Properties of the Matrix in Keratins

Abstract

It has been shown that the initial, so-called Hookean region (0-1.5% strain) in the stress strain curve of keratin is nonlinear, as predicted theoretically for a viscoelastic material such as keratin. Deviations from linearity are confirmed at strains as low as 0.03%. It is speculated that the concept of a Hookean (i.e. linear) region arose as a result of extension tests with wool fibers. In the decrimping phase of extension the stress-strain curve of wool is convex towards the strain axis (a feature referred to as "toe-in"), while the viscoelastic curvature of the material stress-strain curve is concave towards this axis. The combination of these two opposing curvatures results in a point of inflexion, and it is suggested that the apparent linearity of the inflexion region has been accepted uncritically as indicating the presence of a Hookean region. Instead of a Young's modulus, the concept of an inflexion modulus is proposed for stress-strain curves exhibiting toe-in. In compressional stress-strain experiments toe-in is unavoidable at present, but it is estimated, on the basis of extension tests, that the value of the compressional modulus near the origin of the material stress-strain curve may be double that of the observed inflexion modulus. It is proposed that the part of the curve up to ∼2% strain be termed the "pre-yield region" instead of the "Hookean region."