The internal resistance to deformation of the fibers of a body is called stress. Stress can be classified as either simple stress, sometimes referred to as direct stress, or indirect stress. The various types of direct stress are tension, compression, shear, and bearing. The various types of indirect stress are bending and torsion. A third variety of stress is categorized as any combination of direct and indirect stress.
Simple stress is developed under direct loading conditions.
That is, simple tension and simple compression occur when the applied force is in line with the axis of the member and simple shear occurs when equal, parallel, and opposite forces tend to cause a surface to slide relative to the adjacent surface. Indirect stress, or stress due to bending should be properly classified under statics of rigid bodies and not under strength of materials. The bending moment in a beam depends only on the loads on the beam and on its consequent support reactions. Torsion is when a shaft is acted upon by two equal and opposite twisting moments in parallel planes. Torsion can be either stationary or rotating uniformly.