Instability and Plastic Flow of Soils

Materials should according to Drucker's postulate become unstable when the second increment of plastic work is negative. Experimental observations however show otherwise. Based on the condition that a stable material is able to sustain a small perturbation in load, and the requirement that instability requires the second increment of plastic work to be negative, a condition for the mechanical stability of a material with nonassociated flow can be obtained. Experimental data supports the suitability of this condition to predict instabilities created under undrained conditions.