Spontaneous microcracking

In the previous section, the emphasis was on thermal shock, where failure was initiated by a rapid andjor severe temperature change. This is not always the case both single- and multiphase ceramics have been known to spontaneously microcrack upon cooling. Whereas thermal shock can be avoided by slow cooling, the latter phenomenon is unavoidable regardless of the rate at which the temperature is changed. 

Spontaneous microcracking results from the buildup of residual stresses which can be caused by one or more of the following three reasons ... 

To estimate the critical grain size above which spontaneous microcracking would occur, the various energy terms have to be considered. For the sake of simplicity, the grains are assumed to be cubes with grain size d in which case the total energy of the system is " ... 

Spontaneous Microcracking due to Thermal Expansion Mismatches in Multiphase Materials... 

Spontaneous Microcracking due to Phase-Transformation-Induced Residual Stresses... 

Heating or cooling of ceramics for which the thermal expansion is anisotropic. The magnitude of the stresses will depend on the thermal expansion anisotropy, and can cause polycrystalline bodies to spontaneously microcrack. This damage cannot be avoided by slow cooling, but can be avoided if the grain size is kept small. 

One type of pore that is worthy of further consideration is the extreme case of a microcrack. In Section 2.9 it was shown that thermal expansion anisotropy can lead to residual stresses in ceramics and, in some cases, the formation of localized (spontaneous) microcracking. Microcracks may only represent a small fraction of porosity in a body but their ability to concentrate stress can lead to substantial reductions in the elastic constants. For a random array of circular microcracks, radius a, the SC approach shows the elastic constants ix and B of the microcracked material can be approximated by... 

Figure 8.64 Fracture toughness increases with particle size in a microc-rack-toughened ceramic unless the size exceeds the critical size for spontaneous microcracking.

Spontaneous microcracking can be eliminated in ceramics with thermal... 

Residual thermal stresses may lead to spontaneous microcracking of glass matrix composites provided the size of the embedded particles is greater than the so-called critical particle size Dc [9]. Several approaches have been developed to determine Dc. In the model of Davidge and Green, for example, Dc is given by [9] ... 

The elastic modulus (a) and Vickers hardness (b), indented with a 200 N load on polished surfaces, are shown in Fig. 5.55. The lines increase linearly with the increasing fraction of Z1O2. Such a trend may reflect the absence of spontaneous microcrack formation. 

In Fig. 8.9, one sees a 2D crack dislocation with a giant Burgers vector, nb, of length c. Stroh s [52] dislocation model for spontaneous microcrack formation, calculates the elastic energy associated with the wedge deformation of length, c, extending to a barrier by means of ... 

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