Tensile properties of twisted hair fibers.

Hair is routinely twisted during grooming processes, which can cause tangles and lead to breakage of hair fibers. To evaluate the damage caused by twisting hair, the tensile stress-strain properties of single twisted hair fibers were measured by two different experimental procedures: (A) twist at constant length, followed by extension to break (without untwisting); and (B) twist and untwist at constant length, followed by extension to break. In procedure (A), the strength, extension, and initial modulus decreased with increase in twist factor, whereas in procedure (B), the strength and extension did not significantly change from control values, although the initial modulus decreased with increase in twist factor. Furthermore, the degree of recovery from torsional deformation was studied by a variant of procedure (B), where the fiber after untwisting was relaxed for 5 and 10 minutes, respectively, prior to extension to break. The major conclusion from this study was that at low and moderate twist levels, the tensile mechanical properties of human hair are recoverable.