Mechanical Properties of Hair 246 show a large drop in the resistance to the oscillating stress as the fiber is extended into the yield region.15,39 This change in mechanical behavior with extension into the yield region is also completely consistent with α-helix to β-sheet transformation. For a detailed explanation see Feughelman.15 Feughelman first interpreted the behavior of keratin fibers in the yield and post-yield regions in terms of his series-zone model.13 The series-zone model postulates two alternating zones, X and Y, along the microfibrils. The X zones contain the 30% of the a-helices that unfold reversibly in the yield region. The Y zones contain regions of a-helix that cannot be unfolded without the breakdown of disulfide bonds. Thus, unfolding of the Y zones in the post-yield region is irreversible. Chapman and Hearle70,71 proposed an explanation for the post-yield region, based on microfibrils undergoing an a-b transition in parallel with an elastomeric amorphous matrix. Wortman and Zahn72 analyzed Feughelman’s model in light of the structure of keratin and postulated that the 1B regions of the a-helices of the keratin fibers (see Chapter 1, Figure 14) unfold in the yield region. Wortman and Zahn proposed that the 2B regions are more likely to be cross-linked because of their higher cysteine content73 and thus constitute the Y zones. Feughelman16,64 subsequently proposed a new model in which the matrix is considered to be composed of globular proteins with hydrophilic exteriors and hydrophobic interiors. In this model, water surrounding the matrix acts as a continuous three-dimensional polymer network interfacing with hydrophilic sites. The matrix is considered to be in a gel state at equilibrium, and extension into the yield region is treated as a gel-sol transition. Further extension of the α-helices in the post-yield region is opposed by jamming of the matrix proteins between the microfibrils. Hearle74 reviewed all three models, considering the latest information available at the time on keratin microfibrils and the matrix proteins of hair, and argued for use of the Chapman/Hearle model in the investigation of keratin structure and mechanical properties of keratin, pointing out that