Extension experiments are widely recognized to probe the internal properties of the hair, however twisting and bending are believed to contain a considerably higher contribution from the surface (or, at least, the outer portions of the hair). Therefore, surface coatings (or materials that only penetrate into the outer portions of the hair) may be expected to register larger effects in such measurements.
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The torsional modulus of hair can be evaluated using a torsion pendulum.
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In such experiments, individual hair fibers are allowed to twist and untwist in a periodic manner, while the magnitude of the period and amplitude allows for calculations that evaluate torsional mechanical properties. All testing is performed with the equipment in a bench-top humidity chamber.
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Torsion Pendulum Equations:
Shear Modulus (G) of an elliptical fiber
Where L= length, M = moment of inertia for the pendulum mass, T= period, a=semi major axis, and b=semi minor axis