Torsion and Fiber Rigidity

The rigidity R (resistance to twisting) may be calculated from this expression  

Another useful parameter is the logarithmic decrement ( ). This parameter describes the decay in amplitude of the untwisting pendulum with successive oscillations  

Previously, the major drawback to the simple torsion pendulum method was that it could not be used while the hber is immersed in liquids because of the damping effect of the liquid. However, Wolfram and Albrecht [83] have devised an ingenius scheme to overcome this obstacle by inserting the hber into a small glass capillary tube, thus permitting the torsional properties of hair hbers (and other hbers) to be measured in both air and liquids. Other methods are available [79,80,84] for measurement of the rigidity of hbers immersed in liquids. For additional details, see the references indicated and the texts by Meredith and Hearle [84] and Morton and Hearle [76]. 

The torsional modulus for human hair by the pendulum method is lower than either the stretching or the bending modulus at 60% to 65% RH. However, water has a greater effect on torsional properties than on either stretching or bending for both human hair and wool hber (see data describing these three different elastic moduli in Table 8-5 and 8-6). 

For wool hber, the effect of water on the torsional properties is almost 3 to 4 times as great as on the bending or stretching properties, whereas for human hair the effect is nearly a factor of 2 between 65% and 100% RH (see Table 8-6). 

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