Sub critical crack growth

For the engineering use of ceramics, the primary emphasis has been on the threshold and Region 1 behavior. This is because an engineering component will often spend most of its lifetime in these regions. Indeed, it has been become popular to use an empirical power law to describe this behavior for with 

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Ford, F. P., Overview of collaborative research into mechanisms of environmentally assisted controlled cracking in the low alloy, pressure vessel steel/water system , IAEA Specialists Meeting on Sub-Critical Crack Growth, 15-17 May 1985, Sendai, Japan, NUREG/CP-0067 (1986)... 

Further crack growth will only occur if the material can undergo sub-critical crack growth (see Section 8.12). Now, consider the effect of the applied stress, as shown in Fig. 8.45. The additional term in Eq. (8.63) leads to a minimum in the total stress intensity factor. The indentation crack, therefore, undergoes stable... 

To this point, it has been assumed that failure occurs when K =T (or G=R) but, in studies of fracture, it is sometimes found that crack growth can occur at lower values of or G. Thus, kinetic effects must be included in any general formalism. There are several mechanisms that can give rise to sub-critical crack growth, but most attention has been directed to stress corrosion. This behavior has been extensively studied in silicate glasses but it can also occur in many polycrystalline ceramics. Figure 8.72 shows a typical response of ceramics to stress corrosion, with crack velocity v plotted as a function of K (or G). At low values of K, there often appears to be a threshold value of the stress intensity factor below which... 

Figure 8.72 Crack velocity during sub-critical crack growth is a function of the stress intensity factor and is considered to show three different regions of behavior.

Figure 9.3 Schematic of expected static fatigue behavior for cracks experiencing Region I sub-critical crack growth.

To include the effect of sub-critical crack growth into design, it is necessary to determine experimentally the values of B and n, the sub-critical crack growth parameters. These parameters define the behavior of a material in a given active environment. Three types of approach are commonly used for this purpose and these will be discussed below. 

Fignre9.4 Schematic of expected dynamic fatigue behavior for cracks subjected to Region 1 sub-critical crack growth. 

Figure 9.5 Crack front markings showing the position of an indentation crack as it undergoes sub-critical crack growth optical micrograph. (From Dwivedi and Green, 1995, reproduced courtesy of The American Ceramic Society, Westerville, OH.)...

Determination of sub-critical crack growth parameters by in-situ observation of indentation cracks, J. Am. Ceram. Soc., 78 [8] (1995) 2122-8. 

What is the threshold stress intensity factor in sub-critical crack growth ... 

Name two approaches for determining the sub-critical crack growth parameters. 

A cyclic stress is applied to a material that undergoes sub-critical crack growth. Will the number of cycles to failure increase or decrease with increasing frequency ... 

Tlie term static fatigue is used to refer to slow crack growth under long-term steady loading. There appears to be a threshold value of Ki below which no crack growth occurs above this limit, sub-critical crack growth can be represented by ah empirical power law relating the applied Ki and (dn/dt) ... 

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