2. Geometric and kinematic description of processes

Figure 5 shows a schematic diagram of the main processes involving plastic deformation of solid-state metal. It should be remembered that the plasticity of a material corresponds to its ability to exhibit, after application of a state of stress verifying the plasticity criterion $\overline{\sigma }={\sigma }_0$ , whose critical value is defined by the flow stress σ ₀ , a permanent deformation leaving its volume invariant to a first approximation (§