Performance analysis of a reduced cost manufacturing process for composite aircraft secondary structure

Abstract

In the current, environmentally-aware, climate aircraft designers are under increasing pressure to
produce fuel efficient vehicles. Weight reduction is an important method for increasing fuel
efficiency. Fibre reinforced polymer (FRP) composites are known to offer weight savings over
traditional metallic components, due to their excellent stiffness and strength to weight ratios.
However, the major limiting factor for the use of aerospace quality composites is the
manufacturing cost. The costs incurred in the conventional process of prepreg cured in an
autoclave are well documented. The research in this thesis is concerned with reducing the cost of
manufacturing aircraft standard carbon fibre composite sandwich panels, whilst maintaining
mechanical performance.

The overall aim of the EngD is to provide a unified approach for assessing the performance of
carbon fibre sandwich secondary structure that are manufactured using several different
techniques. Cost and performance criteria are defined so that an optimal panel can be produced.
The work has been motivated by the industrial sponsor, GE Aviation Systems. Five combinations
of raw material and processing techniques, manufacturing options (MOs) were considered in
incremental steps from the baseline of unidirectional prepreg cured in an autoclave to the noncrimp
fabric (NCF) infiltrated using resin film infusion (RFI) and cured in a conventional oven.
For cost and performance analysis a generic panel has been designed that is representative of
secondary wing structure on commercial passenger aircraft. The cost was estimated by monitoring
the manufacture of generic panels using each MO, whilst the performance was measured by both
mechanical characterisation tests and by full scale tests on a custom designed rig. The rig applies a
pressure load using a water cushion and allows optical access to the surface of the panel enabling
the use of optical techniques, i.e. thermoelastic stress analysis (TSA) and digital image correlation
(DIC). Feasibility tests on TSA and DIC demonstrated their use on the materials considered in
this thesis, and were used to validate finite element (FE) models.

The RFI out-of-autoclave process was found to reduce generic panel manufacture time by almost
30%, and the material cost was reduced by almost 40%. The mechanical characterisation tests
suggested the ‘new’ process could produce laminates with a similar fibre volume fraction to that of
the original process and similar in and out-of-plane mechanical properties. The in-plane stiffness
was slightly reduced by 7 %, but the strength showed an increase of 12%. Full scale tests on the
generic panels using point out-of-plane deflection measurements and full field TSA demonstrated
the panel produced using the ‘new’ process has adequate performance. Moreover the full-field
tests indicated an improvement in performance. Further work is required to optimise the design of
the panel for weight, in particular the weight of the raw material, and investigating methods for
modelling the NCF for certification.

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