Hair Breakage 256 permanently deformed, and retains a residual strain upon removal of the applied force. As such, without appropriate scrutiny, results from two distinctly different experiments can be mixed: a cyclic fatiguing experiment carried out in the linear-like region of the stress-strain curve, and a complex extension experiment resulting from the application of forces outside of this region. The Dia-stron equipment allows for stress-strain curves to be collected throughout the experiment, and therefore provides knowledge of whether experiments are being performed in the “linear” region. Figure 2 is coded in such a way as to show data from all occurrences. The solid data points represent experiments where the applied stress was within the “linear” region, while the hollow points represent experiments where the stress was above the yield point. This convention will be used throughout this chapter. Therefore, in this instance, it is observed that fatiguing occurs within the “linear” region upon application of stresses below approximately 0.013 g/μm2 while “non-linear” experiments predominantly occur above 0.014 g/μm2 however a mixture of these two behaviors is seen around this range. Despite these distinctly different conditions, it appears (at least over the stress range shown) that all data points lie on a common S-N plot. However, experiments to date have focused predominantly on stress ranges within the “linear” region (presumably where grooming forces occur) and there is still the need to better examine the above presumption. It should also be noted that “linear region” experiments can be performed at stresses that would appear to be above the yield point as obtained from conventional stress-strain experiments. This relates to the viscoelastic properties of hair and is a consequence of the considerably higher strain rate used in these fatiguing experiments. From the linear regression in Figure 2, it becomes possible to estimate the average number of cycles-to-fail for a fiber under a given repeated stress. These results are shown in Table 1, together with a conversion from stress into force, based on an average-sized 70 μm hair fiber. Therefore, as mentioned earlier, it is found that