Design and Analysis of Optimal Braided Composite Lattice Structures
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This work applies to structural optimization within geometric constraints of braided truss structures. Prototypes of the newly developed open-structures have proven useful for their high specific stiffness and rapid manufacturing. Current prototype development has principally been based on trial-and-error and has yielded great improvements. The properties of the structures continue to improve with design changes. To date, no rigorous design tools have been able to predict the structure properties or to suggest advances in the design of optimal structures within the constraints of the braiding process. The results of this thesis provide tools for both those objectives and implement them in software. The computational tools carry the analysis of Open-Architecture Composites Structures (referred to equivalently as O-ACS, open-structures, or simply ‘structures’) from concept and initial constraints, to optimal design. Beginning with the specification of a braiding machine, a geometry model is constructed which can replicate any O-ACS tube the specified machine can manufacture. The geometry model is discretized into finite elements, where the nature of composite yarns is leveraged using beam elements to create an efficient mechanics model of the geometry. The Finite Element (FE) model is validated against a wide range of O-ACS specimens, and is capable of predicting the stiffness of the tubes in bending and torsion to within 5.7% and 8.4%, respectively, across the range tested. For various reasons including manufacturing and testing imprecision, the model cannot accurately predict axial stiffness (60% error across the range tested). Open-structure tubes were found to be significantly more rigid than equal weight composite and aluminum thin-walled tubes in asymmetric loading scenarios such as bending and torsion. Particularly the ability to increase diameter of O-ACS without significant change in mass allows it to exceed the specific stiffness of commercially available, equal weight, competitors. The FE model is wrapped in an optimization routine, in which it can be used to predict the highest specific stiffness structure, given a target weight and known applied loads. The optimized O-ACS tubes’ specific stiffness is predicted to be seven times that of a commercial filament-wound competitor in bending stiffness, and four times in torsional stiffness. All these tools, programmed in MATLAB, have been combined into an accessible Graphic User Interface (GUI) which allows any engineer easy access to the geometry simulation, finite element analysis, and optimization abilities of this work.