Chapter 7 225 the object being strained. For example, extension of a 10 cm length of hair to 11 cm results in 10% strain. Force extension curves for adjacent sections of the same hair at 50% RH and immersed in water are shown in Figure 1. To convert these to a true stress/strain curve, one would divide the force by the cross-sectional area of the hair. The curves can be characterized by three different regions. In the first region (A-B) the curve is approximately linear and a slope can be determined as shown. This part of the curve is often called the Hookean or linear region, and it extends to about 102% of the equilibrium length of the fiber (2% strain). While it can be argued that that slope of the stress-strain curve of hair and other keratin fibers is not “truly” linear in this region, it is close enough over most of the region that a modulus of elasticity can be determined.17,18 The slope in the linear region is higher in the dry hair. Between 2% and 4% strain, the curve “turns over” into the yield region (B-C). In the yield region, very little increase in force is required to increase extension. In the post-yield region (C-D) starting at ~ 30% strain, the force again increases with strain. For this hair, under the conditions tested, the slope in the post-yield region was about 10 times that in the yield region, but only ~ 20% of the slope in the Figure 1. Force/extension curves for hair at 50% RH and in water. Data obtained at 25°C, strain rate 1% per sec using a Dia-stron miniature tensile tester (Dia-Stron Limited, 9 Focus Way, Andover, Hampshire UK).
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