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CRACK LENGTH(mm) FIGURE 6.32 Fiber-bridging degradation curve for 32% SCS-6/Ti-15-3.

If the COD close to crack tip is equal to the CODcr, it is then rational to assume that all the fibers within the crack wake have failed. A predicted lower bound for fatigue failure is presented in Fig. 6.32.

Figure 6.32 provides the so-called unsteady crack growth behavior of the material. Within the limits of the curve the material is expected to behave in a constant manner since the toughening mechanisms are exhibiting limited and therefore predictable fluctuations. Above the curve, values of FB will start to appear significantly degraded pinpointing the onset of unsteady crack growth. The phenomenal great difference between Fig. 6.29 and 6.32 becomes less significant if we considered that experiments have shown that such difference represents only 15-20% of the total life of the material at most stress levels [69]. It is therefore justifiable, in terms of safety, to accept values of final crack length defined by the FB degradation.

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