Mechanical Properties of Hair 242 Feughelman and Robinson18 studied the effect of humidity on the mechanical properties of wool in the linear region of the stress/ strain curve. Both initial stress and stress relaxation were measured. Stress relaxation was characterized by a pseudo-relaxation time, τ, defined as the time required for the stress to relax to one-half of the stress level eventually reached at equilibrium in water. Stress was reported as the ratio of the initial stress at any given RH to the stress in water. Data for 0.8% extension taken from their paper are given in Table 5. Table 5. Effect of relative humidity on initial stress and stress relaxation time at 0.8% extension RH (min) Initial Stress 0 520 2.66 14.4 240 2.61 32 180 2.44 65 120 1.45 91 42 1.45 100(water) 0.66 1.00 The initial stress was reduced by a factor of 2.66 between 0% and 100% RH. The proportional reduction in initial stress for wool between 14.4% and 91% RH is very similar to the data reported by Evans for the variation of the elastic modulus of hair with RH. (See Figure X in Chapter 10 of this text for details.) The effect of water on the rate of stress relaxation in keratin fibers is remarkable. The half-time for relaxation to the equilibrium value of stress was reduced from 520 minutes at 0% RH to less than 1 min in water. Even the transition from 91%RH to water decreased relaxation time more than 60-fold. The shear modulus and shear relaxation in keratin fibers are also strongly affected by the keratin-water interaction.1,51,57 Figure 13 shows the effect of humidity on the torsional (shear) modulus of hair between 20% and 90% RH.