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Techniques for Optimisation and Analysis of Composite Structures for Damage Tolerance and Buckling Stiffness


Reference:

Baker, N., 2012. Techniques for Optimisation and Analysis of Composite Structures for Damage Tolerance and Buckling Stiffness. Thesis (Doctor of Philosophy (PhD)). University of Bath.

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    Abstract

    This thesis explores methods by which carbon fibre reinforced polymers may be fficiently designed with the inclusion of damage tolerance criteria. An efficient method of modelling the compression after impact (CAI) strength of composite materials is selected, and this forms the basis of analysis performed. The CAI model is initially used as the objective in an optimisation routine using a simple genetic algorithm. This indicates features of a damage tolerant composite laminate, namely that plies near the surface are less axially sti® in the loading direction than those nearer the laminate midplane, with a lower Poisson's ratio than the full laminate. This delays sublaminate buckling under laminate uniaxial compression, thus restricting delamination propagation. The designs produced by the optimisation are verified experimentally. In order to improve the computational efficiency of the CAI model a simple surrogate modelling technique for sublaminate buckling is presented. This allows a complete database of results to be produced for a given set of ply angles, in this case standard 0/90/§45± plies. This is used in the full analysis of a collection of layups produced elsewhere to be fully uncoupled, but without the stipulation of midplane symmetry. The surrogate method is shown to reduce computation time by over 99%, and produce results with an average error of less than 0.1% compared to exhaustive analysis. The analysis of the damage tolerance of fully uncoupled laminates shows that the relaxation of midplane symmetry as a design rule gives the designer far more flexibility in layup, and may allow for more damage tolerant laminates to be selected. Finally, the CAI model is incorporated into a stiffened panel design optimisation problem as a constraint. Firstly the panel is optimised using the in¯nite strip analysis tool VICONOPT, with three stiffener geometries. The objective function is minimum mass for a panel subject to compressive and out-of-plane loading, with buckling and strain allowable constraints applied. Damage tolerance constraints are then applied in place of a strain allowable, using a bi-level optimisation approach. This method is shown to allow efficient inclusion of damage tolerance as a constraint in stiffened panel design, although it does not account for interactions in global buckling and local sublaminate buckling which may reduce the strength of the panel. Results indicate that the inclusion of damage tolerance analysis in stiffened panel design shows little benefit for low load panels, but can give significant reductions in mass (up to 30%) for higher load panels.

    Details

    Item Type Thesis (Doctor of Philosophy (PhD))
    CreatorsBaker, N.
    Uncontrolled Keywordscomposites, structures, damage tolerance
    DepartmentsFaculty of Engineering & Design > Mechanical Engineering
    Publisher StatementUnivBath_PhD_2012_N_Baker.pdf: © The Author
    StatusUnpublished
    ID Code32216

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