## 0008

FIGURE 6.28 Predicted threshold stress intensity factor for SCS-6/Ti-15-3.

long cracks can cause failure of the fibers within the plastic zone. Fiber failure at lengths smaller than the critical will result into an immediate reduction of the flow resistance of the material. In [11] it was proposed that the fatigue failure of uMMCs is following a hybrid pattern, constituted by both failure conditions. In the same work it was suggested that as the crack propagates further the significance of the FCE becomes less vital. In other words, there is a particular crack length that could claim the simultaneous debonding of two successive fiber rows [113]. The so-called FCE degradation could deteriorate further to include the simultaneous debonding of a larger number of fiber rows. If a large number of fibers ahead of the crack tip is debonded, the FCE becomes minimum and therefore the crack tip plasticity could follow an unconditional propagation. The term unconditional denotes spread of plasticity controlled only by the material's flow resistance. Maximum spread of plasticity and consequently maximum crack tip opening displacement (CTOD ^ max), will result into fast propagation of the crack and the consequent failure of the fibers ahead of the crack tip.

Considering that at the time of crack instability the crack system is still under equilibrium of the stresses, Eq. 6.18 can be used. In terms of the TZMM maximum plasticity is modeled by assuming n2 ^ 1. This boundary condition yields sin

and cos 1 n2

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