Division of force among layers constituting human hair during bending and tension

Human hair is a three-layered structure comprising the inner medulla, middle cortex, and outer cuticle layer. When a hair is subjected to bending or tensile load, each of these layers absorbs a certain amount of the force applied. However, the magnitude of the force absorbed by each layer is not easy to estimate. This is because, in addition to Young's modulus of each layer, the absorption depends on the area of each layer as seen in the cross-section. This study used a strategic way of combining experiment and theory and found that Young's modulus of the cuticle layer changes in the face of bending and tension. Considering this, the ratio of force sharing inside a human hair was estimated. Bending and tensile tests were conducted on single human hairs to determine the structural elasticity for both deformations which expresses the deformability of the hair independent of its external dimensions. Moreover, Young's modulus of each layer was determined by nanoindentation of hair cross-section. By comparing the structural elasticity determined experimentally with that determined theoretically, Young's modulus of the cuticle layer against tension was found to be 25% of that against bending. Based on this finding, it was found that the cuticle layer bears 35% of the force endured during bending, and the cortex bears the rest; when subjected to tension, the cortex bears more than 90% of the force.