FATIGUE TESTING OF HAIR 611 leading to points of high stress in the structure. Table V shows break stress and break extension results from conventional constant-rate extension curves for both virgin Cauca- sian and single-source Afro hair that back up this supposition. Figure 10 shows fatigue results for single-source Afro hair fi bers plotted in the form of an S-N curve, compared to results for Caucasian hair. Again, all measurements were performed at 60% RH. Two observations arise from these results. First, the regression line for the Afro hair falls below that of the Caucasian hair. But, in addition, there appears to be considerably more variability in the Afro data. That is, while some Afro fi bers survive for an equal or longer duration than the Caucasian hair, a signifi cant number fail after very few fatiguing cycles. Table VI shows Weibull parameters for Afro and Caucasian fi bers that received a 0.010– 0.011 g/μm2 repeated stress. It is seen that the characteristic lifetime for Afro hair is only about half that of Caucasian hair, while a much lower shape factor is obtained. Another function served by the shape factor is to provide an indication of scatter in the experimental data. The smaller the shape factor, the higher the variability. Figure 11 shows survival probability plots produced from these results and illustrates the infl uence of the shape factor. The plots demonstrate a relatively high probability for Afro fi bers to break after application of comparatively few fatiguing cycles. However, if fi bers survive this initial period, their likelihood of surviving higher numbers of cycles dramatically improves. The information obtained from this kind of testing yields more insight into the nature of hair breakage as compared to conventional constant-rate stress-strain testing. Expanding on an earlier point, the highly fragile nature of Afro hair is such an issue that it demands very different habits and practices, a point that would not seem to be refl ected in a relatively meager 13% reduction in break stress and break extension. Such anticipated differences Table V Comparing Conventional Tensile Properties for Caucasian and Afro Hair at 60% RH Hair type Break stress Break extension Caucasian 2.07 g/μm2 45.1% Afro 1.80 g/μm2 39.3% Figure 10. S-N curves for Caucasian and Afro hair at 60% RH. Filled circles represent data for Caucasian hair hollow circles are Afro hair.

JOURNAL OF COSMETIC SCIENCE 612 are perhaps better refl ected in fatigue results, which suggest considerable differences in propensity for breakage for the two hair types. THE EFFECT OF PLASTICIZATION ON THE PROBABILITY OF HAIR BREAKAGE While lubrication reduces the fatiguing force during grooming, there is also a desire to understand whether cosmetic ingredients penetrate into hair to provide any infl uence on the propensity for breakage. In the absence of any chemical reactions, the infl uence of such materials may be expected to involve some degree of plasticization as a result of their incorporation in the hair structure. While well-characterized effects from ingredients may not be available, the plasticizing infl uence of water is extremely well documented. Conse- quently, changes in relative humidity are used to modify the plasticization of hair fi bers. Conventional stress-strain testing results can be generated that show how raising the humidity from 60% to 90% results in an approximate 15% decrease in break stress, while decreasing the humidity from 60% to 20% results in an approximate 10% increase in break stress. Figure 12 shows fatigue data for Caucasian hair at 20%, 60%, and 90% RH in the form of S-N plots. Results show that altering the humidity induces changes in line with predictions, but the magnitude of these effects is considerably larger than anticipated. From the S-N plots, these alterations in relative humidity have produced better than a 10-fold infl uence in the number of cycles-to-fail. From the regression lines, it can be cal- culated that the application of a repeated 0.0125 g/um2 stress at 60% RH results in an average of approximately 2,000 cycles-to-fail, while at 20% RH this value increases to approximately 45,000 cycles. Similarly, the application of a repeated 0.0095 g/um2 stress Table VI Comparison of Weibull Parameters for Caucasian and Afro Hair after Repeated Fatiguing with a 0.010–0.011 g/μm2 Stress at 60% RH Hair type Characteristic lifetime, α Shape factor, β Caucasian 23,200 1.01 Afro 12,600 0.39 Figure 11. Survival probability plots (from Weibull analysis) for Caucasian and Afro hair upon exposure to a repeating 0.010–0.011 g/um2 stress at 60% RH.