고체역학 (Beer) — Transformations of Stress and Strain: Problem 7.161

7.161 Determine the principal planes and the principal stresses for the state of plane stress resulting from the superposition of the two states of stress shown. (Note: The first state shows a square element with shear stress \(\tau_0\) in the standard positive orientation. The second state shows a square element rotated \(30^\circ\) counter-clockwise with shear stress \(\tau_0\) in the same relative orientation.)