The deformation and fracture of simple crystals

Abstract

Under the conditions of elastic deformation a specimen of a simple crystalline solid suffers a definite deformation when a definite load is applied; the deformation does not depend on the duration of application of the load and disappears completely when the applied forces are removed. When the strain is small the stress and strain are linearly related. Elastic behaviour can be readily explained in terms of the intermolecular forces existing within a crystal, as discussed in section 2.10, based on the assumption that the force between neighbouring planes of molecules in a simple crystal varies with the separation of the planes in qualitatively the same way as the force between a pair of molecules, shown schematically in Fig. 1.2(a). This figure also indicates that if F reaches the value —F_m the two molecular planes would fly apart and the crystal would separate into two parts. The applied force F_m, equal to — F_m, would be the tensile force needed to produce rupture between neighbouring molecular planes. In macroscopic terms, the solid would deform elastically in tension and then suddenly split into two parts, a process known as brittle fracture. A number of materials, e. g. rock salt and bismuth, behave approximately in this manner at room temperature.