Benchmark solution
=====================

The values tested are analytical values of a purely spherical problem under perfect plasticity. The pressure varies in the structure according to radius :math:`r` according to the following formula:

:math:`p(r)\mathrm{=}2.{\sigma }_{y}\mathrm{.}\mathrm{log}(\frac{r}{{R}_{\text{int}}})+(\frac{2}{3})\mathrm{.}{\sigma }_{y}\mathrm{.}\frac{{R}_{\text{int}}^{3}\mathrm{-}{R}_{\text{ext}}^{3}}{{R}_{\text{ext}}^{3}}`