28 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table III Angles of Axial Rotation and Extension of Negroid Hair in the Region of Twist (65% RH, 21øC) (Specimen lengths 1-2 mm) Weight (g) Specimen Property 10 20 30 40 50 60 70 80 90 100 A -18 -44 --82 --222 -289 --305 -327 brk. 1 E 15 -- 23 28 38 49 54 A 7 15 6 6 saved 2 E 14 17 17 20 A 22 60 121 188 202 205 209 3 brk. E 7 7 17 42 45 46 55 A -8 -20 --72 --72 --67 4 saved E 1 1 4 12 24 A 3 9 25 25 13 5 saved E 2 7 13 40 42 A 2 8 13 17 35 38 42 42 51 6 brk. E 0 7 7 15 31 35 42 63 90 A 10 18 34 65 89 110 89 7 saved E 3 5 8 21 48 49 83 A 11 14 39 80 46 8 saved E 0 16 26 31 42 A --9 --24 --39 --66 --88 --88 --88 9 --105 brk. E 10 16 23 26 51 57 66 A ? 16 22 49 39 10 brk. E 6 7 2O 2O 36 A = Angle (deg). E = Extension (%). Initial condition: weight of 5 g corresponds to 0 ø. In an extension of this study, fiber elongation was measured along with measurements of the angles of rotation. The results are shown in Table Ill. Although angle of rotation and degree of extension increase together, both level (per unit length) and direction of twist vary considerably in different fibers. For example, very high levels of twist in opposite directions are observed in fibers 1 and 3 at relatively moderate extension levels, while in fiber 6 rather low angles of rotation are found even though the extensibility reaches much higher levels. These high levels of extension may be associated with the absence of critical flaws within the short length of fiber specimen. Fibers 2 and 4, on the other hand, show relatively low extension and twist angles. FRACTURE BEHAVIOR AND MECHANICAL PROPERTIES Fracture ends of fibers from Tables II and III were examined microscopically in an effort to understand the role of torsion in the fracture of these fibers. Obviously, in most cases fracture occurred in the region of twist. The most frequent fracture pattern was the step fracture, only one fiber showing a smooth fracture. There was no simple way of assessing the role of torsional deformation in these fractures except that a considerable number of fibers showed a slanted step fracture such as is shown schematically in Figure 6. A comparison of slanted and level step fracture patterns is

BEHAVIOR OF NEGROID HAIR 29 ] i Figure 6. Schematic of slanted and level step fractures often encountered in twist experiments. shown in the micrographs in Figure 7. It is suggested that slanted step fracture occurs under the action of combined tensile and torsional stresses, and that initiation of cracks occurs under the action of the tensile load and axial splitting is aided by the torsional stresses. Tensile mechanical properties of untreated Negroid hair fibers (Table IV) were obtained from load-elongation curves.'A typical curve is shown in Figure 8 with the characteristic deformation regions of keratin fibers indicated. The pronounced toe-in region prior to the more or less linear Hookean deformation is indicative of the untwisting and bending that occur in the initial stretching of the fiber. The breaking stress reported in Table IV is a nominal stress based on the cross section of the uncleformed fiber. To obtain the true stress at the point of failure, the breaking force would have to be normalized by the cross section at the point of fracture, which is rather difficult to determine since upon fracture the fibers recover from deformation to a considerable extent. Any microscopically determined value of the cross-sectional area of the fiber at the fracture end would have to be corrected taking this recovery into consideration. To establish the extent of recovery, cross-sectional areas of a set of fibers were determined vibroscopically (Ai) , and the fibers were then extended to 35% extension and immediately released. Vibroscopic measurements were repeated on these fibers within 60 s (Af). The central portions of these fibers were then embedded in epoxy resin and cross-sectional areas (Am) were determined on microtomed sections of the fiber after about 86.4 ks (1 day). These results are summarized in Table V. Although the measurements were made on relaxed fibers (treated with a commercial formulation used to straighten Negroid hair), they also apply to untreated fibers as will be seen shortly. Assuming constant specimen volume, 35% extension results in a 26% reduction in cross-sectional area (A35). As shown in Table V, 39% of this reduction is recovered within 60 s, the minimum time required to make a measurement. Recovery continues at longer times, reaching 76% within a day. Similar trends are seen in length recovery, summarized in Table VI. The length recovery of untreated fibers was significantly faster than that of the relaxed fibers, although the recovery of the initial modulus appears to occur at the same rate. The data in Tables V and VI show that the cross-sectional area at the point of fracture cannot be obtained by microscopic measurements on fractured ends. These fibers have a highly developed