What is the reason behind the nonlinearity in the stress-strain curve of elastomers(Molecular Level)

Dear all,

I am trying to code a UMAT subroutine for perfect plasticity with a Tsai-Wu failure surface. I follow the procedure of the closest point projection in Simo & Hughes, Box 3.5, which is quite straight forward. At least I thought so

 The algorithm converges but unfortunately not to the correct point in the (plane-stress) stress space. At first I used an yield surface wich gives a unit sphere in the stress space. It is easy to control the normality of the plastic strain increment and the return-map algorithm is implified to a radial return algorithm.

Abstract

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