Microscopic aspects of fracture: Cohesive stress

Abstract

If we consider an unfaulted material with a crystalline structure, such as the simple cubic cell, which is characterised on the dimensional scale by the lattice parameter ao of the order of 0.5 mn, the possibilities of ordered and bulk deformation (Fig. 1.1), at temperatures that are low compared to the melting point, correspond to the answer to the two components of a force acting on a reticular or crystallographic plane: a stress normal to this plane and a shear stress parallel to this plane. The critical values required to ensure these motions and break the interatomic bond are called the theoretical cohesive stress (or cleavage stress) and the theoretical gliding stress (or shear stress).