38 BEHAVIOR OF NEGROID HAIR Figure 17. Fracture ends of a fiber that failed prematurely at 65% RH at 3% extension. 320). .. . •, .:.: •- .•' . .,•.....• '•1• • -. "•, • ,• x.• -- .....•r-• ß • -•' :•.• --,• ...•,•. Figure 18. Fracture ends of fiber snippets obtained by squeezing a mass of Negroid hair between the fingers. a) 40Ox. b) 112x. c) 128x.

BEHAVIOR OF NEGROID HAIR 39 18. With the exception of a few smooth fiber ends, most of them show damage of some kind. In some fibers (e.g., Figure 18b), fracture seems to have occurred in the region of twist. It should be noted that hand manipulation of the fiber assembly imposes principally torsion and bending stresses on single fibers. In general it can be said that these fiber ends show basically the same patterns shown in Figure 10. The fibrillated and split end fracture patterns show some similarity to fracture patterns for cotton fibers studied by Hearle and Sparrow (2). It should be noted that cotton fibers also have regions of twist reversal, but the general structure of the fiber is considerably different, consisting of a central lumen covered by primary and secondary walls in which fibrils run at an angle to the fiber axis. Fractographic studies of cotton fibers showed that the fibers broke in the vicinity of the twist reversal rather than in the region of twist, indicating that the presence of twist reversal gives rise to a weakness in the fiber structure. Here again direct comparison may not be valid, since the region of twist in the cotton fiber is -•20/xm long (! diameter), whereas in Negroid hair it is several diameters long (-• 500/xm). TENSILE FATIGUING OF NEGROID HAIR To understand fiber damage resulting from grooming procedures, an accelerated test was designed in which fibers were fatigued in the tensile mode under various loads. It should be noted that in the regions of twist, extension of the fibers also results in torsional fatiguing. The lifetime distributions during fatiguing up to a maximum of 11 kc are shown in Table VIII. As would be expected, the total number of failures during Table VIII Lifetime Distributions of Untreated Negroid Hair at Different Fatiguing Loads (65% RH) Percent of Total Failures Total Failures Immediate Load (g) in 11 kc (%) (•0 kc) 0-0.5 kc 0.5-2 kc 2-11 kc 10 19 12 6 40 42 20 20 25 12 25 38 30 41 17 33 20 30 40 61 29 24 27 20 Note: Total number of fibers for each load level = 75-90. 1! kc of fatiguing increases with increasing fatiguing load. An increasing number of these additional failures are either immediate failures or failures in the 0-0.5 kc range, suggesting that this type of fatiguing leads to the accentuation of existing fiber damage. It can be assumed that these loads are probably too small to initiate new damage to the fibers, although cumulative irreversible deformation at the highest load level may produce changes in the fiber structure. Fracture pattern distributions of fibers that were subjected to the fatiguing procedure are shown in Table IX. For the fibers that survived fatiguing for 11 kc and were subsequently broken on the Instron ©, the predominant fracture pattern is the step fracture and the distributions are essentially independent of load. As was pointed out above, the number of survivors decreases with increase in load. Among the fibers that broke before reaching 11 kc, fibrillation was the most commonly encountered fracture