$[2]$ As shown in the figure, torque $T$ is applied to the hollow shaft made of brittle material with an outer diameter $D=100mm$ and an inner diameter $d=60mm.$ When the yield stress $Sy=200$ MPa of the shaft material is obtained, find the maximum torque value that can be applied without causing damage to the shaft. However, use the maximum principal stress theory with a safety factor of $n=2.[25$ points$]$