The relationship between compressive stress and loading cycles in MgO-C bricks with various carbon contents was investigated at room temperature and high temperature. The relationship between fatigue failure and thermal spalling was investigated by estimating a characteristic material constant, which represents the degree of sensitivity of crack growth, in terms of fracture mechanics.The relationship between the ratio of compressive stress for compressive strength and loading cycles in MgO-C bricks is not affected by the temperature of the atmosphere. At the same acting stress ratio, fatigue fracture life decreased as the carbon content in the MgO-C bricks decreased. In the thermal spalling test, the number of heating cycles at which small cracks and large cracks were generated decreased as the carbon content decreased. In the fatigue failure test, the ratio of the dynamic elastic modulus to the initial elastic modulus decreased gradually. This ratio also showed a gradual decrease in the thermal spalling test as the number of heating cycles increased. A characteristic material constant was obtained and compared based on the results of a fracture mechanics discussion. The characteristic material constant obtained from the fatigue failure test was substantially the same as that obtained from the thermal spalling test. From this, a relationship in which crack growth behavior leading to fatigue failure is equivalent to that of thermal spalling in MgO-C brick was recognized.

Fatigue failure tests were firstly carried out. The material constant derived from S-N curve was evaluated and compared with previous study. The compliance method was subsequently used to evaluate the fracture toughness of MgO-C brick and the relationship between the stress factor and crack growth rate. In order to use this method, the effects of brick carbon content on the crack growth rate and crack growth rate in high temperature were investigated.As a result, it was found that the crack growth rate increased when the carbon content in brick decreased. This result was confirmed by X-ray CT scans, which revealed large cracks in bricks with a lower carbon content, even in the middle of fatigue failure tests.Furthermore, the material constants obtained from fatigue failure test and K-V diagram were compared. A material constant was derived by evaluating the relationship between the crack growth rate and stress intensity factor, and the result was found to agree with the value derived from fatigue failure tests. This result confirms that that material constant derived from fatigue failure tests is the inherent property of the material and corresponds to the variation of crack growth rate with changes in stress factor. The effects of carbon content in MgO-C brick on the crack growth behaviour and fatigue failure mechanism were also investigated.

The fatigue failure behaviors of high-alumina brick, agalmatolite brick and silica brick refractory materials were investigated at room temperature and high temperature. As a result, fatigue fracture lives at room temperature are almost the same with the order of high-alumina > agalmatolite > silica. In comparison with other inorganic materials, fatigue failure life in this work was slightly higher than that of silica glass and alumina ceramic.At high temperatures, it should be noticed that fatigue failure lives of the agalmatolite and silica bricks showed significant increases at the temperatures over 1 473 K. On the other hand, the high-alumina brick showed slight increase. The behavior of strain rate at high temperatures proved that the strain rate of the silica brick mainly increased as a result of crack growth, while crack growth in the high-alumina and agalmatolite bricks increased as a result of creep deformation.Moreover, the reason why fatigue fracture at high temperatures increased in the agalmatolite and silica bricks was examined in more detail. It was deduced that the longer fatigue fracture lives at high temperatures were related to the generation of a slight amount of liquid phase. The degree, of how long fatigue fracture life would be, was estimated to be dependent on the wettability between the solid phase and liquid phase; poor wettability elongated fatigue fracture life.

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