Modelling plastic stress relaxation in shaped sapphire crystal growth

Thermally induced stresses in growing shaped sapphire crystals are modelled using transient finite element simulations. Boundary conditions and mass c hanges are: fixed on an expanding remeshed and updated grid. The actual str esses are obtained taking into account plastic relaxation through dislocati on glide along basal, prismatic and pyramidal slip planes. For this purpose , phenomenological creep laws available for this material are implemented i n the frame of thermally activated plasticity. The model is calibrated by c omparison with directional mechanical tests, and validation is performed by growth simulations and dislocation density measurements on as-grown crysta ls. (C) 1999 Elsevier Science B.V. All rights reserved.