Thermal Shock Factors

Thermal shock resistance is not a physical property of the material it depends on many physical characteristics of the material, but also on the shape of the refractory article, its dimensions, the cooling and heating conditions, among other factors. Because of this, there is neither a single criterion or factor, nor is there a single method of its determination. However, people usually use the thermal shock factor (criterion) R, and, additionally, there are many methods of determining the thermal shock, both standard and nonstandard. 

The thermal shock factors (criteria) are used in R D practice and during the R D of materials design. Methods of estimating the thermal shock resistance are required in order to estimate the ability of the material to be used in thermal devices and in furnaces and for production quality control. There are no strict dependencies between thermal shock resistance factors and the values of the thermal shock resistance however, sometimes correlations can be made. 

The classical research on the thermal shock resistance of refractories and brittle solids was made by D. P. H. Hasselman [109, 110]. For samples of a simple shape (thin plate, hollow cylinder, sphere), which are cooled in a liquid medium, the 

Equation (1.27) has a dimension of degrees and reveals the maximum temperature drop in the samples. The first thermal shock factor R reflects the tendency of the thermal shock resistance from strength, thermal expansion, and elastic modulus, but it is impossible to use this factor in order to make a forecast on the service life of the refractory. 

factor R is more applicable when the cooling conditions are not severe (as it takes place at cooling of the sample at air)  

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Data for thermal movement of various bitumens and felts and for composite membranes have been given (1). These describe the development of a thermal shock factor based on strength factors and the linear thermal expansion coefficient. Tensile and flexural fatigue tests on roofing membranes were taken at 21 and 18°C, and performance criteria were recommended. A study of four types of fluid-appHed roofing membranes under cycHc conditions showed that they could not withstand movements of <1.0 mm over joiats. The limitations of present test methods for new roofing materials, such as prefabricated polymeric and elastomeric sheets and Hquid-appHed membranes, have also been described (1). For evaluation, both laboratory and field work are needed. 

Many of these properties depend upon others which may themselves be governed by yet other factors. Thus, as mentioned above, increased porosity usually gives better thermal shock resistance, but it may be necessary for reasons of watertightness to employ a body with a very low porosity. The size of an article is also closely related to the degree of thermal shock which it will withstand. For this reason it is very difficult to give accurate figures for the thermal shock resistance of stoneware bodies. In practice, if precautions are taken to heat up any stoneware articles slowly and evenly no trouble will be experienced. This is a matter on which the ceramic manufacturer should be consulted. 

D. King, Factors Controlling the Thermal Shock Resistance of Enoxv-Metal Systems.. M. S. Thesis, University of Connecticut, 1986. 

A major factor determining the importance of ceramics lies in their usefulness at high temperatures. Two types of experimental techniques are generally used in order to determine the minimum shock required to nucleate fracture (cracking), and the amount of the damage caused by thermal shock 83 the number of quenching cycles, and strength as a function of quench temperature and crack patterns. 

Evans (1975), Evans and Charles (1977), and Emery (1980) performed more refined fracture mechanics studies regarding the onset and arrest conditions Bahr et al. (1988) and Pompe (1993) extended this work and considered the propagation of multiple cracks while Swain (1990) found that materials showing non-linear deformation and A-curve behaviour have a better resistance to thermal shock. More specifically, the behaviour of a crack in the thermal shock-induced stress field was deduced from the dependence of the crack length on the stress intensity factor. Unstable propagation of a flaw in a brittle material under conditions of thermal shock was assumed to occur when the following criteria were satisfied ... 

A recent analysis by Kastritseas etal. (2004c) suggested that in both cases the magnitude of the thermal shock-induced stresses was overestimated as the anisotropic character of the materials was not taken into account. If material anisotropy is accounted for, then both (15.36) and (15.37) cannot predict A Tc accurately even for the largest possible value of the thermal shock-induced stresses (corresponding to a maximum value of the stress reduction factor, A = 0.66). To explain the discrepancy, it was proposed that the interfacial properties may be affected by the shock due to the biaxial nature of the induced stress field, which dictates that a tensile thermal stress component that acts perpendicular to the fibre-matrix interface is present for the duration of the shock. 

Becher, P.F., Warwick, W.H. (1993), Factors influencing the thermal shock behavior of ceramics , in Schneider, G.A. and Petzow, G. (editors), Thermal Shock and Thermal Fatigue Behavior of Advanced Ceramics, Dordrecht Kluwer Academic 37-48 Becher, P.F., Lewis, D.III., Carman, K.R., Gonzalez, A.C. (1980), Thermal shock resistance of ceramics size and geometry effects in quench tests , Am. Ceram. Soc. Bull., 59(5), 542-545... 

Review with the owner his operating procedure, include corrosive elements anticipated query him in detail. Determine low and high temperatures anticipated and their duration. Thermal shock is or may be an important factor. Review the type of cleaning to be used, water-hot, cold, steaming. The type of cleaning agents and solutions or solvents can be a factor. 

Inorganic Silica-Based Mortar The completely compatible mortar is a two-component silica-based mortar used to bed and bond the block when the thermal and chemical environments exceed the capabilities of the adhesive/mem-brane and where vibration and thermal shock are not serious factors. Its thermal characteristics and chemical resistance are identical to that of the block. The cured joints are rigid, dense and abrasion resistant. (See Chapter 22.)... 

Brick spalling is the condition wherein V4 to V2 in. of the brick face breaks away. This is not of concern if it occurs in a few brick in isolated locations since this would probably be a result of individual brick characteristics. However, if the condition occurs in a concentrated area involving a significant number of adjacent brick, there is cause for concern and a specialist should be contacted to analyze the problem. Spalling problems can be caused by thermal shock, excessive compression, receded mortar joints, exposed brick edges, or perhaps other factors. 

Apart from their thermal stability a number of other factors are important in the choice of refractory materials e.g. thermal expansion coefficient, thermal shock resistance, chemical resistance, thermal conductivity and abrasion resistance. These properties depend upon both the microstructure and composition of the ceramic material. 

Conversion of glass to a polycrystalline ceramic is accompanied by increased strength (two to four times), increased fracture toughness (two to four times), increased electrical resistivity (10 times), increased deformation temperature (200-400°C), increased abrasion resistance, and increased thermal shock resistance. All these factors contribute to many applications for glass ceramics dinnerware, cooking utensils, stove tops, radomes, hermetic seals to metals, building materials, and so on. 

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