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Advanced Preformmanufacturing for industrial LCM-Processes

Final Report Summary - APRIL (Advanced Preformmanufacturing for industrial LCM-Processes)

Executive Summary:

Within project APRIL (Advanced Preform Manufacturing for Industrial LCM-Processes), new promising technologies and materials have been investigated for the manufacturing of carbon fiber reinforced aircraft structures. The research work performed focused on developing a green manufacturing process for aerospace products by introducing ecological and economical preform consolidation techniques for liquid resin infusion processes.
By replacing the currently used, highly energy-consuming and waste-producing autoclave process with textile based component manufacturing, the individual components can be pre-shaped separately first. Afterwards, they are combined into integral preforms by using different joining techniques like 3D-stitching, thermal binder activation or ultrasonic welding before they are infused with a thermoset resin system. This gradual process allows for applying different techniques, depending on what is the most ecological and economical solution for each step.
First, a large series of different binder materials in combination with braided or Non-crimp-fabric semi-finished products have been examined with regards to their impact on the mechanical properties and their applicability for component manufacturing. It was found that just a small amount of the investigated materials meet all requirements by the aircraft industry. However, suitable combinations could be determined.
Moreover, the robot-assisted braiding technology was introduced for the manufacturing of stiffener parts (T-stringers), in order to higher the energy-efficiency of skin panel manufacturing. By performing a mechanical characterization of the introduced materials, it could be proved that braided components are able to compete with NCF materials regarding their mechanical properties for stiffeners. Braiding also showed a high manufacturing efficiency by integrating the binder application directly into the process and requires an extremely low scrap rate of less than 1 %.
For preforming and joining the different parts, three different techniques were used: 3D-stitching, thermal binder activation and ultrasonic binder welding. It was found that the thermal activation method leads to the most accurate results for stiffener preforming, while ultrasonic welding proved to be an extremely low energy-consuming and fast technique for connecting separate sub-preforms and integrate them into large preform sets.
To transfer the investigated technologies into serial production, several manufacturing process scenarios have been evaluated with regards to their economical and ecological potential. The most promising approach for full scale manufacturing was found by combining the various technologies and materials in a way that allows for high volume production with accurate preform dimensions, low energy consumption and waste production. A promising technique for serial production is the continuous and automated draping of stiffener preforms. A prototype construction has already been developed within APRIL and tested successfully.

Project Context and Objectives:
Liquid Composite Moulding (LCM) techniques are becoming more and more interesting for aircraft manufacturers due to their advantages against traditional prepreg-autoclave processes (reduction of waste, energy and waste toxicity and economic benefits).
The two steps that are involved in the LCM process are first the production of a dry fibre preform and secondly the resin injection. The focus of this project is on the development of advanced production methods for dry fibre preforms. Technical requirements for a preform are:
 Geometrical tolerances and dimensional stability
 Near-net-shape
 Geometrical complexity to form integral shapes
 Possibility to combine multiple sub-preforms to bigger preforms
 No negative influence on the permeability
The currently available consolidation techniques are stitching and bindering. Stitching techniques are low energy consumption, but are limited to non complex shapes. Novel stitching processes have been developed that enable the processing of 3D preforms by means of one side stitching techniques. Further optimization is needed to enable the processing of complex, integral structures. The consolidation of very complex preforms is possible using bindering techniques. However, bindering shows environmental drawbacks, mainly contamination due to organic volatiles and heating energy, which is needed for the binder activation. Thus, a need for improvement is clear.
For the environmental and economic improvement of the consolidation techniques new techniques have been developed based, on one hand, on novel 3D robotic stitching and, on the other hand, on the use of low temperature activation thermoplastic veils and ultrasonic binder. A demonstration phase followed, consisting of the manufacturing of differently scaled preforms representing skins and stringer sections. Therefore, the braiding technology has also been used to manufacture integral and cost-effective parts. Impregnation tests were performed to evaluate the permeability of the obtained preforms. The objective was to be able to scale the techniques to automated serial manufacturing of big preforms (up to 8x3 m).
The project outline consists of a first review on state-of-the-art technology, material screening and a manufacturing approach (WP1), followed by the development and production of a first small scale preform demonstrator, which will be finished at the end of WP1. Since the design of the demonstrator parts depends on the manufacturing feasibility of the used technologies, it cannot be fixed from the beginning of the project on. Therefore, a milestone was included at the end of month 3 of the project, where the final design of the small scale demonstrator was chosen. The liquid resin infusion (LRI) tests for the different dry fiber textile preforms have been performed by IAI, while the preform production itself was performed by the consortium (USTUTT & Tecnalia). The production of a small scale demonstrator aims at investigating and testing all proposed technologies, in order to estimate their quality in terms of manufacturing feasibility, serial production and ecological benefits. On the basis of the collected experience within WP1, a full scale demonstrator has been developed during WP2.
After demonstrator manufacturing, the used technologies have been investigated with regards to their economical and ecological impact on full serial production within WP3 by comparing the energy consumption and waste production of the different technologies.

Project Results:
Within the first work package, several trade-offs have been performed to screen the materials and technologies and to find the most suitable approach for demonstrator manufacturing.
The following material tests have been performed:
• ILLS tests before and after a hot-wet treatment in boiling water
• Tg characterization with a Dynamic Mechanical Analysis (DMA)
• Material characterization of braid with a high ratio of axial yarns
Moreover, the following technologies have been technically tested and evaluated:
• Thermal binder activation
• 3D-stitching
• Ultrasonic welding
• Braiding and binder integration into braiding process
The binder vs. stitching trade-off shows a tendency towards the following aspects:
• Stitching has a relatively low impact on the environment (no heat transfer necessary), but is disadvantageous in terms of productivity and geometrical flexibility. An advantage regarding ecological manufacturing is that no contaminants are emitted during the stitching process. The low result for stitching as a consolidating technique shows, that due to its low productivity it might not be the preferred option.
• Binder application is more flexible, as the part does not have to be accessible in order to melt the material. Depending on the type of binder used, the energy consumption can be higher due to the applied heat.
• Except for the stitching technique, all applications show a low technological maturity. The consortium still has to investigate if the different solutions are suitable.
Especially in terms of energy consumption, a quantitative analysis has to be elaborated in the course of the project. The consortium will therefore collect data that allows for calculating an overall environmental impact of the technologies.
In order to select the most suitable binder material of interest for the research, a market and literature survey has been carried out among the different materials suppliers. Afterwards, the most promising materials were selected for preliminary testing, which included preforming and shaping trials, ILSS tests with and without hot-wet treatment and thermal DMA testing. In case of the thermal material tests, the results show that for most materials tested, a high Tg-level is reached. Certain materials showed a strong hot-wet knock-down and have been therefore eliminated from further processes. The materials with a sufficient glass transition temperature before and after hot-wet treatment were accepted for demonstrator manufacturing.
Once the preforming trials and the ILSS tests had also been completed, the following binder materials were chosen for further demonstrator manufacturing: PA1541 (veil option), Epikure 05311(precoated fabric option) and Grilon phenoxy yarn (braiding option).
In case of the braided material, it was necessary to obtain the mechanical properties of this new material, as conventional braids do not have a suffcient Young’s modulus for stiffener parts. Within APRIL, new braid configurations were chosen and tested with regards to the requirements for aircraft stiffener parts. Two different braid setups with a high zero degree yarn ratio have been tested and showed a sufficient E-Modulus in axial direction, which was the main criteria for stiffener components. The braids have therefore been included in the further demonstrator manufacturing up to the final stage of the full scale demonstrator.
Within the next work package, preforming tests with 3D-stitching, thermal binder activation and ultrasonic binder welding were performed on small scale demonstrators to determine the most suitable technologies for full scale manufacturing.
The application of 3D-stitching for stiffener preforming showed that the T-stringer can be shaped with a tufting needle. However, only a low preform compaction could be achieved, which is a disadvantage in comparison to processes in which binder material is used for shaping. This led to the decision to focus the work on processes in which binder material is used for shaping the preform.
The whole preform process of the T-stringer was also realized by using thermal binder activation and ultrasonic binder welding. Thermal binder activation proved to be the most reproducible and precise process regarding preform dimensions. Ultrasonic welding can also be used for shaping, but is more advantageous with regards to preform integration, as it is an extremely fast and energy efficient technique.
After testing the different technologies and materials on a small scale, the final demonstrators were produced. Due to the large amount of investigated techniques only the most promising approaches have been used for full scale demonstrator manufacturing. In this case, further work focused on thermal binder activation and ultrasonic welding, while 3D-stitching has been eliminated from the full scale work scope. The reason for this is that small scale results showed a lack of technological maturity of the stitching technique for preforming purposes, which was important for the demanded technology readiness level.
As the evaluation of the required process for T-stringer preforming showed, ultrasonic welding only uses a fraction of the thermal activation method. The reason for this is that the thermally activated preforms are heated up over the whole preform area via heat transmission, while the ultrasonic sonotrode only heats up a very small area in short time by friction. Moreover, the energy is generated directly into the preform and does not have to be transferred over a heated tool over a long time. Therefore, ultrasonic welding is very efficient with regards to a process which aims at low energy consumption.
The NCF material has been prepared with an automated CNC-cutter, which allows for accurately cutting the layers that are stacked and shaped into the preforms. Scrap rates were calculated on the basis of the used cut-drawings. Due to the different cut dimensions for stringers and skin, the scrap rates are not the same.
In case of the braided preforms, rovings have been winded onto spools and fixed on the braided machine. As the braid is directly shaped into the T-stringer, there is no scrap resulting from any cutting. The only scrap generated results from some extra roving material on the spool to initiate the braiding process.
For all preform types, the scrap rates are relatively low (< 20%). NCF stringer cuts generated about 7% scrap in average, the NCF skin about 19%. The lowest scrap rate is achieved with braiding (about 1 %).
The processes with project APrIL were technically evaluated by using the VDI guideline 2225 from the Association of German Engineers. The evaluation process is as follows: First, certain criteria are weighted against each other in order to highlight the importance of specific process attributes. These criteria aim at covering all important aspects of serial production:

• Preform-process time
• Process cost
• Technological maturity
• Reproducibility
• Automation potential
• Energy consumption & waste production

Moreover, the following combinations of materials and technologies have been evaluated separately with respect to the previously determined criteria:
• Stringer preforming with thermal binder activation, ultrasonic welding and stitching
• Stringer materials (NCF and braids)
• Skin preforming with thermal binder activation, ultrasonic welding and stitching
• Assembly of stringers and skin with thermal binder activation, ultrasonic welding and stitching
The results showed, that in case of stringer preforming, the thermal binder activation is the most suitable technique as it allows for an accurate geometrical design of the preform and a high reproducibility. Also for skin preforming, thermal binder activation is the best process for serial production. However, ultrasonic welding proved to be extremely efficient for the assembly of stringers and skin, as it only requires a very low amount of energy and time to activate the binder and connect the separate preforms. Regarding the different material solutions for stringers, NCF and braids, the braiding process showed some advantages due to its high automation potential because there is less waste produced. The NCF solution is more suitable with regards to the mechanical properties of the part and short process-times as no winding/braiding machine configuration is required. However, since the result also depends on the criteria weighting, it can be concluded that both solutions have equal benefits in this case. The decision rather depends on the weighting of the criteria for full scale serial production.
In order to be able to transfer the technologies tested into serial production, an automated draping device for T-stringer preforming has been developed within APRIL. It enables the continuous folding of flat preforms into a T-shape. The device can be used for both NCF and braided materials and works by using a heated tool through which the preform is pulled by electric roll drives. It also allows for the use of various preform thicknesses and binder materials.

Potential Impact:
Research work performed within the scope of the APRIL project refers to a green product life cycle that is one of the main goals within CleanSky, especially on green design and manufacturing techniques.
The results show that a significant reduction of energy in the production of aeronautic composite structures compared to prepreg autoclave technology and state-of-the art preforming techniques can be achieved by using the investigated technologies.
Moreover, the low scrap and energy rates produced would lead to a substantial reduction of environmental pollution and health hazards for production staff in the processing of binder materials.
With regards to an economical process, a lead time reduction resulting from improved consolidation strategies and processes can be achieved. This is due to the fact that all technologies investigated can be automated to a high degree, which will also lead to a high reproducibility of the manufacturing process.
Thus the primary impact of this project is green manufacturing of aerospace products by introducing ecological and economic preform consolidation processes for LRI processes.
In a wider perspective, APRiL also responds to the societal needs through a reduction of fuel consumption and emissions by introducing more lightweight components to aircrafts due to more economic production processes of composite materials. Moreover, it contributes to improved mobility of European Community citizens without compromising comfort and safety.
By implementing the new technologies into large scale manufacturing processes, the project results are able to strengthen the competitiveness of aircraft industry and related SME’s to put Europe to the leading edge in dry fibre textiles and by this to secure jobs in this market segment.
The project results have been presented on one of Germany’s most renowned conferences SAMPE (Society for the Advancement of Material and Process Engineering) in Hamburg 2013. APRIL will also be presented on the 4th International Aerospace Meeting AEROTRENDS in Bilbao. Moreover the partners will seek further collaboration and strengthen their corporate activities, as many of the investigated technologies have to be developed further in order to be fully implemented into large scale serial applications.

List of Websites:
Institute of Aircraft Design, University of Stuttgart (USTUTT)
Pfaffenwaldring 31
70569 Stuttgart, Germany
www.ifb.uni-stuttgart.de

Frieder Heieck (Project Coordinator)
E-Mail: heieck@ifb.uni-stuttgart.de
Phone: +49-711 68561995

Martina Bulat
bulat@ifb.uni-stuttgart.de
Phone: +49-711 68560247

Tecnalia Reserach and Innovation (Tecnalia)
Transport & Industry Division
Mikeletegi Pasealekua, 2
20009 - Donostia-San Sebastian, Spain
www.tecnalia.com

Ricardo Mezzacasa
ricardo.mezzacasa@tecnalia.com
Phone: +34-946 430850

Miguel Segura
miguel.segura@tecnalia.com
Phone: +34 943 105115

Israel Aerospace Industries (IAI)
Engineering Division
Ben Gurion International Airport
70100, Israel

Yaniv Yurovitch (Topic Manager)
yyurovit@iai.co.il
Phone: +972 3 935 5109