No shear stresses τ are exerted on the top or bottom surface of the bulk solid element in Figure 1; i.e., the shear stresses in these planes are equal to zero. No shear stresses are acting at the lateral walls, since the lateral walls were assumed as frictionless. Thus only the normal stresses shown are acting on the bulk solid from outside. Using a simple equilibrium of forces at a volume element with triangular cross-section cut from the bulk solid element shown in Figure 1 (Figure 2, on the left), the normal stress, σα, and the shear stress, τα, acting on a plane inclined by an arbitrary angle α, can be calculated. After some mathematical transformations, which need not be considered here, it follows that