A circular shaft has a diameter D $=20$ mm and a length L $=1\mathrm{.}5$ m and is made of structural steel with elastic perfectly plastic behavior $($G $=95\mathrm{.}98$ GPa and y $=160$ MPa$).$ The shaft fixed at one end and subjected to a torque T at the other end. The angle of twist is measured between the ends as $=0\mathrm{.}2$ rad.