Accelerating production processes with the use of heating equipment that operates under the action of various corrosive agents requires the development of refractory materials that will provide effective and reliable operation. Increased demands are placed on the refractory linings under such conditions. In particular, the refractories should possess a high strength, which is one of the decisive criteria of the working capacity of the heating plant during prolonged operation.The Ukrainian Institute has developed highly refractory, volume-constant corundum concretes [i] which can be recommended for unfired, monolithic linings in various high-temperature units working in reducing, neutral, and oxidizing gaseous atmospheres at service temperatures of 1800-1900~Studies carried out at IPP AN UkSSR showed [2-6] that ceramics and refractories are classified with the nonlinear-elastic systems.This agrees with data in [7][8][9]. In particular, from the results given in [8,9] it follows that ignoring the nonlinearity of the diagrams during their treatment leads to considerable errors in determining the strength.Experimental investigations of the strength properties of corundum concretes were carried out on equipment at IPP AN UkSSR using the methods stated in [2][3][4][5].The specimens were tested according to the four-point bending scheme on equipment designed on the basis of the RM-101M rupture machine.The arrangement of the equipment is shown in Fig. 1. It has special supports with distances of 50 and 30 mm between the internal points of application of the loads, and i00 and 60 mm between the loads. These supports are different from those described in [2,5] in that the rollers i transmitting the force onto the specimen 2 are arranged on the plates (in Fig. I not shown) which can be rotated around cylindrical rods coaxial with the specimen.Electromagnets are used to fix the initial position of the rollers. A rigid plate dynamometer 3 is built into the support~ and the deformations of this are measured with strain gauges, and the bending meter 4, designed on the basis of the small inductive displacement sensor which was designed in IPP AN UkSSR, is built into the support.As a result of improvements made to the metering circuit it is possible to register, on two-coordinate potentiometers, the deformation diagrams not only with the use of the strain gauges (P--a, see Fig. I) glued to the specimen in the compression zone 5 and in the tension zone 6, but also with the aid of the bending meter 4 (P--~, see Fig. 1).In order to study the acoustic emission (AE), diagnosing the development in the specimens of cracks during deformation, use is made of a special instrument [6]; and we developed a metering-information system which ensures the measurement in a stated range of frequencies of the parameters of acoustic emission (total number of impulses, their intensity, amplitude, and the number of elementary events --acts of acoustic emission); in addition, a synchronized signal is transmitted to one of the channels about the change i...

Intensifying the chemical process by increasing the temperature of the thermal decomposition of carbonate raw material in a fast gas stream has made it necessary to develop highly effective refractory materials including high-refractory concretes designed for making the unfired monolithic lining of various zones in thermal aggregates, in particular of the reactors in the production of industrial carbon. The refractory materials used for the lining must be characterized by high strength, thermal-shock resistnace, and constancy of volume during use.On the basis of laboratory and production studies a composition has been developed for a corundum concrete in a hydraulic binder made from high-alumina cement. The unfired specimens of corundum concrete were characterized by high strength and thermal-shock resistance, and an insignificant increase in volume on heating. As the coarse-grained concrete filler we used electrosmelted corundum and the multicomponent, finely dispersed part consisted of hlgh-alumina cement grade VGTs-70, GK alumina, a binding additive, and an addition of highly refractory oxides.With the aim of refining the composition of this corundum concrete, we used the method of mathematical planning of an experiment with quantitative, variable factors to establish the regression dependences between the proportion of individual components of the finely dispersed part and the properties of the corundum concrete mixture and, in the construction of the composition-property diagram, to find the coordinates of the points corresponding to the optimum value of the property to be studied. The method makes it possible to describe a model of the subject of the study by an equation of the form:where y is the function to be studied; x i are variation facotrs (i = i, 2, ..., k); x, --x~ is the concentration in the finely dispersed part of the concrete, respectively, of highalumina cement, GK alumina, binding additive, and the oxide addition.Using a simplex-grid method the choice of the optimal composition of highly refractory corundum concretewhich has the maximum crock strength (y, function) with corresponding limitations on the linear dimensions and the thermal-shock resistance (functions Y2 and ya) can be made. In the planning of the experiment with quantitative factors charaeterizlng the compositions of the mixtures (total concentration of components, 100%), we used the so-called Sheffe simple simplex-grld to optimize the properties of the mixtures and to construct the composition-property diagrams [i].In the k-dimenslonal space where k is the number of components of the mixture xi, the condition of the total concentration of mixture components (100%) in natural units has the form:I where the number of components of the mixture of the system, x i varies from 1 to k.With the transition from the natural to the coded values of the factors the condition of the sum of concentrations of the components of the mixture takes the form:

CONCLUSIONSAll refractory materials are with advantage divided into two groups in terms of the characteristics of their mechanical behavior, viz., brittle refractories for which the degree of brittleness X = i, and relatively brittle refractories with X < i. The method and means of determining the strength of refractories should be decided in accordance with the degree of their brittleness.A criterion is proposed for the resistance of refractories to thermal stresses which takes into account the effective relation between the stresses and deformation.It was shown that in the case of materials of equal strength, rigidity, and coefficients of linear expansion the thermal-shock resistance is greater for the material with the lower degree of brittleness. LITERATURE CITED

The corundum-based concretes having constancy of volume were developed by UkrNllO for accomplishing unfired monolithic lining of various thermal units. In particular, they are intended for service under the action of oxidizing and reducing environments at a temperature of 1800~It is of interest to study their deformation and fracture processes under different mechanical and thermal loads.In order to obtain the most complete understanding of the mechanical behavior of these concretes, we carried out a study of their strength properties by plotting the deformation diagrams, The diagrams were recorded when determining the ultimate bend strength of the specimen using a special device according to the four-point loading scheme [I]. We studied five optimum systems (compositions) of the corundum concretes. Their properties were given elsewhere [2].The nature of the stress-strain diagrams showed that all the concretes belong to the class of nonlinear elastic materials [2]. A distinguishing feature of the deformation diagrams of the experimental concretes is their nonlinearity even under insignificant loads. On increasing the heat-treatment temperature from 120 up to 1400 and 1750~ the nonlinearity is retained, but the magnitude of load required for fracture increases. This is due to the struc, ture evolution that is accompanied by densification and strengthenining of the concretes. This phenomenon occurs during the service of the concretes (in particular, in the working zone).As was shown earlier [2], the magnitude of the limiting deformation e~t of the nonlinearelastic materials is composed of the elastic deformation eel s and the residual deformation (nonelastic strain) eres; the amount of the latter increases with increasing nonlinearity of the diagram [3, 4].The area under the deformation curve corresponds to the density of the total energy of the material undergoing deformation and the shaded triangular area corresponds to the density of the elastic energy of the specimen [2].According to our previous results [4], every material is characterized not only by the total amount of the specific energy, but also by ratio of its constituent parts, i.e., the energy required for elastic deformation (potential energy) and the dissipated and irreversibly consumed energy during deformation. Taking this fact into account, a new characteristic of the mechanical behavior of the difficult-to-deform (low ductility) materials (the brittleness index) was suggested. The brittleness index X proposed in GOST 23132--78 is a ratio of the specific elastic energy S accumulated in the material up to the moment (point) of fracture and the total specific energy W consumed for its deformation up to the same moment [i, 3-6]: ~=s/~ (i)The magnitude of the brittleness index varies from 0 up to i; for brittle materials X = 1 and for relatively brittle materials 0 < X < i.As a characteristic of the mechanical behavior, the brittleness index takes into account the actual stress--strain relationships (laws) valid for a given material. It characterizes ...

One of the main trends in improving the technology and increasing the quality of concrete in recent years in the USSR and abroad has been the use of various plasticizers, reducing the water--cement ratio and thereby increasing the strength and durability of the concrete, and also the use of regulators (accelerators) for the setting process which affect the rate of hydration and setting of the cement material [1-3].