Initiation-growth-fracture process

Crack growth models in monolithic solids have been well document-ed. 1-3,36-45 These have been derived from the crack tip fields by the application of suitable fracture criteria within a creep process zone in advance of the crack tip. Generally, it is assumed that secondary failure in the crack tip process zone is initiated by a creep plastic deformation mechanism and that advance of the primary crack is controlled by such secondary fracture initiation inside the creep plastic zone. An example of such a fracture mechanism is the well-known creep-induced grain boundary void initiation, growth and coalescence inside the creep zone observed both in metals1-3 and ceramics.4-10 Such creep plastic-zone-induced failure can be described by a criterion involving both a critical plastic strain as well as a critical microstructure-dependent distance. The criterion states that advance of the primary creep crack can occur when a critical strain, ec, is exceeded over a critical distance, lc in front of the crack tip. In other words... 

Since Wf = U and A = It, Equation (14) is similar to Equation (12). It has therefore been argued [23] that the impact fracture energy at crack initiation in polymers with ductile behavior G is rather the essential fracture work we. Furthermore, the second parameter representing the variation of the impact fracture energy during stable crack growth, T, has been attributed to the work dissipated in the outer plastic zone and is not related to the fracture process [23]. 

The fracture processes contain aspects of both plane stress and plane strain behaviour. The initial stages of crack growth will always be under plane strain conditions, because the yielded zone must grow to a certain size before plane stress conditions can develop. However, plane stress conditions always apply near the free surfaces. Consequently, plane stress shear lips are often observed at the edges of flat fracture surfaces. 

The term fracture process is meant to describe the whole development of a fracture from its initiation through crack growth and propagation to completion of the fracture. In contrast to thermal or environmental degradation and failure... 

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