On static helical instabilities of screw dislocations in a SmA phase and on their consequence on plasticity

Abstract

The stability of a screw dislocation in a SmA phase submitted to a dilative or compressive strain normal to the layers is investigated. It is shown that the screw line transforms to a helical line, whose handedness depends on the signs of the Burgers’ vector and of the strain. For small strains the process necessitates some activation energy; there is a critical strain above which it is spontaneous. Some consequences are proposed, like : a) the nucleation of edge dislocation lines in the so-called undulation instability, and b) the easy crossing without pinning of edge and screw dislocation lines. The analysis stays entirely in the limit of linear elasticity, and it is