Deflexion, in Mechanics, the bending of any material exposed to a transverse strain. In all bodies so situated deflexion takes place; but whilst the elastic force of the material exceeds the straining force, the deflexion will be directly proportional to the pressure, and will not increase after the load has been on for a second or two, and upon its removal the material will recover its original form; but if the load exceed the elastic force of the material, the extension or deflexion increases with time, a permanent alteration of form ensues, and the deflexion increasing rapidly with slight addition to the load, fracture ensues. The resistance of a material to flexure, as Mr. Tredgold observes, is the only proper measure of its resistance, when it is to be applied in the construction of buildings, and that of its resistance to permanent alteration when it is used for machines. According to Mr.T.'s experiments, a bar of cast iron, 4 feet long and 1.2 inches square, and supported at both ends, sustained a load of 112 lbs. in the middle of its length without permanent alteration.
The deflexion with this load was one-tenth of an inch.