Investigations of liquid resin impregnation and out-of-autoclave curing of composites for the high temperature aerospace applications

Final Report Summary - LRI-HIT (Investigations of liquid resin impregnation and out-of-autoclave curing of composites for the high temperature aerospace applications)

Executive Summary:
The objective of LRI-HiT is to primarily investigate and evaluate the producibility of composite laminates using vacuum assisted process, a resin system that provides high thermo-mechanical properties, and Out-of Autoclave (OoA) QUICKSTEP curing. After the proven technological integration of processes a part is manufactured to demonstrate the technological solution’s readiness at TRL 5-6.
For the combination of OoA process design, existing heated tool technologies and Quickstep’s flexible membrane concepts are evaluated. Considering the resin infusion and fast curing possibilities, the heated tool design and quickstep processing methods were modified. As an output, rigid, heated mould tool that gives a final shape to the part and flexible membrane tool that exerts pressure and temperature during the processing is available. This will help to improve the temperature distribution over the entire tool surface as well as rapid heat transfer to reduce the processing time.
The integrated processes have been tested for their functionality and understood the process implications before manufacturing a demonstrator part. Upon successful initial tests, a 3D, T-stiffened representative part was manufactured which resembles the structure of the section-19 of an aircraft.
High temperature resistance resins operating temperature in the range of 200 °C were screened, tested for their performance and selected for the final demonstrator manufacturing. The goal of selection of a resin system that is less complicated and good for the infusion process has been achieved.
Process integration of rigid heated tool, matching, flexible Quickstep membrane, liquid resin infusion and fast heating concept has been successfully completed and the validation components are manufactured. A full scale demonstrator has also been infused and cured using the newly developed technology. Materials and energy required for the demonstrator manufacturing were well tracked and documented for the purpose of assessing environmental impact and advantages over the state of the art processing.
The developed curing technology is available for the further manufacturing trials as well as for the research activities at Quickstep GmbH.
Project Context and Objectives:
The objective of LRI-HiT was to primarily investigate and evaluate the producibility of composite laminates using vacuum assisted resin infusion process, resin system that provides higher themo-mechanical properties, and OoA curing. The partners gathered know-how of OoA process, resin materials, preforming, tooling and infusion strategies, and evaluated the components made out of the chosen technologies in order to address the objectives of the project:
1. Development of manufacturing concepts for environment friendly and time efficient composite processing
2. Advancement of the energy efficient and cost-effective Quickstep (OoA) process
3. Manufacture of a part to demonstrate one of the processes chosen from first objective (via vacuum assisted resin infusion (VARI) of preform and OoA curing) and process monitoring
4. Develop concepts for the automation of chosen technology
Detailed objectives of individual tasks were:
Screenign and selection of materials and processes)
The objective was to select the fabric and resin materials from the available pool of materials (some of them are already qualified for the Aerospace) for the manufacture of composites and to optimize its processing conditions. Thermosetting resin (Cyanate ester) and woven aerospace qualified 2x2 twill fabric (285 gsm) were selected and near-net shape T-preforms were manufactured using same woven fabric. All the selected materials were converted into composite laminate and delivered to complete the objectives of WP2.

Testing and evaluation of laminates
Manufacture of coupons (from flat laminate as well as representative article) according to the selected materials and processes and its testing and evaluation were the main objectives of this work package. Coupons extracted from flat laminates (2 mm and 4 mm) are tested for dry and wet glass transition temperature, Interlaminar shear stress and T-stiffened laminate was tested for compression after impact.

Tooling and process investigations
This task has objectives of on development of tooling concepts and advancement of OoA technology, especially novel aspects of heated tools and Quickstep fluid distribution and membrane technology. Thin walled, stiff metallic tooling and heated tool concepts are converted into demonstrator tool.
An energy efficient, matching system for Quickstep was selected and developed further for new OoA concept. Tooling for the T profiles to make T-stringers was also developed. Flexible, 3D, net shape membrane was designed and developed for the Quickstep fast curing technology. As a result, there combined processes are tested for their final use.

Validation of selected process
The objective was to validate breakthrough improvements in preforming process, tooling concepts and processing considerations to develop a final fibre reinforced composites and in order to evaluate and solve some demonstrator difficulties, it was essential to manufacture validation elements. The main objectives of the Work package are divided into: definition of validation elements in the light of other WP results, development of cost-effective preforms and manufacturing and testing of those elements for validation of technology and processes.
A T-stiffened laminate (2 mm base and 6 mm stiffener) was manufactured using pre-selected technologies which have validated the processing capabilities and manufacturing concept.

Manufacturing of representative article (demonstrator)
Primary objective of this task was to develop a representative part of aircraft components for the demonstration use and to validate the materials and the processes developed in the previously mentioned tasks.
A conical shaped, T-stiffened demonstrator part representing structural part in the vicinity of auxiliary power unit (APU) is manufacture and it will be used to disseminate the results of the project by presenting them in technical shows or congress.
Project Results:
Material selection (QSG, Topic manager)

For the first stage of material definition and testing various high temperature resins wee screened based on the market available products. Henkel and Lonza provided support of initial screening of benzoxazine(BZ) and cyanate ester (CE) resins respectively. Additionally, bismaleimide (BMI) and polyurathane (PU) resins were also considered as high temperature options.

As a next step, product datasheet (PDS) and material safety datasheet (MSDS) of each resin was screened and analysed for its processing possibilities and safety needs. A differential scanning calorimetry (DSC) instrument from Mettler Toledo was used to analyse thermal characteristics as well as to understand the cure kinetics of the resins. Understanding a trend of dry glass transition temperature (Tg) of cured resin was also the aim of the DSC trials. Based on these results and parallel work on resin blending, resin infusion trials of dry fabric stack were conducted to manufacture the composite laminates for testing. Resin selection to laminate manufacturing work was performed by QSG.

During the stages of this process flow, a constant communication with the Topic manager (TM) was there to ensure that there is no duplicity of work and the project is on the right track to achieve set goals.

Resins available for high temperature applications (infusion grade)
Benzoxazine
Being related to the chemistry of phenolic resins, the benzoxazines offer excellent flame retardancy and very little shrinkage. In contrast to phenolic resins, they cure without elimination of volatiles and show by far better properties of the cured material. The benzoxazines’ decline in mechanical values due to moisture is less pronounced in comparison to the bismaleimides. New efforts in order to make the resins tougher by two-phase modification have shown noticeable success and have dry Tg above 250 °C and wet Tg in the range of 200 °C with GIC values of 350 J/m2. (www.henkel.com).
Cyanate esters
Proven in mars vehicles, satellite antennae and military aerospace components, cyanate esters (CE) are about to find their way into industrial application. They are also favoured because of their low moisture uptake and good dielectric properties (www.lonza.com www.tencate.com). The pot lives are at several hours at 80 °C; even at 200 °C they can reach more than 20 min, depending on the formulation. This type of resins is used for very high temperature applications (Tg: 250-400 °C).
Bismaleimide(BMI)
Technically, bismaleimides also belong to the polyimides, but they are comparably reactive and cure at 150-250 °C in an addition reaction. Military aerospace has used these resins for a long time, e.g. for wing and stabilizer spars, fuselage stiffeners and engine components. They keep their excellent properties up to about 250 °C, exhibit high chemical resistance and good fire properties (www.evonik.de www.cytec.com www.hexcel.com). Normally, the systems are one-component and have to be stored refrigerated.
Systems based on urethanes
The polyurethanes possess properties which are lacked by polyester and epoxy resins. They are less brittle and polymerize quickly. Polyurethanes can be cured below 180 °C and still gets a Tg of above 250 °C. Besides typical PU manufacturing methods such as resin injection moulding (RIM) and fibre spray there are basic approaches towards LCM processing of polyurethane systems that exhibit pot life of more than 5 minutes (www.bayer.de). However, due to the small pot life window for resin infusion, a risk of insufficient fibre wet-out as well as foaming of the resin still exists.

Laminate manufacturing (QSG)

According to the DSC analysis of resins for the fast curing and manufacturer’s recommendations for the standard cure cycle, a Quickstep cure cycle was set and tested on a Quickstep machine. All the gathered information was recorded and analysed for the fine tuning. For the consumable optimization, standard consumable stack was used at the initial stage (release film/agent-preform-perforated film-peel ply-bleeder/resin distribution media -vacuum film). Double vacuum bagging system was used to ensure the continued vacuum pressure during the whole processing cycle of preheating, infusion and curing. Stacking was documented in the form of sketches and photographs.

An aerospace qualified bi-axial Non Crimp Fabric (NCF) is chosen for the manufacturing of laminates. Areal weight of the fabric is 560 g/m2 and expected ply thickness of 0.5 mm. A fabric stack of [+45/-45]4 is used to get a 2 mm thick laminate for initial destructive testing and non-destructive testing (NDT). Vacuum assisted resin infusion (VARI) technique was used for the manufacture of laminates. A free standing post cure was done for all the laminates according to the requirements.

Material definition made based on initial trials
• Defined reinforcement fabric: Hexforce®G0986 (HS carbon fibre, 2x2 twill, 286 g/m2, EP01 binder)
Reasons:
1. NCF is not suitable for above 260 °C post-cured resins
2. Hexforce® G0986 is aerospace qualified material already used in various Airbus programs
3. E01 binder is compatible with cyanate ester resin (topic managers know-how from CfP project “Specimen”)
4. 2x2 twill fabric is good for preforming of 3D shaped parts.
• Selected preforming option for T-Stringer: Quickstep supported binder activated preforming
Reasons:
1. QSG has experience on fast binder activation

Testing and evaluation of laminates (QSG, NCCEF)

Materials and process used to manufacture coupons
An aerospace qualified bi-axial Non Crimp Fabric (NCF) was chosen for the manufacture of laminates. Areal weight of the fabric is 560 g/m2 and expected ply thickness of 0.5 mm. A fabric stack of [+45/-45]4 was used to get a 2 mm thick laminate for initial testing. Vacuum assisted resin infusion (VARI) technique is being used for the manufacture of laminates.

Ultrasonic through transmission C-scan examination, inter-laminar shear strength and Tg measurements have been carried on test coupons received from QSG.

Panel manufactured using CE320x and PES film was found to be of poor quality with significant voids or delaminations when examined by C-scanning. Panel 8 (DT7000+RTM6-2A) showed also some smaller void areas. The other panels appeared to be of relatively good quality.

Interlaminar hear stress (ILSS) tests at room temperature in the dry condition indicated that Panels 5 and 6 had the highest inter-laminar shear strengths.

Panel 6 also exhibited the highest Tg values after boiling water immersion or long time exposure to a combination of high temperature and humidity.

Testing and evaluation of 3D-laminates
Impact test
Drop weight impact test was performed to analyse the VE. Impact testing was performed considering ASTM D5628. A 6 mm ball tip and 50 J energy was used for impact. To compare quantitatively the relative values of the damaged zones, NDT analysis is performed.
Non destructive testing
For NDT testing the following parameters were used:
• Testing probes used for 1 MHz frequency;
• Scan rate: 200 mm/s;
• Raster pitch 0.2 mm;
• Attenuation range (grey scale): 0-40dB;
• Panel ID label at top left of images;
• Impact locations 1-5 marked on image.

Impacts 1 and 2: pulse echo ultrasonic C-scan and B-scan results are monitored and documented. Impact 1 was on the left hand side and Impact 2 was on the right hand side of the laminate.

Delamination along the full length of the sample and extending 40 mm across the width of the T-joint was visible. B-scan of the laminate suggests through thickness plot depth of strongest reflection along the line which is also visible in a C-scan. Delamination at base of the foot of the T-joint is 3 mm.
Delamination along the full length of the sample and extending 50 mm across the width of the T-joint was clearly visible after the testing. B-scan shows through thickness plot depth of strongest reflection along the line shown in C-scan. Delamination at base of the foot of T-joint is 3 mm.

Tooling and process investigations (QSG, APLEX)

Development of thin walled and heated (optional) tooling concept that can be adapted to Quickstep fluid curing and vice-versa

Part design
The part design set for the demonstrator was a double curved skin stiffened with stringers. Part design was considered for the post operations and part trimming to the final size:
• The manufacturing part is 40 mm wider in each direction than the trimmed part.
• Manufacturing part stringer ends 20 mm from each end.
• Stringer slope (up to 45°) done by milling.
• Manufactured stringers milling allowances of 5 mm.

Definition of demonstrator tooling
Proposed LRI-HiT tooling includes on one side a heated tool which acts as a laminating tool as well as source of heat. On the other side a heated flexible membrane exerts heat and additional pressure on the laminate. This system allows double sided, controlled heating of the laminate with high ramp-up rates (8-15 K/min) and additional pressure for the laminate consolidation. In the following sections the metallic heated tool and concept of flexible preformed membrane are described that were implemented in this project.

Metallic heated tool concepts and manufacturing strategies
The demonstrator tooling was made of two parts. Part one was a non-heated aluminium structure which contains the tooling utilities, e.g. vacuum connections, resin channels, stringer positioning points, tooling handling, heat insulation, sensor placements, etc.
Part two was a separate heating system integrated aluminium sub-tool and channels were milled into it for the fluid circulation. Heating and cooling down of the tool was done by Quickstep fluid. Additional electric heating was also considered as an alternate heating source; however, this was not possible to integrate within the project period.

Based on the aforementioned concept, detailed part manufacturing was done according to the following steps:
1. Tool cleaning and release agent application
2. Positioning of skin layers and stiffener layers
3. Assembly of stringer preforms in stringer moulds
4. Stringer positioning inside the mould via locator pins
5. Vacuum bagging
6. Vacuum application at vacuum channels
7. Infusion using VAP
8. Curing
9. Demoulding
10. Cleaning

Quickstep membrane concept
The Quickstep curing system was used from the laminate top (above the rigid metallic tool) as a source of heating and cooling. The preformed flexible membrane was used to transfer heat to cure the laminate.

The preformed membrane was used to transfer heat to the laminate and exert pressure on the laminate. Based on the final geometry of the metallic heated tool (including stiffener and stringer-positioning tool), drawings for the membrane were developed. Dimensional offset was considered for the free positioning of membrane in closed Quickstep chamber. Elastomeric material used for the manufacturing of membrane has very high elongation at break (800 %) which allowed good consolidation of the laminate at given fluid pressure applied by the Quickstep curing system.

Investigation of process and manufacturing concepts

Integration of Quickstep and heated tool
Taking into account the tool heating capacity, insulation, stringer geometry and flat quickstep membrane, an initial Quickstep-heated tool integration trial was performed using Quickstep chamber from the top side and heated mould tool on the bottom side. Both heating systems were connected to the Quickstep fluid heating system.
Temperature monitoring
During the tool integration trials, temperature sensors were placed on the tool surface, on the stringer and beneath the stringer. This way complete understanding of temperature profile was possible.

Validation of selected process (QSG)

Detailed definition and development of validation elements

Geometry of validation element
Geometry of the validation element was chosen from the actual final demonstrator geometry. Two T-stringers attached to the skin is a basic concept for it. Generic concept of the validation element and required tooling for the same were documented. ALPEX manufactured the required metallic tool.

Preform integration

Performing
After the initial Quickstep and heated tool integration trials, another full scale trial was performed to manufacture a complete skin-stiffener assembled preform. This trial also helped to see the repetitiveness of the process and process stability for the infusion trial.
Lay-up selected for the laminate manufacturing
Skin
The skin has an overall thickness of 2 mm except of an area which has a thickness of 4 mm. The taper was adjusted to 1:20 ~ 2.3°.
Material and fibre orientation:
Hexcel Injectex® G0986 (bindered), Area weight 285 g/m2, Cured ply thickness: 0.29 mm
Layup for skin: [0°/90°; +45°/-45°; 0°/90°; +45°/-45°; 0°/90°; +45°/-45°; 0°/90°]
In the thicker area the layup was used twice with a taper of 6 mm between each step of layers.
Stringers
The demonstration part was stiffened with 6 stringers and two of them were 2/3rd of the full length demonstrator. The stringers were positioned 90° to the skin and have a thickness of 6 mm and a height of 40 mm after trimming.
The material for the stringers was the same as one used for the skin. The preform for the stringers consisted of 2 L-Preforms with a symmetrical layup: [0°/90°; +45°/-45°; 0°/90°; +45°/-45°; 0°/90°]S.

Representative article (demonstrator) (QSG)
A representative article was designed and manufactured according to the inputs provided by the topic manager and constant communication between the TM and involved partners for the part design, tool design, tool manufacturing and part manufacturing.
Preform manufacturing was done for skin and stiffeners of the demonstrator part with following particulars:
• Preforms for Stiffeners:
• T-Stiffeners: 4 x full length with lay-up [0/90,+45/-45,0/90, +45/-45,0/90]s
• 2 x 2/3rd of full length
• Skin layers with lay-up [0/90,+45/-45,0/90,+45/-45,0/90,+45/-45,0/90]
• Patched section with lay up same as the skin layers and interleaved in the skin layers
• Preform assembly
• Skin layers with the patch section
• Skin assembly with T-Stinger preforms

Preform assembly
On the top of the skin layers, six T-preforms were placed with the help of stringer tools and positioning bolts. This preform-tool assembly was then vacuum bagged and heat treated to get an assembled skin-stiffener preform ready for infusion. A preform assembly step was incorporated here to demonstrate preformability using the combined heated tool and quickstep system.

Resin infusion and curing
In total four full scale preform infusion trials were conducted and each trial was based on different infusion strategy as per the lessons learned. Trial 1 and trial 3 were conducted using CE resin, Trial 2 and trial 4 using BZ resin. This section gives information about the Trial 4 which is the successful trial to manufacture a demonstrator part.

The assembled preform was weighed before further manufacturing steps in order to calculate the resin required as well as to calculate the theoretical fibre volume content after the laminate curing. The preform was then loaded on the tool and all the stiffener supporting tools were mounted.
To assure the complete resin infusion, a vacuum assisted process (VAP) technique was used. TM gave approval for this processing step and provided a Goretex® membrane as a key consumable for the VAP process.

The preform and tool assembly was bagged using Goretex® membrane. Resin inlet lines, resin flow / distribution medium and other consumables were placed according to the standard resin infusion practices.

The mould tool with the double bag assembly was placed under the Quickstep chamber and pre-heated to 130 ± 5 °C. The resin was preheated in the small oven, at 110 °C. 100 mbar vacuum was applied and the complete lay-up was evacuated. The resin line was then connected and the resin was infused into the reinforcement using a distribution medium. Resin flow was confirmed by monitoring the weighing pan which showed the reduction in the resin weight as infusion progresses. When the pre-calculated amount of resin was infused, the resin inlet port was closed and full vacuum was applied to the outer bag. Total 3650 g resin was infused.

The temperature was then increased to 185 °C at 1-3 °C/min, and held at 185± 5 °C for 120 min. A free standing post cure was then performed by placing the specimen in an oven for 60 min at 232 °C.

After the laminate curing, the cured part was demoulded carefully to avoid any damage to the part. All the demoulding steps are photo documented for further studies if necessary. The part is then cleaned for rest of the peel-ply, tacky tape and other consumables attached to it. The final demonstrator part before trimming and post operations was photo-documented. Data based on the manufacturing of a demonstrator part is supplied to TM for the product life cycle analysis (LCA).

Coordination and Management (QSG)

The objectives of this WP were to coordinate, monitor and ensure the project reporting. Administrative and financial management of the project are performed as well within this work package. The coordinator has been the key player in communication between the partners, the topic manager and the European Commission Project Officer.

The main activities are listed below:
• Information and support towards partners regarding legal, administrative and financial issues,
• Bi-weekly teleconference with the topic manager for technical and management activities,
• Implementation of communication tools,
• Monitoring of compliance by the parties with their obligations,
• Collection, review and submission of reports and deliverables (including financial statements and related certification) to the EC,
• Preparation of agendas of General Assemblies and Executive meetings,
• Chair of these letters, preparation of minutes and at last monitoring of the implementation of decisions,
• Transmission of documents and information between the Parties,
• Administration of the EC funds,
• Providing the parties with certified true copies of any document (upon request).
•
The coordinator organized administrative meetings before or after technical meetings, when it was required. He also provided all the necessary information related to administrative actions and financial guidelines and obligations to the partners, when necessary at the meetings.

Potential Impact:
International passenger demand grew by 5.4% in 2013 compared to 2012 with all regions reporting growth. There is a traffic rise 3.8% in Europe in 2013 compared to 2012. According to Air transport action group, in 2013, over 3 billion passengers were carried by the world's airlines. Worldwide, flights produced 705 million tonnes of CO2 in 2013. Considering the forecast of airtrafic predicted by Eurocontrol, Europe has 14.4 millions of flights in 2035, 50 % more than 2012.

The LRI-HiT project has intended to provide/contribute to an effective answer to the issue of global energy/fuel consumption, proposing technologies to lower down carbon footprint, planes global weight, thus global fuel consumption. To address this challenge, out-of-autoclave process was modified to manufacture composites structures dedicated to replace already existing aircraft metallic parts in the vicinity of power units. Another major tool to address this challenge is the use of techniques which can allow suppressing a number of metallic items which contribute to airplanes global weight thanks to highly integrated parts. At the end of the project, partners gathered expertise gained through the development – as proof of concept – a representative aircraft part of section 19 is manufactured.

To correlate societal implications to a more scientific field, LRI-HiT studied different high temperature resin options that are environmentally friendly and easy to process with reduction in wastage for high temperature aircraft components. This was one of project’s targets and is a major advancement as future researchers will save time as they will have at disposition tools to go a step further than the project. On that first basis, LRI-HiT developments will largely serve community in the future.

More strategically, it was identified at the beginning of the project that there are worldwide ongoing investigations on replacement of high temperature metallic / ceramic parts as one of their priorities in their research programs. Although many efforts were made at the European level to enhance competitiveness and niche research, combined efforts on environment friendliness, cost effectiveness and new processing technologies were missing. LRI-HiT, by integrating two OoA technologies and preform resin infusion process fills in the gaps and links laboratory research and industry. Selection of technologies was drastically led and took into account feasibility of a larger scale manufacturing, to appeal industrials and make them consider concretely the use of these processes and materials. Steps to follow to a possible qualification of these materials were considered as well.

The spread of use of innovative processes and their combination with dry fibre preforms results in time saving and part weight saving, which lead to lower fuel consumption due to diminution of aircraft parts weight and reduction of industrial production steps (less energy required as less parts produced separately). Impacts on society will thus be the development of new industries, producing these advanced processes, thus, employments will be created. Regarding the reduction of production costs and costs of use of aircraft vehicles, this will reduce the operational costs of aircraft. Furthermore, it can be highlighted that lower carbon emission targeted are reached for products developed in the frame of the project is in line with European policy objectives.

The LRI-Hit project allows the development of a strong European expertise in both composites materials processing and innovative materials development which propels European citizens at the edge of innovation and expertise in these fields. This will guarantee to maintain high added value parts manufactured in Europe, ensuring an effective competitiveness.

Detailed exploitable foreground foreseen

Cost effective cowl doors:
• Purpose is to improve the manufacturing cost and the performances (high working temperature) of a shell stiffened fan cowl door for an aeroplane using the Quickstep and heated tool process.
• Depending of the result of the demonstrator manufacturing along with the material work packages results, Airbus and QSG could develop a door offer with some cost effective stiffener preforms and/or tooling improvements providing a competitive advantage compared to the existing product by around 2020.
• If the demonstrator results are deemed valuable, a patent could be initiated around late 2015.
• On top of the LRI-HiT demonstrator, some additional testing can be done to fully quantify the results & improvements. These activities could be shared between QSG, ALPEX and Airbus.
• If impact results in capturing a new cowl door project on new airliner, it could generate a 5 to 15 M€ turnover by 2025 for Quickstep.

Cost effective primary structural parts:
• Purpose is to improve the manufacturing cost and the performances of aircraft primary structural components in the vicinity of engine, power units and auxiliary power units (APU) using new Quickstep-Heated tool-LRI process.
• Depending of the result of the demonstrator manufacturing along with the material work packages results, QSG could develop generic components and could develop an offer for the production equipment (tool, Quickstep machine and injection system) for such application.

Industrial solution of new OoA process
• Purpose is to improve the manufacturing process developed in the LRI-Hit Project. Quickstep curing process in combination with heated tooling will be further improved for the serial production of high end composites in aerospace, automobile and railway applications.
• Upon further screening of potential improvements QSG and ALPEX will develop their core technologies and provide an energy efficient solution of Out of Autoclave (OoA) composite processing.
• If impact results in capturing a new equipment sales, it could generate a 10 to 15 M€ turnover by 2025 for Quickstep and Alpex.
List of Websites:
Contact details:
Dr.-Ing. Amol Ogale
Rolf-Engel-Strasse 6.5
85521 Ottobrunn
Germany

Final Report Summary - LRI-HIT (Investigations of liquid resin impregnation and out-of-autoclave curing of composites for the high temperature aerospace applications)

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