Discrete Material Optimization (DMO) is a method, which was originally developed for designing composite structures via multi-material topology optimization principles. Current study applies DMO to sandwich structures with variable thickness in the core and face-sheets. Each layer contains design variables for available materials. Materials are combined through interpolation schemes to define properties of the layer. The objective function (mass of the structure) and the failure constraints are interpolated via Rational Approximation of Material Properties (RAMP) in order to calculate with smooth variables, but achieve discrete results. This enables gradient optimization via Interior Point Optimizer (IPOPT) with constraints on maximum stress, wrinkling, and crimping. Structure is modeled by the finite element method, which calculates element forces and moments repeatedly as the stiffness of the structure changes during optimization. Element loads are used by the first-order shear deformation theory to evaluate the stresses in the layers to obtain failure constraints requested in each iteration by the gradient optimizer. Solution is demonstrated on the plate examples showing material distribution and discreteness level. In addition, constraint aggregation by Kreisselmeier-Steinhauser (KS) function was utilized to decrease the number of constraints in the optimization.

One of the typical sandwich ending is tapered transition to a solid laminate, which causes significant stress distribution changes. The reviewed articles show that tapered area causes increase in the shear stress in the core, increase of the axial forces in the facesheets and local bending at the fork point, at points of the tapering angle change, and at ply drop positions. Most of the studies gave attention to the endings without reinforcing. During Erasmus+ internship at KTH 2D model of the tapered ending with reinforcing plies, various geometry and resin filler in the core tip was investigated to see the influence on the stress distribution. It was found that tension load case is not as critical as bending load case. Increasing of the solid laminate thickness, adding plies and inserting a short resin or adhesive filler into the core tip area lead to significant stress reduction, whereas in the transition point, from tapering to constant thickness sandwich, increasing radius is more efficient than reinforcing plies in regard to reduce stress concentration.

A miniaturized heat switch is a device under the development dedicated to control automatic cooling of a space instrument box. The design study presented in this paper focuses on a stiffness issue of the switch base plate which should assure good thermal contact with the instrument box. Due to inner deformations of the switch, base plate deforms which might decrease contact area needed to the heat transfer. Thus several design concepts of the copper base plate were investigated and multimaterial additive manufacturing was reviewed. One approach was to decrease deformations of existing geometry by multi-material design by local exchange of copper with steel, first by engineering intuition, then by the stiffness based topology optimization, and finally with the layer-wise topology optimization better respecting manufacturing possibilities. Another approach was to change the geometry and use the topology optimization constrained with a flatness of the base plate but with single material. Finally, deformation changes of all the variants were compared.

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