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. Indirect stress will be discussed in detail in later sections. In order for the engineer to effectively design any item, whether it is a frame which holds an object or a complicated piece of automated machinery, it is very important to have a strong knowledge of the mechanical and physical properties of metals, wood, concrete, plastics and composites, and any other material an engineer is considering using to construct an object. Strength of materials deals with the relationship between the external forces applied to elastic bodies and the resulting deformations and stresses. In the design of structures and machines, the application of the principles of strength of materials is necessary if satisfactory materials are to be utilized and adequate proportions obtained to resist functional forces.