Kinetic genesis of some statistic regularities of fracture

The statistics of strength and longevity of solids has been the subject of investigation for many years. Some common regularities in fracture statistics have been experimentally revealed. These are Jhurkov's exponential and power-type longevity equations, Weibull's statistics of strength, and different types of size effect. However, the present notions on the nature of the observed statistic regularities are far from being complete. Here the object is to examine fracture statistics on the basis of a previously suggested kinetic approach. We consider the evolution of the statistical ensemble of cracks under static and steady-rate loading of a solid. The growth of an individual crack is therewith treated as a spasmodic stochastic process and presented by a time-dependent distribution density of a crack size. That function is used to construct the conditional longevity distribution with respect to the growth of an individual crack with the prescribed initial size. The final longevity distribution is found as the distribution of a minimum of conditional longevities. That procedure has been realized for both brittle and quasi-brittle cases. The obtained longevity and strength distributions were found to be in qualitative agreement with the observed statistic regularities. However, the conventional `weak link concept' and proposals for the statistics of dangerous defects are in conflict with the obtained results. It is moreover shown that, within the limits of the wide-spread concept, Jhurkov's longevity equation and Weibull's statistics of strength are mutually exclusive. Although the results were obtained from simple models, there is a good probability of kinetic origin for the basic statistic regularities of fracture.     

Kinetic genesis of some statistic regularities of fracture

The statistics of strength and longevity of solids has been the subject of investigation for many years. Some common regularities in fracture statistics have been experimentally revealed. These are Jhurkov's exponential and power-type longevity equations, Weibull's statistics of strength, and different types of size effect. However, the present notions on the nature of the observed statistic regularities are far from being complete. Here the object is to examine fracture statistics on the basis of a previously suggested kinetic approach. We consider the evolution of the statistical ensemble of cracks under static and steady-rate loading of a solid. The growth of an individual crack is therewith treated as a spasmodic stochastic process and presented by a time-dependent distribution density of a crack size. That function is used to construct the conditional longevity distribution with respect to the growth of an individual crack with the prescribed initial size. The final longevity distribution is found as the distribution of a minimum of conditional longevities. That procedure has been realized for both brittle and quasi-brittle cases. The obtained longevity and strength distributions were found to be in qualitative agreement with the observed statistic regularities. However, the conventional `weak link concept' and proposals for the statistics of dangerous defects are in conflict with the obtained results. It is moreover shown that, within the limits of the wide-spread concept, Jhurkov's longevity equation and Weibull's statistics of strength are mutually exclusive. Although the results were obtained from simple models, there is a good probability of kinetic origin for the basic statistic regularities of fracture.     

Experimental investigation of electron guns for THz microwave vacuum amplifiers

Single-sheet and multiple beam electron emitters based on thermionic minicathodes for terahertz traveling-wave tubes have been studied. Data are presented for impregnated blade thermionic cathode with dimensions 0.1 × 0.7 mm and a maximum current density of 114 A/cm² in a pulsed mode. A variant of the five-beam electron gun with 0.25-mm-diameter cylindrical minicathodes in cells of a control grid is proposed that provides a current density of 85.5 A/cm² at a grid potential of 900–1000 V.

     

Equation of state,composition, and conductivity of dense metal-vapor plasma

The caloric and thermal equations of state, composition, and conductivity of dense vapor plasma of different metals (Cu, Ag, Au, Ti, Ni, and B) have been calculated for the temperature range of 10000–50000 K and for pressures up to 20000 atm. An ion-molecular chemical model of a nonideal gas-plasma mixture, which was previously proposed for an aluminum vapor plasma, has been used in the calculations. Results obtained for the equation of state and conductivity (resistance) of metal vapor in the range of applicability of the model are in fairly good agreement with the experimental data. A comparison of the calculation and experimental results for different metals suggests that the model proposed is universal for metal-vapor plasmas.     

On hypervelocity penetration of the mesh-bumper strings into a projectile

An elastic–plastic model describing hypervelocity penetration of a mesh-bumper string by an arriving projectile is presented. An analytical solution for the case of rigid penetration is derived whereby the dependence between penetration depth and impact velocity is established. A comparative analysis between rigid penetration and penetration with a spreading string is performed. The mesh strings interact between each other with the aid of flow arising during penetration. The model which is developed allows us to estimate destruction depth of the projectile during its interaction with the mesh strings.     

Testing a 32-channel integrated circuit for recording signals of silicon detectors

A 32-channel integrated microcircuit for microstrip silicon detectors of the ??Nuklon?? project for studying ionizing radiation in space conditions is tested. The microcircuit allows one to record signals of relativistic particles and nuclei with a charge from Z = 1 up to Z > 50 from silicon detectors. A structural diagram of the microcircuit and its main experimental characteristics are given. The power consumption does not exceed 1.5 mW/channel, and the dynamic range of recorded signals is not smaller than 100 pC.