JOURNAL OF COSMETIC SCIENCE 404 Second, the end of the Hookean region is normally categorized by the Hookean limit, indicated by A on the chart. Third, the region between A and B is known as the yield region and the end of the yield region (point B) is the turnover point. Fourth, the region bound by points B and C is the post-yield region. Finally, point C represents the force to break and the percent extension to break. One may also calculate the post-yield modulus, which is the slope of the stress–strain curve in the post-yield region. As already noted, Figure 18 clearly demonstrates the infl uence of H2O on the tensile properties of hair. The stresses in the load-elongation curve for hair containing greater amounts of water are much lower than that for a dryer hair sample. This phenomenon is due to the disruption by H2O of hydrogen bonds and salt bridges that stabilize the alpha keratin structure, making the hair more extensible at lower applied forces. Not surprisingly, treatments that infl uence the chemical structure of hair will ultimately lead to greater/ less H2O sorption/desorption, thereby infl uencing the tensile properties of the fi ber. It should be noted that in normal hair water can access the amorphous region, but is unable to ubiquitously penetrate the crystalline phase alpha keratin in normal, healthy hair. In an extraordinary text written by Max Feughelman, entitled Mechanical Properties and Structure of Alpha-keratin Fibres, many of the fi ne details of wool structure are elucidated through interpretation of mechanical testing data (42). Feughelman demonstrates the utility of tensile strength studies to go beyond providing a simple measurement of fi ber strength. He describes the cortex in terms of a simplifi ed two-phase system—a crystalline Figure 18. Illustrative load-elongation curves for dry and wet hair illustrating the different regions: Hookean, Yield, and Post-yield region. The end of the Hookean region is indicated by, A, the Hookean limit while the end of the Yield region is denoted by B, the Turnover point. The point at which the fi ber breaks due to overwhelming strain is delineated by C.

HAIR SHAPE AND DAMAGE FROM RE-SHAPING HAIR 405 component composed of IFs embedded in an amorphous matrix of disulfi de-rich globular proteins. During the extension of a keratin fi ber, reversible and irreversible changes occur throughout the various phases of the stress–strain curve. For example, alpha keratin structure is conserved throughout the Hookean region. This means that the fi ber can be extended up to the end of the Hookean region without infl icting permanent damage to the alpha keratin structure. Once we extend the fi ber beyond the Hookean limit (A), conformational changes are introduced and alpha keratin is converted to beta keratin. Fortunately, this process is reversible throughout the yield region. Therefore, stretching a fi ber up until the turnover point (B) will not lead to permanent changes in the crystal structure. It should be noted that approximately 30% of the alpha helix is converted to beta sheet in the yield region (42). When the alpha helix opens up, this changes the water management properties of hair—now, water can penetrate into the crystalline phase ma- terial. In the post-yield region, the fi ber stiffens with increasing elongation. It is believed that disulfi de cross-linking provides the major opposition force to extension in this re- gion of the load–elongation curve. By this point, changes in protein secondary structure (conversion of alpha keratin to beta keratin) are permanent along with other major irre- versible changes. One should also bear in mind that tensile measurements performed on a virgin, undam- aged fi ber are much different than those completed on hair with a rich history of abuse. Further, conducting one load–elongation procedure (through the break point) is distinc- tive from a series of cycles carried out in the Hookean or yield regions (extension followed by returning to the state of origin). Such repetitive insult studies may offer a more realis- tic approach to mimicking hair’s daily experience with brushing or combing. Alterna- tives to tensile strength measurements include mechanical fatigue and cyclical extension testing, which probably more accurately describe the forces encountered in daily groom- ing (41,43,44). Moreover, the forces generally encountered in tensile testing can often exceed that required to pull a fi ber, including the follicle, from the scalp (45). Neverthe- less, tensile testing provides us with greatly needed information on the overall physical condition of the fi ber. EFFECT OF DAMAGING TREATMENTS ON HAIR MECHANICAL PROPERTIES A great deal of work has been conducted on the effects of bleaching and its infl uence on the tensile properties of hair. Robbins gives a review of this material where he outlines several important concepts (41). Findings suggest that wet tensile strength is more sus- ceptible to bleaching than dry tensile strength. Bleaching damages disulfi de bonds, con- verting cystine (disulfi de bonds) to cysteic acid. More than likely, the hair fi ber is more accessible by H2O (especially in the matrix), in the absence of disulfi de bonds, thereby disrupting hydrogen bonds and making the fi ber more extensible (weaker tensile strength). We should also expect that the porous structure of bleached hair, due to de- composition of surface and internal lipids, might also play a role in facilitating H2O ac- cess to the interior of the fi ber. Likewise, a considerable amount of work has been completed to better understand the mechanical properties of permanently waved hair. Similar to the case of bleaching, permanent waving affects the dry tensile properties of hair less than the wet tensile properties. During the permanent waving process, disulfi de bonds (cystine) in hair are reduced (cleaved), and then reformed by oxidative treatment