Showing papers by "Vadim V. Silberschmidt published in 2000"

Analysis of cracking in rock salt

Abstract: ¶The use of underground mines in salt-rock formations for waste repositories makes the precise analysis of isolation properties of rock salt very important. One of the main mechanisms responsible for a degradation of isolation ability of the rock salt is the generation and development of cracks under influence of mining processes. Various aspects of cracking in salt rocks are studied – both in situ and in laboratory tests – for rocks of the world's largest Verkhnekamskoe potash-ore deposit (Russia). It is shown that not all the discontinuities (fractures) are closed in the course of the creep deformation of rocks. Parameters of cracking are determined for various conditions: age of pillars, mining technology, etc. Different methods – filtration, electrometric, nuclear magnetic resonance, mechanical tests – are used for crack observation and characterisation, both in specimens and in situ. Traditional investigations (partly with specially designed equipment) are accomplished by the testing methods of fracture mechanics for determination of critical values of stress intensity factors KIc and KIIc. The problems and ways of the use of the obtained results in analysis of isolation properties for areas in vicinity of underground mines and/or waste repositories are discussed.

Dynamics and Scaling Characteristics of Shear Crack Propagation

Abstract: A model for a description of a shear crack (fault) propagation is proposed on the basis of the unification of continuum damage and fracture mechanics with ideas of fracture models for random media. Energy release linked with local failures of elements as a result of evolution of fracture processes at lower scale levels (stimulated by the stress concentration in the vicinity of the fault) is treated as a seismicity source. 2d simulations are performed for analysis of the effect of the size of pre-existing fault on characteristic features of rupture evolution. Scaling (including multifractal) character of crack propagation and of respective energy release is shown.

Enhanced fracture energy and effects of temperature at spalling in ceramics : continuum damage modelling

Abstract: Enhanced fracture energy losses at spalling and the temperature dependence of the spalling strength of alumina ceramic bars are analysed on the basis of the experimental tests conducted both in room temperature and within the temperature range up to 1500°C at strain rates of some 500 s−1. The experimental method and the measurements are first shortly outlined. The mechanical response of ceramic bars is modelled then as a heterogeneous distribution of brittle-elastic mesoelements undergoing continuum damage at the known strain history, corresponding to that registered in the experiments. The mesoelements are characterised by the values of initial damage randomly fluctuating within a given band-width superposed on a deterministic distribution, which corresponds to the fabrication conditions of the ceramic bars. The model has been tested in the evaluation of room-temperature experiments, its parameters: the average value of the initial damage, Young's modulus of the undamaged material and the energy absorption capacity in continuum damage are taken from the calibration fitting the experimental data. The registered energy losses at spalling, which exceed the static values of fracture energy by almost an order of magnitude, can be explained thus by the enhancement of the dissipation due to bulk damage, which is computed on the basis of the above parameters. The temperature change of the Young's modulus of the matrix material is taken as corresponding to the measured change of the uniaxial wave velocity in the bar, and corrected by the temperature change of the mass density. The analysis of the model shows that the drop in the spalling strength of the specimens with the increase of the temperature is phenomenologically related to the falling energy absorption capacity within the continuum damage mechanism. An explanation of this phenomenon is attempted, based on the grain-size-related mechanisms of the microfracture from pre-existing intergranular flaws distributed over the bulk of ceramics.