The strength of unfired refractories when the lining is warmed up and during use at high temperatures is quite significant [i]~The destruction of brittle materials during use occurs most often under bent [2] and therefore the ultimate strength of unfired refractories is determined in this case from the deformation using the instrument designed by us at the Ukrainian Refractory Institute.The instrument for studying the ultimate strength of refractories when they are bent after heating to 1450=C (Fig. I) consists of a loading device, a heating chamber for the specimens, and energy-supply and monitoring systems.The loading device includes a lever i and a weight 2 which moves along the lever. This makes it possible to smoothly increase the load on the specimen 3 from 0 to 15 kgf (with additional loading up to 26 kgf).The force on the specimen is applied by means of two alumina rods 4 which are arranged at a distance of 40 mm from each other and 20 mm from the axis of the fulcrum.The load is moved along the lever i by means of the beam 5 with a screw thread which is rotated by the electric motor 6. When the specimen is destroyed, the lever rotates freely and breaks the electric circuit which supplies the motor.This makes it possible both automatically and accurately to record the position of the load at the moment of destruction and to determine the force which destroys the specimen.The specimen is heated in the resistance furnace 7 which has silicon carbide heaters. The temperature in the furnace is measured by a Pt--Pt thermocouple whose junction is beneath and in contact with the specimen.The voltage across the heater is supplied via a Variac 8 of the RNO-10 type and thus the temperature in the furnace can be controlled.The design of the furnace makes it possible simultaneously to load 20-22 specimens of cross section 8 x 8 and length i00 mm~ The specimens are placed on prism fulcrums with a distance between each of 80 mm.The method used to measure the ultimate bend strength consists of the following: the specimens are placed in the furnace and heated to a specified temperature with a dwell period of i0 min; the specimen is placed under the loading rods and a load increasing from zero to the load at which the specimen is destroyed is applied smoothly to the specimen.After destruction, the specimen is removed from the testing point, the next specimen inserted, and the whole process repeated.Three tests are made at each temperature and the specimens are then heated to the next temperature and again three tests are made, and so on.The stress Obend at which the specimen is destroyed is determined from the following equation,where M is the bending moment~ kgfocm; W, moment of resistance of the specimen, cm3; P, destructive load, kgf; 2, arm of the application of the force, equal to 2 cm; b. width of the specimen, cm; and h, its height, cm~The error Aabend in the determination of the destructive stresses was calculated using the followingequation:
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