Cuticle decementation and cuticle buckling produced by Poisson contraction on the cuticular envelope of human hair

Cyclical extension stresses in dry hair at strain levels between 7% and 15% were seen to produce localized cuticle decementation and buckle formation at the cuticle edges. The experiments show that this type of damage results from circumferential compression stresses on the dry cuticular envelope that acts as a rigid thin-wall pipe during the process of Poisson contraction as the fiber is being elongated. Higher cyclical extensions (>25%) were seen to result instead in cuticle lifting with no buckle formation. This latter type of cuticle damage is similar to the one observed by Reutsch et al. and was already ascribed to the action of longitudinal shear stresses. When the hair was moistened or solvent-swollen, no damage was observed at each individual cuticle; instead, long deep transversal cracks and decementation of the whole cuticular envelope from the cortex were observed. The experiments indicate that the lack of individual cuticle damage under swollen conditions results from a strengthened hydrogen-bonding crosslinked endocuticle and from a lesser fiber Poisson contraction when the fiber is swollen. Cuticle buckling and decementation at the cuticle edges like chose described above were also frequently found in che hair of a panel of 100 individuals. The prevention and repair of this type of damage is also discussed.