Example calculations crack growth

The elastic region can release strain energy to drive crack growth an example of such a calculation is given in Chapter 14 in the gas pipe case study. However, if the yielded zone extends across the specimen width before crack growth occurs, or the polymer is anisotropic, the stress field implied by the use of Ki no longer exists. Other methods must be used to characterise fracture resistance. 

As a second comphcation, crack tip stress, strain, and strain rate within the process zone are more fundamental than AK or AK, amd govern CF crack growth kinetics. It is not presendy possible to unambiguously calculate the stress intensity dependence of these more fundamental parameters [76,22-24]. Shoji and cowoikers argue that the time-based rate of mechanical FCP (da/dtf) in an inert environment is proportional to the rate of dislocation emission from the crack tip, or equivalently, to the crack tip strain rate [98]. The value of ck/diy for FCP in vacuum, or more jpically moist air, is therefore an indirect crack tip driving force parameter for correlating CF do/dAI data which are also often stated with respect to a time-rate. In this approach, da/dtf is the product of da/dN and f. An example of this correlation is shown in Fig. 20 which represents the CF enhancement in da/dN relative to the air... 

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