Discrete Stiffness Tailoring of Aerospace Composite Structures: Analysis, Optimisation and Testing

Abstract

Variable Stiffness (VS) designs allow variation of the bre angle within a single ply layer, enabling a significant expansion in the design space available for stiffness tailoring of composite laminates. Tailoring is typically achieved through continuous steering of bres, which maintains transverse structural continuity, but manufacturing methods capable of fabricating such designs are unable to achieve industrial manufacturing rates, and also impose minimum bre steering radii constraints, limiting performance improvements. Discrete Sti ness Tailoring (DST) is a novel manufacturing concept where sti ness tailoring is achieved using discrete changes in ply angle to favourably redistribute stresses. Resulting performance increases can be exploited to potentially achieve rapidly manufacturable lightweight structures, uninhibited by the minimum tow-turning radii which limit continuous bre steering approaches.

In this thesis, the Discrete Stiffness Tailoring concept is initially demonstrated through the simple redistribution of material within a quasi-isotropic laminate, and is shown both analytically and experimentally to improve buckling stress by 16% with no failure observed in regions of discrete angle change. Discrete tailoring introduces discontinuities, ply seams, within a laminate and the reduced tensile strength of these seams is investigated. Although a marked reduction in tensile strength is observed with greater numbers of discontinuous plies, it is found that for uni-axial compressive loading with seams parallel to the load, the decrease in transverse strength is not found to be critical.

An efficient two-stage optimisation routine is implemented to design a DST minimum mass T-stiffened aircraft wing panel subject to buckling and manufacturing feasibility constraints. The panel is manufactured and compression tested to failure, extending the DST design concept to component level for the first time. A weight reduction of 14% is achieved compared to a constant stiffness optimum, through redistribution of load to the stiffener region. The optimum design removes material from the skin, between stiffeners. Experimentally, the optimised tailored panel achieved a buckling load, without failure, within 4% of that predicted, validating both the methodology and modelling.

The validated optimisation routine is used to perform a parametric study of infinitely wide stiffened panels under varying uni-axial compressive loads, representative of those experienced by commercial aircraft. Amendments to the original optimisation methodology allow for the selection of non-standard angle designs, and a blending constraint is added to maximise the arrangement of continuous plies between regions. Greater mass reductions due to tailoring are obtained with smaller in-plane loads, and the same level of material efficiency is able to be maintained for wider stiffener spacings with the application of stiffness tailoring.

Date of Award	17 Nov 2021
Original language	English
Awarding Institution	University of Bath
Supervisor	Andrew Rhead (Supervisor) & Richard Butler (Supervisor)