Consumers place a significant emphasis on hair strength as one of the primary indicators of its health. In the first part of this column series, the use of conventional tensile testing experiments was described to generate stress-strain curves that serve in characterizing the structural integrity of individual hair fibers. It was shown that wear and tear associated with various everyday treatments and conditions does affect these properties, and physically weakens hair strands. However, it was also noted that this approach does not necessarily represent a realistic simulation of the stimuli experienced by fibers on a daily basis, nor does it seem to represent the manner by which consumers judge “hair strength.” Instead, this consumer perception likely arises from some self-appraisal of the tendency for their hair to break—for example, by noticing broken fibers in a brush or comb after grooming, or perhaps in the base of the shower after washing.
When hair is groomed, individual fibers are repeatedly exposed to the application of relatively minor forces as opposed to the one-time catastrophic deformation imparted in conventional tensile experiments. In the mechanical testing world, the tendency for materials to fail under a repeated stimulus is termed fatigue testing, and this article discusses this topic in relation to hair breakage. It will be shown that this alternative testing approach provides considerable insight into the cause of hair breakage, and subsequently allows for the identification of strategies for its minimization; it will also be demonstrated how learning this provides the underlying theory by which anti-breakage and even “strengthening” claims are crafted.
In the previous column, it was shown that healthy, average-sized, 70-µm hair fibers require the one-time application of approximately 80 g of force to induce breakage at 60% relative humidity. It may therefore seem reasonable to presume that breakage would not occur unless everyday grooming habits and practices produce conditions that exceed this threshold. However, this is not correct, and it is well-recognized that materials will break in a predictable manner upon repeated application of stresses and strains considerably below the “break point” identified by stress-stain experiments. This happens because all materials fail at their weakest point; therefore, the presence of any structural flaw represents a nucleation site that can be propagated by a repeating external stimulus. In short, any chip, crack, or asperity in the hair structure will be progressively worsened by the repeated action of grooming and other mechanical manipulation until breakage ultimately occurs.