A dynamical model of a crystal structure. II

Abstract

A quantitative study of the mechanical properties of two-dimensional bubble rafts has been made. The elastic properties have been used to check the theory of the interaction of the bubbles, and reasonable agreement is obtained. The plastic properties are the main subject of this paper. Plastic deformation proceeds by the motion of dislocations, and we have taken advantage of the possibility of making absolutely perfect crystalline rafts to study the production of dislocations. They are produced either in pairs in the body of the rafts, or singly from the edges. The shear strain required to produce them varies from 7° for bubbles of 2 mm. diameter to a fraction of a degree for bubbles of less than 0.5 mm. diameter. The analogy with metals is best for bubbles of about 1.2 mm. diameter; for these the critical shear strain is about 3°. The observed shear strains can be explained in terms of the forces between the bubbles, and so it seems that the very low shear strengths of metallic single crystals cannot be explained in terms of dislocation creation, but must be a consequence of the initial presence of faults of some kind.