Chapter 8 265 stress that imparts failure after a single application. This seemingly represents the equivalent of the break stress obtained from a conventional extension experiment (albeit, at a different extension rate). Therefore, there is the possibility of anchoring an S-N curve on the x-axis using the conventional approach, and then estimating the remainder of the curve from knowledge of this relationship. This hypothesis requires further investigation, although it is noted that additional smaller data sets have been collected at 5%, 40% and 75% relative humidity, with the resulting S-N curves appearing where predicted in relation to the existing data. The approximate parallel alignment of S-N curves for virgin and relaxed Afro hair is also noted (Figure 7), while it is possible that the Caucasian and Afro curves (Figure 6) may better align as additional data is collected over a broader stress range. The above results make it possible to extend the modeling shown in Figures 4 and 5 to include the sizeable effect of environmental conditions. Figure 10 shows the effect of both stress and humidity on the cycles-to-fail, while Figure 11 combines the effect of humidity and fiber dimensions. Figure 10. Average cycles-to-fail for Caucasian hair at different RHs