As shown in the figure, forces F can be resolved perpendicular to the plane and parallel to the plane. perpendicular componenet gives tensile stress to the plane. parallel component gives shear stress. Though perpendicularly resolved components of left side force and right side force are opposite to each other, they give combined tensile stress to the plane. similar thing happening for shear stress also.
Tensile stress is independent of angle α. Shear stress is maximum when α = 90º