FRACTOGRAPHY OF HUMAN HAIR 461 50tm "4 lbigure 9. Fracture of natural brown hair in ethyl acetate different from those of virgin hair. Defatting virgin hair by soaking overnight in 2:1 chloroform/methanol had little or no effect on the fracture pattern in either water or air at 50% RH. STRESS/STRAIN BEHAVIOR VS. RELATIVE HUMIDITY Force and elongation measurements were made on natural brown hairs at several rela- tive humidities from 20 to 90%, and also immersed in water. To avoid introducing the variable of hair diameter, the dimensionless ratio of the force at the end of the Hookean region (plateau force) to the break force has been used for comparison of be- havior at different relative humidities. This ratio was found to decrease continuously as relative humidity increased, as shown in Figure 11. Also shown is the increase in percentage of elongation at break. The trend is opposite that of the plateau force/break force ratio. Instead of the steady decline exhibited by the ratio data, the percentage-of- elongation trend indicates a change in fracture mechanism above about 80% RH, where a sharp increase in extensibility begins. DISCUSSION These studies indicate that there are qualitatively distinct mechanisms involved in the tensile fracture of wet and dry virgin hair. In wet fracture we have observed the cuticle

10a SO•m ..: '"'.,.•.• •.•"•-"7 ..... :t ß: •. ß 10b 5-0tam • .:• Figure 10. Fractures of 3-hr bleached hair in water showing cuticle peeling