Figure 11. The number of required cycles for growing a flaw from an initial to critical size for two different far-field stresses (Eq. 27). The solid line is for a far-field stress of 20 MPa; it corresponds to the stresses of mastication. The dashed line corresponds to a slightly elevated stress level (30 MPa). An initial flaw 0.3 mm long will grow to catastrophic size in roughly 10⁶ cycles at 30 MPa (approximately one year); at 20 MPa, a pre-existing flaw would have to be 0.9 mm long to grow to catastrophic size in the same number of cycles. Lifetime models such as Eq. 27 depend critically on the Paris law exponent, m (Eq. 26), the critical flaw size (Eq. 25), and the stress intensities at the head of the crack tip.