's surface during 1501 days in outer space did not, in general, affect the mechanical parameters of the composite materials. Most of the observed effects of exposure to conditions in outer space were less pronounced for plastics protected by aluminum foil or other plastic coatings. The data obtained can be used for designing external elements of spacecraft by selecting materials with specified and predictable properties for long-term service.Translated from Mekhanika Kompozitnykh Materialov, Vol. 29, No. 4, pp. 457–467, July–August, 1993.In conclusion we thank I. G. Zhigun and R. P. Shlits for assisting in determining the mechanical properties of PCM as well as the crew of the Salyut-6 and Salyut-7 space stations for setting up, monitoring, and delivering samples to earth.     

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.     

Investigation of fatigue damage in polymer films by the method of light scattering

Conclusions In cyclic loading of the investigated polyurethane there originates a polydisperse system of microdefects with nonspherical shape, in consequence of which the light scattering increases irreversibly. The method of light scattering is sufficiently sensitive for determining the relative concentrations and dimensions of microdefects that are smaller than the wavelength of light. It was shown that the predominant mechanism of failure of polyurethane is the increased concentration of microdefects whereas their dimensions change only imperceptibly.Translated from Mekhanika Kompozitnykh Materialov, No. 5, pp. 910–917, September–October, 1984.