Chapter 7 241 not simple to make, and apart from the balanced fiber method of Robbins (Figure 10), require specialized equipment that may not be widely available. Dia-stron now makes a device to measure bending by the cantilever beam method, but the equipment is very new and this author has no experience with it, nor could I find any data from its use in the literature as of the time of this writing. Models for the Mechanical Behavior of Hair The “Hookian” region: Models for the mechanical behavior of keratin fibers must account for the effect of water on viscoelastic properties. Hair and wool are plasticized by water and the properties of dry hair are very sensitive to relative humidity as previously shown in Figure 1 and Table 4. This author is old enough to remember having a home humidity gauge that contained a horse hair working against a spring to indicate RH. (See Chapter 10 for further mention.) Successful mechanical models must also account for viscous damping as seen in the DMA and the torsion measurements discussed previously, as well as the related phenomenon of stress relaxation (Figure 11). When a keratin fiber is extended and held at a fixed length, the stress relaxes with time toward an equilibrium value. Figure 12 shows stress relaxation of a hair extended to 102% of its initial length (2% strain) at a rate of 10% per minute in water plotted as the apparent modulus value. Figure 12. Tensile stress relaxation of a hair in water at 2% strain after extension at 0.5% strain/sec