Extended limit design theorems for continuous media

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Prager, William, 1949, Recent developments in the mathematical theory of plasticity, J. Appl. Phys., 20, 235, 10.1063/1.1698348

Drucker, D. C., 1950, Some implications of work hardening and ideal plasticity, Quart. Appl. Math., 7, 411, 10.1090/qam/34210