358 JOURNAL OF COSMETIC SCIENCE Figure 7. Beta-delta failure from extension of hair fibers in the dry state (21). Reprinted with permission of the Journal of Cosmetic Science. unaltered hair generally provides step fractures rather than smooth fractures. Under this condition, crack initiation is often in the cortex and is initiated at lower strain levels than in the wet state. Therefore, crack propagation occurs at a slower rate. Flaws such as the presence of the medulla (more frequent in thicker fibers) and the intercellular cement can play a role in directing crack propagation in the axial direction to provide step fractures (12). At very low humidity, smooth radial fractures are also observed, with crack initiation in the cortex and brittle failure along the fiber cross section (Figure 4B). CUTICLE CHIPPING VIA EDGE FAILURE IN THE DRY STATE Chipping away of the cuticle edges is a common occurrence in everyday grooming actions (Figure 8). In the d ry state, edge failure most likely begins as outer beta-delta failure, between 18-MEA and the delta layer in the monolayer model (as described), because of weak hydrophobic bonding (Figure 2A), followed by scale edge lifting (Figure 7). Shearing off of scale fragments then results from rubbing actions, as occurs in combing, brushing, or even shampooing. WET-STATE OR HIGH-HUMIDITY FAILURE Deformation in the wet state first leads to deep transverse cracks across the cuticle, and then with continued deformation fiber breakage occurs.

ADHESION FAILURE IN HAIR 359 Figure 8. Cuticle chipping from dry combing hair fibers. DEEP TRANSVERSE CRACKS ACROSS THE CUTICLE Hydrophilic regions are swollen more at high humidity or in water and are more deformable than in the dry state. The swollen cortex exerts pressure on the cuticle-cortex CMC (Figure 1), thus we conclude that the hydrophilic "contact zone" (Figures 2A, 3) of the delta layer between the cortex and the cuticle becomes the weak link from Poisson contraction and swelling pressure, as suggested earlier (12). Therefore, crack initiation occurs in the cuticle-cortex CMC very likely near the interface of the contact zone (hydrophilic protein layer) and the globular protein layer (see the monolayer model of Figure 2A). Under the influence of swelling pressure, crack propagation during defor­ mation extends across all cuticle layers perpendicular to the fiber axis to produce deep transverse cracks (Figure 9), and virtually no scale lifting or buckling occurs if the fibers are not previously damaged (13). EXTENSION TO BREAK OR CATASTROPHIC FAILURE Continued extension or deformation leads to fiber breakage, with high moisture levels providing mainly smooth fractures for Caucasian hair, especially hair with little to no damage (see Figure 4A) (12). In the wet state, especially when the hair is not damaged or does not contain flaws, hydrophilic layers near hydrophobic layers are swollen and more extensible than in the d ry state and, to some extent, relieve strain in the hydro-