Abstract
Any program of research on airplane structures must give consideration to the design and stress-analysis requirements of the Army, the Navy, and the Aeronautics Branch of the Department of Commerce. The requirement that the breaking strength of the structure or of its component parts must be the basis of strength calculations places a difficult problem before research men. Hooke’s law of proportionality of stress to strain forms the basis of the mathematical theory of elasticity, and above the proportional limit of the material (which for aircraft metals may be taken as the elastic limit), formulas based on this law do not apply. Formulas are derived from the standpoint of the mathematical theory of elasticity which fit the data quite accurately for stresses within the elastic limit. For stresses above the elastic limit an empirical formula will be necessary. In so far as the practical use of the data is concerned, since stresses above the proportional limit are involved, a derived formula may not be as desirable as the experimental curve. The value of the experimental curve depends very greatly upon the proper selection of the coordinates and the parameters.
