Structural characteristics and mechanical behaviour of beard hair

Abstract

The structural characteristics of beard hair have been analysed using optical microscopy, scanning electron microscopy and atomic force microscopy. The cross-sectional profile of beard hair is found to be broadly elliptical. The three main morphological features cited in previous literature for scalp hair, namely the cuticles, cortex and medulla were observed. A novel and efficient method has been developed in order to characterise the cross sectional area of an elliptical hair accurately. Quantitative data are presented for the variation of average cross-sectional area across different facial sites (cheek, chin and neck) for three different subjects. Tensile tests have been conducted on a variety of specimens to study the tensile stress-strain behaviour of beard hair in both wet and dry state at a range of cross-head speeds. Application of the area characterisation method significantly reduced the scatter in the mechanical data. The Young's modulus and yield stress values of beard hair are affected significantly by the presence of moisture but only to a limited extent by the strain rate. Repeat tensile tests have been conducted on beard hair samples which were kept in storage for nine months. A drop in the Young's modulus of up to 30% has been observed indicating an “ageing” effect (due to prolonged storage) on the properties of the hair.