Hair Breakage 252 irradiation, etc.) can significantly decrease the tensile properties of hair. However, while these measures provide a convenient means of characterization, it may be argued that the approach does not provide a particularly accurate simulation of how consumers make an assessment of “hair strength.” Instead, it appears likely that consumer appraisal involves viewing the number of broken fibers in a brush or comb after grooming, noting the number of fibers at the bottom of the tub after showering, or by observation of split ends in a mirror. In fact, in the consumer vernacular, it seems likely that “hair strength” represents a self-assessment involving the ease of hair breakage. Moreover, the reason for this breakage relates to wear and tear accumulated over the lifetime of the fiber, as opposed to a single, one-time catastrophic stimulus. By means of illustration, conventional stress-strain experiments show how an average-sized 70 μm hair fiber at 60% relative humidity requires approximately 80 grams of force to induce breakage. Consequently, it may be presumed that failure would not occur unless forces of this magnitude are encountered. This value appears quite sizable, with such forces presumably only being encountered upon detangling an especially persistent knot, which, hopefully, represents a relatively rare occurrence. Moreover, it is has been noted that fibers will be plucked from the follicle upon application of forces considerably less than the “break force.”1 Nonetheless, the presence of split ends is evidence enough that significant breakage does indeed occur on the head, and so another mechanism must explain their formation. In fact, it is widely recognized that materials will fail in a relatively predictable manner upon repeated application of stimuli significantly lower than those required to cause failure from a single application. That is, a sample is progressively weakened by the propagation of cracks and flaws, which ultimately leads to failure. Thus, in the hair care world, it becomes possible to envisage how everyday stresses and strains associated with grooming (and possibly other practices) will produce the same occurrence. This mode of deformation constitutes fatigue testing and often represents stimuli