Effect of testing conditions on the tensile properties of a single human hair and the in-situ observation of cuticles during the tensile deformation by an optical microscope

Abstract

The purpose of this study is to investigate the effect of testing conditions, i.e., the strain rate and the gauge length, on the tensile properties of a single human hair. Moreover, the changes in the surface morphology of the hair under the tensile deformation is reported. Human hair has been found to hold various information on our body such as disease, aging, etc. And therefore, the information of hair is expected to be used in various fields for application in the future, e.g., as a sample for medical inspection. Because the mechanical properties of human hair are greatly affected by its internal structure, theses must be suitable parameters for above mentioned application. To conduct a tensile test of a single human hair under the optical microscope observation, a compact testing apparatus was developed, and the tensile test of a hair was performed under various conditions of strain rate and the gauge length. Young’s modulus was independent with the strain rate, and its average value was 3.45 GPa. Although the fracture stress showed the strain rate dependency, it was independent with the strain rate under 2 × 10^-4 s^-1. The average value of fracture stress without depending on the strain rate was 192 MPa. The fracture stress increased with decreasing the gauge length. This was considered that the less defects are included in the cortex for shorter testing section. Moreover, from the in-situ observation of the surface of the hair under the tensile deformation, the lift up of the edge of the cuticle was observed, and this behavior was quantitatively monitored as the delamination area. The delamination area of the outer cuticle monotonically increased with increasing the strain up to the fracture point.