Chapter 8 263 Effect of relative humidity: As seen in Chapter 10, water acts as a plasticizer for hair, with the water content (and consequently the mechanical properties) being highly dependent on the relative humidity of the environment. Figure 8 demonstrates this effect by showing break stress results from conventional stress-strain testing as a function of the relative humidity. Therefore, changing the environmental conditions from the 60%RH baseline is observed to increase or decrease the break stress by 10-20%. Figure 9 shows S-N plots for fatigue data collected on Caucasian hair under these same three humidity conditions. As seen with the work on Afro hair, fatigue results suggest the presence of considerably larger effects than those predicted by stress-strain testing. Indeed the magnitude of these differences is startling with elevated humidity reducing the cycles-to-fail by around two orders of magnitude. The above findings would seem to have rather profound implications. To put these results into perspective, Table 2 showed the average number of cycles-to-fail for Caucasian, Afro and relaxed Figure 8. Break stress results for Caucasian hair as a function of relative humidity