In English. Summaries in Estonian

Proceedings of the Estonian Academy of Sciences.



Volume 49 No. 3 September 2000


Wave interaction for characterization of nonlinear elastic materials; 171–185


Abstract. A method for ultrasonic nondestructive testing of nonlinear elastic materials (structural elements) is elaborated. Simultaneous propagation of two longitudinal waves, their reflection and interaction data are utilized. Convenient choice of the wave frequency makes it possible to analyse the data recorded on the boundaries of the specimen in terms of wave harmonics. A detailed  analysis of the harmonics evolution and interaction is presented. It is shown that amplitudes of the harmonics and phase shifts are sensitive to material properties.

Key words: nondestructive testing, elastic material, nonlinearity, longitudinal waves, interaction.


A model for cyclic shear in plasticity; 186–197

Kalju KENK

Abstract. A model for describing the plastic behaviour of a material in cyclic shear is proposed. It comprises a set of elements connected in parallel, each of which is built up of a linear spring and viscosity and slip subelements, all connected in series. Appropriate choice of subelement properties enables us to describe cyclic hardening and ratchetting processes of the material without the use of the hypothesis of kinematic hardening. The capabilities of the model are illustrated with numerical simulation results.

Key words: plasticity, structural model, cyclic shear.


Treatment of agricultural wastes using disintegrator; 198–206

Dmitri GOLJANDIN, Boris TAMM, and Aleksei TYMANOK

Abstract. Treatment of agricultural wastes spreading over large territories after harvest is an increasing problem. This concerns particularly woody and herbaceous wastes which need some refining on the spot before any transportation. This paper discusses the problem of grinding those wastes with a disintegrator that assures the output size and density, suitable for transportation and further application, e.g., as fuel for small furnaces. The grinding process and the construction of the milling machine, disintegrator, is described, emphasizing its modularity and possibility of optimizing the machine’s configuration and the process parameters, based on the notion of specific energy.

Key words: disintegration, size-grading, recycling of wastes, specific energy.


Stochastic approach to room-and-pillar failure in oil shale mining; 207–216


Abstract. The stability of the mined areas in an oil shale deposit may decrease in the course of time. As a result, large areas may collapse after underground mines are abandoned. The traditional approach to calculating the stability of underground rooms is to use average parameters and to ignore the variability of rock properties. In addition, the formal method of calculating the bearing capacity of pillars takes into consideration short-term safety and overlooks the weakening of the water saturated rock. This paper describes the results of stochastic modelling of the long-term decrease in the bearing capacity of the pillars, including the case of drowned mines. Continuous pillar weakening and the increase in the probability of room collapse are considered.

Key words: room-and-pillar mining, collapse, bearing capacity, moisture, stochastic approximation method.


Experimental investigation of turbulence modulation by rough solid

particles in a grid-generated flow; 217–229



Abstract. Experimental results relating to the modulation of grid-generated turbulence by rough particles (glass beads of the size 700 mm) in vertical downward flow are presented. Two grids with mesh sizes of 4.8 and 10 mm generated isotropic turbulence. The grid Reynolds numbers were about 3000 and 6300, and the particle Reynolds number was about 80. Histograms of the particle velocity, distributions of the turbulence intensity along the axis of flow, and spectral density of the turbulent energy of flow in different cross-sections obtained by LDA measurements are presented. It was found that the presence of rough particles leads to the attenuation of the turbulence intensity of the flow and to the reduction of the energy at high frequencies.

Key words: vertical two-phase flow, grid-generated turbulence, turbulence modulation, decay curves, energy spectra.


Mechanics at the Institute of Cybernetics; 230–251


Abstract. A review of the research in the field of mechanics at the Institute of Cybernetics from 1960 to 2000 is presented. A selection of most important publications is given in the references.

Key words: theory of plates and shells, acoustic echo-signals, wave scattering by elastic objects, nonlinear wave motion, nonlinear dynamics, mathematical modelling of physiological processes, integrated photoelasticity.