There is no hookean region in the stress-strain curve of keratin (Or other viscoelastic polymers)

Abstract

Abstract Viscoelastic polymers exhibit stress relaxation, and it is therefore predicted that their stress-strain curves will be nonlinear at all strains. It is shown in this report that this behavior is observed only with straight and properly clamped specimens; when a specimen is crimped or badly clamped, the initial part of the curve exhibits “toe-in” (caused by straightening of the specimen), which obscures the viscoelastic curvature. In this case, a pseudo-linear region, strictly an inflection region, is observed at low strains, which arises from opposing viscoelastic and toe-in curvatures. There is thus no Hookean region in the stress-strain curve of (straight or crimped) viscoelastic polymers, and the concept of a Young's modulus is inapplicable; however, the inflection modulus, derived from the slope of the inflection region has limited applicability, particularly when the toe-in is not excessive. Instead of a Young's modulus, a unique stiffness parameter is proposed, termed the initial secant modulus, which is based on the secant near the origin of the “material stress-strain curve,” i. e., the curve in the absence of toe-in. It is proposed that in the case of keratin the term “Hookean region” be replaced by the term “pre-yield region.”