20 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Figure 4. i-l. Extension-induced failure in the endocuticle facilitates scale lifting and chipping away, thus exposing the endocuticle as surface structure. properties after 30% extension. The tip section shows a slightly lower recovery than the root section, especially in properties such as initial modulus and yield stress, although the differences here are small and not statistically significant. Figure 5. Development of shear forces between cuticular layers during fiber extension and failure in endocuticle leading to surface scale lifting.

STRESS RELEASE IN HAIR CUTICLE 21 3000X 3000X • 2400X 3000X Figure 6. a: Root section of untreated hair fiber after 32-36% extension and overnight relaxation in deionized water. b: Tip section of untreated hair fiber after 32-36% extension and overnight relaxation in deionized water. c, d: Cuticles of untreated hair fiber that appear frozen in the lifted position after 32-36% extension and overnight relaxation in deionized water. In an effort to establish to what extent the visual cuticular damage that still exists after the recovery process affects the various extension phenomena, we carried out a second extension of these fibers. In Figure 7 the damage phenomena in the root section during first and second extension are compared, and it is clearly seen that the damage phe- nomena occur at much lower extension levels during the second extension. In other words, the first extension has caused irreversible damage, especially in the endocuticular Table I Effect of Extension on the Mechanical Properties of Human Hair: Mechanical Properties of Root and Tip Break properties Yield stress Energy Mod. (GN/m 2) Stress (MJ/m 2) Ext. (%) Root Tip Root Tip 3.73 9.8 x 10 -2 20.2 x 10 -2 3.01 3.90 9.7 x 10 -2 19.3 x 10 -2 2.84 Relative recovery(%)ofproperties a•er 30% extension 95 92 95 97 90 90 95 94 46.6 45.2 104 102