Understanding Curly Hair Mechanics: Fiber Strength

Abstract

The relationship between the geometric and mechanical profiles of hair fibers has been studied, with special focus on curly samples. Incidental observations pointed to a significantly different viscoelastic character with varying curliness. Further investigations confirmed initial observations, showing an initial distinct toe region behavior for curly fibers on the stress-strain plot, which is absent for straight fibers. This behavior suggested a difference in the viscoelastic nature of the curly fiber that is linked to mechanical energy stored in the fiber. Results also suggest that the strength of hair depends on two main components, and further pointed out that de facto methods of tensile testing may erode curly fiber strength during preparation. The main outcome of this study is that the tensile strength (σT) of hair fibers is composed of two (rather than one main) components, namely the toe region (σt) and the elastic region (σε), so that: σT=σt+σε. For noncurly fibers, the greatest part of fiber strength is derived from σε, while σt ≈ 0. For curly fibers, σt (i.e., springiness) adds significantly to the overall strength, even though σε remains the major contributor. Although these results require validation in larger studies, they are significant in the current understanding of curly hair. Also, they may represent a fundamental shift from the current understanding of tensile testing of human hair in general.