PLASTIC YIELDING IN HAIR CUTICLES 71 deformation recovery of the fiber upon stress release. SEM surface analysis of hair fibers subjected to a total of 20 of these cycles showed that their cuticles did not suffer any damage at all. As the number of turns per inch per cycle increased to 10-15, the deformation became partially irreversible and the first signs of cuticular damage ap- peared on the hair. Figure la shows patterns of cuticle damage fully developed after the application of 20 of these cycles. It can be seen in this figure that the hair damage effects appear as long helicoidal regions of damaged and undamaged cuticles around the lon- gitudinal axis of the hair fiber. Such a helicoidal pattern results from the damage occurring at the regions of maximum stress when the fiber is twisted. Figure lb shows a higher magnification of the damaged areas with abundant microcrack and microvoid formation. At higher levels of torsion, 20 cycles and 19-25 turns per inch per cycle, the microvoids and microcracks coalesced and gave rise to the formation of thin, shallow, longitudinal cracks that propagated along the cuticular envelope. Cracks frequently penetrated into the cortex but without splitting the fiber into two sections (see Figures 2a and 2b). At 25-30 turns/inch/cycle thin cracks developed into macroscopic longitudinal fractures that split the fiber into two portions (see Figure 3a). The appearance of such cracks is akin to those found in hair with "split ends." It is important to mention that the formation of longitudinal cracks and longitudinal fractures such as those depicted in Figures 2a, 2b, and 3a was found to be exclusively the result of torsion. The application of high tension or bending stresses to a hair fiber resulted in cracks perpendicular to its longitudinal axis. A more detailed description of hair fracture under high-tension stresses is given elsewhere (11,12). Analysis of hair from a panel of 100 individuals showed that plastic deformation of cuticles and long longitudinal cracks also occur commonly in panelists' hair. The analysis showed that approximately 80% of the panel population presented similar damage ..= Figure l. Helicoidal patterns of cuticle damage produced after the application of 20 torsion cycles, 15 turns/inch per cycle at 10% RH. (a) x0.42k (b) x2.6k.

72 JOURNAL OF COSMETIC SCIENCE ß 2b Figure 2. Long, thin, longitudinal cracks produced after the application of 20 torsion cycles, 25 turns/inch per cycle at 10% RH. (a) x0.42k (b) xl.21k. patterns--in particular, at the tip sections of hair. For instance, a panelist hair fiber with a typical long longitudinal macrocrack is shown in Figure 3b. The micrograph shows that the crack has occurred in the middle of a wide and long surface zone partially devoid of cuticles. This damage characteristic is similar to that produced artificially by fiber twisting as displayed in Figure 3a. Microvoids, microcracks, and long cracks were also found to occur in hair fibers that had been abraded with a combing wheel, even at smaller deformations. Whether the cuticle damage patterns were found in panelists' or artificially damaged hair, the observed damaged regions were found to form helicoidal strips approximately 30 microns wide and 2 to 3 millimeters long. Figures 1, 2, and 3 show that their main feature is the lack of cuticles, suggesting that they may have been abraded before or during the fatigue experiments. This possibility was ruled out because the tested hair was collected from hair sections close to the roots and was characterized by undamaged cuticles. Also, cuticle wear experiments using a combing wheel revealed that the apparent lack of cuticles in fibers subjected to torsion is not due to wear. For instance, two hair surfaces are shown in Figures 3c and 3d, one of a hair fiber subjected to 120,000 combing strokes and another subjected to 20 cycles of torsion with 15 turns/inch/cycle. As illustrated by Figure 3c, in the case of the hair fiber subjected only to combing wear, a narrow region is fully worn up to the cortex. At the periphery of this region one can perceive about four or five cuticle layers overlapping each other in a stepwise descendant fashion, i.e., cuticles and regions without cuticles are not at the same topographic level. In contrast, the fiber subjected to torsion experienced no cuticle overlapping instead, the surface of the top cuticles is at the same topographic level and fused with the regions devoid of cuticles (see Figure 3d).