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IMac-Pro
Industrialisation of Manufacturing Technologies for Composite Profi les for Aerospace Applications

Tags: Air

State of the Art - Background

Today, most of the profiles used for the structure of an aircraft and for stiffening the skins on the wings and fuselage are made of aluminium, even partly of titanium. In modern aircraft design these materials are increasingly substituted by carbon fibre reinforced plastics (CFRPs). CFRPs have a very high potential for lightweight design by offering excellent specific stiffness and strength. They also allow an optimised design regarding geometry, local thicknesses and local fibre direction.

The market for cost-effective manufacturing of CFRP profiles is expected to grow significantly in the future, due to a rapidly growing aircraft market and the demand for lightweight designs based on CFRPs, which improve the ecological compatibility of planes and helicopters, and reduce aircraft production and development costs.

CFRP parts already in use are mostly produced using pre-impregnated (prepreg) technology. Automated production of prepreg parts is currently limited to two-dimensional geometries with moderate curvature. Complex shapes still need extensive manual work with hand-lay-up of the material, layer by layer, and a manual vacuum bagging. Resin curing in the autoclave is also a very expensive step of prepreg production.

IMac-Pro will focus on 'textile preforming' in combination with 'out-of-autoclave curing', which would provide promising alternatives to prepreg.

Objectives

The technological objective of IMac-Pro is the development of a complete integrated process chain for the cost-effective serial production of optimised CFRP stiffener profiles (e.g. frames, stringers, struts, floor beams, drive shafts, etc.) for all kinds of aircraft (passenger and freighter planes, helicopters) based on textile technologies in combination with advanced injection and curing technologies.

Depending on the geometrical requirements and the loads, the targeted profiles might be straight with a constant cross section (e.g. fuselage stringers), straight with a varying cross section (e.g. wing spars) or complex curved (e.g. fuselage frames).

In IMac-Pro, net-shaped textile preforming techniques with a high potential for automation and cost saving will be the baseline for the production of the profiles, with the following challenging goals to reach:

- a weight saving of at least 20% compared to aluminium design;

- a weight saving of up to 5% compared to prepreg design for curved profiles;

- a cost saving of more than 45% compared to prepreg design for complex profiles.

Description of Work

The project addresses different types of aircraft stiffener profiles. The common characteristics of these parts are an enormous length in comparison to the dimensions of the cross section. They can be divided into two main categories due to the significant impact on the manufacturing technology necessary for the production:

1. Massive profiles like floor beams and frames with edge dimensions in the range of 50 to 400 mm and wall thicknesses from 0.5 to 8 mm. These profile types have the additional challenge that their cross section shape and their wall thicknesses may change along the profile continuously or periodically.

2. Stringer profiles with relative small cross section dimensions in the range of 50 mm, no changing of the cross section, but partly single or double curved and of even greater length (up to 30 metres).

For the massive profiles the braiding technology is the baseline for preform production. Profile type 2 will be addressed by continuous forming (similar to pultrusion) of textile semi products and by fibre patch preforming (FPP).

Different techniques of curing will be investigated: RTM with adaptable elements to compensate for the settling (thickness reduction) of the preform, continuous injection and microwave heating.

Expected Results

The expected results of IMac-Pro comprise mostly of machine prototypes, devices and measurement systems, which will be adapted to already existing equipment:

- a circular braider at Kümpers will be equipped with the 0° and 90° lay-up devices;

- a circular braider from USTUTT, which can be opened, will be used to address the braiding of closed frames;

- a laboratory stringer preform machine at SECAR, which will be combined with FPP lay-up units;

- a microwave oven at DLR which will be used for the fast stringer curing.

Using the developed machines and tools, three demonstrator structures are planned:

- a stiffened panel with four pre-cured stringers on a prepreg skin;

- a stiffened panel with four stringer preforms and textile skin cured at the same time;

- a cargo floor unit, consisting of a curved frame profile, a straight crossbeam and z-struts.

A consequent application of the project results will lead to a significant weight saving of the whole airframe and by this to an improvement of environmental compatibility without loss of performance. At the same time, a significant reduction in aircraft acquisition costs can be expected. The main cost-cutting issues compared to prepreg are: lower basic material costs, reduced waste due to net shaping, a high degree of automation for complex shapes, and the potential for cheaper tools and non-autoclave curing.

Manufacturing of the Carbon Fibre Preform for a Curved Profi le by Braiding
Manufacturing of the Carbon Fibre Preform for a Curved Profi le by Braiding

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