32 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS memory for the original structure which they can regain even after being extended into the yield region. PREMATURE FAILURE IN TENSILE DEFORMATION The findings thus far suggest that a significant number of Negroid hair fibers have weak spots along their lengths. Under "dry" (65% RH) conditions, stress concentrations build up at these spots during tensile deformation which can lead to failure at low levels of extension. The origin of the weak spots is not clearly understood but might be associated with mechanical damage produced by grooming procedures. Premature failure (arbitrarily defined as failure at extensions less than 20%) seems to be a unique characteristic of Negroid hair and seems to be responsible for the breaking of hair into half-wavelength sections during manipulation as was mentioned earlier. The distributions of breaking elongations are shown in Figure 9 for 65% RH and for the NUMBER FR•E, TION .12 - .10 .08 .06 .04 .02 0 .14 .12 .10 .08 .06 .04 .02 untr., 65 % R H 8 16 24 32 40 48 56 64 72 80 untr., wet EXTENSION-TO- BREAK { % ) Figure 9. Distributions of extension to break for hair fibers as a function of relative humidity. wet condition. At 65% RH about 22% of the fibers break at elongations under 20%, whereas in water almost all fibers break above 30% extension. It appears that in the wet condition, plasticization of the keratin leads to a relaxation of stress concentrations in the immediate vicinity of flaws, thus delaying fiber fracture until a higher level of extension is reached. Microscopic examination of fracture ends frequently provides clues to the mechanisms

BEHAVIOR OF NEGROID HAIR 33 Smooth '1 i ] I Step Fibrillated Figure 10. Principal fracture patterns observed in tensile failure of Negroid hair fibers. by which fracture occurs. With this objective in mind, the fracture ends of all fibers which had been extended to break were examined in an optical microscope to discern the different modes of fracture and to establish the frequency of their occurrence. Five different modes were identified and are shown schematically in Figure 10. Detailed scanning electron micrographs of fracture ends typical of each of the five modes are Figure 11. Matching fracture ends of a fiber with smooth fracture faces after fracture in water. 80Ox. Fracture seems to have initiated in the region marked.