Bending of Beams
When a 'beam' experiences a bending moment it will change its shape and internal stresses (forces) will be developed. The photograph illustrates the shape change of elements of a beam in bending. Note that the material is in compression on the inside of the curve and tension on the outside of the curve, and that transverse planes in the material remain parallel to the radius during bending

The pure bending shown in the diagram can be produced by applying four forces to the beam, two of opposite direction at each end. This configuration is known as 'four point bending' and produces a uniform bending moment over the center section of the beam as illustrated in (b) opposite.

From: McMahon & Graham, "The Bicycle & the Walkman," Merion (1992)