Stresses on Oblique Sections:
We already have seen that the equilibrium of a rigid body must be satisfied overall and additionally if we divide it into a number of small rigid bodies each of these small elemental bodies must also be in equilibrium individually. The satisfaction of equilibrium does not base on the way elements are divided. Even if the solid is divided by inclined or even curved surfaces, equilibrium ought to be satisfied for each of the elements therefore divided.
Consider the solid illustrated in given figure where the solid is divided into small elements by inclined planes. The inclination (or orientation) of a plane is described by its aspect angle, explained as the angle made by its normal to the longitudinal axis of the original bar. Assume the aspect angle of the plane be θ. As the width of the plane b is unaltered & length of the plane h is enhanced to h/ cos θ, the area of the inclined plane is A / cos θ, where A is area of cross section of the original solid.
If σx be the stress working on the plane normal to x axis (longitudinal axis), after that the axial force P = σx × A. This force working on the inclined plane might be resolved into normal and tangential components.