Hair anisotropy and damage: Understanding hair cracking and fracture via the moving loop test

Hair can become damaged and break as a result of mechanical actions such as brushing. Individual hairs (known as hair fibres) are usually tested to failure in tension but this does not reflect the type of loading to which they are normally subjected. Previously, we proposed a new test - the Moving Loop fatigue test - which simulates the extreme bending of tangled hair during repeated brushing. Previous results showed that this test is capable of generating longitudinal splits, simulating the phenomenon of split ends. In the present paper we report further results from this test method, expanding the number of hair types investigated and demonstrating the dependence on applied force and effects of environmental treatments (combinations of hydration and heating). In addition to recording the number of cycles to failure we also used interrupted testing to investigate the mechanisms of damage initiation and propagation. We found that cracks can initiate in one of three interfaces - cuticle/cuticle, cuticle/cortex and cortex/cortex. The first two result in splits which start at or near the hair surface and propagate across the hair fibre to cause fracture, whilst the cortex/cortex-initiated splits propagate along the hair to macroscopic lengths. We attribute these differences in behaviour to differing anisotropy of hair strength, due to varying bond strengths in the cell-membrane complexes in these three interfaces. Computer simulation using finite element analysis provided insights into the distribution of tensile and shear stress and the effects of anisotropy on failure modes.