Abstract
Thick or stiff feeling hair is thought to be due to a complex combination of several physical properties. Torsional stress and bending stress play an important role in the physical properties of human hair, Rigidity, as modulus of rigidity from a material dynamics viewpoint, was evaluated, Based on the estimated diameter of the minor and major axes of hair, torsional stress was evaluated as a modulus of rigidity. Evaluation of the modulus of rigidity of intact hair and delaminated hair, from which cuticles were removed by physical stripping, enabled estimation of the modulus of rigidity of the cuticle. The results indicated that the modulus of rigidity of the cuticle was approximately 3.5 times than that of the cortex. The contribution of the cuticle on the torsional stress of whole hair was also evaluated. The contribution of the cuticle on the torsional stress of whole hair was found to be as much as 60%. Since the cuticle contributes to a large part of torsional stress as well as bending stress, it is assumed that the cuticle plays an important role in the thick or stiff feeling of hair. The ratio of Young's modulus versus the modulus of rigidity of hair was calculated to be 11.94, which is larger than that of uniform material. Since the cuticle and cortex can play the part of composite material, it is quite reasonable that the larger value of the ratio of Young's modulus versus the modulus of rigidity of hair was found in hair, which is a complicated structure combining both cuticle and cortex.
