Dear colleagues,
Consider a simple non-linear elastic material with stress given as
σ = D(εdev) εdev + B εiso
where εdev is the norm of εdev, D is a function of εdev and B is constant. The material is isotropic since the principal directions of σ and ε will coincide.
If we differentiate σ wrt ε to obtain the material stiffness the form of the stiffness tensor will be
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