The procedure for determining the force lines consists of two stages: 1) Defining the internal surface.
Stress is a measure of the average amount of force exerted per unit area.
The stress distribution can be obtained from known theoretical[1] or numerical (Finite element method) analysis.
Figure 3 shows the case of pure bending of a beam with rectangular cross section.
The tensile and compressive force lines are symmetrical and are denser at the beam’s edge.