Hair Breakage 274 Table 8. Weibull parameters for Afro hair at 60% RH as a function of applied stress Stress Range Characteristic lifetime, Shape factor, 0.014‑0.015 g/μm2 128 0.40 0.013‑0.014 g/μm2 301 0.43 0.012‑0.013 g/μm2 3,090 0.40 0.011‑0.012 g/μm2 5,350 0.35 0.010‑0.011 g/μm2 9,990 0.37 0.009‑0.010 g/μm2 52,600 0.34 Comparison of these findings to the Caucasian hair results in Table 5 shows how this Afro hair gives rise to substantially lower characteristic lifetimes, but also notably lower values for the shape parameter. The significance of the shape parameter can be seen in Figure 15 which compares Survival Probability plots for Afro and Caucasian hair under the same conditions. As will be recalled from Figure 6, the S-N plot for Afro hair showed substantially more scatter as the consequence of a relatively high number of premature failures. As such, the Survival Probability plot for Afro hair shows a much steeper initial slope, as there is considerably higher likelihood of breakage after application of fewer cycles. Therefore, the shape parameter provides a measure of this variability, with smaller values indicating higher scatter within the data. Another way to appreciate the significance of this term is demonstrated in Figure 16 which shows the shape for Survival Probability plots with a common characteristic lifetime and a varying shape parameter. As seen with Afro hair data, a lower shape parameter indicates a higher tendency for premature failure and yields a steeper initial slope in the Survival probability plot. Conversely, a higher value indicates lesser tendency for early failure, and instead constitutes a mild wear-out mechanism. Therefore, as pointed out in the earlier discussion about S-N curves, we do indeed observe different breakage mechanisms in Caucasian and Afro hair, with the ethnic hair demonstrating a considerably higher tendency for premature failure.