ideal stiff-plastic body, flat deformation, motion of particles, free surface, analytical decisions, energy dissipation, destruction.

Within the limits of the ideal stiff-plastic bodies model the quasistatic problem about the sample stretching with the rounded angular cuts is considered. The construction of possible plastic currents is realised and the form of a free surface in which the rounded angular cut is deformed as a result of stretching a sample is defined. Motion of the particles in a plastic area and change of the geometry of the field with time of time are investigated. The energy transition criterion from the continuous forming of a free surface to destruction in the top of the rounded angular cut is formulated.