Subcritical crack growth in ceramics

a power law is used to describe the crack-growth rate in region (T)  

If a time-dependent stress a(t) is applied, the life time of the component can be calculated by integration of equation (7.1). If the load is static, it is 

The effect of time-dependent loads on subcritical crack growth will be discussed in section 10.3. In exercise 24, an example for designing ceramic components against subcritical crack growth is given. 

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The techniques and test geometries that have been used to measure subcritical crack growth in ceramics are several, but they share a common principle, namely, the subjection of a well-defined crack to a well-defined stress intensity Ki, and a measurement of its velocity v. The technique considered here, the advantages of which are elaborated upon below, is the double torsion geometry shown in Fig. 12.8. For the double torsion specimen, K is given by... 

S. M. Wiederhorn. Subcritical Crack Growth in Ceramics. Fracture Mechanics of Ceramics, Vol. 2. Plenum, 1974, p. 613. 

The reduction of strength after short time corrosion in acids (pit formation) cannot be correlated with the weight loss [511], whereas after intensive corrosion a correlation exists between thickness of the corroded layer and strength [507]. Acid corrosion appears to influence subcritical crack growth in Si3N4 ceramics [507, 523]. 

Hoffman, M., Rodel, J., Stemitzke, M. et al., Fracture toughness and subcritical crack growth in alumina/silicon carbide nanocomposites , Fracture Mechanics of Ceramics, 1996, 12 179. 

Gonzaga CC. Subcritical crack growth in dental ceramics effect of the material (microstructure) and the test method. Doctoral thesis. University of Sao Paulo 2007. 

Gonzaga CC, Yoshimura HN, Cesar PF, Miranda Jr WG. Subcritical crack growth in porcelains, glass-ceramics and glass-infiltrated alumina composite for dental restorations. Journal of Materials Science Materials in Medicine 2009 20(5) 1017-1024. 

Muraoka, M. and Abe, H. (1996) Subcritical crack growth in silica optical fibers in a wide range of crack velocities. J. Am. Ceram. Soc., 79 51-57. 

Oates, W.S., Lynch, C.S., Rddel, J., Lupascu D. and Aulbach, E., 2003, Investigation of subcritical crack growth in lead zirconate titanate, submitted to the Journal of the American Ceramic Society. 

K. D. McHenry and B. G. Koepke, Electric Field Effects on Subcritical Crack Growth in PZT pp. 337 352 in Fracture Mechanics of Ceramics, Vol. 8. Edited by F. F. Lange. Plenum Press, New York, 1983. 

Ohji T (1994) Tensile Creep Rupture And Subcritical Crack Growth of Silicon Nitride. In Hoffmann MJ, Petzow G (eds) Tailoring of Mechanical Properties of Si3N4 Ceramics. Kluwer Academic Publishers, Netherlands, p 339... 

High-temperature failure of ceramics typically occurs by either subcritical crack growth or creep. In an attempt to summarize the data available so... 

The subcritical crack growth rate in silicon nitride materials is relatively low in comparison to other ceramic materials. The growth exponent is in the range of... 

As mentioned at the start of this section, the J-integral concept was originally applied to metals, in which the linear elasticity concept might include the plastic observations, i.e., plastic flow in the close vicinity of the tip. Reservations regarding the use of the J-integral in brittle materials, such as ceramics, are a consequence of the presence of microcracks or rather subcritical crack growth before fracture sets in, making it not strictly applicable. However, in ceramics, in which extensive inelastic processes are active in the crack-tip process zone, the use of J is not more restrictive than the use of the Kic criterion in linear elastic firacture mechanics. 

Stress corrosion cracking (SCC) of ceramics is generally recognized as a type of subcritical crack growth. Relative humidity is an important factor in SCC crack growth for many ceramics. Water generally increases SCC crack growth rates. 

Ceramics can also fail by subcritical crack growth due to stresses and localised chemical reactions. In glasses, for instance, water can enter surface defects and can attack the bonds between the silicon and the oxygen atoms if... 

If a ceramic can fail by subcritical crack growth, its life time when loaded with a certain stress can be calculated from equation (7.2). In section 7.2.6, we used a deterministic approach to do so, but by now we have learned that the failure stress values scatter and the probability of failure follows a Weibull distribution Pf(failure stress, must therefore also be distributed stochastically. 

It is accepted that ceramics under load fail as a result of subcritical crack growth from preexisting flaws, and as shown in Sections 5.9.2 and 5.2 and Table 5.1 these are relatively easily generated. Hence ceramic lifetime predictions require evaluation of crack growth rates when subjected to an applied stress just as much as estimations of the initial flaw sizes. 

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