CUTICLE DECEMENTATION AND BUCKLING 217 ..'. %. • . .. . 77kx 8kv Figure 3. Surface of a hair fiber after 200 strain cycles at 25% extension and 10% RH. drated fiber is being elongated, its cuticular envelope acts as a laminar rigid thin-wall pipe under a process of radial compression. Such a compression process is the result of the fiber's Poisson contraction as the fiber is elongated the term "Poisson contraction" is used in mechanics of solids to describe the lateral contraction experienced by a slab of material when subjected to tensile deformations. The compression stresses can usually be calculated from the Poisson's ratio, which is the ratio of the strain in the lateral direction to that in the axial direction (15). In the case of hair, calculations reveal that when 7%, 15%, or 30% tensile strains are imposed on the hair fiber, its cortex radius decreases by 4.6%, 7.1%, and 12.0%, respectively. Such radial contractions acting on the dry and rigid cuticular envelope will, certainly, generate circumferential compression stresses capable of decementing and buckling the cuticle. As is well known, a typical form of failure in thin-wall pipes under compression is wall buckling, a phenomenon that takes place after the circumferential compression stress reaches a certain limit (15,16). Since cuticle lifting and buckling are mostly observed at low extensions (see Figures 2a, 2b, and 2c), it can be inferred that compression and shear stresses in the fiber circum- ferential direction cause more cuticle damage at low elongations (7-15%), while at higher extensions (25-40%) the predominant longitudinal shear stresses with a small component of circumferential compression stresses will produce the massive cuticle lifting observed in Figure 3. Thus, when a dry fiber starts to be elongated, its cuticle will first experience strong circumferential compression stresses and later at higher extensions will be subjected mostly to the type of longitudinal shear stresses discussed by Reutsch et al. (10).

218 JOURNAL OF COSMETIC SCIENCE 4a 4b 4c • xl.2k .. xl.2k Figure 4. Surfaces of hair fibers after 200 strain cycles at different relative humidities and different extensions as follows: 4a, 65% RH and 15% extension 4b, 80% RH and 15% extension 4c, 100% RH and 30% extension. 5a 5b El. E:2k x :- O . 77k x $k:.• 43E -- -- Figure 5. Surface of a hair fiber after 200 strain cycles at 30% extension at 100% RH showing deep transversal cracks and cuticular envelope detachment from the cortex. 5a x220 5b x770. In contrast, when the volume of a hair fiber is increased by solvent or water swelling, the cortex and cuticular envelope expand radially. Under these conditions, two main phe- nomena will occur: first, the endocuticular regions of the fiber will act as a soft swollen gel highly crosslinked by hydrogen bonding, and second, the Poisson contraction will be considerably less than when the fiber is not swollen. The first phenomenon will