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Title: Buckling of stiffened variable stiffness panels
Author: Coburn, Broderick Howard
ISNI:       0000 0004 5923 6755
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2015
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Over the past five decades the aerospace industry has seen the increased use and development of composite materials and structures, largely due to their superior specific strength and stiffness in comparison with metals. These two major attributes, coupled with their increased tailorability, enable the design of lighter, more efficient structures. Traditional tailoring is limited to modifying the orientation of laminate plies to achieve the desired performance. However, recent advancements in tape laying and fibre placement technologies, have led to the possibility of laminates with varying fibre orientations in the plane of a ply, thereby creating variable stiffness (VS) structures. These VS laminates greatly expand the design space and provide designers with additional degrees of freedom and tailorability. In recent years, analytical methods and finite element analysis (FEA) have shown that VS laminates can deliver a significant improvement in buckling and post-buckling performance for plates and shells. A potential application, which exploits this enhanced buckling performance, is the use of VS laminates as the skin of a stiffened panel. Here, the VS skin is designed to redistribute in-plane loads towards the stiffeners, which act as panel breakers, providing an expected increase in buckling performance, thereby facilitating the design of lighter and more efficient structures. The present work was conducted with the main aim of developing fast, robust and accurate tools for the linear buckling analysis of novel stiffened VS panels. The Ritz energy method was used to develop semi-analytical models for various plate configurations subject to uniaxial and biaxial compression; firstly, plates exhibiting discontinuously varying stiffness terms with bend-stretch coupling, a feature commonly present in the skin-flange region of stiffened panels; secondly, thick plates and sandwich panels with continuously varying stiffness terms, through fibre-steering, by including a first-order shear deformation theory; and thirdly, blade stiffened VS panels by utilising the previously developed methods to capture the important features of discontinuous and continuous stiffness variations and transverse shear flexibility. Benchmarked against a commercial FEA package, the semi-analytical models are shown to be both an accurate and computationally efficient alternative, and thus well suited for design and optimisation. To conclude, design and heuristic optimisation case studies for civil airliner blade stiffened panels, with both straight fibre and VS skins, were performed applying practical design and failure constraints. Optimal VS designs showed improvements in structural efficiency compared to standard configurations, with weight savings over 5% and 20% for VS skins satisfying and neglecting the 10 % rule, applied only for 00 and 900 plies, respectively. Furthermore, the mass reductions were shown to be achievable utilising relatively few VS plies in some cases, thus providing an avenue for application without the need for significant changes in stiffener or skin design, rates of deposition, or design rules and guidelines.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available