Chapter 7 245 circumstances.65-67 Discussion of these models is beyond the scope of this chapter. The role of stress relaxation in the development of temporary set was discussed previously in Chapter 5. The two-phase model also explains the greater effect of RH on the shear modulus compared to the extensional modulus discussed previously. As the connection between the microfibrils through the matrix is weakened by hydration and swelling of the matrix proteins, the microfibrils can move by each other more easily as the fiber twists. This mechanism is not available to relieve stress in extension.15,57,58 By way of analogy, think of a bundle of wires held together by glue that can soften with hydration. When the glue is hard, it is very hard to twist the wire bundle, but when it is soft the bundle can be easily twisted, as the wires will move by each other more easily. However, softening of the glue will have little or no effect on the force to extend the wires. The two-phase model of the cortex of keratin fibers is consistent with fiber structure and successfully accounts for the effects of water on the mechanical properties in shear and extension, the dynamic mechanical properties as long as strains are restricted to the linear region of the stress/strain curve. Other models are required to account for behavior in the yield and post yield regions. The yield and post-yield regions: X-ray diffraction results have demonstrated that there is a progressive loss of α-helical content with a corresponding increase in β-sheet in the yield region.4 By the end of the yield region about 30% of the original α-helix has been reversibly unfolded. Mechanical behavior of keratin in the yield region is modeled using a Burte-Halsey model.68 The fiber is considered to contain a continuum of units which can exist in a short state, A (α-helix), or an extended state, B (β-sheet), with an energy barrier between the states. In the yield region, helices unfold cooperatively, leading to a phase transition between state A and state B at approximately constant stress.69 This length change at constant stress is thermodynamically equivalent to the transformation of water to steam with a volume change at constant temperature and pressure. DMA measurements with Lincoln wool at 118 Hz also