Stresses on Oblique Sections:

We have already seen in which the equilibrium of a rigid body should be satisfied whole and in addition if we divide it within a number of small rigid bodies each of these little elemental bodies should also be in equilibrium individually. The satisfaction of equilibrium does not depend on the way elements are divided. Even if the solid is divided through inclined or even curved surfaces, equilibrium must be satisfied for every of the elements therefore divided.

Consider the solid shown in Figure while the solid is divided within small elements through inclined planes. The inclination (or orientation) of a plane is described through its aspect angle, described as the angle made through its normal to the longitudinal axis of the original bar. Now let the aspect angle of the plane be θ. Because of the width of the plane b is unaltered and length of the plane h is increased to h/ cos θ, the area of the inclined plane is A/ cos θ, whereas A is area of cross section of the original solid.