## Problem 1: Part 4: Consistency condition
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The consistency condition during plastic flow requires that

- ${\dot {f}}({\boldsymbol {\sigma }},\alpha ,T)=0~.$

Write down an expression for the time derivative of $f({\boldsymbol {\sigma }},\alpha ,T)$ using the chain rule.

The time derivative is

- ${\dot {f}}({\boldsymbol {\sigma }},\alpha ,T)={\frac {\partial f}{\partial t}}={\frac {\partial f}{\partial {\boldsymbol {\sigma }}}}:{\frac {\partial {\boldsymbol {\sigma }}}{\partial t}}+{\frac {\partial f}{\partial \alpha }}{\frac {\partial \alpha }{\partial t}}+{\frac {\partial f}{\partial T}}{\frac {\partial T}{\partial t}}~.$

In shorter form

- ${\dot {f}}=f_{\boldsymbol {\sigma }}:{\dot {\boldsymbol {\sigma }}}+f_{\alpha }~{\dot {\alpha }}+f_{T}~{\dot {T}}~.$