The deformation and fracture of simple crystals

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 —the two molecular planes would fly apart and the crystal would separate into two parts. The applied force equal to —Fj, 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. 

Not all materials behave in this way at room temperature many common metals, e.g. copper and aluminium, are ductile, i.e. they are able to undergo large amounts of permanent extension and, when failure occurs, it does not do so by a simple brittle fracture. Rather, when a specimen of such a material in the form of a wire or rod is extended at room temperature, the behaviour 

The evidence presented in this section indicates that there are two alternative processes that can occur when the strain in a crystal is raised to a sufficiently high value. These processes are 

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It was shown previously that dynamic numerical simulation can be used successfully to analyze adsorption and crystallization [80,81]. The method was further developed to cover the processes of the deformation and fracture of a crystal [71,72]. Here, we will present the results of a study on the deformation and fracture of a crystal in the presence of the foreign atoms of the adsorption-active medium. A reasonably simple system was used in an attempt to cover a broad range of variations of the experimental conditions so that various cases of the medium influence could be observed. 

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