Determination of sub-critical crack growth parameters

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. 

An alternative approach, termed dynamic fatigue, is to measure strength as a function of stressing rate. Equation (9.11) can be written as 

From a logarithmic plot of the dynamic strength versus stressing rate, the value of n is obtained from the slope [= /( + )] and B is obtained from the intercept. An attractive feature of this approach is that failure occurs from natural cracks. 

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

The final approach is called static fatigue, in which lifetime for different levels of constant applied stress are determined. Writing Eq. (9.10) as 

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

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