Final Report Summary - EXPECT (Examination of Practical Aspects of Innovative Bonded Composite Repair Techniques)
Executive Summary:
Even though adhesively bonded composite repairs exhibit significant advantages in terms of mechanical efficiency compared to mechanically fastened ones, they are at the same time extremely sensitive to process parameters variations. Small deviations against the repair specifications (e.g. surface preparation) may lead to disproportionally larger consequences to the final mechanical performance of the repair and, consequently, to the integrity of the structure. Moreover, the inevitable differences between laboratory and repair shop conditions, may lead to the cancellation of promising technological achievements for “practical” reasons, because the required conditions could not be easily met in standard practice.
The scope of this project is to examine from a practical application point of view the innovations achieved within GRA Task 1.3.6-02 for bonded composite repairs, in order to facilitate their transfer to production conditions. For this reason, the whole chain of process steps which have to be followed for the performance of a typical repair will be executed in standard repair shop conditions, in terms of facilities, equipment, tooling and personnel training. Potential contradictions of innovations developed by the GRA to standard repair practices will be identified and alternative solutions will be proposed, to facilitate their efficient transfer to production standards and requirements.
More specifically, EXPECT will focus on estimating the potential of vacuum sand blasting as a surface pre-treatment method in composite aircraft repair. Vacuum sand blasting is an abrasive method like grinding, but it may be that it will provide better process reliability than a manual grinding process. Compared to conventional sand blasting, it takes care that the blasted material will not pollute the surroundings.
A surface pre-treatment with a vacuum sand blasting permits to replace the conventional manual grinding, improves bonding strength and permits to remove the eventual surface contamination. Its big advantage in respect to conventional sand blasting methods is the possibility to perform the treatments without the need for any special protective equipment. Thanks to the system design, blasting dust cannot escape even in case of leaks or other defects. The attractiveness to use it in repair process is based on a possibility of solvent and water-free treatment that could be done automatically, without hand contact with a repair zone.
After theoretical analysis and calculation of “process sensitivity factors and constraints”, as derived from an experimental campaign, an assessment of the practical application issues of innovative process was performed within EXPECT. This included the application of all the required procedures (innovative or standard practices) in small scale representative repair cases. According to these results, further evolution of developed solutions and change proposals were recorded and implemented, and a final demonstrator repair was performed.
Project Context and Objectives:
The main steps which need to be followed for the performance of a “typical” repair to a composite structure, generally include:
a. Initial Non Destructive Testing (NDT) of the damaged area, in order to define in detail the external borders of the damage.
b. Removal of the damaged composite material by cutting, drilling and milling, in order to get a scarfed or stepped configuration.
c. Composite patch preparation (lay-up) / application (preparation of vacuum bagging and application of heat)
d. Final NDT of the repair, to identify potential debondings, delaminations, moisture, foreign inclusions or other anomalies.
In order to enable the performance of these repair steps in situ, a series of special portable equipment has been developed, fulfilling the repair specifications requirements and overcoming the numerous constraints of repair performance within hangars or repair workshops.
Using this kind of portable equipment, the application of an extensive variety of composite repairs can be supported. These repairs range from typical small or large composite to composite repairs, up to composite to metal repairs, as, for example, the recent repair of an ATR-72 aluminium floor beam using a bonded carbon patch, performed in situ by GMI Aero, in cooperation with the ATR company.
The main concept driving this proposal is to perform the whole chain of required steps for the performance of typical composite repairs, as listed above, using the same equipment existing in repair workshops of airline companies and MROs, and applying the same conditions / constraints (environmental, personnel training level etc.) usually existing in typical workshops, with the following main objectives:
a. To evaluate the performance of bonding strength and examine practical application aspects of bonded composite repairs using innovative materials and, potentially, modified surface preparation processes as developed within the JTI-GRA WPs, along the whole range of repair application procedures (surface preparation of composite substrate (stepped / scarfed), lay-up of the repair patch, curing of the patch and the adhesive at elevated temperature (~120 °C) and NDI.
b. To identify areas of potential contradiction of new processes to existing repair practices and procedures.
c. To propose alternative solutions (changes of procedures, development of special equipment etc.) in order to overcome such potential contradictions / constraints
By applying these steps, the main objective of the proposal will be fulfilled, which is to guarantee that the technological solutions developed within JTI-GRA ITD corresponding WPs will not only work under conditions of academic research, but will also work reliably if applied in commercial aircraft maintenance. The practical applicability of the new methodology will also be demonstrated, by performing repairs to larger panels, within the frame of WP1.3.7 of the GRA ITD.
For the achievement of the above mentioned objectives, there were 2 technical Work packages in order to perform the research and development activities of this project, together with 2 additional Work packages supporting the research activities, dealing with dissemination of results and project management , as follows:
WP1 contained the definition of the innovative repair process parameters and their elaboration against the process sensitivity factors and constraints. Moreover, an evaluation of the standard equipment appropriateness for the new techniques will be performed, before applying the new technique and assessing its practical application aspects.
Within WP2, the demonstrator repairs were performed, within the frame of WP1.3.7 of the GRA ITD. Moreover, the documentation of these demonstrator repairs was prepared, based on the overall process evaluation results. The documentation was prepared in two steps, namely an intermediate and a final one. This WP closed with the final evaluation of the practicability of the new methods.
WP3, dealt with the activities concerning dissemination and exploitation of research results produced within this project, attempting to maximize the knowledge transmitted from this research project to the scientific and the industrial world working on aeronautics. This Work package was considered of top importance, as the overall impact of the project and its effect to the European competitiveness largely depends on it.
The Project Coordinator, GMI, was mainly responsible for the implementation of WP4, which contains the project management and coordination activities as well as for the prompt periodical reporting towards EU and the “Topic Manager”. Playing a key-role in the project, the Project Coordinator, was responsible for the adequate performance of the required management and coordination activities.
More specifically, EXPECT will focus on estimating the potential of vacuum sand blasting as a surface pre-treatment method in composite aircraft repair. Vacuum sand blasting is an abrasive method like grinding, but it may be that it will provide better process reliability than a manual grinding process. Compared to conventional sand blasting, it takes care that the blasted material will not pollute the surroundings.
A surface pre-treatment with a vacuum sand blasting permits to replace the conventional manual grinding, improves bonding strength and permits to remove the eventual surface contamination. Its big advantage in respect to conventional sand blasting methods is the possibility to perform the treatments without the need for any special protective equipment. Thanks to the system design, blasting dust cannot escape even in case of leaks or other defects. The attractiveness to use it in repair process is based on a possibility of solvent and water-free treatment that could be done automatically, without hand contact with a repair zone.
After theoretical analysis and calculation of “process sensitivity factors and constraints”, as derived from an experimental campaign, an assessment of the practical application issues of innovative process was performed within EXPECT. This included the application of all the required procedures (innovative or standard practices) in small scale representative repair cases. According to these results, further evolution of developed solutions and change proposals were recorded and implemented, and a final demonstrator repair was performed.
Project Results:
EXPECT project will focus on estimating the potential of vacuum sand blasting as a surface pre-treatment method in composite aircraft repair. Vacuum sand blasting is an abrasive method like grinding, but it may be that it will provide better process reliability than a manual grinding process. Compared to conventional sand blasting, it takes care that the blasted material will not pollute the surroundings.
The hardware used for vacuum sand blasting is not available by the Consortium (GMI/NTUA) but at the Topic Manager facilities (Fraunhofer IFAM). As a first step a technical expert of GMI will visit Fraunhofer IFAM to see the technique and get an impression of restrictions and practical problems in its application to repair and perform the repair of a simple composite plate where the vacuum sand blasting will be applied. The main characteristics of the technique are summarized in the following Paragraph. The results produced within this project will be attempted to be applied to the JTI-GRA demonstrator repair.
- Vacuum sand blasting methodology description
As far as the methodology application constraints are concerned, the proposed maximum sand blasted area dimensions, according to currently available by the Topic Manager equipment, are 400x200mm.
- JTI-GRA Demostrator repair
According to the current plans of GRA WP 1.3.7 (not officially confirmed yet) the following demonstrator repair is envisaged:
- Substrate nature: Stringer reinforced panel.
- Substrate dimensions: 1200 x 900 mm².
- Substrate material: Nano-modified CFRP. Lamination sequence not defined, yet.
- Substrate radius of curvature: Flat, or very slightly curved.
- Substrare configuration: Monolithic (no honeycomb).
- Damage size (hole of material to be removed): TBD, but complying with the maximum vacuum sand blasting application constraints, as listed above.
- Scarf angle: Approximately 2-3°.
- Adhesive material: TBD
- Repair patch dimensions: TBD, but complying with the maximum vacuum sand blasting application constraints, as listed above.
- Repair patch scheme: Circular / rectangular / Ellipse, TBD.
- Repair patch material: The type of repair still has to be chosen considering the properties of the nano-CFRP in its current non-commercial state and the practical importance of the different repair methods. The following alternatives have been identified:
- A prepreg-repair on a scarfed hole using a conventional repair prepreg.
- A prepreg-repair on a scarfed hole using a prepreg made of the nano material.
- A wet layup on a scarfed hole using the nano material.
- A pre-cured patch made of nano-CFRP bonded using a paste adhesive.
- A pre-cured patch made of nano-CFRP bonded using a film adhesive.
Curing will be performed according to the patch material and adhesive film curing specifications, but not exceeding 180°C. No autoclave conditions will be assumed
- Repair patch lamination sequence: TBD, according to final specifications of demonstrator repair case and selected repair patch material, as listed above.
- External flushness requirements: Standard requirements for bonded composite patches. Even though this is not a primary objective, the number of layers positioned externally to the substrate should be kept to a minimum.
Finishing materials will be standard aeronautical paint and primer.
- Other Requirements
The following general requirements should be taken into consideration:
- Safety Requirements: Standard, applied to maintenance hangars and flight line.
- Repair application environment: Base (depot) repair conditions are assumed (clean & controlled air-conditioned environment etc.) There are no restrictions on electricity, water, gas etc...
- Accessibility: Only one-side access to the repair area is allowed assumed for application purposes.
- Personnel training assumptions: Skilled personnel with standard aeronautical training is assumed to be available.
- Transportability: All the equipment and tools should be transportable and suitable in so far that a repair at the wing on a large aircraft is feasible (e.g. lift/gantry available)
- Equipment MMI requirements: Standard for use by skilled technical personnel
Innovative features compatibility with standard repair elements
As already described, the EXPECT project will focus on estimating the potential of vacuum sand blasting as a surface pre-treatment method in composite aircraft repair. Vacuum sand blasting is an abrasive method like grinding, but it may be that it will provide better process reliability than a manual grinding process. Compared to conventional sand blasting, it takes care that the blasted material will not pollute the surroundings.
- Preparation of repair specimens
The hardware used for vacuum sand blasting is not available by the Consortium (GMI/NTUA) but at the Topic Manager facilities (Fraunhofer IFAM). According to the decision taken during the Kick Off Meeting, two (2) GMI experts (GMI President and GMI Training Manager) on bonded composite repairs have visited Fraunhofer IFAM, to see the technique and get an impression of restrictions and practical problems in its application to repair and perform the repair of a simple composite plate, where the vacuum sand blasting will be applied.
In order to evaluate in detail the innovative features compatibility with standard repair elements, it was agreed that certain composite parts, representative of most commonly encountered repair cases, should be prepared by GMI, in order to be sand blasted by Fraunhofer IFAM team.
According to that, GMI has used two sandwich panels made of composite materials (portions of landing gear doors of an Airbus A320 aircraft) for the preparation of areas usually encountered in repairs.
More specifically, the composite panels included four (4) processed areas, which were surface prepared using the Leslie tooling for machining of composite materials.
Processing included both techniques commonly applied in surface preparation of composite materials (i.e. scarfing and stepping). Moreover, in certain cases a hole was opened and the honeycomb core material was removed.
Process sensitivity factors and constraints
A surface pre-treatment with a vacuum sand blasting permits to replace the conventional manual grinding, improves bonding strength and permits to remove the eventual surface contamination. Its big advantage in respect to conventional sand blasting methods is the possibility to perform the treatments without the need for any special protective equipment. Thanks to the system design, blasting dust cannot escape even in case of leaks or other defects. The attractiveness to use it in repair process is based on a possibility of solvent and water-free treatment that could be done automatically, without hand contact with a repair zone. However, the vacuum sand blasting installation, constructed in IFAM, permits to treat only the flat surfaces. A sample holder that fixes the sample in horizontal position was developed in IFAM to treat flat scarfed CFRP and shows promising results in bonding and decontamination experiments. Consequently, treatment of a round scarfed samples, representative of repair zones provided to IFAM by GMI ("a real live" case) met some difficulties mainly due to the curvature of a surface and difficulties of keeping the same working distance between the treatment head and the sample.
In order to evaluate in detail the innovative features compatibility with standard repair elements, certain composite parts, representative of most commonly encountered repair cases, were prepared by GMI, in order to be sand blasted by Fraunhofer IFAM team.
According to that, GMI has used two sandwich panels made of composite materials (portions of landing gear doors of an Airbus A320 aircraft) for the preparation of areas usually encountered in repairs, as has already been described. Processing included both techniques commonly applied in surface preparation of composite materials (i.e. scarfing and stepping). Moreover, in certain cases a hole was opened and the honeycomb core material was removed.
- Experimental results
According to the experimental campaign performed by IFAM, the following main process sensitivity factors have been identified:
- Variation of working “Distance 1”: Keeping the working distance 2 at 18 mm, the distance 1 was varied from 0.5 to 2 mm. This caused the variation of differential pressure (pressure difference between atmospheric pressure and the pressure, measured in the vacuum sand blasting system) from 100mBar to 50 mBar. As a result, the efficiency of blasting, measured as a depth of hole blasted in 4 minutes of blasting, reduces from 100 microns (at differential pressure of 100mBar) to 0 microns (50mBar). Due to this reason in all future experiments, the distance between treatment head and sample surface (distance1) was kept at 0.5mm.
- Variation of working “Distance 2”: Fixing the distance 1 at its optimal value of 0.5mm working distance 2 was varied. It appears that for both too small (2-5mm) and too big (more than 10 mm) distances, the efficiency of treatment (material removal rate) is sub optimal. While it reaches its maximum at working distance about 8mm.
- Role of differential pressure: The variation of differential pressure from 100 to 20 mbar causes the decrease of the removal speed from 200 to less than 10 microns in 4 minutes. The differential pressure of ~70mbar is currently used in bonding experiments to assure a reasonable speed of treatment (approximate speed of 400cm2/hour has been achieved in configuration that removes surface contaminations).
Moreover, a couple of additional remarks have been noticed, as follows:
- Differential pressure was not enough to assure the correct treatment: on a flat surface it is measured to be about 70 mBar +/- 7%, on the curved panels it was between 25 and 45 mBar.
- Variations of differential pressure were too high to insure the homogeneity of treatment.
- The holes edges could not be treated because of downfall of a vacuum in this zone.
According to the above mentioned results, it has been concluded that certain areas where improvements could be implemented are the following:
- Development of an adaptor that permits to treat "real live" curved round scarfed surfaces.
- Development of solution to treat the edges of the hole within the same method.
- Minimization of the dust on the sample after the process.
- Increase of the method treatment speed.
- Increase homogeneity of treatment (i.e. to minimize the variations of differential pressure).
Concluding, within Task 1.3 an extensive study was performed, in order to trace the optimum way to treat the non compatibilities of innovative repair processes against standard repair practices, and a sensitivity analysis was performed, in order to evaluate the effect of several parameters in the process implementation efficiency and justify the selection of appropriate solutions and change proposals to be implemented within following steps of the project.
First Assessment of practical application issues of innovative process
Further to the experience gained during the performance of Vacuum Grit Blasting (VGB) operations by Fraunhofer IFAM, in combination with the execution of actual composite repair applications by GMI and NTUA within the frame of JTI projects, and taking into consideration practical applicability issues (time, equipment, cost, precision, etc...) the following two VGB application methods were conceived:
- Accurate VGB application method (AVGB).
For the accurate application of Vacuum Grit Blasting, special equipment will need to be developed, together with the appropriate templates to be positioned on the composite part to be repaired. The application methodology together with the required equipment and templates will be based on the GMI Leslie surface treatment series of equipment operating principle.
More specifically, the operating principle will be based on the controlled (in terms of location and distance from the part) dragging of the VGB orifice above the area to be repaired. In order to achieve this controlled operation, the developed equipment should have the following characteristics:
- The orifice diameter should be defined according to the stepping width and taking into consideration the efficiency pattern achieved using the VGB method. For example, if the actual step width is 5 mm, then a 7mm orifice should be used in order to achieve a 5mm homogeneously treated area at the centre. The orifice should be equipped with an external plate of relatively big diameter (i.e. significantly overlapping the maximum processing diameter) which will act as distance of treatment regulator, in combination with the templates, in order to achieve the Optimum Efficiency Distance (OED). Therefore the distance between the orifice edge and the lower side of the external plate (D) should be defined according to the following equation:
D = (Template Height + 2 x Step Height) – OED (1)
Moreover, a spacer of appropriate thickness should be attached to the external side of the vacuum line, so that the total width of the spacer and the vacuum part of the orifice should be 4mm. More specifically, the width (W) of the spacer should be defined according to the following equation, for a 5mm stepping repair treatment:
W= 7.5 - (Vacuum Line External Diameter/2) (2)
- The templates produced should form co-centric cycles, having as basis the maximum surface treatment diameter increased by 10mm, and in steps of 5mm width (reducing. The number of co-centric templates should be equivalent to the number of steps formed within the composite part to be repaired, while their height should be defined in order to keep the distance between the orifice and the composite surface at the required Optimum Efficiency Distance (OED), according to (1).
Gross VGB application method (GVGB)
Given that, according to experimental measurements, the material removal rate (i.e. reduction of thickness per “passage”) is an order of magnitude less than the typical stepping height a less accurate but simpler application methodology could be alternatively proposed. Such method would be based on the standard sand blasting application method and would be manually applied by means of a pressure gun. Consequently, the need for development of new equipment would be significantly limited, while increasing the method application range, suffering a penalty on the quality of the achieved results, however.
Documentation of demonstrator repairs - Intermediate version
As detailed in the EXPECT DoW, within WP2 the demonstrator repairs will be performed, within the frame of WP1.3.7 of the GRA ITD. More specifically, WP 2 is centred around a demonstration repair performed at GMI facilities. This repair was not considering special surface preparations, but was supposed to show a successful repair using traditional grinding as a pre-treatment and the nano-resin as matrix material. Within Task 2.1 the main steps to be followed for the “Panel 14” scarfing and repair application have been defined.
The demonstrator repair concerns the repair of "Panel 14" of deliverable "GRA-1.3.7-DL(1.3.7-27)-FHG-TECH-212282 B". The resin used for the fabrication of the panel was a basic epoxy resin modified with hard and with elastic nano-particles to increase the toughness. The main panel characteristics were the following:
• Panel width 370 mm
• Total panel length 560 mm,
• Free length 400 mm
• 2 omega-stringers
• Stringer width 136 mm
• Stringer foot width 40.5 mm
• Skin and stringer thickness 3 mm
• Quasi-isotropic layup (0/90,+-45,0/90,+-45)s
• Fabric HexForce G0926 D 1304 TCT
Performance of GRA ITD W.P.1.3.7 demonstrator Repairs
“Panel 14” has been processed using the GMI “Leslie” tooling for machining of composite materials. As a first step, a “through-thickness hole” was opened to the composite structure, to remove the damaged material.
In a second phase, grinding was performed in successive steps, in order to achieve the required stepping pattern.
The composite repair patches were prepared according to instructions defined in Deliverable 2.1. Two patches have been prepared, namely one (smaller) for the rear side of the panel and one (bigger) for the front side of “Panel 14”. The dimensions of the front patch were exactly matching the stepping configuration created on “Panel 14” through milling.
The repair patches which have been prepared in the previous step have been subsequently applied to the rear, initially, and then to the front sides of the milled “Panel 14”. An appropriate curing cycle has been performed after each patch installation.
Documentation of demonstrator repairs - Final version
The final version of the documentation of demonstrator repairs has been defined within Task 2.3. The main steps for the “Panel 14” routing, milling and repair application have been described in previous Paragraphs of this report. Moreover, further to the experience gained during the performance of Vacuum Grit Blasting (VGB) operations by Fraunhofer IFAM, in combination with the execution of actual composite repair applications by GMI and NTUA within the frame of JTI projects, and taking into consideration practical applicability issues (time, equipment, cost, precision, etc...) the following two VGB application methods were conceived, namely :
a. Accurate VGB application method (AVGB).
b. Gross VGB application method (GVGB)
The main application steps of each VGB methodology have been described in previous Paragraphs of this report.
Final assessment of practicability issues of Innovative Repair Process
Further to the experience gained during the performance of Vacuum Grit Blasting (VGB) operations by Fraunhofer IFAM, in combination with the execution of actual composite repair applications by GMI and NTUA within the frame of JTI projects, and taking into consideration practical applicability issues (time, equipment, cost, precision, etc...) two VGB application methods, namely AVGB and GVGB, have been proposed for the surface treatment of composite components to be repaired using bonded composite patches, in order to achieve higher bonding strength and increased bond durability. According to the results of the EXPECT project, the final assessment of practicability issues related to VGB together with the advantages and disadvantages of each proposed application method is presented below:
a. Accurate VGB application method (AVGB).
For the accurate application of Vacuum Grit Blasting, special equipment will need to be developed, together with the appropriate templates to be positioned on the composite part to be repaired. The application methodology together with the required equipment and templates will be based on the GMI Leslie surface treatment series of equipment operating principle. The advantages and disadvantages of the AVGB application methodology could be summarized as follows:
- AVGB Advantages:
• Rather accurate method, achieving homogeneous surface preparation.
• High quality of surface prepared area.
• Reliable method, in terms of application and achieved results.
• Repeatable results.
• Non-operator dependent results.
- AVGB Disadvantages:
• High cost of developed equipment.
• Relatively low speed of application.
• Application limited to stepped repair profiles
• Difficulties in application to curved structures
• Relatively complicated method
b. Gross VGB application method (GVGB)
Given that, according to experimental measurements, the material removal rate (i.e. reduction of thickness per “passage”) is an order of magnitude less than the typical stepping height a less accurate but simpler application methodology could be alternatively proposed. Such method would be based on the standard sand blasting application method and would be manually applied by means of a pressure gun. Consequently, the need for development of new equipment would be significantly limited, while increasing the method application range, suffering a penalty on the quality of the achieved results, however. The advantages and disadvantages of the GVGB application methodology could be summarized as follows
- GVGB Advantages:
• Low cost of developed equipment.
• High speed of application.
• Applicable to both stepped and scarfed repair profiles
• Applicable to curved structures
• Relatively simple method
- GVGB Disadvantages:
• Less accurate method, in term of homogeneous surface preparation.
• Low quality of surface prepared area.
• Non-reliable method, in terms of application and achieved results.
• Non -repeatable results.
• Operator dependent results.
Potential Impact:
Economic growth around the world has led to a continuous increase of air-traffic numbers during the past decades. This increase is expected to continue at an even stronger pace for the next two decades. As the operating fleet grows, the costs and hazard exposure will also increase. Despite the recent difficulties faced by the industry, the market forecast over the next twenty years for commercial aircraft is expected to be of the order of €1.6 Trillion. The Aerospace Market remains a highly competitive one and any aspect of commercial advantage must be sought. EXPECT will address a key element of competitive advantage for the industry, those of aircraft reliability during operation and maintenance costs.
Replying directly to JTI’s call for proposals, the primary drivers for EXPECT relate to safety, economic and societal issues. The application of the proposed advantages to the composite repair process will improve reliability during operation, improve performance and minimise the time the aircraft needs to spend on the ground for repair, which are among the main targets of Cleansky JTI. This will permit increased aircraft availability and lower maintenance costs to be incurred by the operating company. The increase in reliability will lead to a reduction in accidents, loss of life and associated compensation costs resulting from failure of critical aircraft structural components. It is expected that this project will lead to a major change in the development of bonded composite repair procedures, thereby strengthening the EU position within the global Aerospace Market, whilst maintaining the competitive advantage of the EU companies over its US and Chinese rivals.
It is planned that full industrialization of the results of the project, in terms of application of the new processes to various cases where bonded composite repair will be engaged, could take place very shortly after the end of the project, according to the requirements of the “Topic Manager” and CSJU.
3.1.1 Savings Generated as a Result of this Project
Reduced Maintenance Costs:
The inspection of aircraft is carried out during periods of maintenance activity. During this period the aircraft is decommissioned from service. For an Airbus A320 minor checks take place every 600 flight hours for the newly manufactured aircraft and every 500 flight hours for the older ones. Medium planned maintenance normally takes place every 20 months for the new aircraft and every 15 months for the old ones. Major planned maintenance during which the aircraft is taken apart is carried out every 6 years for the new A320 and every 5 years for the old ones. Major planned maintenance can result in aircraft being taken out of service for well over 30 days. According to Airbus in the first 5 years of operation an A320 requires 564 man-hours in maintenance, for 10 years of operation 1,344 man-hours and for 12 years of operation 1,981 man-hours. The total average cost of maintenance for an A320 over a period 15 years is €5.2 million, a significant burden for the operating airline. Total maintenance costs for Europe amount to €615 million per year. The successful implementation of the EXPECT project development is expected to reduce these maintenance, repair and inspection costs significantly. Finally, potential practical applications of the innovative repair processes will concern the new Green Regional Aircraft, leading to significant reduction of maintenance costs and increase of reliability in maintenance.
Contribution to Community Social Objectives
If this project will have no direct impact on employment in the aeronautic sector, it will contribute to the renown of European technology and will induce future related research developments. The project is an applied research project with ambition to provide a technology that can be directly industrialized within the JTI and promoted to the worldwide market of repair stations. It deals with a subject, bonded composite repairs, which is a challenging issue between Europe and USA and where large scale research activities take place worldwide.
a. Employment prospects and level of skills in the EU
The European aerospace industry directly employs some 429 thousand people whilst the second tier suppliers employ a further 500 thousand people. European industry has taken bold steps to use an increasing amount of advanced aerospace materials in their aircraft structures. Consequently, their aircraft have on average considerably more advanced light-weight materials than US suppliers, such as Boeing, making European aircraft significantly lighter than US manufactured aircraft of similar seat capacity. This gives European aircraft manufacturers a considerable competitive advantage over US manufacturers. This advantage is threatened by a loss of confidence in the use of advanced light-weight materials following recent air disasters caused by undetected defects in such components. The results of this project, combined with the overall improvement of bonded composite repairs, are expected to help in restoring confidence in the use of advanced aerospace materials. This will result in safeguarding and increasing the employment prospects in the European aerospace industry.
b. Life extension of aircraft:
This project will deliver technology, which will help in improving safety and operational capability of aircraft, leading to an increase in operational life. The countries of Eastern Europe have ageing air fleets that include 30-35 year old aircraft. In the absence of appropriate techniques to restore their structural integrity and to support life extension programs, these ageing aircraft will need to be decommissioned in the near future to meet EU safety standards. This will seriously affect the East European airlines that are currently struggling to be competitive and survive in the global market. The EU predicts strong growth in the market which could be exploited by airlines that increase the operational life of their aircraft whilst meeting EU safety standards, leading to significantly better returns on investment and profitability. EXPECT will contribute to European wide sustainability and growth, particularly enhancing employment prospects in the new Member States.
c. Level of skill in EU:
This project will lead to an improvement in the level of skills for European citizens, as it will assist in implementing new bonded composite repair processes. The project will help in developing and sustaining EU expertise in these new technologies, with direct positive effects to the level of EU in the global composites market.
d. Environmental impact
Through enabling life extension of existing aircraft structures and improving their structural integrity, as well as by supporting of the Green Regional Aircraft development process, greening of aircraft operation and maintenance is achieved, thus helping in reduction of the environmental impact of aviation. Considering that approximately 600kg of CO2 are emitted per kilowatt hour of energy generated by fossil fuels with existing technology, this significant improvement in maintenance activities is directly matching with the targets of the Cleansky JTI.
List of Websites:
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Even though adhesively bonded composite repairs exhibit significant advantages in terms of mechanical efficiency compared to mechanically fastened ones, they are at the same time extremely sensitive to process parameters variations. Small deviations against the repair specifications (e.g. surface preparation) may lead to disproportionally larger consequences to the final mechanical performance of the repair and, consequently, to the integrity of the structure. Moreover, the inevitable differences between laboratory and repair shop conditions, may lead to the cancellation of promising technological achievements for “practical” reasons, because the required conditions could not be easily met in standard practice.
The scope of this project is to examine from a practical application point of view the innovations achieved within GRA Task 1.3.6-02 for bonded composite repairs, in order to facilitate their transfer to production conditions. For this reason, the whole chain of process steps which have to be followed for the performance of a typical repair will be executed in standard repair shop conditions, in terms of facilities, equipment, tooling and personnel training. Potential contradictions of innovations developed by the GRA to standard repair practices will be identified and alternative solutions will be proposed, to facilitate their efficient transfer to production standards and requirements.
More specifically, EXPECT will focus on estimating the potential of vacuum sand blasting as a surface pre-treatment method in composite aircraft repair. Vacuum sand blasting is an abrasive method like grinding, but it may be that it will provide better process reliability than a manual grinding process. Compared to conventional sand blasting, it takes care that the blasted material will not pollute the surroundings.
A surface pre-treatment with a vacuum sand blasting permits to replace the conventional manual grinding, improves bonding strength and permits to remove the eventual surface contamination. Its big advantage in respect to conventional sand blasting methods is the possibility to perform the treatments without the need for any special protective equipment. Thanks to the system design, blasting dust cannot escape even in case of leaks or other defects. The attractiveness to use it in repair process is based on a possibility of solvent and water-free treatment that could be done automatically, without hand contact with a repair zone.
After theoretical analysis and calculation of “process sensitivity factors and constraints”, as derived from an experimental campaign, an assessment of the practical application issues of innovative process was performed within EXPECT. This included the application of all the required procedures (innovative or standard practices) in small scale representative repair cases. According to these results, further evolution of developed solutions and change proposals were recorded and implemented, and a final demonstrator repair was performed.
Project Context and Objectives:
The main steps which need to be followed for the performance of a “typical” repair to a composite structure, generally include:
a. Initial Non Destructive Testing (NDT) of the damaged area, in order to define in detail the external borders of the damage.
b. Removal of the damaged composite material by cutting, drilling and milling, in order to get a scarfed or stepped configuration.
c. Composite patch preparation (lay-up) / application (preparation of vacuum bagging and application of heat)
d. Final NDT of the repair, to identify potential debondings, delaminations, moisture, foreign inclusions or other anomalies.
In order to enable the performance of these repair steps in situ, a series of special portable equipment has been developed, fulfilling the repair specifications requirements and overcoming the numerous constraints of repair performance within hangars or repair workshops.
Using this kind of portable equipment, the application of an extensive variety of composite repairs can be supported. These repairs range from typical small or large composite to composite repairs, up to composite to metal repairs, as, for example, the recent repair of an ATR-72 aluminium floor beam using a bonded carbon patch, performed in situ by GMI Aero, in cooperation with the ATR company.
The main concept driving this proposal is to perform the whole chain of required steps for the performance of typical composite repairs, as listed above, using the same equipment existing in repair workshops of airline companies and MROs, and applying the same conditions / constraints (environmental, personnel training level etc.) usually existing in typical workshops, with the following main objectives:
a. To evaluate the performance of bonding strength and examine practical application aspects of bonded composite repairs using innovative materials and, potentially, modified surface preparation processes as developed within the JTI-GRA WPs, along the whole range of repair application procedures (surface preparation of composite substrate (stepped / scarfed), lay-up of the repair patch, curing of the patch and the adhesive at elevated temperature (~120 °C) and NDI.
b. To identify areas of potential contradiction of new processes to existing repair practices and procedures.
c. To propose alternative solutions (changes of procedures, development of special equipment etc.) in order to overcome such potential contradictions / constraints
By applying these steps, the main objective of the proposal will be fulfilled, which is to guarantee that the technological solutions developed within JTI-GRA ITD corresponding WPs will not only work under conditions of academic research, but will also work reliably if applied in commercial aircraft maintenance. The practical applicability of the new methodology will also be demonstrated, by performing repairs to larger panels, within the frame of WP1.3.7 of the GRA ITD.
For the achievement of the above mentioned objectives, there were 2 technical Work packages in order to perform the research and development activities of this project, together with 2 additional Work packages supporting the research activities, dealing with dissemination of results and project management , as follows:
WP1 contained the definition of the innovative repair process parameters and their elaboration against the process sensitivity factors and constraints. Moreover, an evaluation of the standard equipment appropriateness for the new techniques will be performed, before applying the new technique and assessing its practical application aspects.
Within WP2, the demonstrator repairs were performed, within the frame of WP1.3.7 of the GRA ITD. Moreover, the documentation of these demonstrator repairs was prepared, based on the overall process evaluation results. The documentation was prepared in two steps, namely an intermediate and a final one. This WP closed with the final evaluation of the practicability of the new methods.
WP3, dealt with the activities concerning dissemination and exploitation of research results produced within this project, attempting to maximize the knowledge transmitted from this research project to the scientific and the industrial world working on aeronautics. This Work package was considered of top importance, as the overall impact of the project and its effect to the European competitiveness largely depends on it.
The Project Coordinator, GMI, was mainly responsible for the implementation of WP4, which contains the project management and coordination activities as well as for the prompt periodical reporting towards EU and the “Topic Manager”. Playing a key-role in the project, the Project Coordinator, was responsible for the adequate performance of the required management and coordination activities.
More specifically, EXPECT will focus on estimating the potential of vacuum sand blasting as a surface pre-treatment method in composite aircraft repair. Vacuum sand blasting is an abrasive method like grinding, but it may be that it will provide better process reliability than a manual grinding process. Compared to conventional sand blasting, it takes care that the blasted material will not pollute the surroundings.
A surface pre-treatment with a vacuum sand blasting permits to replace the conventional manual grinding, improves bonding strength and permits to remove the eventual surface contamination. Its big advantage in respect to conventional sand blasting methods is the possibility to perform the treatments without the need for any special protective equipment. Thanks to the system design, blasting dust cannot escape even in case of leaks or other defects. The attractiveness to use it in repair process is based on a possibility of solvent and water-free treatment that could be done automatically, without hand contact with a repair zone.
After theoretical analysis and calculation of “process sensitivity factors and constraints”, as derived from an experimental campaign, an assessment of the practical application issues of innovative process was performed within EXPECT. This included the application of all the required procedures (innovative or standard practices) in small scale representative repair cases. According to these results, further evolution of developed solutions and change proposals were recorded and implemented, and a final demonstrator repair was performed.
Project Results:
EXPECT project will focus on estimating the potential of vacuum sand blasting as a surface pre-treatment method in composite aircraft repair. Vacuum sand blasting is an abrasive method like grinding, but it may be that it will provide better process reliability than a manual grinding process. Compared to conventional sand blasting, it takes care that the blasted material will not pollute the surroundings.
The hardware used for vacuum sand blasting is not available by the Consortium (GMI/NTUA) but at the Topic Manager facilities (Fraunhofer IFAM). As a first step a technical expert of GMI will visit Fraunhofer IFAM to see the technique and get an impression of restrictions and practical problems in its application to repair and perform the repair of a simple composite plate where the vacuum sand blasting will be applied. The main characteristics of the technique are summarized in the following Paragraph. The results produced within this project will be attempted to be applied to the JTI-GRA demonstrator repair.
- Vacuum sand blasting methodology description
As far as the methodology application constraints are concerned, the proposed maximum sand blasted area dimensions, according to currently available by the Topic Manager equipment, are 400x200mm.
- JTI-GRA Demostrator repair
According to the current plans of GRA WP 1.3.7 (not officially confirmed yet) the following demonstrator repair is envisaged:
- Substrate nature: Stringer reinforced panel.
- Substrate dimensions: 1200 x 900 mm².
- Substrate material: Nano-modified CFRP. Lamination sequence not defined, yet.
- Substrate radius of curvature: Flat, or very slightly curved.
- Substrare configuration: Monolithic (no honeycomb).
- Damage size (hole of material to be removed): TBD, but complying with the maximum vacuum sand blasting application constraints, as listed above.
- Scarf angle: Approximately 2-3°.
- Adhesive material: TBD
- Repair patch dimensions: TBD, but complying with the maximum vacuum sand blasting application constraints, as listed above.
- Repair patch scheme: Circular / rectangular / Ellipse, TBD.
- Repair patch material: The type of repair still has to be chosen considering the properties of the nano-CFRP in its current non-commercial state and the practical importance of the different repair methods. The following alternatives have been identified:
- A prepreg-repair on a scarfed hole using a conventional repair prepreg.
- A prepreg-repair on a scarfed hole using a prepreg made of the nano material.
- A wet layup on a scarfed hole using the nano material.
- A pre-cured patch made of nano-CFRP bonded using a paste adhesive.
- A pre-cured patch made of nano-CFRP bonded using a film adhesive.
Curing will be performed according to the patch material and adhesive film curing specifications, but not exceeding 180°C. No autoclave conditions will be assumed
- Repair patch lamination sequence: TBD, according to final specifications of demonstrator repair case and selected repair patch material, as listed above.
- External flushness requirements: Standard requirements for bonded composite patches. Even though this is not a primary objective, the number of layers positioned externally to the substrate should be kept to a minimum.
Finishing materials will be standard aeronautical paint and primer.
- Other Requirements
The following general requirements should be taken into consideration:
- Safety Requirements: Standard, applied to maintenance hangars and flight line.
- Repair application environment: Base (depot) repair conditions are assumed (clean & controlled air-conditioned environment etc.) There are no restrictions on electricity, water, gas etc...
- Accessibility: Only one-side access to the repair area is allowed assumed for application purposes.
- Personnel training assumptions: Skilled personnel with standard aeronautical training is assumed to be available.
- Transportability: All the equipment and tools should be transportable and suitable in so far that a repair at the wing on a large aircraft is feasible (e.g. lift/gantry available)
- Equipment MMI requirements: Standard for use by skilled technical personnel
Innovative features compatibility with standard repair elements
As already described, the EXPECT project will focus on estimating the potential of vacuum sand blasting as a surface pre-treatment method in composite aircraft repair. Vacuum sand blasting is an abrasive method like grinding, but it may be that it will provide better process reliability than a manual grinding process. Compared to conventional sand blasting, it takes care that the blasted material will not pollute the surroundings.
- Preparation of repair specimens
The hardware used for vacuum sand blasting is not available by the Consortium (GMI/NTUA) but at the Topic Manager facilities (Fraunhofer IFAM). According to the decision taken during the Kick Off Meeting, two (2) GMI experts (GMI President and GMI Training Manager) on bonded composite repairs have visited Fraunhofer IFAM, to see the technique and get an impression of restrictions and practical problems in its application to repair and perform the repair of a simple composite plate, where the vacuum sand blasting will be applied.
In order to evaluate in detail the innovative features compatibility with standard repair elements, it was agreed that certain composite parts, representative of most commonly encountered repair cases, should be prepared by GMI, in order to be sand blasted by Fraunhofer IFAM team.
According to that, GMI has used two sandwich panels made of composite materials (portions of landing gear doors of an Airbus A320 aircraft) for the preparation of areas usually encountered in repairs.
More specifically, the composite panels included four (4) processed areas, which were surface prepared using the Leslie tooling for machining of composite materials.
Processing included both techniques commonly applied in surface preparation of composite materials (i.e. scarfing and stepping). Moreover, in certain cases a hole was opened and the honeycomb core material was removed.
Process sensitivity factors and constraints
A surface pre-treatment with a vacuum sand blasting permits to replace the conventional manual grinding, improves bonding strength and permits to remove the eventual surface contamination. Its big advantage in respect to conventional sand blasting methods is the possibility to perform the treatments without the need for any special protective equipment. Thanks to the system design, blasting dust cannot escape even in case of leaks or other defects. The attractiveness to use it in repair process is based on a possibility of solvent and water-free treatment that could be done automatically, without hand contact with a repair zone. However, the vacuum sand blasting installation, constructed in IFAM, permits to treat only the flat surfaces. A sample holder that fixes the sample in horizontal position was developed in IFAM to treat flat scarfed CFRP and shows promising results in bonding and decontamination experiments. Consequently, treatment of a round scarfed samples, representative of repair zones provided to IFAM by GMI ("a real live" case) met some difficulties mainly due to the curvature of a surface and difficulties of keeping the same working distance between the treatment head and the sample.
In order to evaluate in detail the innovative features compatibility with standard repair elements, certain composite parts, representative of most commonly encountered repair cases, were prepared by GMI, in order to be sand blasted by Fraunhofer IFAM team.
According to that, GMI has used two sandwich panels made of composite materials (portions of landing gear doors of an Airbus A320 aircraft) for the preparation of areas usually encountered in repairs, as has already been described. Processing included both techniques commonly applied in surface preparation of composite materials (i.e. scarfing and stepping). Moreover, in certain cases a hole was opened and the honeycomb core material was removed.
- Experimental results
According to the experimental campaign performed by IFAM, the following main process sensitivity factors have been identified:
- Variation of working “Distance 1”: Keeping the working distance 2 at 18 mm, the distance 1 was varied from 0.5 to 2 mm. This caused the variation of differential pressure (pressure difference between atmospheric pressure and the pressure, measured in the vacuum sand blasting system) from 100mBar to 50 mBar. As a result, the efficiency of blasting, measured as a depth of hole blasted in 4 minutes of blasting, reduces from 100 microns (at differential pressure of 100mBar) to 0 microns (50mBar). Due to this reason in all future experiments, the distance between treatment head and sample surface (distance1) was kept at 0.5mm.
- Variation of working “Distance 2”: Fixing the distance 1 at its optimal value of 0.5mm working distance 2 was varied. It appears that for both too small (2-5mm) and too big (more than 10 mm) distances, the efficiency of treatment (material removal rate) is sub optimal. While it reaches its maximum at working distance about 8mm.
- Role of differential pressure: The variation of differential pressure from 100 to 20 mbar causes the decrease of the removal speed from 200 to less than 10 microns in 4 minutes. The differential pressure of ~70mbar is currently used in bonding experiments to assure a reasonable speed of treatment (approximate speed of 400cm2/hour has been achieved in configuration that removes surface contaminations).
Moreover, a couple of additional remarks have been noticed, as follows:
- Differential pressure was not enough to assure the correct treatment: on a flat surface it is measured to be about 70 mBar +/- 7%, on the curved panels it was between 25 and 45 mBar.
- Variations of differential pressure were too high to insure the homogeneity of treatment.
- The holes edges could not be treated because of downfall of a vacuum in this zone.
According to the above mentioned results, it has been concluded that certain areas where improvements could be implemented are the following:
- Development of an adaptor that permits to treat "real live" curved round scarfed surfaces.
- Development of solution to treat the edges of the hole within the same method.
- Minimization of the dust on the sample after the process.
- Increase of the method treatment speed.
- Increase homogeneity of treatment (i.e. to minimize the variations of differential pressure).
Concluding, within Task 1.3 an extensive study was performed, in order to trace the optimum way to treat the non compatibilities of innovative repair processes against standard repair practices, and a sensitivity analysis was performed, in order to evaluate the effect of several parameters in the process implementation efficiency and justify the selection of appropriate solutions and change proposals to be implemented within following steps of the project.
First Assessment of practical application issues of innovative process
Further to the experience gained during the performance of Vacuum Grit Blasting (VGB) operations by Fraunhofer IFAM, in combination with the execution of actual composite repair applications by GMI and NTUA within the frame of JTI projects, and taking into consideration practical applicability issues (time, equipment, cost, precision, etc...) the following two VGB application methods were conceived:
- Accurate VGB application method (AVGB).
For the accurate application of Vacuum Grit Blasting, special equipment will need to be developed, together with the appropriate templates to be positioned on the composite part to be repaired. The application methodology together with the required equipment and templates will be based on the GMI Leslie surface treatment series of equipment operating principle.
More specifically, the operating principle will be based on the controlled (in terms of location and distance from the part) dragging of the VGB orifice above the area to be repaired. In order to achieve this controlled operation, the developed equipment should have the following characteristics:
- The orifice diameter should be defined according to the stepping width and taking into consideration the efficiency pattern achieved using the VGB method. For example, if the actual step width is 5 mm, then a 7mm orifice should be used in order to achieve a 5mm homogeneously treated area at the centre. The orifice should be equipped with an external plate of relatively big diameter (i.e. significantly overlapping the maximum processing diameter) which will act as distance of treatment regulator, in combination with the templates, in order to achieve the Optimum Efficiency Distance (OED). Therefore the distance between the orifice edge and the lower side of the external plate (D) should be defined according to the following equation:
D = (Template Height + 2 x Step Height) – OED (1)
Moreover, a spacer of appropriate thickness should be attached to the external side of the vacuum line, so that the total width of the spacer and the vacuum part of the orifice should be 4mm. More specifically, the width (W) of the spacer should be defined according to the following equation, for a 5mm stepping repair treatment:
W= 7.5 - (Vacuum Line External Diameter/2) (2)
- The templates produced should form co-centric cycles, having as basis the maximum surface treatment diameter increased by 10mm, and in steps of 5mm width (reducing. The number of co-centric templates should be equivalent to the number of steps formed within the composite part to be repaired, while their height should be defined in order to keep the distance between the orifice and the composite surface at the required Optimum Efficiency Distance (OED), according to (1).
Gross VGB application method (GVGB)
Given that, according to experimental measurements, the material removal rate (i.e. reduction of thickness per “passage”) is an order of magnitude less than the typical stepping height a less accurate but simpler application methodology could be alternatively proposed. Such method would be based on the standard sand blasting application method and would be manually applied by means of a pressure gun. Consequently, the need for development of new equipment would be significantly limited, while increasing the method application range, suffering a penalty on the quality of the achieved results, however.
Documentation of demonstrator repairs - Intermediate version
As detailed in the EXPECT DoW, within WP2 the demonstrator repairs will be performed, within the frame of WP1.3.7 of the GRA ITD. More specifically, WP 2 is centred around a demonstration repair performed at GMI facilities. This repair was not considering special surface preparations, but was supposed to show a successful repair using traditional grinding as a pre-treatment and the nano-resin as matrix material. Within Task 2.1 the main steps to be followed for the “Panel 14” scarfing and repair application have been defined.
The demonstrator repair concerns the repair of "Panel 14" of deliverable "GRA-1.3.7-DL(1.3.7-27)-FHG-TECH-212282 B". The resin used for the fabrication of the panel was a basic epoxy resin modified with hard and with elastic nano-particles to increase the toughness. The main panel characteristics were the following:
• Panel width 370 mm
• Total panel length 560 mm,
• Free length 400 mm
• 2 omega-stringers
• Stringer width 136 mm
• Stringer foot width 40.5 mm
• Skin and stringer thickness 3 mm
• Quasi-isotropic layup (0/90,+-45,0/90,+-45)s
• Fabric HexForce G0926 D 1304 TCT
Performance of GRA ITD W.P.1.3.7 demonstrator Repairs
“Panel 14” has been processed using the GMI “Leslie” tooling for machining of composite materials. As a first step, a “through-thickness hole” was opened to the composite structure, to remove the damaged material.
In a second phase, grinding was performed in successive steps, in order to achieve the required stepping pattern.
The composite repair patches were prepared according to instructions defined in Deliverable 2.1. Two patches have been prepared, namely one (smaller) for the rear side of the panel and one (bigger) for the front side of “Panel 14”. The dimensions of the front patch were exactly matching the stepping configuration created on “Panel 14” through milling.
The repair patches which have been prepared in the previous step have been subsequently applied to the rear, initially, and then to the front sides of the milled “Panel 14”. An appropriate curing cycle has been performed after each patch installation.
Documentation of demonstrator repairs - Final version
The final version of the documentation of demonstrator repairs has been defined within Task 2.3. The main steps for the “Panel 14” routing, milling and repair application have been described in previous Paragraphs of this report. Moreover, further to the experience gained during the performance of Vacuum Grit Blasting (VGB) operations by Fraunhofer IFAM, in combination with the execution of actual composite repair applications by GMI and NTUA within the frame of JTI projects, and taking into consideration practical applicability issues (time, equipment, cost, precision, etc...) the following two VGB application methods were conceived, namely :
a. Accurate VGB application method (AVGB).
b. Gross VGB application method (GVGB)
The main application steps of each VGB methodology have been described in previous Paragraphs of this report.
Final assessment of practicability issues of Innovative Repair Process
Further to the experience gained during the performance of Vacuum Grit Blasting (VGB) operations by Fraunhofer IFAM, in combination with the execution of actual composite repair applications by GMI and NTUA within the frame of JTI projects, and taking into consideration practical applicability issues (time, equipment, cost, precision, etc...) two VGB application methods, namely AVGB and GVGB, have been proposed for the surface treatment of composite components to be repaired using bonded composite patches, in order to achieve higher bonding strength and increased bond durability. According to the results of the EXPECT project, the final assessment of practicability issues related to VGB together with the advantages and disadvantages of each proposed application method is presented below:
a. Accurate VGB application method (AVGB).
For the accurate application of Vacuum Grit Blasting, special equipment will need to be developed, together with the appropriate templates to be positioned on the composite part to be repaired. The application methodology together with the required equipment and templates will be based on the GMI Leslie surface treatment series of equipment operating principle. The advantages and disadvantages of the AVGB application methodology could be summarized as follows:
- AVGB Advantages:
• Rather accurate method, achieving homogeneous surface preparation.
• High quality of surface prepared area.
• Reliable method, in terms of application and achieved results.
• Repeatable results.
• Non-operator dependent results.
- AVGB Disadvantages:
• High cost of developed equipment.
• Relatively low speed of application.
• Application limited to stepped repair profiles
• Difficulties in application to curved structures
• Relatively complicated method
b. Gross VGB application method (GVGB)
Given that, according to experimental measurements, the material removal rate (i.e. reduction of thickness per “passage”) is an order of magnitude less than the typical stepping height a less accurate but simpler application methodology could be alternatively proposed. Such method would be based on the standard sand blasting application method and would be manually applied by means of a pressure gun. Consequently, the need for development of new equipment would be significantly limited, while increasing the method application range, suffering a penalty on the quality of the achieved results, however. The advantages and disadvantages of the GVGB application methodology could be summarized as follows
- GVGB Advantages:
• Low cost of developed equipment.
• High speed of application.
• Applicable to both stepped and scarfed repair profiles
• Applicable to curved structures
• Relatively simple method
- GVGB Disadvantages:
• Less accurate method, in term of homogeneous surface preparation.
• Low quality of surface prepared area.
• Non-reliable method, in terms of application and achieved results.
• Non -repeatable results.
• Operator dependent results.
Potential Impact:
Economic growth around the world has led to a continuous increase of air-traffic numbers during the past decades. This increase is expected to continue at an even stronger pace for the next two decades. As the operating fleet grows, the costs and hazard exposure will also increase. Despite the recent difficulties faced by the industry, the market forecast over the next twenty years for commercial aircraft is expected to be of the order of €1.6 Trillion. The Aerospace Market remains a highly competitive one and any aspect of commercial advantage must be sought. EXPECT will address a key element of competitive advantage for the industry, those of aircraft reliability during operation and maintenance costs.
Replying directly to JTI’s call for proposals, the primary drivers for EXPECT relate to safety, economic and societal issues. The application of the proposed advantages to the composite repair process will improve reliability during operation, improve performance and minimise the time the aircraft needs to spend on the ground for repair, which are among the main targets of Cleansky JTI. This will permit increased aircraft availability and lower maintenance costs to be incurred by the operating company. The increase in reliability will lead to a reduction in accidents, loss of life and associated compensation costs resulting from failure of critical aircraft structural components. It is expected that this project will lead to a major change in the development of bonded composite repair procedures, thereby strengthening the EU position within the global Aerospace Market, whilst maintaining the competitive advantage of the EU companies over its US and Chinese rivals.
It is planned that full industrialization of the results of the project, in terms of application of the new processes to various cases where bonded composite repair will be engaged, could take place very shortly after the end of the project, according to the requirements of the “Topic Manager” and CSJU.
3.1.1 Savings Generated as a Result of this Project
Reduced Maintenance Costs:
The inspection of aircraft is carried out during periods of maintenance activity. During this period the aircraft is decommissioned from service. For an Airbus A320 minor checks take place every 600 flight hours for the newly manufactured aircraft and every 500 flight hours for the older ones. Medium planned maintenance normally takes place every 20 months for the new aircraft and every 15 months for the old ones. Major planned maintenance during which the aircraft is taken apart is carried out every 6 years for the new A320 and every 5 years for the old ones. Major planned maintenance can result in aircraft being taken out of service for well over 30 days. According to Airbus in the first 5 years of operation an A320 requires 564 man-hours in maintenance, for 10 years of operation 1,344 man-hours and for 12 years of operation 1,981 man-hours. The total average cost of maintenance for an A320 over a period 15 years is €5.2 million, a significant burden for the operating airline. Total maintenance costs for Europe amount to €615 million per year. The successful implementation of the EXPECT project development is expected to reduce these maintenance, repair and inspection costs significantly. Finally, potential practical applications of the innovative repair processes will concern the new Green Regional Aircraft, leading to significant reduction of maintenance costs and increase of reliability in maintenance.
Contribution to Community Social Objectives
If this project will have no direct impact on employment in the aeronautic sector, it will contribute to the renown of European technology and will induce future related research developments. The project is an applied research project with ambition to provide a technology that can be directly industrialized within the JTI and promoted to the worldwide market of repair stations. It deals with a subject, bonded composite repairs, which is a challenging issue between Europe and USA and where large scale research activities take place worldwide.
a. Employment prospects and level of skills in the EU
The European aerospace industry directly employs some 429 thousand people whilst the second tier suppliers employ a further 500 thousand people. European industry has taken bold steps to use an increasing amount of advanced aerospace materials in their aircraft structures. Consequently, their aircraft have on average considerably more advanced light-weight materials than US suppliers, such as Boeing, making European aircraft significantly lighter than US manufactured aircraft of similar seat capacity. This gives European aircraft manufacturers a considerable competitive advantage over US manufacturers. This advantage is threatened by a loss of confidence in the use of advanced light-weight materials following recent air disasters caused by undetected defects in such components. The results of this project, combined with the overall improvement of bonded composite repairs, are expected to help in restoring confidence in the use of advanced aerospace materials. This will result in safeguarding and increasing the employment prospects in the European aerospace industry.
b. Life extension of aircraft:
This project will deliver technology, which will help in improving safety and operational capability of aircraft, leading to an increase in operational life. The countries of Eastern Europe have ageing air fleets that include 30-35 year old aircraft. In the absence of appropriate techniques to restore their structural integrity and to support life extension programs, these ageing aircraft will need to be decommissioned in the near future to meet EU safety standards. This will seriously affect the East European airlines that are currently struggling to be competitive and survive in the global market. The EU predicts strong growth in the market which could be exploited by airlines that increase the operational life of their aircraft whilst meeting EU safety standards, leading to significantly better returns on investment and profitability. EXPECT will contribute to European wide sustainability and growth, particularly enhancing employment prospects in the new Member States.
c. Level of skill in EU:
This project will lead to an improvement in the level of skills for European citizens, as it will assist in implementing new bonded composite repair processes. The project will help in developing and sustaining EU expertise in these new technologies, with direct positive effects to the level of EU in the global composites market.
d. Environmental impact
Through enabling life extension of existing aircraft structures and improving their structural integrity, as well as by supporting of the Green Regional Aircraft development process, greening of aircraft operation and maintenance is achieved, thus helping in reduction of the environmental impact of aviation. Considering that approximately 600kg of CO2 are emitted per kilowatt hour of energy generated by fossil fuels with existing technology, this significant improvement in maintenance activities is directly matching with the targets of the Cleansky JTI.
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