This viscoplastic behaviour is fully determined by the evolution of the equivalent viscoplastic strain \(p\) as a function \(f\) of the Von Mises stress \(\sigma_{\mathrm{eq}}\) : \[\dot{p}=f\left(\sigma_{\mathrm{eq}}\right)=A\,\sigma_{\mathrm{eq}}^{E}\]
where :
\(A\) and \(E\) are declared as material properties .
The return-mapping algorithm used to integrate this behaviour requires the definition of \(f\) and \({\displaystyle \frac{\displaystyle \partial f}{\displaystyle \partial \sigma_{\mathrm{eq}}}}\) (see [1] and [2] for details).
We introduce an auxiliary variable called tmp to limit
the number of call to the pow function