Structural Design of Variable Stiffness Composites Laminated on Doubly Curved Surfaces

  • Matthew A Thomas

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)


The continual need to improve aircraft performance and efficiency has driven aircraft manufacturers to develop ever more efficient aircraft. The majority of these efficiency savings are due to the increasing usage of composite materials, which exhibit excellent strength and stiffness to weight ratios, making them attractive for application in lightweight aerospace structures. Fan blades in jet engines are a component in which composite materials, such as carbon fibre reinforced epoxy, have recently been utilised. Fan blades rotate at thousands of revolutions per minute creating large centrifugal and aerodynamic pressure loads. As the centrifugal force is directly related to the density of the material used in the blade, the adoption of stronger, stiffer and lighter materials produces great benefits in structural and aerodynamic performance. This research investigates the development of methods for variable stiffness composites which could be used to further improve the structural design of composite fan blades.

Variable stiffness laminate design in the literature has been limited to structures of zero to little curvature in only one direction, such as cylinders. Here, a novel method for designing fibre paths of variable angle tow (VAT) plies on to doubly curved surfaces has been developed. Firstly, a piece-wise Bézier curve, defined using a set of control point angles, is projected onto the surface of interest. Adjacent tow paths are then calculated by finding the correct distance over the surface to ensure the arc length is a tow width in length. This ensures that tows tessellate correctly. Once all adjacent tow paths are calculated and the surface is sufficiently covered, the local fibre angle can be calculated using the angle difference between the tow vectors and a reference 0° ply.

Lamination parameters convert laminate stiffnesses to a convex and continuous design space, while also decreasing the number of design variables needed to define laminate stiffness. Lamination parameters were spatially varied over pre-twisted plate and fan blade shell models using spline surface interpolation of a set of control points. A method for constraining lamination parameter variation over a part in relation to manufacturing constraints of typical automated fibre placement machines, notably minimum turning radii, has been developed. The optimal spatially varying lamination parameter distribution for a variable stiffness and thickness fan blade were found through gradient-based optimisation. Finite element analysis results show a reduction in strain energy compared to a quasi-isotropic (QI) and the optimal constant stiffness laminates, respectively. The tow paths and ply shapes of every VAT ply are subsequently found using a two-stage optimisation process to minimise the difference between the VAT laminate and the optimal lamination parameter variation.
Date of Award14 Nov 2017
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorPaul M Weaver (Supervisor) & Stephen R Hallett (Supervisor)

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