The failure of refractories under the action of thermal stresses has a complex character.To explain this phenomenon it is necessary to investigate not only the physicomechanical properties of the material but also the distribution of thermal stresses and also to obtain reliable information on the processes of occurrence and development of cracks in thermal failure.To investigate the process of failure of the structure in thermal loading, the authors of this article used the acoustic-emission method [i, 2].To establish the sensitivity of the method to changes in structure, an investigation was made of the behavior of a cordierite and mullite base refractory material known to be high-temperature resistant with specified slow cooling.The sample preparation conditions are given in [3, 4]. By changing the final firing temperature the phase composition of the mullite--cordierite materials was controlled (Table i).The block plan of the unit for measuring the acoustic-emission signals is shown in Fig. I. The unit measures the quantity and intensity of the acoustic-emission signals accompanying the processes of destruction of the material.The elastic wave excited within the sample 1 in thermal loading created by the high-temperature furnace 2 with the programmed controller 3 passes along the waveguide 4 and is converted by the piezoceramic sensor 5 into an electrical signal.The output signal of the sensor, amplified by the preamplifier 6, is fed to the spectrum analyzer 7, from which after selection it is transmitted to the oscillograph 8 and the former 9. The information is recorded by the digital printer i0 and the recorder 11.The sensor is a piezoelectric element of type TsTS ceramic with a single output. The resonant frequency is 50 kHz.The unit operates in the 20-30-kHz frequency range, the input sensitivity is 50-100 ~V, and the degree of suppression of the external interferences is 70-80%.The cooling rate of the samples was controlled by an appropriate mechanism for raising the furnace by a certain height.The samples were heated to 1000~ held at this temperature for I0 min, and cooled to room temperature with an average rate of 15 deg C/min. In cooling the rate of appearance (intensity AN/AT) and the sum of the impulses N were measured. As may be seen from Fig. 2, samples of different phase composition have dissimilar kinetics

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Many refractory articles undergo repeated thermal and mechanical loadings in service. As a result of this, cyclic thermomechanical stresses develop which lead to the destruction of the articles. The questions of predicting fatigue (durability) and ensuring the operational reliability of the articles during cyclical loading in most cases at present remain open [i].

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