BEHAVIOR OF NEGROID HAIR 25 RESULTS AND DISCUSSION FIBER CONFIGURATION IN THE REGION OF TWIST In order to understand the nature of the twist and its implications for the mechanical and fractographic behavior of the Negroid hair fiber, it was decided to examine the region of twist microscopically. Scanning electron micrographs of the twisted regions of two typical fibers are shown in Figure 3. It appears that substantial changes in the cross-sectional shape of the fiber occur in the twist region, involving a flattening or collapse of the fiber structure. This was further confirmed by examining cross-sectional shapes in these regions (Figure 4) as a function of distance from an arbitrary reference 0 y.m 250 $OO :370/.m 530 Figure 4. Cross-sectional shapes of Negroid hair fiber in the region of twist. Indicated distances measured from arbitrary reference point. 200 x. point (0/am). Such changes in cross section within relatively short distances can lead to stress concentrations during tensile deformation which may result in premature failure. The major and minor axes of the fibers were determined by optical microscopy on cross sections of several fibers, assuming that the contour of the fiber cross section approximates an ellipse. The average ellipticity at 65% RH was found to be e = 1.895 _+ 0.083 (95% confidence level). This value is much larger than that found for Caucasian

26 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table I Angle of Rotation (Degrees) About the Fiber Axis Under the Action of a Tensile Load (65% RH, 21øC) Weight (Specimen Lengths 65-95 RR) Added (g) • I 5 10 20 30 40 50 60 70 80 SpeciRen 1 2 3 --16 --30 --30 --21 30 92 120 88 brk. 20 - 7 --18 -18 -45 -41 5 -70 -187 7 -15 -33 -53 -70 -77 103 165 brk. brk. hair (e -- 1 to 1.4). The high ellipticity is a reflection of the flattened collapsed appearance which seems to be a typical characteristic of Negroid hair. The influence of increasing load on torsional untwisting of fiber specimens 65 to 95 mm in length is shown in Table I. The angles of rotation change in sign during the course of a single measurement, suggesting the presence of at least two twists in the specimen in opposite directions. Indeed, the twisted configuration shown in Figure 3 does not appear to be helical or unidirectional over an extended length of the fibers. Table II Angles of Axial Rotation (Degrees) of Negroid Hair Under Tensile Loading (65% RH, 21øC) (Specimen lengths 1-2 RR) Weight (g)• 5 10 20 30 40 50 60 70 80 90 100 110 SpeciRen 1 2 3 4 5 6 7 8 9 lO 11 12 13 14 15 16 17 18 19 0 --11 --16 --20 --29 --33 --40 --46 0 --7 --24 --47 --81 --174 brk. 0 0 -8 -17 -29 -51 -41 brk. 0 3 13 16 26 42 42 42 0 -9 -15 -43 -61 -109 -117 -136 0 -5 -12 -21 -21 -2 11 11 0 -4 -17 -51 -47 brk. 0 --4 --6 --10 8 58 a (brk.) 0 6 23 45 97 69 69 brk. 0 -6 -19 -27 -65 -94 -107 -115 0 4 12 24 25 33 33 36 0 -24 -62 -128 -177 -207 -193 saved 0 -118 -147 -147 -164 -143 saved 0 4 19 9 29 50 57 66 0 0 18 7 -5 brk. 0 --4 --9 --87 --70 --70 saved 0 19 30 42 73 85 90 0 4 10 13 31 17 25 0 4 4 4 14 14 11 20 0 0 8 19 36 60 brk. 21 0 2 4 8 16 40 53 -34 brk. 59 57 brk. -133 -133 brk. brk. brk. 33 33 -43 57 57 brk. 192 216 brk. saved 11 li b (brk.) 42 saved brk. aBroke during weight change. bBroke during the angle measurements.