Figure 7. Matching ends obtained in torsional untwisting of Negroid hair fibers under the influence of a tensile load. In most cases the fracture occurred in the region of twist. a) Slanted step fracture. b) Level step fracture. Table IV Tensile Mechanical Properties of Untreated Negroid Hair Test No. of Cross-Sect. Area Init. Mod. Breaking Stress Breaking Elong. Condition, RH (%) Specimens (10 -9 m •) (109N/m •) (109N/m •) (%) 65 32 5.8 _+ 0.33 2.85 _+ 0.16 0.123 _+ 0.016 27 + 5 Wet 38 5.4 4- 0.25 1.22 4- 0.07 0.119 _+ 0.010 44 _+ 3

BEHAVIOR OF NEGROID HAIR 31 LO•D YIEED REGION HOO KEAN REGION .._. TOE- IN REGION EXTENSION POST-YIELD REGION Figure 8. Typical load-elongation curve for Negroid hair fiber. Table V Cross-Sectional Areas of Extended (35%) Relaxed Negroid Hair Fibers (65% RH, 21øC, 95% Confidence Level) Cross-Sectional Area (10-9m 2) Recovery (%) h i hf(60 s) Am(86.4 ks) R•(60 s) R2(86.4 ks) 5.21 _+ 0.66 4.39 -+ 0.48 4.89 _+ 0.52 39 76 R• = (Af- A35)/(A • - A35) X 100. R 2 = (A m - A35)/(A i -- A35 ) x 100. Here, A35 = A,/1.35 = 3.86. Table VI Length Recovery of Negroid Hair After 35% Extension at 65% RH, 21øC Recovery from Extension (%) Recovery of Initial Modulus (% of first extens.) Recovery Time•* Instantaneous 60 s 86.4 ks 60 s 86.4 ks Untreated 41 + ! 62 + 2 81 + 1 80 _+ 10 93 + 12 Relaxed 30 + 2 47 + 3 67 + 2 74 + 10 95 + 7