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.     

Comparison of the reliefs of fracture surfaces in PMMA under static and cyclic loading

Investigations of the relief of fracture surfaces in PMMA with the help of optical and interference microscopes have shown that there are no qualitative differences in the appearance of the fracture surfaces obtained under static and cyclic loading conditions. Quantitative studies have established that the size of the specular zone increases linearly with increase in the logarithm of the lifetime at a given test temperature. Other conditions being equal, in cyclic tests the specular zone is larger than in static tests. The results obtained are considered to be further confirmation of the validity of the theory that under any loading conditions fracture is based on the same thermo-fluctuation mechanism.Mekhanika Polimerov, Vol. 2, No. 1, pp. 52–59, 1966     

Generalized temperature-time characteristic of the strength and hardness of amorphous polymer materials

A new generalized characteristic of the strength and hardness of amorphous polymer materials is developed using two geometric surfaces, viz., of strength and stress relaxation. The combination of these surfaces determines the service range of the polymer material. For the polyarylate of isophthalic acid and phenolphthalein the service range is determined exclusively by the strength surface.Mekhanika Polimerov, Vol. 1, No. 4, pp. 89–91, 1965     

Effect of the starting structure of a polymer on the time dependence of its strength under cyclic loading

The stress dependence of the logarithm of the cyclic life for different modifications of the same polymer is represented by a fan of straight lines that converge upon extrapolation to zero stress at a single point log A_cy, which differs from the analogous extrapolation point log A_st. The possible causes of this effect are discussed.Ioffe Physico-Technical Institute, AS USSR, Leningrad. Translated from Mekhanika Polimerov, Vol. 4, No. 6, pp. 1008–1013, November–December, 1968.     

Estimation of the long-term strength of polymer materials under arbitrary loading histories

A nonlinear version of the phenomenological theory of long-term strength of polymer materials (viscoelastic bodies) is proposed. It is based on the introduction of a function accounting for the damage accumulation connected with changes in the load intensity. The form of this function may be determined from the results of testing the material with a load changing with time in a certain way, for instance, periodically. As a parameter, the function contains the rate of the changing load or the frequency for periodic loads. For a quasi-isotropic material, the basic relationships of the theory proposed are generalized to the case of combined stresses. The durability (failure time) calculations of the material based on this theory are compared with experimental data for a number of polymer and composite materials in a wide range of loading modes. Translated from Mekhanika Kompozitnykh Materialov, Vol. 35, No. 5, pp. 585–594, September–October, 1999.     

Variation in the optical properties of polymeric film materials under static and cyclic loading (review)

Conclusion At the present time, use of optical methods in analyzing the structure of polymeric materials reduces, with rare exception, only to the method of the low-angle scattering of polarized light. Orientation phenomena and supramolecular structures, whose dimensions exceed the length of the light wave, are studied using this method. The potentials of optical methods are not fully utilized, however, in studying heterogeneities commensurate with, or smaller than the wavelength, and it is precisely microcracks, micropores, and the dispersed phase in a polymeric mixture that exhibit these dimensions. The high sensitivity of optical methods to these heterogeneities, and the possibility of using the Burger-Beer law for polymeric films ensures the widespread use of light-scattering and spectral-transparence methods for the structural analysis of heterogeneous materials (polymeric mixtures, filled composite materials), and also in studying pores and fatigue damage in polymeric films, and in exposing the most heavily loaded zones in articles.Translated from Mekhanika Kompozitnykh Materialov, No. 1, pp. 35–56, January–February, 1989.     

Strength of reinforced plastics under static loading

The question of the static fatigue of reinforced plastics is considered for plane stress. Relations that predict the static fatigue are given for very simple types of loading. Experimental data are presented for thin-walled tubes of SVAM (5:1) glass-reinforced plastic subjected to long-time tests at constant internal pressure.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, Vol. 5, No. 2, pp. 265–273, March–April, 1969.     

Self-heating and failure of plastics under cyclic loading

It has been shown that plastics heated by cyclic deformation have two characteristic temperatures—a critical temperature T_hc, at which the most heavily stressed part of the test piece is intensely heated, and a temperature T_h, at which the test piece fails. The values T_hc and T_h are determined not only by the physicomechanical properties of the material, but also by the state of stress and strain, by the scale factor, and by the heat transfer conditions. It has likewise been shown that the form of the fatigue fracture surface of plastics is determined by the type of deformation and the temperature T_h.Mekhanika Polimerov, Vol. 3, No. 3, pp. 483–492, 1967     

Behavior of rigid plastics under cyclic loading

Research progress on the dynamic fatigue of plastics is briefly reviewed. Attention is concentrated on the problems of damage accumulation and self-heating. The effect of various factors on the fatigue of plastics is considered. The possibility of predicting the cyclic life-time from the results of long-time static strength tests is examined. The prospects for the construction of a theory of fatigue strength in complex states of stress are weighed.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, Vol. 5, No. 1, pp. 97–107, January–February, 1969.     

Molecular mobility in polymers under cyclic loading

Using the NMR method the authors have investigated thermal molecular movement in polymers under the conditions of fatigue tests. They have found that the increase in molecular mobility under cyclic loading (cycle frequency 1–10 Hz) is due only to general self-heating of the polymer. Heating of the specimens as a result of hysteresis heat release takes place uniformly throughout the volume.A. F. Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 6, pp. 1035–1038, November–December, 1972.     

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)