Molecular dynamics analyses of defect-free Al single crystals subjected to bending were carried out in order to investigate dislocation nucleation from free surfaces. A principal object of the analyses was to provide a background for the development of dislocation nucleation criteria for use in discrete dislocation plasticity calculations. The molecular dynamics simulations used an embedded atom potential for Al. Bending was imposed on a strip by specifying a linear variation of displacement rate on opposite edges. The overall bending response was determined and the character of the dislocations nucleated was identified. It was found that the stress magnitudes at the instant of dislocation nucleation were nearly an order of magnitude smaller than for homogeneous bulk dislocation nucleation. The characterization of dislocation nucleation in terms of various phenomenological nucleation criteria was explored. In particular: (i) a critical resolved shear stress; (ii) the onset of an elastic instability; and (iii) a critical stress-gradient criterion. It was found that dislocation nucleation was not well-represented by a critical value of the resolved shear stress but was reasonably well-represented by the critical stress-gradient criterion.

An Analysis of Dislocation Nucleation near a Free Surface. Y.Liu, E.Van der Giessen, A.Needleman: International Journal of Solids and Structures, 2007, 44[6], 1719-32