The aim of this first chapter is to give nonspecialists in physics and metallurgy a general idea of the structure and mechanisms of deformations and fracture of the principal materials used in ordinary structures. Therefore, it consists of a brief summary of the classical knowledge as found in specialized works. The main references among these are given in the bibliography. Knowledge of these basic physical mechanisms is necessary for the formulation of hypotheses upon which the macroscopic phenomenological theories of deformation and fracture can be based. These theories, presented in Chapters 4–8, must indeed integrate the phenomena associated with discrete entities such as atoms, crystals, molecules, cells, etc., to the level of homogeneous continuum models. In metals, the introduction of the concept of dislocations by Taylor and several others in 1934 marked the first decisive step in explaining the phenomenon of plastic deformation. In a more general way, the invention of transmission and scanning electron microscopes has permitted, during the 1960s, an understanding of the main mechanisms of deformation and fracture.

It must, however, be remembered that in the field of fracture where these phenomena occur at the much larger scale of crystal or molecular arrangements, there are several unresolved questions, and for this reason the schematic treatment given in this chapter remains very rudimentary.