Mechanical Properties of Hair 234 Dynamic Mechanical Measurements Dynamic Mechanical Analysis (DMA) is very commonly used to study the properties of polymers. To perform DMA on fibers, a fiber is placed under tension and subjected to oscillating stress. DMA has been used to investigate the mechanical properties of both wool and hair.18,35, 39-43 Most frequently the tension is small and the oscillating stress is such that the total extension is less than 1% to remain in the linear region, though higher strains have been used.15, 39 When oscillating stress is applied to a viscoelastic material such as hair, the resulting strain is out of phase with the stress by an angle δ as illustrated in Figure 6. The relevant equations for the strain, and the stress, are where ω is the frequency of the oscillation and t is time as illustrated in Figure 6 with ε 0 and σ 0 both normalized to 1.0. For a pure elastic solid, δ is 0 degrees and for a pure liquid δ is 90 degrees. We can define two moduli from the stress, one from stress that is in phase with the strain and one from the stress that is out of phase.44 The in phase modulus, E’, is called the “storage modulus” Figure 6. Normalized stress and strain for a hypothetical DMA experiment with phase angle