Kinetics of the failure of loaded materials at variable temperature

The possibility of predicting the lifetime of loaded materials at variable (increasing) temperature is demonstrated on the basis of a kinetic approach to the problem of the failure of such materials. Zh. Tekh. Fiz. 68, 76–81 (November 1998)     

Thermal expansion of the skeleton of chain molecules in polymer crystals

The expansion of the carbon skeleton of molecules in crystallites and the longitudinal contraction of the crystallite lattice in poly(ethylene) (PE) and poly(caproamide) (PCA) with an increase in the temperature are measured using IR and Raman spectroscopy and x-ray diffraction. The thermal expansion of the carbon skeleton due to transverse vibrations is theoretically calculated within the atomic chain model. The theoretical and experimental data on the thermal expansion of the carbon skeleton are in good agreement.     

Local breaking stresses in loaded microporous SiC ceramics

The ballistic strength and hardness have been measured for a series of SiC ceramic samples with variable (from 1 to 20%) porosity. It is shown that interpore bridges are stress concentrators, and the dependence of the factor of stress concentration on porosity is determined. The conclusions based on the experimental results are confirmed by the finite-element calculations of stress fields in porous systems.     

Hardness of microporous SiC ceramics

Samples of silicon carbide ceramics with variable microporosity and hardness are studied. By measuring the hardness of the ceramics as a function of the integral porosity, it is established that the hardness is proportional to the mean length of bridges connecting pores. Microporosity is found to influence the hardness via stress concentration at the bridges, as a result of which they break up. Indentation causes extensive breakup of the bridges. The mean stress coefficient as a function of the integral porosity of the ceramics is estimated. It is shown that the micromechanisms of quasi-static indentation and high-speed indentation (penetration of a striking bar) are in many ways similar to each other.     

An opportunity for directly estimating the characteristics of zero-point dynamics in polyethylene crystals

For large polyethylene crystallites (100 × 60 × 60 nm), the width of the Raman band at 1129 cm^?1 and the angular position of the x-ray equatorial 110 reflection were measured as a function of temperature over the range 5–300 K. It is found that the Raman bandwidth has an athermic (zero-point) component at low temperatures. This component is used to estimate the zero-point energies of torsional and bending vibrations of polyethylene molecules. These energies are close to those obtained from analyzing the x-ray diffraction data. It is concluded that the characteristics of zero-point dynamics can be determined directly from measuring the zero-point width of a Raman band.     

Comparison of the shapes of macro- and submicrocracks in loaded polymers

Small-angle x-ray scattering has been used to estimate the longitudinal (L) and transverse (L) dimensions of the submicrocracks formed in loaded polymers. The growth under load of macrocracks, initiated in the same polymers by a slit or a circular hole, has also been observed. The ratios L/L for the macro- and submicrocracks are found to coincide.A. F. Ioffe Physico-Technical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 6, pp. 1013–1018, November–December, 1971.     

Characteristics of a high-power RF oscillator based on a pulsedhollow-cathode discharge

The characteristics of a very simple high-power pulsed RF oscillator are presented. This oscillator is based on the use of a low-pressure pulsed hollow-cathode discharge during the periods when its cathode sheath becomes unstable. A set of four different oscillators has been studied in different operating regimes for various values of load resistance. These oscillators were tested and found to produce pulsed power of over 100 kW with efficiency exceeding 20% at frequencies up to 50 MHz. The tests were performed with pulse duration of 14 μs and repetition rate up to 100 Hz. Equivalent circuits of the oscillator for RF loading and for DC current are described. The requirements for a power supply to drive the device as well as recommendations for the oscillator usage are presented     

Energy of the elastic loading of anharmonic solids

This paper reports on the results of measurements of changes in the temperature of a solid under the adiabatic elastic loading (thermoelastic effect), the coefficient of thermal expansion, and the Young’s modulus of a rigid-chain vitrified polymer, namely, polyimide. It has been found that there are differences in the sign and magnitude of the changes in the energy of thermal origin in samples and the work of the external force. The energy of the thermoelastic effect has been explained in terms of the influence exerted by the anharmonic expansion of a solid, with the separation of the quasi-static potential and dynamic components of the thermal energy of the solid. The loading with an external force causes a redistribution of the thermal energy. A change in the temperature of the solid corresponds to a change in the dynamic component. An energy analysis of the adiabatically loaded anharmonic oscillator has confirmed the conclusion regarding the mechanism of energy transfer and revealed that, under loading, there is a redistribution of the kinetic and potential components of the internal energy of the oscillator.     

Characteristics of elementary acts in the mechanical fracture kinetics of polymers

The main characteristics of elementary acts in the mechanical fracture kinetics of oriented polymers, namely, the initial potential barrier U ₀ and the activation volume V _A , are determined. The values of U ₀ And V _A are obtained from analyzing the temperature and force dependences of the life of polymers with allowance for the actual stresses (overstresses) applied to chain molecules, which are found by IR spectroscopy. The values obtained (U ₀ ≈ 4 eV, V _A ≈ 0.03 nm³) agree satisfactorily with the dissociation energy and activation volume of the fluctuation breaking of covalent skeleton bonds in polymer molecules.     

Mechanisms of reversible thermal deformation of oriented polymers

Reversible thermal deformation coefficients (TDCs) of oriented samples of a flexible-chain polymer (polyethylene) and of a number of rigid-chain polymers were measured in the longitudinal and transverse directions near room temperature. The same samples were used to measure the TDCs of crystallites by x-ray diffraction. The magnitudes of the TDCs of macroscopic oriented samples and of constituting crystallites and the characteristics of the thermal deformation of flexible-chain and rigid-chain polymers are compared. A conclusion is made that the mechanisms that determine thermal deformation in the longitudinal and transverse directions for the flexible-chain and rigid-chain polymers are different.