Final Report Summary - COMPARE (Comparative evaluation of NDT techniques for high-quality bonded composite repairs)
Executive Summary:
Adhesively bonded composite repairs exhibit significant advantages in terms of mechanical efficiency compared to mechanically fastened ones but unfortunately they are extremely sensitive to process parameter variations. Small deviations from the repair specifications and subsequent flaws, could lead to disproportionally large consequences to the final mechanical performance of the repair and to the integrity of the structure. Moreover, the inevitable differences between laboratory and repair shop conditions could induce additional 'problematic' areas, which need to be reliably traced before the certification of methods and the release of individual aircraft to flight.
Consequently, the existence of reliable and easy to apply non-destructive testing (NDT) techniques is of capital significance to the repair reliability and the safety of flight. For this reason, existing NDT principles need to be adapted to the specificities of bonded composite repairs, in order to guarantee high-quality and highly durable bonded repairs; therefore, achieving certification of their flightworthiness. The scope of this project was to answer this need by a comparative evaluation of three different NDT techniques (conventional ultrasonic, shearography and laser ultrasound), in terms of flaws detectability (e.g. delaminations, debondings, porosity, foreign object inclusion, etc.), functional reliability, repeatability of results, operational constraints, overall performance and applicability to the typical bonded composite repair cases of the aeronautical industry. To achieve this comparative evaluation, the latest NDT methodology and equipment was used, which was provided by the consortium members. Moreover, as the majority of aircraft structural repairs is performed in aircraft hangars, the comparative evaluation went beyond the limits of detecting artificially induced flaws in a laboratory environment, by including verification of these methodologies in real-life environments (i.e. in situ) to detect flaws in actually repaired aircraft structural components.
The project resulted in the identification of strengths and weaknesses of each of the three NDT methods applied and to a detailed comparative evaluation of each method. The implementation of results in composite repair inspection will assist the correct selection of NDT methods.
Project Context and Objectives:
As detailed in COMPARE description of works (DoW), within work package one (WP1), the methodology that is being followed is defined in detail as follows. This included the definition of typical composite repair parameters, NDT requirements and constraints, detailed specifications of NDT methods to be evaluated as well as development of strategies for mobile NDT solutions. More specifically, task 1.1 (Definition of design parameters / constraints), which is the 'source' for the generation of this deliverable, contains the definition of the main design parameters and constraints that need to be taken into consideration for the inspection of bonded composite repairs. These parameters generally include:
1. definition of patch geometrical characteristics (dimensions, shape, overlap length etc.)
2. selection of patch material, thickness and plies orientation
3. selection of adhesive type and thickness.
Within this task, deliverable 1.1 was submitted.
Finally, as the main objective of this call for proposals (CfP) was to perform comparative evaluation of certain NDT methods, the criteria of this comparison effort are considered of capital importance for the success of the project. Consequently, it was decided to devote a specific task for this procedure, i.e. task 1.2. Criteria for NDT methods comparative evaluation, in order to ensure coverage of all the aspects of the evaluation procedure that are critical to the aeronautical industry. The outcome of this task was a 'matrix', including evaluation criteria and specific weight factors per criterion, tailored to the needs of bonded composite repairs for aeronautical applications, in order to produce a final ranking of the examined techniques, both in Lab Scale and in 'real-life' environment. Consequently, according to the COMPARE DoW, within task 1.2 the main parameters that affect the selection of NDT method have been considered and a corresponding 'Table of criteria for NDT methods comparative Evaluation' has been formulated. Deliverable 1.2 was submitted within this task.
Additional work was performed to make a preliminary assessment of laser ultrasound parameters. Ultrasonic measurements were performed on composite samples before damage or patch repair. Laser power levels were found to be low enough so that no visual damage was produced on samples and, at the same time, ultrasound was detected. At these power levels, medium to low ultrasound sensitivity was achieved. Rayleigh surface waves were generated on 3mm samples and Lamb waves were generated on 2mm samples. Lamb waves attenuate much slower compared to other ultrasound waves and hence are good for long range inspection, but Lamb waves have very complex wave forms and are multi-mode. Therefore, it is preferable to use Rayleigh waves to detect features on samples. To enhance Rayleigh wave generation on samples thinner than 2mm, the generation laser is focused to a line. Additionally, measurements also allowed the determination of both longitudinal and shear wave velocities. Finally, it is recommended to have samples with either a smooth, shiny finish or to be coated to create such a finish, to emulate the final product.
Apart from the main objective, which is the comparison of NDT methods, within this project new strategies for in situ damage identification, going beyond the state of the art and overcoming the inevitable problems arising from mobility of NDT methods, have been investigated. This task was split into two parts, namely theoretical examination of the practical problems to be solved and lab scale implementation of the developed solutions. Deliverable 1.3 was submitted within this task.
According to the COMPARE DoW the appropriate number of composite repair specimens with artificial flaws (calibration specimens etc.) was manufactured to support the comparative evaluation activities. Deliverable 1.4 was submitted within this task.
The implementation of the support activities in WP1, enabled the comparative evaluation of the NDT techniques, according to the criteria defined in task 1.2. These criteria have been used in task 2.1 to generate a comparative evaluation of samples with representative flaws and without repair patches as described in task 1. Non-destructive testing was carried out to the composite repaired specimens using all the three aforementioned methods and according to the test plan and the evaluation criteria defined in task 1.2. The specimen was tested with:
1. conventional ultrasound method using two scanning techniques: immersion and manual
2. laser ultrasound method
3. laser shearography (LS) method.
A comparative evaluation of the three NDT methods was produced for the lab scale representative sample and a comparison table was used to highlight their performances. Deliverable 2.1 was produced within this task.
Following the results of task 2.1 the comparative evaluation of the NDT methods was performed on a real-life sample, having a repair patch. Apart from the NDT techniques themselves, portable NDT strategies were studied, using laptop and universal serial bus (USB) data acquisition, developed in task 1.3. Advantages and disadvantages of each NDT method for bonded composite repairs of aeronautical structures were noted; and combining results of tasks 2.1 and 2.2 a final assessment of the examined NDT methods was presented, which was the main target of this project. Deliverable 2.2 was submitted within this task.
Within task 3.1 activities related to publication and project results, attempting to maximise the knowledge transmitted from this research project to the scientific and the industrial world working on aeronautics, have been carried out. The results of such activities have been presented within deliverable 3.1.
Within task 4.1 the Project Coordinator, TWI, was mainly responsible for the implementation of the project management and coordination activities as well as for the prompt periodical reporting to the Topic Manager and the Cleansky joint technology initiative (JTI). It is considered that such activities have been adequately implemented in both cases. Deliverable 4.1 was produced within this task.
Project Results:
Specification requirements
Composite repair parameters and the criteria for NDT methods comparative evaluation were defined in WP1. The selected repair parameters were divided in to the following categories that are described below in detail:
Materials:
1. Adhesive: Hysol EA 9695.050 PSF NW
2. Substrate: HexPly M21-34-268T800S-300
3. Repair patch: HexPly M20/34%/134/IM7 (12K).
Geometrical considerations - Lamination Sequence:
1. Damage size (hole of material to be removed): circle with diameter 30 to 50 mm
2. Substrate radius of curvature: Flat
3. Substrare configuration: Monolithic (no honeycomb)
4. Repair patch scheme: Circular.
5. Repair patch diameter: 30cm.
6. Scarf angle: 2-3°. Scarf ratio 1:20 to 1:30
7. Substrate lamination sequence: [0/0/+45/0/-45/90/90/-45/0/+45/0/0], thickness equals approximately 3 mm.
8. Substrate material configuration: No stringer repair considered.
9. External flushness requirements: No more than 200mm (i.e. one ply) at the external surface is acceptable, due to aerodynamic constraints.
Other requirements:
1. Safety Requirements: Standard, applied to maintenance hangars and flight line.
2. Repair application environment: Base (depot) repair conditions are assumed.
3. Accessibility: Only one-side access to the repair area was allowed assumed for application of non-destructive inspection purposes.
4. Personnel training assumptions: Skilled personnel with standard aeronautical training were assumed to be available.
5. 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).
6. Equipment man machine interface (MMI) requirements: Standard for use by skilled technical personnel.
For the preparation of deliverable 1.2 in terms of defining the appropriate comparison criteria, the following considerations have been made:
Defect type:
The type of defects determines which method is most suitable, for example conventional ultrasound testing (UT) cannot detect surface/sub-surface defects, but laser UT (LUT) is suitable for these defect types.
Defect orientation:
The type of determines which method is most suitable for example shearography cannot detect out of plane defects.
Inspection speed
It is expected that that comparative speed of the three methods will be as follows: Shearography > Piezoelectric (PZ) UT > LUT.
Characterisation of defects
The characterisation of the defects (i.e. type, size, depth) in the repair patch is expected to be difficult for all methods, especially shearography.
Inspection sensitivity:
This refers to the smallest defects that can be detected. Shearography has really low sensitivity, e.g. couple of mm. For all UT techniques, it depends on the frequency used. However, the signal-to-noise (S/N) ratio decreases as the frequency increases; and the cost of equipment may increase.
Surface preparation:
PZ UT requires a couplant such as a gel or water jet; shearography sometimes requires the surface to be painted to improve the S/N; LUT normally needs no surface preparation, but rough sample surfaces require higher laser power to be used.
Required skills from operator:
For inspection set up, data interpretation, equipment set up a skilled operator with suitable training and experience is required to carry out the testing according to the predetermined testing requirements.
Health and safety measures:
Appropriate health and safety measures are required when a laser is operational at the test area. The lasers used in LUT are considered high risk and therefore suitable safety precautions must be in place prior to the testing such as the use of eye protection or laser barriers.
According to the previously mentioned main considerations, a table of criteria has been formulated in D1.2. NDT assessments carried out on the manufactured samples.
WP2 aimed to describe the assessment of the different NDT methods that were carried out on the manufactured samples. Initially, representative lab scale samples without patch repairs were used and reported in deliverable 2.1. The NDT assessments were also carried out on realistic samples with patch repairs in order to compare the results with the results obtained from the previous testing, which were included in a follow up report D2.2.
For the testing, a square carbon reinforced fibre polymer (CFRP) panel with dimensions 300 x 300mm was manufactured by means of prepreg/autoclave technology.
For testing without a repair patch, four artificial delaminations of different sizes were induced at predefined locations using the following method. This method used intercalation of plastics bags containing small amounts of ammonium carbonate during the laminating process. The ammonium carbonate transforms to ammonia and carbon dioxide gases when reaching temperatures of about 80° C, increasing its volume and producing a local delamination during the curing process.
During the repair patch process, artificial defects were created by inserting 55µm thick disks of Flashbreaker tape, which consists of a 25µm polyester film and 30µm silicon adhesive, during the patch repair process.
Conventional ultrasound method
Due to additional availability of scanning ultrasonic systems in the consortia, two types of scans were produced water immersed automated conventional ultrasound (ACU) and manual conventional ultrasound (MCU).
Immersion automated scanning (for test piece without patch):
ACU scan was performed in a raster scan manner with a step of 0.5 mm. An immersion tank was used with a single 5mm diameter, 5MHz probe in a pulse-echo configuration. Receiver amplification was set to 44 dB using Hilgus software. After scanning the sample, which took over one hour, a time of flight C-scan was generated. To remove direct current (DC) a high-pass filter was set to 0 MHz and to remove high frequency noise a 20MHz low-pass filter was used.
Immersion automated scanning (for test piece with patch):
The ACU scan was performed in a raster scan manner with a step of 0.3 mm on the side opposite to the repair patch since the repair side was too rough and wrinkled to get reasonable results. An immersion tank was used with a single 5mm diameter, 5MHz probe in a pulse-echo configuration. Receiver amplification was set to 32.5 dB using Hilgus software. After scanning the sample, which took over one hour, a time of flight C-scan was generated. To remove DC a high-pass filter was set to 0 MHz and to remove high frequency noise a 20MHz low-pass filter was used.
Manual scanning (for both test pieces):
As originally planned in this project, a MCU inspection was performed, using a 6.25mm diameter, 10MHz probe in a pulse-echo configuration.
Laser ultrasound method (for both test pieces):
Unlike ACU and MCU, which used bulk waves, this method used surface waves to identify inserted features in material. LU generation was performed using a Q-switched Nd:Yag laser, with 1064 nm wavelength, 10 ns pulse duration, energy approximately 20mJ/pulse and repetition rate of 20Hz. The laser light was fed through an optical fibre and focused into approximately a 10 x 1mm line to enhance surface wave amplitudes in a normal direction to the line. Material damage was avoided by using the thermoelastic regime, where laser energy densities of about 2 mJ/mm2 were used, but required 64 averages to have a reasonable S/N ratio. The ultrasound detector was a state-of-the-art LU system from IOS, with a 0.5mm laser spot diameter.
LS method:
LS was applied by means of an LTI-6200S portable thermal shearography inspection system, manufactured by Laser Technology Inc. The LTI-6200S shearography system is a portable phase-stepped laser interferometer designed for the shearographic non-destructive testing of composite and metallic structures using thermal stressing. The SC-6200S Inspection Head contains the laser, shearing optics and video camera, which include the system's laser interferometer. This optical system is designed to compare the out of plane motion of adjacent points across the surface of the test article when the test article is stressed. The LTI-6200S is designed to work with thermal stressing. Within the SC-6200S, integrated halogen lamps are used to generate a thermal gradient across the area of the test article being inspected. This thermal stressing generates strains within the material caused by the thermal expansion characteristics of the structure. These strains can manifest themselves as small distortions in the surface of the test article that can be detected by the SC-6200S Shearography Camera.
Results and Conclusions:
All methods showed good indication of feature locations and similar sizes, for a sample without a repair patch, but LU and ACU showed smallest feature dimensions, i.e. 10x10mm generated feature. Additionally, ACU and MCU were able to indicate depth of features since they were in pulse-echo mode. Inspection sensitivity is expected to be superior with LU method due to its small footprint, although in one dimension only since the other dimension was limited by distance between generation and detection. None of the methods required surface preparation, but only LU and LS where non-contact and non-invasive, other methods did required a gel or water which create contamination issues in particular for repair patch samples. As far as safety is concern LU has the highest level, requiring eye protection and if performed on a real sample it would require a special shielding around inspection zone. LS although safe to use, it is recommended to avoid direct eye contact with beam, all other methods do not require particular health and safety issues. Both LU and LS require a level of expertise to generate and interpret data, but CU methods require a minimum of ultrasound testing (UT) level 1. Inspection speeds were much higher with LS, but its level of detail was the lowest. If both speed and detail are required, it is recommended to have a two level approach. Use LS as a first rough estimate and if an indication is detected, then use LU or CU to extract detailed information.
LU and CU have both shown similar capabilities in identifying artificial delaminations and they both require some level of expertise or training. However, LU has the obvious advantage, over CU, of being non-contact and non-invasive. LS as LU has the advantage of being non-contact and non-invasive, in addition possessed fastest inspection speeds, but had poorest sensitivity. As far as safety is concern LU has the highest level requiring eye protection and if it were performed on an in-service sample, it would require a special shielding around inspection zone. LS although safe to use, it is recommended to avoid direct line of sight with beam. Inspection speeds were highest using LS, but its level of detail was the lowest. If both speed and high resolution are required, it is recommended to have a two level approach. Use LS as a first rough detection system and if an indication is found, then use LU or ACU to acquire additional detail. Feature depth information may be extracted from LU data using additional signal processing and interpretation. Finally, LU inspection speeds can significantly increase by using a higher repetition rate laser, but at a higher cost.
All methods showed good indication of a general area where defects were inserted, for a sample with repair patch, but only MCU and LU were able to identify individual flaws, where smaller indications were 10mm and 6mm, respectively. These flaw dimensions fall within the aerospace industry requirements of reporting for repair a delamination size which is greater or equal to 8mm diameter or 8x8mm. In this comparison, although ACU and MCU are expected to achieve such resolution due to sensor diameters of 5mm and 6.25mm LU method showed higher resolution capability and therefore better compliance. Additionally, only LU was capable of detecting 15mm intended flaw. However, ACU and MCU were able to indicate depth of features and only ACU and LU were capable of detecting patch details.
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 EUR 1.6 trillion. The aerospace market remains a highly competitive one and any aspect of commercial advantage must be sought. COMPARE aimed to address a key element of competitive advantage for the industry, those of aircraft reliability during operation and maintenance costs.
Replying directly to JTI call for proposals, the primary drivers for COMPARE 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 European Union (EU) position within the global aerospace market, whilst maintaining the competitive advantage of the EU companies over its United States of America (USA) and Japanese rivals.
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 six years for the new A320 and every five 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 EUR 5.2 million, a significant burden for the operating airline. Total maintenance costs for Europe amount to EUR 615 million per year. The successful implementation of the COMPARE project development is expected to reduce maintenance, repair and inspection costs significantly, through increase of repair reliability and reduction of time required for the performance of repairs. Finally, potential practical applications of the innovative repair processes will concern the new Smart Fixed Wing Aircraft, leading to significant reduction of maintenance costs and increase of reliability in maintenance. The project will assist in the development of high technology small and medium sized enterprises (SMEs), where job opportunities will be developed for the industrialisation and series production of the projects results, contributing to the renown of European technology and inducing future related research developments. By providing state of the art techniques to European companies for the years to come, this project will maintain the orientation of airlines and repair stations, including Asian maintenance repairs and overhauls (MROs), towards European sources. In particular GMI, being currently leader in its market, will maintain its prominent role in the maintenance equipment and service business and will be equipped with additional important tools for further exploitation towards the USA and Asian markets.
This project delivered knowledge for improving safety and operational capability of aircraft that can lead to an increase in operational life. The countries of former Eastern Europe, which have recently entered the EU, have aged aircraft fleets, which include 30 to 35 year old aircraft. In the absence of efficient repair solutions for their composite parts, these ageing aircraft may 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. COMPARE can contribute to European wide sustainability and growth, particularly enhancing employment prospects in the new Member States. This greater level of sophistication represents a step change over current technology, which is based on less efficient methods. The project developed and sustained the EU expertise in these new technologies, particularly in versatile application of pre-fabricated patches, for the repair of thick components or parts with complex geometries.
Through enabling more extensive use of the pre-fabricated composite patch repair methodology and consequently extending the economic life of aircraft components made of composite materials, greening of aircraft fabrication and maintenance is achieved, thus helping in reduction of the environmental impact of aviation, which is one of the main targets of the CleanSky JTI.
Dissemination:
At the end of the project there were a number of publications in professional literature and at conferences, press releases and exhibition of literature at appropriate trade fairs. Time and effort was spent on publishing these articles in the journals and preparing presentations for the conferences.
The dissemination activities included:
1. Participation to the JTI 13th Call Info Day: Following an invitation by the organisation committee, GMI Aero has agreed to participate in the Clean Sky Info Day for Call 13, which took place in Paris on 6 July 2012. Within this Info Day, GMI was invited to present itself, together with the benefits coming from its participation in the Clean Sky project. Within the presentation prepared, special reference to the COMPARE project was made.
2. Participation to the 'Composites repair monitoring and validation. Dissemination of innovations and latest achievements to key players of the aeronautical industry - Aeroplan' project, Seventh Framework Programme (FP7) AAT-2011- RTD-1 (CSA-SA) 285089). All three COMPARE Consortium partners (TWI, GMI Aero and NTUA) are simultaneously participating to the AEROPLAN Coordination and Support Project. Within this project, several dissemination activities are taking place, including the preparation of a book of abstracts and presentations to specific target groups together with a website devoted to this project, namely http://www.aeroplanproject.eu. Moreover, within the frame of the Aeroplan project, the participation in a major aeronautical event has been achieved through the presentation of the innovations developed within the 'Aeroplan background project', including COMPARE, namely to the 'Aircraft composite repair management forum', organised by the 'Aviation Week Magazine', which took place on 9 October 2012 at RAI, Amsterdam, Netherlands.
3. Participation in the 'BINDT2012 Conference'. All three COMPARE Consortium partners (TWI, GMI Aero and NTUA) presented and published a paper titled 'Comparative evaluation of NDT techniques for high quality bonded composite repairs' at the British Institute of Non-destructive Testing (BINDT) 2012 conference. Presentation can be downloaded from: http://www.bindt.org/downloads/NDT2012_3B3.pdf
Use and dissemination of foreground:
The individual aspects of development have been and will continue to be, disseminated by publication and presentation at technical events. A description of the overall project technical undertaking and results will be presented in an appropriate international publication (e.g. Aerospace Testing International). However, since the development of composite inspection techniques carried out within the COMPARE project, further dissemination will not occur until intellectual property (IP) is suitably protected and agreement has been requested by all the relevant partners.
Adhesively bonded composite repairs exhibit significant advantages in terms of mechanical efficiency compared to mechanically fastened ones but unfortunately they are extremely sensitive to process parameter variations. Small deviations from the repair specifications and subsequent flaws, could lead to disproportionally large consequences to the final mechanical performance of the repair and to the integrity of the structure. Moreover, the inevitable differences between laboratory and repair shop conditions could induce additional 'problematic' areas, which need to be reliably traced before the certification of methods and the release of individual aircraft to flight.
Consequently, the existence of reliable and easy to apply non-destructive testing (NDT) techniques is of capital significance to the repair reliability and the safety of flight. For this reason, existing NDT principles need to be adapted to the specificities of bonded composite repairs, in order to guarantee high-quality and highly durable bonded repairs; therefore, achieving certification of their flightworthiness. The scope of this project was to answer this need by a comparative evaluation of three different NDT techniques (conventional ultrasonic, shearography and laser ultrasound), in terms of flaws detectability (e.g. delaminations, debondings, porosity, foreign object inclusion, etc.), functional reliability, repeatability of results, operational constraints, overall performance and applicability to the typical bonded composite repair cases of the aeronautical industry. To achieve this comparative evaluation, the latest NDT methodology and equipment was used, which was provided by the consortium members. Moreover, as the majority of aircraft structural repairs is performed in aircraft hangars, the comparative evaluation went beyond the limits of detecting artificially induced flaws in a laboratory environment, by including verification of these methodologies in real-life environments (i.e. in situ) to detect flaws in actually repaired aircraft structural components.
The project resulted in the identification of strengths and weaknesses of each of the three NDT methods applied and to a detailed comparative evaluation of each method. The implementation of results in composite repair inspection will assist the correct selection of NDT methods.
Project Context and Objectives:
As detailed in COMPARE description of works (DoW), within work package one (WP1), the methodology that is being followed is defined in detail as follows. This included the definition of typical composite repair parameters, NDT requirements and constraints, detailed specifications of NDT methods to be evaluated as well as development of strategies for mobile NDT solutions. More specifically, task 1.1 (Definition of design parameters / constraints), which is the 'source' for the generation of this deliverable, contains the definition of the main design parameters and constraints that need to be taken into consideration for the inspection of bonded composite repairs. These parameters generally include:
1. definition of patch geometrical characteristics (dimensions, shape, overlap length etc.)
2. selection of patch material, thickness and plies orientation
3. selection of adhesive type and thickness.
Within this task, deliverable 1.1 was submitted.
Finally, as the main objective of this call for proposals (CfP) was to perform comparative evaluation of certain NDT methods, the criteria of this comparison effort are considered of capital importance for the success of the project. Consequently, it was decided to devote a specific task for this procedure, i.e. task 1.2. Criteria for NDT methods comparative evaluation, in order to ensure coverage of all the aspects of the evaluation procedure that are critical to the aeronautical industry. The outcome of this task was a 'matrix', including evaluation criteria and specific weight factors per criterion, tailored to the needs of bonded composite repairs for aeronautical applications, in order to produce a final ranking of the examined techniques, both in Lab Scale and in 'real-life' environment. Consequently, according to the COMPARE DoW, within task 1.2 the main parameters that affect the selection of NDT method have been considered and a corresponding 'Table of criteria for NDT methods comparative Evaluation' has been formulated. Deliverable 1.2 was submitted within this task.
Additional work was performed to make a preliminary assessment of laser ultrasound parameters. Ultrasonic measurements were performed on composite samples before damage or patch repair. Laser power levels were found to be low enough so that no visual damage was produced on samples and, at the same time, ultrasound was detected. At these power levels, medium to low ultrasound sensitivity was achieved. Rayleigh surface waves were generated on 3mm samples and Lamb waves were generated on 2mm samples. Lamb waves attenuate much slower compared to other ultrasound waves and hence are good for long range inspection, but Lamb waves have very complex wave forms and are multi-mode. Therefore, it is preferable to use Rayleigh waves to detect features on samples. To enhance Rayleigh wave generation on samples thinner than 2mm, the generation laser is focused to a line. Additionally, measurements also allowed the determination of both longitudinal and shear wave velocities. Finally, it is recommended to have samples with either a smooth, shiny finish or to be coated to create such a finish, to emulate the final product.
Apart from the main objective, which is the comparison of NDT methods, within this project new strategies for in situ damage identification, going beyond the state of the art and overcoming the inevitable problems arising from mobility of NDT methods, have been investigated. This task was split into two parts, namely theoretical examination of the practical problems to be solved and lab scale implementation of the developed solutions. Deliverable 1.3 was submitted within this task.
According to the COMPARE DoW the appropriate number of composite repair specimens with artificial flaws (calibration specimens etc.) was manufactured to support the comparative evaluation activities. Deliverable 1.4 was submitted within this task.
The implementation of the support activities in WP1, enabled the comparative evaluation of the NDT techniques, according to the criteria defined in task 1.2. These criteria have been used in task 2.1 to generate a comparative evaluation of samples with representative flaws and without repair patches as described in task 1. Non-destructive testing was carried out to the composite repaired specimens using all the three aforementioned methods and according to the test plan and the evaluation criteria defined in task 1.2. The specimen was tested with:
1. conventional ultrasound method using two scanning techniques: immersion and manual
2. laser ultrasound method
3. laser shearography (LS) method.
A comparative evaluation of the three NDT methods was produced for the lab scale representative sample and a comparison table was used to highlight their performances. Deliverable 2.1 was produced within this task.
Following the results of task 2.1 the comparative evaluation of the NDT methods was performed on a real-life sample, having a repair patch. Apart from the NDT techniques themselves, portable NDT strategies were studied, using laptop and universal serial bus (USB) data acquisition, developed in task 1.3. Advantages and disadvantages of each NDT method for bonded composite repairs of aeronautical structures were noted; and combining results of tasks 2.1 and 2.2 a final assessment of the examined NDT methods was presented, which was the main target of this project. Deliverable 2.2 was submitted within this task.
Within task 3.1 activities related to publication and project results, attempting to maximise the knowledge transmitted from this research project to the scientific and the industrial world working on aeronautics, have been carried out. The results of such activities have been presented within deliverable 3.1.
Within task 4.1 the Project Coordinator, TWI, was mainly responsible for the implementation of the project management and coordination activities as well as for the prompt periodical reporting to the Topic Manager and the Cleansky joint technology initiative (JTI). It is considered that such activities have been adequately implemented in both cases. Deliverable 4.1 was produced within this task.
Project Results:
Specification requirements
Composite repair parameters and the criteria for NDT methods comparative evaluation were defined in WP1. The selected repair parameters were divided in to the following categories that are described below in detail:
Materials:
1. Adhesive: Hysol EA 9695.050 PSF NW
2. Substrate: HexPly M21-34-268T800S-300
3. Repair patch: HexPly M20/34%/134/IM7 (12K).
Geometrical considerations - Lamination Sequence:
1. Damage size (hole of material to be removed): circle with diameter 30 to 50 mm
2. Substrate radius of curvature: Flat
3. Substrare configuration: Monolithic (no honeycomb)
4. Repair patch scheme: Circular.
5. Repair patch diameter: 30cm.
6. Scarf angle: 2-3°. Scarf ratio 1:20 to 1:30
7. Substrate lamination sequence: [0/0/+45/0/-45/90/90/-45/0/+45/0/0], thickness equals approximately 3 mm.
8. Substrate material configuration: No stringer repair considered.
9. External flushness requirements: No more than 200mm (i.e. one ply) at the external surface is acceptable, due to aerodynamic constraints.
Other requirements:
1. Safety Requirements: Standard, applied to maintenance hangars and flight line.
2. Repair application environment: Base (depot) repair conditions are assumed.
3. Accessibility: Only one-side access to the repair area was allowed assumed for application of non-destructive inspection purposes.
4. Personnel training assumptions: Skilled personnel with standard aeronautical training were assumed to be available.
5. 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).
6. Equipment man machine interface (MMI) requirements: Standard for use by skilled technical personnel.
For the preparation of deliverable 1.2 in terms of defining the appropriate comparison criteria, the following considerations have been made:
Defect type:
The type of defects determines which method is most suitable, for example conventional ultrasound testing (UT) cannot detect surface/sub-surface defects, but laser UT (LUT) is suitable for these defect types.
Defect orientation:
The type of determines which method is most suitable for example shearography cannot detect out of plane defects.
Inspection speed
It is expected that that comparative speed of the three methods will be as follows: Shearography > Piezoelectric (PZ) UT > LUT.
Characterisation of defects
The characterisation of the defects (i.e. type, size, depth) in the repair patch is expected to be difficult for all methods, especially shearography.
Inspection sensitivity:
This refers to the smallest defects that can be detected. Shearography has really low sensitivity, e.g. couple of mm. For all UT techniques, it depends on the frequency used. However, the signal-to-noise (S/N) ratio decreases as the frequency increases; and the cost of equipment may increase.
Surface preparation:
PZ UT requires a couplant such as a gel or water jet; shearography sometimes requires the surface to be painted to improve the S/N; LUT normally needs no surface preparation, but rough sample surfaces require higher laser power to be used.
Required skills from operator:
For inspection set up, data interpretation, equipment set up a skilled operator with suitable training and experience is required to carry out the testing according to the predetermined testing requirements.
Health and safety measures:
Appropriate health and safety measures are required when a laser is operational at the test area. The lasers used in LUT are considered high risk and therefore suitable safety precautions must be in place prior to the testing such as the use of eye protection or laser barriers.
According to the previously mentioned main considerations, a table of criteria has been formulated in D1.2. NDT assessments carried out on the manufactured samples.
WP2 aimed to describe the assessment of the different NDT methods that were carried out on the manufactured samples. Initially, representative lab scale samples without patch repairs were used and reported in deliverable 2.1. The NDT assessments were also carried out on realistic samples with patch repairs in order to compare the results with the results obtained from the previous testing, which were included in a follow up report D2.2.
For the testing, a square carbon reinforced fibre polymer (CFRP) panel with dimensions 300 x 300mm was manufactured by means of prepreg/autoclave technology.
For testing without a repair patch, four artificial delaminations of different sizes were induced at predefined locations using the following method. This method used intercalation of plastics bags containing small amounts of ammonium carbonate during the laminating process. The ammonium carbonate transforms to ammonia and carbon dioxide gases when reaching temperatures of about 80° C, increasing its volume and producing a local delamination during the curing process.
During the repair patch process, artificial defects were created by inserting 55µm thick disks of Flashbreaker tape, which consists of a 25µm polyester film and 30µm silicon adhesive, during the patch repair process.
Conventional ultrasound method
Due to additional availability of scanning ultrasonic systems in the consortia, two types of scans were produced water immersed automated conventional ultrasound (ACU) and manual conventional ultrasound (MCU).
Immersion automated scanning (for test piece without patch):
ACU scan was performed in a raster scan manner with a step of 0.5 mm. An immersion tank was used with a single 5mm diameter, 5MHz probe in a pulse-echo configuration. Receiver amplification was set to 44 dB using Hilgus software. After scanning the sample, which took over one hour, a time of flight C-scan was generated. To remove direct current (DC) a high-pass filter was set to 0 MHz and to remove high frequency noise a 20MHz low-pass filter was used.
Immersion automated scanning (for test piece with patch):
The ACU scan was performed in a raster scan manner with a step of 0.3 mm on the side opposite to the repair patch since the repair side was too rough and wrinkled to get reasonable results. An immersion tank was used with a single 5mm diameter, 5MHz probe in a pulse-echo configuration. Receiver amplification was set to 32.5 dB using Hilgus software. After scanning the sample, which took over one hour, a time of flight C-scan was generated. To remove DC a high-pass filter was set to 0 MHz and to remove high frequency noise a 20MHz low-pass filter was used.
Manual scanning (for both test pieces):
As originally planned in this project, a MCU inspection was performed, using a 6.25mm diameter, 10MHz probe in a pulse-echo configuration.
Laser ultrasound method (for both test pieces):
Unlike ACU and MCU, which used bulk waves, this method used surface waves to identify inserted features in material. LU generation was performed using a Q-switched Nd:Yag laser, with 1064 nm wavelength, 10 ns pulse duration, energy approximately 20mJ/pulse and repetition rate of 20Hz. The laser light was fed through an optical fibre and focused into approximately a 10 x 1mm line to enhance surface wave amplitudes in a normal direction to the line. Material damage was avoided by using the thermoelastic regime, where laser energy densities of about 2 mJ/mm2 were used, but required 64 averages to have a reasonable S/N ratio. The ultrasound detector was a state-of-the-art LU system from IOS, with a 0.5mm laser spot diameter.
LS method:
LS was applied by means of an LTI-6200S portable thermal shearography inspection system, manufactured by Laser Technology Inc. The LTI-6200S shearography system is a portable phase-stepped laser interferometer designed for the shearographic non-destructive testing of composite and metallic structures using thermal stressing. The SC-6200S Inspection Head contains the laser, shearing optics and video camera, which include the system's laser interferometer. This optical system is designed to compare the out of plane motion of adjacent points across the surface of the test article when the test article is stressed. The LTI-6200S is designed to work with thermal stressing. Within the SC-6200S, integrated halogen lamps are used to generate a thermal gradient across the area of the test article being inspected. This thermal stressing generates strains within the material caused by the thermal expansion characteristics of the structure. These strains can manifest themselves as small distortions in the surface of the test article that can be detected by the SC-6200S Shearography Camera.
Results and Conclusions:
All methods showed good indication of feature locations and similar sizes, for a sample without a repair patch, but LU and ACU showed smallest feature dimensions, i.e. 10x10mm generated feature. Additionally, ACU and MCU were able to indicate depth of features since they were in pulse-echo mode. Inspection sensitivity is expected to be superior with LU method due to its small footprint, although in one dimension only since the other dimension was limited by distance between generation and detection. None of the methods required surface preparation, but only LU and LS where non-contact and non-invasive, other methods did required a gel or water which create contamination issues in particular for repair patch samples. As far as safety is concern LU has the highest level, requiring eye protection and if performed on a real sample it would require a special shielding around inspection zone. LS although safe to use, it is recommended to avoid direct eye contact with beam, all other methods do not require particular health and safety issues. Both LU and LS require a level of expertise to generate and interpret data, but CU methods require a minimum of ultrasound testing (UT) level 1. Inspection speeds were much higher with LS, but its level of detail was the lowest. If both speed and detail are required, it is recommended to have a two level approach. Use LS as a first rough estimate and if an indication is detected, then use LU or CU to extract detailed information.
LU and CU have both shown similar capabilities in identifying artificial delaminations and they both require some level of expertise or training. However, LU has the obvious advantage, over CU, of being non-contact and non-invasive. LS as LU has the advantage of being non-contact and non-invasive, in addition possessed fastest inspection speeds, but had poorest sensitivity. As far as safety is concern LU has the highest level requiring eye protection and if it were performed on an in-service sample, it would require a special shielding around inspection zone. LS although safe to use, it is recommended to avoid direct line of sight with beam. Inspection speeds were highest using LS, but its level of detail was the lowest. If both speed and high resolution are required, it is recommended to have a two level approach. Use LS as a first rough detection system and if an indication is found, then use LU or ACU to acquire additional detail. Feature depth information may be extracted from LU data using additional signal processing and interpretation. Finally, LU inspection speeds can significantly increase by using a higher repetition rate laser, but at a higher cost.
All methods showed good indication of a general area where defects were inserted, for a sample with repair patch, but only MCU and LU were able to identify individual flaws, where smaller indications were 10mm and 6mm, respectively. These flaw dimensions fall within the aerospace industry requirements of reporting for repair a delamination size which is greater or equal to 8mm diameter or 8x8mm. In this comparison, although ACU and MCU are expected to achieve such resolution due to sensor diameters of 5mm and 6.25mm LU method showed higher resolution capability and therefore better compliance. Additionally, only LU was capable of detecting 15mm intended flaw. However, ACU and MCU were able to indicate depth of features and only ACU and LU were capable of detecting patch details.
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 EUR 1.6 trillion. The aerospace market remains a highly competitive one and any aspect of commercial advantage must be sought. COMPARE aimed to address a key element of competitive advantage for the industry, those of aircraft reliability during operation and maintenance costs.
Replying directly to JTI call for proposals, the primary drivers for COMPARE 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 European Union (EU) position within the global aerospace market, whilst maintaining the competitive advantage of the EU companies over its United States of America (USA) and Japanese rivals.
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 six years for the new A320 and every five 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 EUR 5.2 million, a significant burden for the operating airline. Total maintenance costs for Europe amount to EUR 615 million per year. The successful implementation of the COMPARE project development is expected to reduce maintenance, repair and inspection costs significantly, through increase of repair reliability and reduction of time required for the performance of repairs. Finally, potential practical applications of the innovative repair processes will concern the new Smart Fixed Wing Aircraft, leading to significant reduction of maintenance costs and increase of reliability in maintenance. The project will assist in the development of high technology small and medium sized enterprises (SMEs), where job opportunities will be developed for the industrialisation and series production of the projects results, contributing to the renown of European technology and inducing future related research developments. By providing state of the art techniques to European companies for the years to come, this project will maintain the orientation of airlines and repair stations, including Asian maintenance repairs and overhauls (MROs), towards European sources. In particular GMI, being currently leader in its market, will maintain its prominent role in the maintenance equipment and service business and will be equipped with additional important tools for further exploitation towards the USA and Asian markets.
This project delivered knowledge for improving safety and operational capability of aircraft that can lead to an increase in operational life. The countries of former Eastern Europe, which have recently entered the EU, have aged aircraft fleets, which include 30 to 35 year old aircraft. In the absence of efficient repair solutions for their composite parts, these ageing aircraft may 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. COMPARE can contribute to European wide sustainability and growth, particularly enhancing employment prospects in the new Member States. This greater level of sophistication represents a step change over current technology, which is based on less efficient methods. The project developed and sustained the EU expertise in these new technologies, particularly in versatile application of pre-fabricated patches, for the repair of thick components or parts with complex geometries.
Through enabling more extensive use of the pre-fabricated composite patch repair methodology and consequently extending the economic life of aircraft components made of composite materials, greening of aircraft fabrication and maintenance is achieved, thus helping in reduction of the environmental impact of aviation, which is one of the main targets of the CleanSky JTI.
Dissemination:
At the end of the project there were a number of publications in professional literature and at conferences, press releases and exhibition of literature at appropriate trade fairs. Time and effort was spent on publishing these articles in the journals and preparing presentations for the conferences.
The dissemination activities included:
1. Participation to the JTI 13th Call Info Day: Following an invitation by the organisation committee, GMI Aero has agreed to participate in the Clean Sky Info Day for Call 13, which took place in Paris on 6 July 2012. Within this Info Day, GMI was invited to present itself, together with the benefits coming from its participation in the Clean Sky project. Within the presentation prepared, special reference to the COMPARE project was made.
2. Participation to the 'Composites repair monitoring and validation. Dissemination of innovations and latest achievements to key players of the aeronautical industry - Aeroplan' project, Seventh Framework Programme (FP7) AAT-2011- RTD-1 (CSA-SA) 285089). All three COMPARE Consortium partners (TWI, GMI Aero and NTUA) are simultaneously participating to the AEROPLAN Coordination and Support Project. Within this project, several dissemination activities are taking place, including the preparation of a book of abstracts and presentations to specific target groups together with a website devoted to this project, namely http://www.aeroplanproject.eu. Moreover, within the frame of the Aeroplan project, the participation in a major aeronautical event has been achieved through the presentation of the innovations developed within the 'Aeroplan background project', including COMPARE, namely to the 'Aircraft composite repair management forum', organised by the 'Aviation Week Magazine', which took place on 9 October 2012 at RAI, Amsterdam, Netherlands.
3. Participation in the 'BINDT2012 Conference'. All three COMPARE Consortium partners (TWI, GMI Aero and NTUA) presented and published a paper titled 'Comparative evaluation of NDT techniques for high quality bonded composite repairs' at the British Institute of Non-destructive Testing (BINDT) 2012 conference. Presentation can be downloaded from: http://www.bindt.org/downloads/NDT2012_3B3.pdf
Use and dissemination of foreground:
The individual aspects of development have been and will continue to be, disseminated by publication and presentation at technical events. A description of the overall project technical undertaking and results will be presented in an appropriate international publication (e.g. Aerospace Testing International). However, since the development of composite inspection techniques carried out within the COMPARE project, further dissemination will not occur until intellectual property (IP) is suitably protected and agreement has been requested by all the relevant partners.