Chapter 7 227 The post-yield slope decreased linearly with the fraction of S-S bonds cleaved (R2 = 0.88) showing the importance of the disulfide bond network to hair strength in the post-yield region. At point D in Figure 1, the hair breaks. The break force has a linear dependence on the cross-sectional area of the hair as seen in Figure 3 below. Figure 2. Normalized post-yield slope (arbitrary units) versus fraction of S-S bonds cleaved Figure 3. Break force versus cross-sectional area (curve provided by Trefor Evans)