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

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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 proposed that interesting non-linear phenomena should appear for high strain values