The derivation of a strain hardening modulus from true stress-strain curves for thermoplastics

Abstract

Some problems in the measurement of stress-strain curves for thermoplastics are considered and the importance of measuring true stress-strain curves is emphasized. However, an increasing number of suitable curves have now been published of which the large majority have been found to comply with the following Gaussian equation: σ true =Y+G p (λ 2− 1 λ ) where σ true is the true stress, Y is the extrapolated yield stress, G P is the strain hardening modulus and λ is the extension ratio. It is shown that the Considere condition for necking requires that Y G P > 3 and the available results support the proposition that this is a necessary condition for necking to occur. High values of G P are observed in polymers with an extended chain conformation.