## Problem 1: Part 3: Rate form constitutive relation
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Write down the rate form of the elastic stress-strain law. Assume that deformations are small so that objectivity of the rates is not a concern.

The rate form of the elastic constitutive equation is

- ${\dot {\boldsymbol {\sigma }}}={\boldsymbol {\mathsf {C}}}:{\dot {\boldsymbol {\varepsilon }}}^{e}={\boldsymbol {\mathsf {C}}}:({\dot {\boldsymbol {\varepsilon }}}-{\dot {\boldsymbol {\varepsilon }}}^{p})$

Plugging in the expression for ${\dot {\boldsymbol {\varepsilon }}}^{p}$ we get

- ${\dot {\boldsymbol {\sigma }}}={\boldsymbol {\mathsf {C}}}:\left({\dot {\boldsymbol {\varepsilon }}}-{\dot {\gamma }}{\frac {\partial f}{\partial {\boldsymbol {\sigma }}}}\right)$