Fractography of Human Hair

The study investigated the fracture behavior of human hair under tensile loading, identifying three main fracture types: smooth, step, and undefined fibrillated ends. Moisture content significantly influenced fracture type, with low (∼0%) and high (90%) relative humidity leading to smooth fractures, and intermediate humidity causing step fractures. Surface treatments with polymers or surfactants did not affect hair strength or fracture behavior. Fracture initiation varied with moisture content, occurring in the cortex at low moisture and at the surface at high moisture, indicating the role of cortex swelling pressure. Larger cross-sectional fibers tended to split along the axis due to flaws or medullary cells. Unlike synthetic fibers, hair fibers followed the Griffith criterion of brittle fracture, with secondary cracks forming due to stress concentrations. The rate of stress transfer between cortical cells was also important in the fracture mechanism.