A thin rectangular section of bar with thickness $\text{'}t\text{'}$ and width $\text{'}b\text{'}$ subjected to torque T$\text{'}.$ Obtain an appropriate stress function and angle of twist for the section. Determine maximum shear stress and angle of twist of section for $700Nm$ torque. Section parameters are $b=90mm,t=10mm$ and modulus of rigidity $G=80{10}^3\frac{N}{m}{m}^2.$