Fig. I$-3$

$4.$ A torsionally$-$loaded solid shaft has a cross$-$sectional diameter of $d,$ subjected to a pair of torques of $T$ at both ends. The maximum cross$-$sectional shear stress is $q,$ and the principal stresses associated with this point are $=q,{}_2=q,$ and ${}_3=q,$

$5.$ According to the failure phenomena of specimens in uniaxial tension tests, materials can be categorized into two types, including $$ materials and $q,$ materials.