34 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS shown in Figures 11-15. Matching parts of the fracture surface of a smooth fracture are indicated in Figure 11. The point of initiation of fracture seems to be at the edge of the cortex. Matching ends of a step fracture are shown in Figure 12, with matching parts Figure 12. SEM micrographs of step fracture of untreated fiber at 65% RH. a) and b) The matching ends. 240 x. c) Fracture end in (a) enlarged. Note pulling out of intercellular cement from cortical cells. 800x. d) Step in (b) enlarged with a crack indicated. 800 x. indicated. Several cracks can be seen in the vicinity of the fracture. The angle fracture shown in Figure 13 seems to occur due to the presence of torsion during tensile fracture. The fibrillated fracture end in Figure 14 suggests that an extensive loss of intercellular cohesion in the cortex preceded failure (see Figure 12c) and that torsional modes of deformation might have been involved in the fracture. Figure 15 shows a typical split end. Fracture pattern distributions are given in Table VII. The numbers in parentheses in Table VII are for Caucasian hair (1) and clearly indicate that smooth fracture, especially at higher relative humidity, is the predominant mode of fracture in contrast to that for Negroid hair. The predominant fracture mode for Negroid hair, the step fracture, results from initiation of multiple cracks along the fiber axis (1), and it would therefore be reasonable to conclude that extensive fiber damage is responsible for this behavior.

BEHAVIOR OF NEGROID HAIR 35 Figure 15. Angle fracture of a Negroid hair fiber in the tensile mode. 54x. Fibrillation also seems to be a rather common type of fracture in Negroid hair, while it was observed only occasionally in Caucasian hair (1). An examination of the fracture pattern distribution among premature failures reveals a shift towards fibrillated or split ends as the predominant fracture mode. It appears that a significant number of the fibers that fail prematurely have rather poor cohesion of the cortical cells. This observation is supported by the fracture pattern distribution of fibers that failed during fatiguing, discussed below. Step fractures involve axial splitting of the fiber, the length of the split apparently influenced by the water content of the fiber (1). In an effort to establish the effect of Figure 14. Fibrillated end of a fiber fractured at 65% RH. 240x.