$2\mathrm{.}69$ A bar $1$ m long is bent and then stress relieved. The radius of curvature to the neutral axis is

$0\mathrm{.}50$ m $.$ The bar is $25$ mm thick and is made of an elastic, perfectly plastic material with

$S_y=500$MPa and $E=207$GPa. Calculate the length to which this bar should be stretched so

that, after unloading, it will become and remain straight.