A rigid-plastic material is dened as a material in which no deformations occur (at all) for stresses up to a certain limit, the yield point. For stresses at the yield point, arbitrarily large deformations are possible without any change in the stresses. In the uniaxial case, a tensile or compressive rod, this corresponds to a stress-strain curve as shown in Figure 1.1. The stress, the yield stress, for which arbitrarily large strains are possible, is denoted fY. In the gure the yield stresses for tensile and compressive actions are assumed equal. A rigid-plastic material does not exist in reality. However, it is possible to use the model when the plastic strains are much larger than the elastic strains.