Suggest using the latest ESDU's listed below for calculating inelastic beam stresses, etc.
COZZONE, Frank P. (Lockheed): "Bending Strength in the Plastic Range," Jour. of the Aeronautical Sciences, Vol. 10, No. 5, pp. 137-151, May 1943. The basic paper.
BRUHN, E. F.: Analysis and Design of Flight Vehicles, Tri-State Offset Co., hardcover, 4th printing, 1965 (out-of-print). See Chapter C3: "Yield and Ultimate Strength in Bending," pp. C3.1-C3.12. Chapter C3 is included, see below.
BROEK, J. A. Van Den: Theory of Limit Design, John Wiley and Sons, hardcover, 73 illustrations, 144 pages, index, 1948.
SMITH, James O. and SIDEBOTTOM, Omar M.: Inelastic Behavior of Load-Carrying Members, John Wiley and Sons, 447 pages, index, 1965. See their references. pp. 100-102. See SMITH's Advanced Mechanics of Materials below.
SMITH, James O. and SEELY, Fred B.: Advanced Mechanics of Materials, 2nd Ed., John Wiley and Sons, hardcover, 1952. This is a well read classic textbook. (Don't consult the revised edition by other authors -- the older text is better.)
SECHLER, Ernest E.: Elasticity in Engineering (Galcit Aeronautical Series), John Wiley and Sons, hardcover, 419 pages, index, 1952. See his and Louis G. DUNN's Airplane Structural Analysis and Design.
ESDU 01.06.00: "Information on the use of Data Sheets 01.06."
ESDU 01.06.01: "Form factors for circular sections under combined bending and axial load."
ESDU 01.06.02: "Form factors for flanged sections under combined bending and axial loads."
ESDU 01.06.01: "Form factors for angles, tee and channel sections under combined bending and axial loads."
ESDU 88035: "Maximum inelastic bending moments and form factors for symmetric bending of beams." Has FORTRAN computer program for calculating the bending moments associated with the maximum allowable extreme fiber stress in a beam under symmetric bending. Also see: " ESDU 76016: "Generalization of smooth continuous stress-strain curves for metallic materials."
ESDU 90002: "Struts: strength under flexural, local and inter-rivet buckling (computer program)."
ESDU 90004: "Maximum inelastic bending moments and end loads for beam sections of general polygonal shape (computer program)."