Given

$[{}_i{]}_{?}=\left[\begin{array}{ccc}\frac{3}{2}& -\frac{1}{2\sqrt[\phantom{2}]{2}}& -\frac{1}{2\sqrt[\phantom{2}]{2}}\\ -\frac{1}{2\sqrt[\phantom{2}]{2}}& \frac{11}{4}& -\frac{5}{4}\\ -\frac{1}{2\sqrt[\phantom{2}]{2}}& -\frac{5}{4}& \frac{11}{4}\end{array}\right]$ units,

find the principal stresses and the associated principal directions,