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.     

Seismoacoustic prediction of the fracture processes in glass-reinforced plastics

A technique and equipment for investigating the seismoacoustic emission associated with the fracture of a loaded glass-reinforced plastic are described. It is shown that the emission is present from the beginning of loading and increases regularly up to failure. Glass strands consisting of 100 plain elementary fibers of varying diameter and resin-impregnated strands have been investigated in tension and specimens of glass-reinforced plastic and unreinforced epoxy resin in tension and bending. Experimental data on short-time, static and repeat loading are presented for the glass-reinforced plastic. It is shown how the integral pulse distribution and the pulse amplitude frequency function depend on the applied load level.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 3, pp. 421–429, May–June, 1971.     

Problems in the prediction of the long-term strength of polymer materials. A review

Conclusions As in the case of the selection of the simplified equations used in the theory of nonlinear viscoelasticity, variants of the equations of the temperature-time dependence of the strength of polymer materials are constructed on the basis of some similarity condition of the experimental lifetime curves. Principles according to which it would be possible to make a prior assignment of a material to some class on the basis of similarity criteria only using, for example, information on chemical composition and structure without testing for long-term strength are lacking. The equations presented in this review may be used for predicting the lifetime only when the nature of the resistance to long-term destruction over time in a given temperature range exactly corresponds to the assumptions inherent in some equation. Thus, the selection of the boundaries of the temperature ranges, in which we may assume uniformity for the deformation and destruction mechanisms leading in the final analysis to identical types of destruction, becomes most significant. Hence, the parametrical methods have a series of advantages since they do not place rigid limitations on the nature of the long-term strength curves at a given temperature.The temperature-time analogy method is the most favored method for predicting long-term strength since this method, without negating other approaches, is their generalization. The use of this method is most promising for cases of a complex stressed state and complex stress pathways. In this case, the temperature -time shift function is most conveniently introduced into the surface strength functionals.Translated from Mekhanika Kompozitnykh Materialov, No. 4, pp. 694–704, July–August, 1979.     

Approximation of relaxation spectra

Relaxation spectra of polyneoprene (PNP) were determined by the Schwarzl-Staverman method [15] using experimental creep data. The real spectrum distribution was approximated by the Charlier function constituting a more general distribution. The application of Kolmogorov's and Pearson's criteria showed that the empirical spectrum distribution is not substantially different from the Charlier distribution and that the deviations are stochastic in character. Varying the temperature produces a displacement of the entire spectrum set along the time scale. If the temperature dependence of the spectrum is known, the mechanical reaction of PNP can be calculated for a given temperature. Creep curves constructed with the aid of an electronic computer by a method that accounts for a continuous relaxation spectrum fit into an experimental confidence interval.     

Fatigue of a glass-fiber-reinforced plastic under sonic-frequency vibrations

A procedure is described for fatigue tests of glass-fiber-reinforced plastics specimens under conditions of symmetrical extension and compression with a steady peak of dynamic stress at a vibration frequency of approximately 500 Hz. As a test for fatigue failure we chose the steep increase in the mechanical loss angle of the vibrating system which occurs when a macrocrack is formed in the specimen. The time dependence has been plotted for the mechanical loss angle, the resonance frequency of the vibrations system, a parameter proportional to the peak deformation of the specimen, and the temperature of vibrational heating-up in its cross section where there is the greatest stress. The progress of these dependences has been analyzed in the build-up of fatigue damage in the material. An experimental relation has been obtained between the resultant dissipative function of the material and the peak dynamic stress. Results are given of fatigue tests on specimens of a glass-reinforced plastic winding cut in the weft direction of the reinforcing fabric.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 6, pp. 1039–1043, November–December, 1972.     

Graphoanalytical determination of rheological coefficients

A graphoanalytical method is proposed for determining the most probable discrete values of the relaxation times. This method consists in the quantile partition of the relaxation spectrum distribution function.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, Vol. 4, No. 4, pp. 754–757, July–August, 1968.     

Thermal creep of polyethylene in the presence of thermal cycling

The temperature-time dependence of Poisson's ratio and the volume relaxation of LDP is investigated in the case of uniaxial tension. It is established that at elevated temperatures and loads the material becomes denser and Poisson's ratio exceeds 0.5. It is shown that the (t) curves and the temperature dependence of the instantaneous Poisson's ratio are nonmonotonic, while the nature of the (t) relaxation changes qualitatively as a certain temperature region is entered and the stress is increased.Mekhanika Polimerov, Vol. 4, No. 3, pp. 421–427, 1968     

Static and vibrational creep of a fabric-reinforced plastic in shear in the plane of the reinforcement

The results of a comparative experimental investigation of the short-term static and vibrational creep of a fabric-reinforced plastic in shear in the plane of the reinforcement are presented. The experimental procedure is described in detail. It is shown that on the investigated ranges of temperature, stress, and amplitude-frequency parameters the effect of an additional vibrational load on the creep process is unimportant.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 4, pp. 605–610, July–August, 1971.