Chapter 7 243 We can see that the average shear modulus at 20% RH is only ~1.5 GPA compared to ~4.0 GPA for the extension modulus. G decreases from ~1.5 GPa (109N/m2) at 20% RH to ~0.4 GPa at 90% RH, a decrease of ~70% compared to an ~20% decrease in E over this RH range. (See Figure X in Chapter 11). Wolfram and Albercht51 reported torsion pendulum data on hair at 65% and 100% RH and found that log decrement increased from ~0.17 at 65% RH to 0.40–0.56 at 100% (in water) indicating a large increase in torsion damping on hydration. The effects of water on the shear and elastic modulus, dynamic mechanical properties, and stress relaxation in the “Hookian” have all been successfully explained by the two-phase model proposed by Feughelman.12,18,58 The two-phase model is consistent with the molecular structure of keratin fibers and in fact helped to shed light on the molecular structure as it was formulated. It considers the mechanical properties of the fiber to be determined by a water- impenetrable phase, C, the microfibrils, and a water permeable phase, M, the matrix. The microfibrils consist of α-helical proteins (keratins) aligned approximately parallel to the fiber axis59-62 and Figure 13. Effect of RH on the shear modulus of hair measured with a torsion pendulum (courtesy of Trefor Evans)