24 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Experiments were done with loads of 10, 20, 30, and 40 g. Fibers surviving this fatigue test were subjected to tensile fracture on an Instron © tensile tester, and the extension at break was determined. In the course of this work it was observed that some fibers had a tendency to assume a permanently twisted configuration produced by a unidirectional twisting movement of the weight as it lifted from the lower disc. This twisting action was prevented by inserting restraining clamps between the weights. MICROSCOPIC TECHNIQUES Throughout this work, optical and scanning electron microscopic (SEM) techniques have been used extensively to examine various aspects of fiber structure. Fiber cross sections were prepared by embedding the fibers in an epoxy resin matrix and using a microtome with a glass knife. Fractured ends of fibers after various mechanical tests were sputtered with gold prior to SEM examination. Figure 3. Negroid hair fibers showing regions of twist. 300x.

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