4. In the OXYZ Cartesian coordinates, a stress sta...

4. In the OXYZ Cartesian coordinates, a stress state of a point is(1) Draw the stress unit cube with the stresses labeled on X, Y and Z planes; (6 points)(2) One of the X, Y and Z planes is a principal plane. Which is the principal plane, and what is the principal stress on it？ (4 points) (3) Find the characteristic equation of the stress state, and solve the other 2 principal stresses at this point; (6 points) (4) Calculate the maximum shear stress and the mean stress at this point. (4 points)