36 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS . •, ß ,. '•. :2:: ß '? •t• .5".' .. ,lb •.. •:. .:ax •' • • i•-•'. Figure 15. Typical split end in the tensile fracture of a fiber. 160x. this variable, the lengths of axial splitting after fracture at 65% RH and in the wet state were measured microscopically, and the distributions of these lengths are plotted in Figure 16. The number fraction in these figures is based on smooth and step fractures only, the cross-hatched region on the first bar representing the fraction of smooth fractures. It is clear from Figure 16b that the increase in water content does not have the drastic effect of producing almost exclusively smooth fractures as was observed in the case of Caucasian hair, but on the contrary leads to a considerable increase in the length of axial splitting. This reflects the weakening effect of increased moisture content on the intercellular domains, which seem to play a significant role in the mode of fracture. The fracture ends of fibers that failed at low elongation were examined by scanning electron microscopy. A typical fiber which failed at 3% extension, shown in Figure 17, seems to have been damaged considerably before it was subjected to tensile stress. As has been mentioned earlier, Negroid hair tends to break in half-wavelength sections during gentle manipulation. Fiber ends resulting after squeezing a mass of Negroid hair between the fingers were examined by SEM typical micrographs are shown in Figure Table VII Fracture Pattern Distribution for Negroid Hair Fibers in the Tensile Mode No. of Fracture Type (%) Specimens RH (%) Smooth Step Angle Fibril. Split End 36 65 6 (56) 56 (44) 6 22 10 38 wet 8 (91 •) 42 (9 •) 21 24 5 8 • 65 0 25 12 38 25 Numbers in parentheses for Caucasian fibers. •At 90 ø RH instead of wet. •Premature failures.

BEHAVIOR OF NEGROID HAIR 37 NUMBER FRACTION 0.3 a) UNTREATED, 0.2 65 % RH 0.1 0 .2 .4 .8 I.O 4 mm LENGTH OF AXIAL SPLITTING (ram) NUMBER FRACTION O.Z I 0.! b) UNTREATED, WET .2 ß 6 .8 1.0 1.2 1.4 LENGTH OFAXIAL SPLITTING (ram) Figure 16. Axial splitting length distributions after fracture (a) at 65% RH and (b) wet. Cross-hatched bars represent smooth fractures.