FIGURE 6.27 Comparison between experimental and predicted crack arrest for SCS-6/ Ti-15-3. Curve has been created by considering Vf = 39%.

Equation 6.31 in terms of stress intensity factor can be written as:

where Kh is the threshold stress intensity factor for MMCs. In Eq. 6.32 the effect of the closure stress a1 is disregarded. In Fig. 6.28 the effect of different values of fiber volume fraction on Kth is presented.

From Fig. 6.28 it is clear that, for cracks of approximately 1 mm of length, the value of Kth asymptotically tends to a constant value. This distinction could represent a transition between a short and a long crack. At this stage we have to make clear that optimization of the crack arrest capacity of the uMMC through o3d(Vf) is limited by the ultimate tensile strength of the material and the desired transverse properties.

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