The Migration of Stress Concentrations in Integrated Composite Structures.

Lead Research Organisation: University of Bath
Department Name: Mechanical Engineering


Constant demand from the aerospace industry to reduce weight, and to increase both component performance and the manufacturing rates of composite structural parts has led to interest in the use of discrete stiffness tailoring (DST), in order to realise these targets. Traditionally, the fibre angle within a single ply layer is constantly orientated in a single direction, and it is the stacking sequence of plies that is optimised for the required stiffness and strength. Discrete stiffness tailoring however, seeks to alter the fibre angle and thickness across the geometry of a part in a discontinuous fashion.

A previous ESPRC sponsored project found that a maximum weight saving of 40% was achievable if both the ply angle and thickness were simultaneously optimised for composite structures. This optimisation could be achieved with a minimum of three discrete strips or zones across the width of a flat panel, where the orientation of the fibre is constant but different within each strip. DST laminate structures can theoretically be manufactured using Automated Tape Laying (ATL) machines, which are able to deposit material at a high-rate when compared to other manufacturing methods. This technology, combined with the use of discrete bi-axial composite tapes (i.e. Non Crimp Fabrics) has in theory the twofold advantage of increasing the deposition rate by as much as eight times currently achieved, and creating structures with a 40% improvement in performance.

However, structures created using the DST technique may be critically affected in terms of strength due to the vulnerability of the joints between the adjacent strips. The stress concentrations developed as a result of the stiffness changes between strips, the joint itself, and any possible taper introduced as a result of the optimisation have not yet been investigated fully. The aim of this project is to initially characterise the stress concentrations between the fibre strips, and then to investigate the role of taper on stress transfer, and to be able to define this analytically. The final objective is to develop design strategies for strip deposition taking into account minimising the manufacturing time, material wastage etc. and optimising the component strength. These aims will be met through a combination of experimentation and analytical work.

This PhD is part of the ADAPT ESPRC sponsored project, the overall aim of which is to produce a fourfold increase in the deposition of stiffness tailored material, in order to meet demand for new short-range aircraft in the next 20 years, as well as improving component performance. The ESPRC have identified Manufacturing the Future as a key theme for current research; a focus for which this project is highly relevant, in terms of aiming to develop current production processes. The project is supported by an industrial partner, GKN; and also will incorporate some involvement from Chomarat (thin-ply manufacturer) and Airbus.


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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509589/1 01/10/2016 30/09/2021
1789271 Studentship EP/N509589/1 01/10/2016 30/09/2020 Lucie Culliford
Description The use of Discrete Stiffness Tailoring (DST) has been shown experimentally to increase the buckling performance of composite panels subjected to compression, parallel to the seams created by the regions of alternative stiffness. The seams introduced a potential weakness, but the panels did not fail due to the seamed region. The performance benefit can be predicted by commercially available software, as well as efficient analytical models. An optimisation methodology incorporating the previously validated analytical model has been developed in order to design minimum mass structures with DST for buckling. An optimised blade stiffened compression panel with DST has been designed and manufactured.

The inclusion of a seam significantly decreases the strength transverse to that seam when compared to a continuous composite specimen. This strength, however, is sufficient to deal with loading parallel to the seams, as shown in the experimental buckling panels. As such this loss of transverse strength should not preclude this technique from being used in certain cases.
Exploitation Route Lower mass, discretely stiffened structures can be designed to replace higher weight standard composite structures for the same buckling performance.

The design of the seam for strength is yet to be investigated.
Sectors Aerospace, Defence and Marine