Influence of prestressing on the service lives of polymers

The authors have investigated changes in the lifetimes of polymethylmethacrylate, polyethylene, and polystyrene resulting from prestressing. They show that, according to the temperature, stressing can cause either an increase or a decrease in the lifetime.A. F. Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 1, pp. 176–178, January–February, 1972.     

Interfaces between nanostructures and stepwise creep in highly oriented polymers

A comparative laser-interferometric study of steady creep in oriented ultrahigh-molecular-weight polyethylene films differing in the structure of interfaces between nanosized structural units has been carried out to gain a better understanding of the creep mechanism in oriented polymer materials. In contrast to conventional methods, laser interferometry permits measurement of creep rates from very small strain increments (0.3 μm) to within 1%. This technique made it possible to detect the stepwise nature of plastic deformation in creep. The data obtained suggest that the creep rate and its periodic changes are controlled by the structure of the interfaces, and that the plastic deformation itself occurs to a considerable extent through shear of nanosized structural units relative to one another by an “acceleration-deceleration” type. It is proposed that the “deceleration” phase is due to a glide resistance created by some “stoppers” having either physical or chemical nature, which become destroyed and reappear again in the course of creep. Fiz. Tverd. Tela (St. Petersburg) 41, 1788–1791 (October 1999)     

Inhomogeneity of the strain rate of polymer materials with different supramolecular structures

The characteristics of rate oscillations in submicron deformation increments in the course of creep under compression of polymer materials of different classes, namely, amorphous poly(vinyl butyral), amorphous-crystalline poly(tetrafluoroethylene), and a composite consisting of polyimide with graphite particles, have been investigated. The strain rate has been measured using an interferometer on a deformation base of 300 nm. The periods of rate oscillations have been used to determine the deformation jumps, and the amplitude of rate oscillations has been used to determine the jump sharpness. It has been demonstrated that the radical differences in the structure of materials manifest themselves in the parameters of deformation jumps at different stages of creep. The type of jumps makes it possible to reveal the type of molecular packing in the starting polymer or the packing formed during deformation.     

Inhomogeneity of deformation of solids at the nanometer level

A new method for processing interferometrically recorded deformation data has been implemented for studying an inhomogeneity in the rate and parameters of deformation jumps at the nanostructure level, which provides detection of deformation jumps of less than 300 nm. It is shown that the lower limit for deformation jumps lies in the range 10–30 nm for aluminum and is 130 nm for amorphous polymer (poly(methyl methacrylate)). It is assumed that the sizes of jumps correspond to scales of ordered structures, as was previously established for higher level structures. The results obtained make it possible to investigate more thoroughly the multilevel character of deformation and to evaluate the sizes of the nanostructural units, their evolution during deformation and under the effect of external fields, as well as their relation to the microscopic and macroscopic inhomogeneities of deformation.     

Deformation of poly(methyl methacrylate) after exposure to radiation and magnetic fields

It is demonstrated that prolonged treatment in a constant magnetic field considerably increases the creep rate under compression of poly(methyl methacrylate) samples irradiated with gamma-ray doses as high as 100 kGy. For higher irradiation doses, the effect of the magnetic field on the creep rate is insignificant.     

Rate spectra of small deformations in solids

The method of constructing the temperature dependence for the rate of small creep deformation (strain-rate spectra) is applied to determine relaxation transitions in solids. This method is based on precision measurements of the rate using a laser interferometer and is distinguished by its high resolution. The possibility of using the spectra for predicting critical temperatures in the fracture kinetics of polymers and metals, as well as changes in the electrical properties (for example, the superconducting transition in Y-Ba-Cu-O ceramics) is pointed out. Fiz. Tverd. Tela (St. Petersburg) 41, 848–850 (May 1999)     

Interferometric measurement of displacements and displacement velocities for nondestructive quality control

It is shown that the interferometric measurement of small displacements and small-displacement velocities can be used to determine internal stresses or the stresses induced by an applied load in solids and to control structural changes in them. The interferometric method based on the measurement of the reaction of a solid to a small perturbation in its state of stress is applied to determine stresses from the deviation of the reaction to perturbations from that in the standard stress-free case. For structural control, this method is employed to study the specific features of the characteristics of microplastic deformation that appear after material treatment or operation and manifest themselves in the temperature and force dependences of the rate of a small inelastic strain.     

Nonmonotonic creep in LiF crystals containing Mg

Precision measurements of the creep rate using interferometric recording of the process in LiF crystals containing 0.002 to 0.03 wt % Mg using interferometric recording of the process show that nonmonotonic episodes in the form of alternating segments with relatively high and low creep rates periodically appear against a background of overall creep attenuation. The accumulation of creep strain takes place mostly on the segments with increased rates, which causes kinks to appear in the creep curve that are noticeable when shifts are resolved at a level of fractional microns. Measurements were made of the height of steps L based on “rate-strain increment” curves for stresses smaller and closer to the yield point. Increasing the stresses causes the value of L to fall off, while increasing the Mg content causes it to increase. It is proposed that the nonmonotonic behavior of creep is connected with structural nonuniformity of the material, which is determined by the impurity content and the nonuniformity of the strain process itself. For small strains in the neighborhood of the yielding area, where strain comes about as a result of a broadening of a pre-existing slip band, the impurity atoms enhance the nonuniformity of the strain (L increases). As the strain (stress) increases, L decreases, which corresponds to a smoothing out of the nonuniformities. It is emphasized that the step-like character of the strain accumulation is a general property. Fiz. Tverd. Tela (St. Petersburg) 40, 690–693 (April 1998)