2. Introduction to elementary deformation processes

Even if all materials are treated with the tools of continuum mechanics, stresses result from the perturbation of their internal energy, induced by the modification of their microstructure. In this way, all or part of the material's constituent elements are brought out of their equilibrium position to "accommodate" the applied macroscopic deformation, resulting in an increase in the material's energy (enthalpy or entropy). The postulates of continuum mechanics – at any point occupied by the body there is a material point; each point moves continuously; two points that are infinitely close remain close; physical properties are continuous – contradict this physical reality, which is that the material's response involves discrete, discontinuous elementary deformation processes. It is in these elementary processes that polymers stand out from other materials, inducing specific macroscopic behaviors....