ABSTRACT
Theories in the mechanics of solids, as a practical matter, are simplified descriptions of actual material behavior. Plasticity theories describe the response of model solids strained beyond an elastic limit. A plasticity theory is completed when the elastic limit and the stress-strain relationship beyond the elastic limit are specified. Plasticity is sometimes thought of rather casually as a material that flows slowly or squeezes like toothpaste. Time-independent plasticity requires an attempt at additional loading for additional straining. Time-dependent plasticity implies possible creep, increasing strain at constant stress, and possible relaxation, decreasing stress at constant strain. Motion of hard, well-jointed rock masses under light loads may appear to defy a continuum description and thus require mechanics of a different kind, certainly not a plasticity theory. An important complication that arises in rock mechanics is the generation of new fractures and cracks and the extension of existing joints.
