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Industrialisation of Out-of-Autoclave Manufacturing for Integrated Aerostructures

Final Report Summary - IRIDA (Industrialisation of Out-of-Autoclave Manufacturing for Integrated Aerostructures)

Executive Summary:
One of the major challenges of the aeronautical industry is the reduction of manufacturing cost, while maintaining high standards of safety and supporting the “greening” of the air transport sector. Currently the industry is looking into alternatives to autoclave processing in order to cut down cost and reduce the environmental footprint. The term Out-of-Autoclave manufacturing was introduced to describe this industrial need.
The IRIDA project answers to this need by developing on a technology that is proven and reliable yet innovative. The IRIDA concept is based on the FibreTemp© technology for heated CFRP moulds for LCM/LRI techniques. According to this technology the mould is heated through its carbon fibre reinforcements that act as heating elements. The results are reduced energy consumption as the thermal mass of these tools is very low, uniform temperature distribution and decreased process cycles (due to fast heating and cooling). Furthermore, an additional proven advantage of the proposed technology is the improved dimensional stability of both the mould and the manufactured components as the low CTE of CRFP minimised thermal stresses and spring-in effects.
IRIDA implemented a workplan aimed to investigate of all aspects of composite processing in order to achieve a reliable, accurate and repeatable process leading into the design and manufacturing of a prototype tool and a demo part. Among the results of the IRIDA efforts:
― a study of various material configurations in respect to the endurance of the tools under thermal cycling
― the design of a LRI CFRP tool with self heating capabilities and integrated cooling system
― simulation of the infusion process for various resin flow strategies
― manufacturing of a large complex aeronautical part in the form of an aircraft engine nacelle
Project Context and Objectives:
Since the early days of the introduction of composite materials to the aeronautics industry there has been a constant strive to balance performance and safety with production cost. Today the most common material for the fabrication of composite aerostructures, especially load bearing ones, is carbon fibre reinforced polymers (CFRP) and the “mainstream” production method is autoclave curing. This method had been for a long time the state-of-the-art concerning the properties of the produced material. Although very efficient in terms of part quality, autoclave curing requires a lot of energy and time to heat, cool and pressurise the heavy machinery. Nowadays the trend is to turn to Out-of-Autoclave (OoA) techniques, able to produce cost effective, yet equivalent in material properties, components.
Production techniques, collectively characterised as Liquid Composite Moulding (LCM), are rapidly penetrating the industry especially for the production of secondary structures or less critical parts.These techniques also require energy for heating and cooling their tooling,, although significantly less compared to autoclave curing of prepregs. LRI or Liquid Resin Infusion which is one of the mostwidely used OoA methods often utilises tools made of CFRP. As the thermal mass of these tools is significantly less compared to autoclave tools, heating and cooling is much faster leading to quicker process cycles. Energy consumption is also reduced which is also beneficial in terms of CO2 footprint.
The main goal of IRIDA was to demonstrate the production of large aeronautical parts using an Out-of-Autoclave method based on LRI according to top industry standards by utilising a series of high-end technologies. The key objective is to accomplish fast and cost-efficient production while assuring the accuracy and reliability of the process through advanced monitoring and control systems. The main innovation behind the IRIDA concept is the FibreTemp technology in mould/tool making. This technology allows the fabrication of CFRP moulds that are heated through their carbon fibrereinforcement taking advantage of the electrical resistance of carbon. This allows for heat generation exactly where it is needed which is on the surface of the tool. In IRIDA the combination of the Fibretemp technology with state-of-the-art temperature control systems and curing monitoring sensors aims to the achieve a novel CFRP production system with great potential for industrialisation.
The detailed technical objectives of the project are listed below:
― Determination of specifications and manufacturing goals - Functional analysis of the proposed technologies
― Durability studies and material selection
― Small scale manufacturing trials (stiffener integration, sensor application)
― Application of numerical tools for electrothermal analysis and the simulation of the infusion process
― Detailed design of a prototype mould for an engine nacelle component
― Manufacturing and inspection of the prototype mould
― Production of a demonstration part
― Assessment of part quality
― Overall evaluation of the IRIDA technology




Project Results:
Thermal cycling study
The determination of the operating life and durability of the tools expressed in the number of cycles (parts) is essential for the industrialization of the technology. To this end a study of the behaviour of tool materials (mainly the resin system) in respect to thermal cycling was performed. Thermal cycling sustained by the tool (up to 180°C) often leads to microcracks (for a standard epoxy resin after about 200 parts) that affect the vacuum tightness and cause deterioration of the surface finish. The study was performed using actual self-heated tool plates which were connected to a power source and a temperature control system. Five tool plates were prepared by Fibretech and tested by UoP using the setup described above. The five plates were made from the following candidate materials:
- standard epoxy resin
- PU resin
- cyanate-ester resin
- standard epoxy with Ni coating
- standard epoxy with 3 layers of Hextool prepreg
The result of the first phase of the test was that the epoxy and the cyanate ester resin lasted longer than the other candidates. The limiting factor for all plates was the failure of the electrode connection after some cycles. This is attributed to the CTE mismatch between copper and CFRP which leads to resin cracks and loss of electrical contact between the electrode and the carbon fibers. This lead into a subsequent study by Fibretech for alternative electrode materials with lower CTE than copper (tungsten and molybdenum).
The outcome of the study was that molybdenum was selected as an optimum material choice for high-temperature tooling applications.
In addition auxiliary materials (vacuum bags, peel plies, breathers, tooling boards etc) were selected based on temperature range capabilities and suitability to the manufacturing process and materials.

Geometric features – Integration of stiffeners
The design for the demo part included three types of stiffeners. For their integration into the part, appropriate techniques and tooling components were developed and evaluated. Forming of the open stiffeners was effected through the use of silicone supports and cores and hard tools which can incorporate heating elements. Flexible heating elements were also tested to provide extra heat from the exterior of the tool (vacuum bag side). The closed geometry stiffeners were produced with the aid of aquacore supports and flexible silicone cores.

Heat management system
Due to the intricate design of the prototype tool and in order to minimize the measurement errors it was decided to position thermocouples and sensors directly at the surface of the tools. To accomplish that, very thin, Type K temperature sensors were laminated between the heating layers and the surface of the mould. To test this configuration a test plate with such a sensor was build and tested. The result was that the temperature measured by the sensor and by an infrared (thermal) camera and an infrared thermometer were almost the same.
According to the Topic Manager requirements, rather high cooling rates must be achieved. Although the CFRP cools down easily due to its low heat capacity, an active cooling system was developed in order to reach higher cooling rates. The first concept was to utilize a perforated honeycomb core in the construction of the tool which would allow the circulation of cool air beneath the tool surface. The concept has been successfully tried with a test plate which incorporated a perforated aluminum honeycomb core. A second concept for the cooling system was proposed which allows air circulation under the thermal insulation cover. During the heating phase the circulating air will ensure a homogenous temperature distribution. In the cooling mode the system will vent hot air from underneath the insulation cover and thus allow for faster cooling.

Analysis and simulation
The parameters of the heating elements required for the operation of the tool were determined by a preliminary thermal analysis based on calculations of the heat generated due to Joule effect on the carbon fibers.
A more detailed thermal analysis using FEA was performed in order to verify the temperatures developed in the tool. The FE model for this analysis was build based on the initial configuration of the heating elements and the geometry of the tool.
The infusion strategy (location of resin gates and vents) was decided after analysis and simulation of the resin flow during the process.The PAM-RTM software was applied for this task. Fluid flow through porous media is a very demanding problem. The problem is a moving boundary problem and the CV/FEM method is a quasi – steady state approach. The pressure field was solved for each time step for a specific numerical domain. The pressure-field numerical domain is different for every time step. In moving boundary problems the number of elements should be as small as possible. Even the simplest geometrically moving boundary problem requires high computational power. For this reason, a simplification of the geometry was necessary, yet the main dimensions (location and cross sections of the stiffeners, overall size etc) were kept unchanged.

Final design
The system presented is designed for the production of the IRIDA demonstrator part in an one-shot vacuum assisted infusion process. The demonstrator part is representative of an nacelle structure and is essentially a curved panel stiffened by three integrated stringers. As it is a rather complex geometry the tool is designed with the necessary variability in mind. The design allows for different infusion set-ups to be studied.
The complete tool comprised of different parts and components. The tool consists of:
- the main mould with the integrated fibretemp heating system
- the stringer forms with integrated heating systems
- the lid for thermal insulation
- the catwalks necessary for the workers to place the fibres and
- the control unit

Manufacturing of the prototype tool
Following the standard fabrication procedure for self heated tools from mockup to the final infusion, the main body of the nacelle tool was manufactured. The main body consists the tooling surface that will form the outer side of the part. It includes the main heating elements (6 zones) and the backside insulation.
The heating fields were tested and found to operate perfectly producing a very homogeneous temperature distribution in the main heating field with a clear switchover to the colder areas, which is necessary for the placement of the stringers.
Different temperature zones are realised by the different amount of carbon layers. The areas on which the stiffeners are placed later have to be configured colder than the rest of the surface.

Manufacturing of the demonstrator part
After the inspection of the tool and the verification of the good operation of all components, the manufacturing of the demonstrator nacelle part has commenced.
All the fibre reinforcements and other materials (cores, foams etc) that consist the skin of the part were layed on the tool and a preforming step was applied. Then the support, heated cores and other auxiliary tooling features for the forming of the stringers were carefully introduced in the preform. Thermocouples, resin flow and curing sensors were placed in critical areas of the part to allow for the detailed logging of data, necessary for the assessment of the tool operation.
Finally resin was injected and the infusion process was performed. The tools was closed with the thermal insulation lid and the part was allowed to cure following the designated temperature profile.

Inspection of the demonstrator part
The part was shipped to the premises of the University of Patras for the overall inspection and quality assessment.
A series of tests and measurement was performed. More specifically:
1. Visual inspection - Documentation of visible flaws and defects
2. Dimensional accuracy measurements
3. Surface roughness
4. Ultrasonic testing
5. Macrographs of cross sections - Fiber volume fraction
6. DSC - Degree of curing

The Fiber Volume fraction was determined using the Cured Ply Thickness technique. For the areas that were well formed and consolidated the FVf ranged from 40 to 60%. The average surface roughness was about 0.6μm.
The Degree of cure was determined from DSC tests performed on samples taken from the areas of the thermocouples to have a good correlation to the curing temperature. The Degree of cure was found to be over 90% for all the part. A somewhat lower value was obtained for an area where the max temperature was 15-20 degrees lower than the average curing temperature.


Potential Impact:
IRIDA project has presented CSJU a demonstration of an effective and efficient Out-of-Autoclave manufacturing process meeting many of the strict requirements dictated by the topic. Furthermore IRIDA perfomed a step-bystep verification of all the individual features of the manufacturing process. Apart from the delivery of a finished prototype, the most important contribution of IRIDA is the experiences and the lesson learned from the implementation of the workplan which can lead to a roadmap towards the industrialisation of the proposed Out-of-Autoclave manufacturing technique.
In the process of attaining its objectives IRIDA has assessed a series of technologies and has contributed to the advancement of knowledge in several areas like process monitoring and control, analysis and simulation integration techniques for protruding features like stiffeners, thermal cycling durability of CFRP tools, electrode materials etc. All the above present individual opportunities for exploitation.

The most important potential impact of the IRIDA project is the adoption of an energy efficient Out-of Autoclave manufacturing method for composite structures in the aeronautical industry. Although large integrated structures may still remain a challenge, despite the progress achieved in IRIDA, application in less complex components is within reach.
The main benefits for the industry would be the reduction of the manufacturing cost and the compliance with environmental targets. In a global stage where cost efficiency is a vital parameter of economy the main expected benefit of the IRIDA project for the European Aerospace industry is the strengthening of competitiveness in an area where emerging players (e.g. China) already pose a threat to market balance.
Once proven in the demanding field of aeronautics, spill-over to other industrial sectors can comfortably predicted. First of all, the rail and automotive industry which have already adopted several LRI and LCM processes present ground of application for the manufacturing of large structures like vehicle chassis, fairings etc. In these sectors aspects like production cost and amortization of tool cost is very important and the durability of self heated composite tools will be challenged.
The naval-marine industry can also utilise the IRIDA technologies especially in what concerns introduction of advanced material systems that require curing in elevated temperatures. Monitoring and control systems may also improve the performance in this sector. Last but not least in the energy sector, where wind turbine blades of even larger size are developed and infusion is a prerequisite the application of self heated tools can allow the utilization of advanced materials towards the weight and strength optimisation of the blades.

List of Websites:
Contact details

Fibretech Composites GmbH
Tel: +49 (0) 421 30 38 519
Fax:+49 (0) 421 38 01 975
website: http://www.fibretech-composites.de/
Contant persons:
Frank Freyer: frank.freyer@fibretech-composites.de
Jens Brandes: jens.brandes@fibretech-composites.de

Applied Mechanics Laboratory/University of Patras
Tel: +30 2610 969443
Fax: +30 2610 969417
website: http://www.aml.mech.upatras.gr
Contant persons:
Prof. Vassilis Kostopoulos: kostopoulos@mech.upatras.gr
Dr Dimitris Vlachos: vlahos@mech.upatras.gr