Viscoelastic effects crack models

The zero velocity limit, .q, represents the threshold value required for crack motion, and is equal to the thermodynamic work of adhesion under true equilibrium conditions. In developing a description of the effects of small-scale viscoelasticity, the aim is to develop a viscoelastic cohesive zone model that gives reasonable predictions for o. i and ... 

Craze growth at the crack tip has been qualitatively interpreted as a cooperative effect between the inhomogeneous stress field at the crack tip and the viscoelastic material behavior of PMMA, the latter leading to a decrease of creep modulus and yield stress with loading time. If a constant stress on the whole craze is assumed then time dependent material parameters can be derived by the aid of the Dugdale model. An averaged curve of the creep modulus E(t) is shown in Fig. 13 as a function of time, whilst the craze stress is shown in Fig. 24. 

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