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Experimental methods for determining thermal stability of high-temperature materials under dynamic and static thermal loading conditions are considered. Experimentally, thermal stability is characterized by the destructive temperature difference or critical (destructive) rate of change of the temperature field in the bulk or at the surface of the material. It is shown that, with the observance of special conditions, the thermal-shock method, based on sharp cooling of heated specimens in water, provides an acceptable destructive temperature difference for a particular material, in agreement with theory. At present, however, no unique method for experimental determination of the thermal stability of high-temperature materials has been developed. A route towards standardizing static methods for determining the destructive temperature difference is proposed. THERMAL STABILITY OF HIGH-TEMPERATURE MATERIALS. GENERALITIESHigh-temperature materials 2 determine technological, economic, and environmental conditions and the dynamics of development of the leading research and industrial complexes of the world community [1]. While differing in chemical and phase composition, structure and properties, manufacturing technology, and application, these materials share general characteristics of which the thermal stability is of prime importance.The high-temperature materials [2] are divided into the following groups:1. Whisker-type single crystals. 2. Bulky single crystals.3. High-density (with a density close to theoretical) fine-grained polycrystalline materials.4. Porous fine-grained polycrystalline materials. 5. Porous coarse-grained polycrystalline materials. 6. High-porosity polycrystalline materials. 7. Fibrous (filamentary) polycrystalline materials. Except for materials of the 1st and 2nd groups (which are single-phase), the rest of materials may be single-or multiphase, quasi-homogeneous of heterogeneous.Knowledge of the relations between fundamental properties of materials, principles of design of engineering structures intended for service under thermal shock conditions is of importance both for theory and practice. A theory of thermal stability that has been developed within the framework of the maximum stress theory and. quantum field theory of thermal states provides an adequate description of the behavior of materials in a nonstationary temperature field [3,4]. Thermal stability was substantiated as a physical property of materials related to the fundamental properties of matter. However, numerical analysis of the thermally stressed state of high-temperature materials in a high-intensity nonstationary temperature field and associated therewith technological implications is an arduous task; for this reason, experimental determination of thermal stability is frequently a preferable alternative.As an example of an approximate numerical analysis one may refer to the determination of the critical heating rate of a refractory lining by solving the nonstationary heat conduction equation using destructive temperature grad...
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Methods are considered for resolving dynamic problems of thermoelasticity and thermoelastoplasticity. It is shown that as a result of incorrect physical hypotheses, lying at the basis of thermal conductivity and wave formation mechanisms in a solid with thermal shock in the region of acoustic waves of stresses, well known methods for resolving dynamic problems cannot lead to universal strength and heat resistance criteria. This complicates development of principles for designing high-temperature, and especially refractory and ceramic materials, resistant to thermal shock.The solution of dynamic (transient) problems of thermoelasticity and thermoelastoplasticity has important practical applications for designing and preparing high-temperature materials and objects resistant to thermal shock in the range of acoustic or shock-wave stresses.A widespread group in the class of materials are silicate and refractory non-metallic materials, including refractories, i.e. refractory materials and objects, engineering and structural ceramics. By exhibiting unique properties these materials are extremely sensitive to thermal shock in the range of acoustic stress waves as a result of brittleness, high elasticity modulus and reduced thermal conductivity, that limit the range of their application and reduce life under conditions of the operation of transient thermal fluxes.Since 1950 the dynamic problem of thermoelasticity and thermoelastoplasticity and problems of preparing hightemperature thermally stable materials has been the subject of 17000 publications 1 , that points to the importance, and in a number of cases, the irresolution of these problems. It is surprising for more than 50 years of the history of the development of contemporary solid mechanics and physics unified model representations have not been developed in strength and thermoelasticity theories 2 .Dynamic problems of thermoelasticity are resolved within the framework of the mathematical body of solid mechanics using the generalized Hooke's law, differential equations for movement, a differential equation for Fourier transient thermal conductivity, geometric relationships, initial and boundary conditions, and also a series of assumptions whose basis is either not clear or it is not generally considered incorrect. In particular, the last affirmation concerns the mechanism of wave formation with thermal shock and the mechanism of heat propagation that are definitive in constructing an adequate practical physical model of thermal shock, for example in the range of acoustic waves and stresses.In order to resolve a set of dynamic of thermoelasticity equations the Fourier transient thermal conductivity equation is written taking account of the interconnection of temperature and stress fields. There is also consideration of the problem of dynamic thermoelasticity that separates the problem of thermal conductivity and the problem of determining thermal stresses according to temperature field found. The problem is resolved by both mathematical and analytical methods. Po...
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