Mechanical Properties of Hair 224 content increases,1 while resistance to extension decreases with water but to a much lesser extent.2 The X-ray diffraction studies of Astbury and co-workers3-5 revealed that keratin fibers such as wool and hair and even porcupine quills have a characteristic X-ray pattern, indicating a structure oriented roughly parallel to the fiber axis, called the α-keratin pattern. Stretching the fiber in steam led to the appearance of a second pattern, called the β-keratin pattern. Linus Pauling received the Nobel Prize for solving these protein structures, the α-helix and β-sheet.6 Subsequent work on the mechanical properties of keratin fibers, spearheaded by Max Feughelman but with contributions from many other workers, has led to better understanding of the mechanical properties of these fibers and of the structure of the fibers as well.7-16 For instance, the mechanical properties of the hair cortex can be modeled using the two phase model of Feughleman12,15 which consists of the α-keratin micro-fibrils embedded in a matrix of keratin associated proteins. This model has implications for the structure of the fiber cortex and its interactions with water. This chapter will review the mechanical properties of hair starting with a review of the basic stress/strain curve in tensile extension and the methods commonly used to make tensile measurements of hair. Tensile properties have been widely used to measure damage to hair from cosmetic treatments and some of this work will be summarized. Other methods for measuring hair mechanics, including dynamic mechanical analysis, torsional and bending methods, will be considered. The various models of mechanical behavior in each region of the stress/strain curve will be summarized and discussed. Tensile Properties of Hair The mechanical behavior of hair is most frequently studied in extension by obtaining a stress/strain curve. In tensile analysis, stress is defined as the average force per unit area. Currently stress is most commonly given in either units of N/m2 or gmf/cm2. Strain is the change in dimension relative to the relevant dimension of