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Executive Summary:

Overall, IAPETUS is proposing the development of multifunctional systems that will be used for critical repair of structural components providing out of autoclave on-aircraft curing capabilities, tailored coupling of dissimilar interfaces, enhanced mechanical properties and finally sensing / life monitoring abilities for the lifetime of the repaired component. The final aim of IAPETUS is the application of the proposed methodology on structural components, where all the aforementioned functionalities will be demonstrated.

Within the scope of IAPETUS is the exploitation of state of the art technology know how that consortium members possess, such as alternative curing technologies and advanced curing monitoring technologies such as wireless impedance curing monitoring, or revolutionary NDT monitoring techniques such as lock-in transient thermography that will help in the assessment and validation of the proposed technologies.

IAPETUS is expected to have a major impact in the field of aircraft repair as it proposes repair technologies that are applicable on both older and new generation aircrafts, readily addresses issues that arise from the current repair technologies, such as on aircraft repair, galvanic corrosion or increased damage tolerance, and is introducing novel health monitoring concepts.

IAPETUS aims at revolutionizing aircraft repair processes with composite materials, (mainly the hot bond repair approach used in field repair) by using novel hybrid composites. These materials are expected to fully exploit the unique properties of nano-scaled fillers in the matrix and adhesive systems in order to step change the hot bond field repair by:

• Introducing an innovative curing methodology (either via direct resistance heating or by induction heating) of the patch/adhesive system, with the benefits of homogenous heating and curing and the minimization of the developed thermal stresses. In addition the introduction of an on line curing monitoring system will allow for a precise control of curing concluded to enhanced quality of the resulted repair.
• Providing direct inspection of the bonding/repair integrity and continuous health monitoring of the repaired site in service.
• Offering increased mechanical and bonding performance in the repair itself

Project Context and Objectives:

The main target of IAPETUS is to develop novel repair technologies and materials for both metallic and composite aircrafts. This will be realised via the usage of novel hybrid composite systems, which offer the multi-functionality that will lead to the development of innovative (non conventional) repair technologies and life cycle health monitoring capabilities together with the enhancement of repair efficiency.

• General Objective

The main target of IAPETUS is to develop novel repair technologies and materials for both metallic and composite aircrafts. This will be realised via the usage of novel hybrid composite systems, which offer the multi-functionality that will lead to the development of innovative 9non conventional) repair technologies and life cycle health monitoring capabilities together with the enhancement of repair efficiency.

• Strategic Objectives

The goals of the IAPETUS project comply with the top-level objectives of the aeronautical work-programme, identified in the Strategic Research Agenda and the Vision 2020 report:

IAPETUS directly contributes to the two Top-Level Objectives for European aeronautics, identified in the Strategic Research Agenda and the Vision 2020 Report:

1. To meet society’s needs for a more efficient, safer and environmentally friendly air transport.
2. To achieve global leadership for European aeronautics, with a competitive supply chain capable of exploiting all of the expertise in Europe, turning new technologies into competitive products.

IAPETUS contributes to the creation of a more efficient, safer and environmentally friendlier air transport, by addressing the issue of novel repair technologies in aero-structures and forwarding cost effective performance and sustainable international development, directly meeting the challenge posed in the Strategic Research Agenda.

IAPETUS promotes global leadership for European aeronautics by creating a competitive supply chain that includes small, medium and large size enterprises. In this project, top level research organisations from the aeronautical field develop repair technologies that are well in demand in European Aeronautics and can be readily used by industrial end users.

• Scientific and technical objectives:
• To develop an innovative, easy to apply, direct heating composite bonding patch repair technology for the maintenance of aerostructures, serving both the cases of ageing aluminium aircrafts as well as the new generation of Composite aircrafts.
• To develop a system with a cure monitoring sensor embedded in the conductive patch, which controls the heating profile in order to result in an optimally cured bonding patch repair.
• To develop new electrically conductive composite repair patches with improved mechanical performance based on the appropriate use of nanotechnology. The composite patches will consist of modified graphite / epoxy system. The modified epoxy matrix material contains a controlled amount of Carbon Nanotubes (CNTs) that gives to the graphite/epoxy patch homogenized electrical and thermal conductivity properties, thus reducing the electrical and thermal anisotropy of the patch, improved fracture toughness characteristics
• To develop a new generation of adhesives, modified by using CNT additives that offer improved mechanical properties, increased peeling and shear strength, controlled coefficient of thermal expansion and high electric and thermal conductivity, permitting, thus, the application of the direct heating polymerization process. Two heating/curing alternatives for the new patch repair system will be investigated and evaluated against technologies currently in use such as the heating blankets, (a) direct resistance heating and (b) induction heating
• To overcome the problem of galvanic corrosion in the case of aluminium Aerostructures. The electrically conductive graphite/epoxy patch repair system, although ideal for the repair of composite components, interacts electrochemically with Al. IAPETUS will address the issue of galvanic corrosion, either by minimizing Red Ox potential through the proper control of CNT types and content, or by overcoming it with the use of modified but electrically non-conductive adhesive films.
• To develop approaches that exploits the proven sensing properties of the integral CNT network in the patch and the adhesive for the continuous monitoring of the structural integrity of the repaired site. This will be performed via the measurement of the conductivity changes and the mapping of variations that are due to deformation of the percolated CNT network and/ or irreversible changes due to the in service degradation of the patch and the adhesive.
• To integrate and validate all the above technologies for the innovative repair and the smart patch for damage monitoring in coupon level.
• To implement the above technologies for the repair of small scale, aluminium and composite aerostructures using innovative curing technologies and achieve enhanced damage tolerance and fatigue performance smart patch repair, with life monitoring capabilities.
• Specific objectives:

Pioneering The Air Transport of the Future: Break through and emerging technologies

The European aeronautics industry has to be in the forefront of technology. This is a prerequisite for its viability against overseas competition. The research and development of revolutionary methods and technologies will strengthen the competitiveness of the European Aerospace industry, ensuring long term viability and the readiness to face challenges in the near future.

A fundamental issue in this approach is the investment in technologies that will extend the airworthiness of current aerostructures, while ensuring that emerging technologies will be able to support aerostructures of the near future. This is made even more imperative with the introduction of aircrafts such as the AIRBUS 380, which make extensive use of advanced materials that require more stringent and efficient supporting technologies.

IAPETUS targets directly to the core of these requirements since it revolutionalizes repair technology, by exploiting state of the art materials and techniques as well as by introducing new technological concepts. These novel concepts start from the patch design and its interface with the parent structure, continue to propose novel concepts regarding the repair process with a view to developing and validating efficient on-aircraft repair processes and finally offer the ability of continuous assessment of the repair efficiency. It is worth noting that all the above are performed non intrusively, via the use of nano-sized reinforcing phases, which provide the multi-functionality to the doubler/ substrate system.

To this respect, IAPETUS is in total accordance with the life cycle priority of Breakthrough and emerging technologies with respect to Life cycle (AAT.2008.4.2.2) which is concerned with:

• the investigation of new approaches to the maintenance of air vehicles: IAPETUS will introduce new concepts regarding the composite patch design and application,
• the application of advanced technologies in existing aircraft: IAPETUS will focus on tailoring the interphase between the composite doubler and current aluminum structures to minimizing/eliminating electrochemical corrosion while achieving advanced damage tolerance properties.
• self-monitoring; built in all systems: IAPETUS will exploit the on-going research in using the conductivity of the isotropically conductive hybrid doubler to monitor the patch efficiency, by mapping the changes in the stress state underneath the doubler that mirror unwanted crack propagations as well as the integrity of the patch itself.
• increased use of nano-technologies: IAPETUS will achieve the above priorities through the usage of state of the art materials that are in the forefront of the current European and International research, such as carbon nanotubes.



TECNALIA ES RTD Nanocoatings, Nanocomposites based on polymer, ceramic or metallic matrixes

Nanotechnologies for structural applications

Nanotechnologies for sensors Coordinator

SP0&1 Leader/WP0.1 Leader/WP1.1 Leader

Material (CNT) development, Laboratory testing,

Electrochemical behaviour, numerical analysis, NDT techniques

PZL-Swidnik PL IND Engineering and manufacturing of aircraft structures, numerical analyses, NDT , ground and flight testing WP3.1 Leader

Manufacturing, Repair application, Testing of large scale components

HUN SW IND Manufacturer and marketer of advanced epoxy resins, adhesives, coating systems, electrical insulating materials, tooling materials and structural composites Development & supply of materials (resins & adhesives)

UoP EL HE Design and optimization of composite structures

NDE techniques

Smart materials and structures

Sensors / actuators technology SP2 Leader /WP1.4 Leader/WP2.2 Leader

Technology development, Laboratory testing,

Sensing & monitoring technologies, numerical analysis

INASCO EL SME Technology provider on advanced materials and industrial processes

Sensor / actuator manufacturer WP0.2 Leader

Curing monitoring system development, numerical analysis, Economic evaluation

DAHER* FR IND Aerospace manufacturer, maintenance services SP3 Leader (until August 2011)

WP3.2 Leader

Manufacturing, Repair application,

GMI FR SME On-site repair application specialist, manufacturer of repair equipment WP1.3 Leader

Repair equipment, modelling & thermal management of processes

UoI EL HE Composite testing

Advanced NDT techniques

Electrochemical behaviour of bimetallic interfaces WP1.2 Leader/WP2.3 Leader

Technology development, Laboratory testing,

Sensing & monitoring technologies

USFD UK HE From microscopy to design via fundamental understanding of materials behaviour WP2.1 Leader

Laboratory testing,

Sensing & monitoring technologies, numerical analysis

HAI EL IND Aerospace manufacturer, maintenance SP3 Leader/WP3.3 Leader

Manufacturing, Repair application, Testing of large scale components

(*) DAHER announces its withdrawal on31/08/2011 as it is reflected in Amendment nº2 to the project Contract

The consortium gathers 3 Universities and 1 research centre from 3 different European Countries, 3 aircraft manufacturers/aircraft part suppliers (DAHER, HAI, PZL), and 1 global manufacturer and marketer of differentiated chemicals (HUN). All research organizations have long-standing, established collaboration to the entire European Aircraft Industry. In addition, they have worked together previously in several projects.

Description of work

The project has been organized into three technical Sub-projects (SP). Each SP will focus on a different aspect of the overall research effort and is also broken down into Work-packages and Tasks.

SP1 will deal with the development of the two novel curing methodologies (direct resistance and induction heating) and patch material improvement through the use of CNT additives. Possible solutions for the issue of galvanic corrosion will also be investigated and a curing monitoring system suitable for use in the field of composite repair and adapted to CNT doped materials will be implemented. Finally, the proposed curing methodologies will be experimentally evaluated in terms of curing efficiency and patch bonding integrity. Based on the outcome of this study, the best of the two proposed processes will be selected to continue in the final technology integration phase in SP3.

SP2 will demonstrate the applicability of the smart patch concept based on the approach of electrical resistance measurements and will provide both hardware and software for a real life application of the method (off-line and on-line). In this sense, the electrical resistance mapping performance for the detection of various types of damage, both critical and sub-critical, in repaired aero-structures will be critically evaluated against typical ultrasonic inspection.

A second NDT technique that will be also validated against ultrasonics and will be used for on-line damage monitoring will also be adopted: The Flash Thermography method.

SP3 will provide the framework of integration of the technologies of innovative curing, curing control and monitoring (developed in SP1) and the smart patch technologies (developed and validated in SP2). As a first step the integration of technologies will be made in small scale components and validated through appropriate testing (quasi-static and dynamic/fatigue loading conditions). Provided the successful small scale validation, the next step of the present SP concerns the validation of the integrated system in a suitable aeronautical structure, which selection will be based on a thorough review of the common damage occurrences and standard practices of the industry. A structure that is believed to present a challenging case of damage repair while it accommodates the planned activities like manufacturing and testing is a wing-type structure. The envisaged construction will consist of thin (skin) and thick (web) components. Both Aluminium and Composite structures will be manufactured.

Project Results:

SP0 management and exploitation planning

The coordinator and the partners involved in project and subprojects management have normally dealt with all the day to day administrative aspects:

• Management of funding
• Distribution of information from the EC to the consortium
• Organisation of consortium progress review meetings
• Preparation of periodic report
• Distribution of project reports and further deliverables to the EC
• Resolving of problems occurring between the partners during the project

More in detail, management activities are listed:

• Website of the project with the update information of the project. The website is divided in two different areas, one for public dissemination of the project and, the second one, only for project members.
• Organization of progress project meetings (KOM, 6 months, 12 months, 18 months and 24 months, 30months and final meeting (43 months)
• Organization of several technical meetings (several teleconferences to prepare SP3 activities and 35 months progress teleconference). Organisation of two workshops for the implementation of the two Innovative curing technologies in composite and aluminium demostrators at UoI (Greece) and PZL (Poland) in October and November 2012
• Organization of several technical teleconferences
• Preparation of the official reporting documents.
• Distribution of the funding between partners.
• Coordination of the diffusion actions in different forums (Aeronautics Days 2011, Matcomp 2011, JEC Composites Show 2013)
• Preparation of two Amendments to the Contract (June 2012 and March 2012)

SP1 Innovative curing methodologies for composite patch application

Main objectives achieved during this SP are summarized hereunder:

• Based on a review on the state-of-the-art technologies the following fundamental elements were determined: The representative patch design, patch application process and quality control procedure.
• Efficient fabrication techniques that support the incorporation of CNTs in the composite patch laminates were provided. Electrochemical corrosion phenomena associated with the application of CFRP patches on aluminium substrate were also investigated. The work also included the deployment of a dielectric curing monitoring system adapted to the CNT doped resin.
• Development and deployment of the induction heating and resistance heating set-ups. A major part of the task dealt with the integration of the curing and temperature monitoring systems. The influence of the CNTs in the effectiveness of the processes was also investigated.
• Modelling schemes of the direct resistance heating/curing process were developed. The heating/curing concepts were studied by numerical simulations to provide a step-by-step confirmation of the applicability of the proposed techniques and subsequently to optimise them for use in the field of patch repair.
• Workshop between UoP, GMI, INASCO and PZL to train PZL personnel on the novel patch application techniques like the Induction Heating process and Direct Resistance Heating, so that PZL could assume all manufacturing activities related to patch application in SP3.
• Investigation on the effectiveness of the innovative patch application processes was accomplished.Each of the innovative technology was related to a specific substrate for further repair implementation: Induction curing for composite patch repair on Aluminium substrates and Resistance curing for composite pacth repair on Composite substrates

SP2 Innovative smart composite patches for damage detection in repair

• Definition of the “smart patch” approach within IAPETUS and the most promising sensing technologies for smart patch implementation were evaluated.
• Provision of the design requirements and implementation strategy of a model smart repair system were provided.
• Implementation off-line of the smart patch technologies on model repair configurations with 6 artificially induced damage specifically defined.
• Application of Damage monitoring technologies off-line and verification, calibration and benchmarking of the proposed smart patch concepts were outlined.

SP3 Integration and validation of innovative curing methodologies and smart patch damage monitoring in a multipurpose composite patch repair system

• Definition of the coupons for the test campaign.
• Development of two different types of coupons to investigate the behaviour of two representative joints in multiple regions: different areas of structural integrity are examined in a bonded repair to a cracked plate.
• Dynamic mechanical testing at coupon and demonstrator level
• To describe the validation of the integrated innovative curing technology into large scale applications. Results and conclusions of previous WPs were incorporated as inputs to WP3 in order to facilitate the development of the patch from the design to implementation phase. Within this subproject not only innovative curing techniques were recognized but also smart patch sensing methods were identified. This scaling up process from coupon to demonstrator level assisted the maturation and the industrialization of both technologies, setting the specifications and the guidelines for further development.
• Selection of the large scale structures (Aluminum and composite) that would be manufactured, in order to introduce and compare the two different patching technologies (conventional vs innovative method).

Composite Demostrator. RESISTANCE HEATING

The aeronautical structure which was selected for the scaling up process of the innovative curing technique (Resistance Heating) along with the application of smart patch method was a representative wing panel of width 550mm and length 550mm. The dimensions were restricted due to lack of larger testing machines, available among the partners.

The structure consisted of a skin having the before mentioned dimensions and three “T” stringers.

Three demonstrators were developed, 3-stringer stiffened panels to be the representative article of a lower wing part, one for the application of conventional curing technique utilizing the heating blanket method (implemented by HAI), the second one for the application of the innovative curing technique utilizing the resistance heating method (implemented by UoP) and the third one was considered as spare

Aluminium demonstrator INDUCTION CURING

The helicopter vertical stabilizer was selected as an Aluminium demonstrator. The vertical stabilizer is sandwich structure, the core of which was made of HexWeb CRIII-5/32-5052-.001N-3.8 and the skin was made of AL-2024 T3 alloy sheet, 1.5mm to 0.6mm thickness. Prior to the selection of the critical area to be repaired, fatigue test was performed and the most critical region was identified to be around the holes at the centre of the stabilizer. The artificially introduced crack that would be repaired by the panel was 35mm long close to the hole. The repair patch consisted of 4 plies Hexcel 43280S fabric with a stacking ply sequence [90/0]2. Each ply was impregnated with CNT doped Epocast 52

Two demonstrators – the composite patch repaired stabilizer, the first using reference conventional curing (Anita equipment) and the second using innovative curing (induction heating with vacuum) were manufactured by PZL. The second one was prepared with GMI help, using induction heating

Main results

IAPETUS is focused in two approaches:

• The first approach is related to the direct resistance heating of the system via the use of the appropriate current density. The direct resistance heating concept is based on the application of electrical current through the carbon fiber bundles as shown in Figure 3-11 and uses them as heating elements. A typical carbon fibre epoxy system doped with nanotubes has electrical resistance of the order of Ohms. As is reported in IAPETUS DoW and based on initial studies performed by UoP, a current of the order of 10 A may raise the temperature of such a system up to 100 °C. With the CNT network functioning together with the carbon fibers as the heating element within the structure, (a) any unwanted effects resulting from inhomogeneous heating by the external heating sources will be eliminated, and (b) there will be minimum need for complicated heating and control systems to provide a homogeneous heat flow.
• The second approach is based on the thermal activation of the carbon fibers together with the CNT network in the patch by means of induction heating. Induction heating occurs in ferromagnetic materials or, in this case in the CNT, when exposed to a varying magnetic field. This is the result of the development of Eddy Currents in the material. Heat generation is mainly the consequence of the Joule effect. In general, an induction system consists of a power generator, an induction coil (Figure 3-12), and a susceptor. The magnetic field is generated by a current varying with time, passing through the Induction Coil. The material to be bonded (composite patch) and the susceptor are in a fixed position relative to the Coil (the coil could also be moved above the susceptor in the case of a large surface to be heated by “induction”). The power generator creates the alternating current in the coil. This voltage has a predetermined power not exceeding 50 – 70 Watts for a typical application, while the optimum frequency lies in the area of 5 KHz to 40 KHz.

Consequently, the two repair methodologies developed within SP1 where considered to be implemented in SP3. According to the results each of the innovative technology was related to a specific substrate for further repair implementation:

- Induction curing for composite patch repair on Aluminium substrates

Aluminum Substrate → induction heating (employed by GMI)

Aluminum Substrate → conventional methodology (employed by PZL /AgustaWestland)

- Resistance curing for composite pacth repair on Composite substrates

Composite Substrate → resistance heating (employed by UoP)

Composite Substrate → conventional methodology (employed by UoP)

In IAPETUS program, it is shown that the induction heating (IH) and direct resistance heating (DRH) can be successfully used to cure the composite patch to repair carbon composite or aluminum aeronautical structure.

Both novel methods are good choice for field repair applications.

The advantage of both techniques are the following:

 Accurate temperature control (direct control through continuous power control),
 Localized volumetric heating,
 Minimized distortion because site-specific process delivers heat exactly where it is needed, so the parts are not exposed to heat.
 Energy-efficient and rapid process,
 One-step process for multilayer repair,
 Portable equipment,
 Easy to operate units,
 Vacuum consolidation,
 Variable damage area capability.

The following guidelines for applying innovative curing process together with curing process monitoring should be presented:

1. The areas of usual metal or composite repair are generally small: 200 x 200 [mm], large repair are for areas of 600 x 600 [mm], geometrically complex surface.

The patch to be cured using the DRH method must have some size limits. These limits have to do with the available electrical power, produced heat and temperature distribution that can be achieved. The maximum dimensions for the patch to be cured with the DRH method are 300 x 300 [mm], geometrically complex, while the thickness is limited to maximum 6 layers. The IH technique generally has not the above limitation because of the size of coil and possibility to move the coil to heat large areas, geometrically complex surface, thin or thick. The coil must be adapted to the surface of interest before heating.

It is possible to apply: Pancake coil, Butterfly Coil with bent ‘wings’, Butterfly Coil with separated ‘wings’.

2. The DRH system uses the resistivity of carbon fibers within the patch to heat and cure the liquid resin. Copper electrodes are placed at the edges of the patch in contact with the carbon fibers. The IH heating system does not require such elements in the composite patch.
3. In order to use the system efficiently and avoid unwanted faults like overheating the material, the electrical properties of the carbon fibers must be known. The resistivity of the material and its dependence on temperature is a very important parameter as it directly affects the temperature developed and the power requirements. Therefore a short testing procedure for system calibration should be performed in order to identify critical properties.
4. For the control and the data logging of the curing cycle in the IH method the MAXIM User Interface was used. The control software allows the user to define the voltage and current limits, set the working frequency of the coil and record the measurements from up to 6 thermocouples. The DRH curing system was designed to dynamically control the curing process based on thermocouple readings (3 thermocouples for a specific setup).
5. The IH system can generate energy to heat certain quantity of adhesive material up to a temperature: 95 to 120 °C, sometimes 180 °C. Control of the penetration depth of the heat in the substrate can be achieved through adjustment of the frequency.

The DRH system is capable of reaching curing temperatures up to 180oC.

6. Using the DRH technique for composite patch repair, because patch and substrate materials are electrically conductive, it is clear that contact between the fibers under electric current and the substrate should be avoided. As a remedy an insulating layer between the substrate and the patch is proposed in the form of a very thin glass fiber layer. Such layers are often used in the repair of aluminum structures in order to avoid contact with the carbon fibers and prevent galvanic corrosion. The thickness of the layer is not expected to affect the strength of the patch adhesion.
7. The carbon composite patch repair of aluminum parts needs application of BR 127 corrosion inhibiting primer especially for the IH technique (in case of the DRH technique, a thin glass fabric between patch and aluminum part is used), because of the galvanic corrosion effect.
8. The vacuum bagging must also be used in both methods as the typical procedure used in all composite repairs.
9. The cost of equipment involved in the DRH technique is relatively small as compared to conventional and alternative curing techniques.
10. Introduction of an on-line curing process monitoring system will allow for precise control of curing process to enhance the quality of the resulted repair.
11. Dielectric sensors (thin film interdigital electrodes) of innovative curing process monitoring are placed in contact with the curing material and provide information on material properties in the vicinity of the sensor surface. The sensor comprises two copper comb electrodes. The prong width is about 30 35μm and the inter-electrode space is about 350μm. The sensor substrate is a soft, porous, polymer film. The thickness of this film is 130μm, the width is 10mm and the length of comb is 25mm.
12. The sensor response is limited above 210°C as the substrate become black, indicating the onset of degradation process.
13. As electrodes are conductive, carbon fabric should not come in direct contact to prevent sensor short-circuit. In this case, a small cloth of glass fabric or peel ply should cover the sensor surface.

The application of electrical resistance mapping as well as IR thermography and Lamb wave technique for the monitoring of critical and sub-critical damage requires for each specific application the loop operation of the HMS for its optimization. This will be performed through intensive analysis in order to conclude to the necessary spatial and temporal resolution of the system and establish the application envelop of such a system providing:

• Final assessment of the damage monitoring capabilities of smart patch. Number of sensors (conductive spot arrays), data handling, placement and optimization.
• Development of generic guidelines for the application of quality assessment and the damage monitoring of smart patch in different repair configurations

Damage monitoring techniques were applied on conventional and innovative patch repaired aluminium and composite demonstrator. All demonstrators were subjected under fatigue loading testing. Results from Lamb’s wave and electrical monitoring methods were compared to thermal camera recordings for verification.

Maximum fatigue loading for the conventional aluminum demonstrator reached out 580,000 cycles until the patch had totally been debonded, while for the innovative one 30,000cycles at the same frequency (f=20Hz) was set as the maximum load due to a computer failure. Both damage monitoring techniques could detect patch debonding on critical and sub-critical regions, which was made obvious only at high cycles of fatigue loading. Lamb waves method could detect degradation only after 405,000cycles where off-line electrical change monitoring recorded severe changes over 350,000cycles. Below 15,000cycles loading, both methods were able to detect changes that were corresponded to changes undergone due to stabilization mechanisms until the whole system reached a stable condition. Electrical mapping illustrated the cumulative damage throughout the loading history. Damage propagation could be detected clearly at regions near patch edges and crack position. Indications from both methods were verified by comparison to thermal camera recordings. For innovative demonstrator though, very few conclusions can be derived from both techniques and not valid assumptions can be obtained since fatigue loading test could be completed. The same trend as with the conventional patch repair is exhibited up to 30,000cycles, where in the initial stages of loading the resistance drops until it reaches an ‘equilibrium’ (here nearly after 10kCycles). As the test remained under 50kCycles, it is believed that no damage degradation has been recorded.

Innovative and conventional composite demonstrators were subjected to the same fatigue loads and cycles but at different frequencies as shown in Table 3-1 and Table 3-2. On conventional patch repaired demonstrator, both Lamb waves technique and electrical monitoring have ended up with the same conclusions: below 30,000cycles there are recorded some changes which have been attributed to stabilization issues. Up to 120,000cycles no significant changes have been obtained while above 120,000-150,000 a more distinctive area of changes is obvious indicating damage propagation. Post processing of the results showed that the debonding started around the damage area (artificial hole) verified by NDE scan images. On innovative patch repaired component, Lamb waves results analysis cannot be considered conclusive in terms of structural integrity of the repaired region due to inconsistency of the baseline reference set of data. Few results from path 1 that could be analysed showed an indication of a developed damage in the repaired region but not valid assumptions can be derived. Electrical monitoring technique on the other hand has been recorded a similar behaviour with the conventional one. Small changes in resistance are present after 120,000cycles while above 190,000cycles significant changes occur. Alternative electrical monitoring technique was less sensitive to loading and in terms of damage no major point can be identified indicating a clear damage initiation or development.

Conventional (thermal blanket) and innovative heating methods (induction and resistance heating) were applied for patch curing on composite and aluminum demonstrators. A 3-stringer stiffened panel, representative of a lower skin part of a wing box was selected as the composite aerostructure while a vertical stabilizer of a helicopter was manufactured as the aluminum demonstrator. Both structures were subjected to fatigue testing under damage monitoring techniques. The following conclusions can be derived:

- Damage monitoring techniques that were applied on patch repaired demonstrators were efficiently detected damage propagation or dispatching, as their final results were verified with NDE methods used to evaluate patch repairing.
- Lock in thermography was assumed to be a validated NDE method, efficient to detect damage propagation and dispatching
- Lamb waves and on/off line electrical resistance methods cannot be considered as “live” methods since data obtained should be post processed before any result could be derived.
- Not validated assumptions can be obtained on novel curing methods used for patch repair. Lamb waves recorded data were insufficient for further analysis for the innovative composite demonstrator while he only results derived from electrical current monitoring had a similar behavior with the conventional one instead that damage propagation started at somehow higher loading cycles. Due to computer failure during fatigue testing on aluminum stabilizer not enough data were recorded and none assumption can be obtained.
- Results from both damage monitoring techniques on conventional demonstrators were in great accordance and were verified from IR thermal images. Dispatching started taking place at high loading cycles from critical regions.
- Damage propagation started from critical and sub-critical regions (around the hole for the composite demonstrators, at the edges and around an insert for aluminum demonstrator).
- There cannot be any comparison between innovative and conventional patch repaired demonstrators so an evaluation of novel curing methods cannot be assessed.
- It would be of great interested results from bonded repairing to be compared with ones of bolted repairing so that a more efficient evaluation of novel repair methods could be achieved.

IAPETUS project concentrated on “Smart patch repair” approach on composite and aluminium aerostructures, which proved as a very promising method for repairing aerostructures at low cost with high efficiency

As proper identification and disposition of defects is essential to the correct choice and performance of repairs the following steps (also presented in Figure 4-1) in repair process should be completed:

 Non-destructive inspection (NDI) of damage. Identification damage including its location and dimensions

• Assessment of damage and reparability study by composite patching. This study is based on parameters like the criticality of the damage, the geometrical limitations, the accessibility for the repair application, the materials selection for the repair and the stress analysis of the area.
• Designing the composite patch repair is the most crucial aspect as the following quantities must be determined:

o Patch material
o Patch shape and dimensions
o Number of plies
o Stacking sequence
o Stepping of plies
o Adhesive material and thickness

• Application of the composite patch repair, which involves the surface preparation and the bonding of the patch.
• Monitoring requirements involves a diagnostic system detecting crack and debond damages

Sensing technologies are involving the detection of disband growth and the monitoring of the growth damage. For this, the following have been investigated:

 Sensor inventory, where a number of sensors (optical fiber with FBGs, triboluminescent, embedded optical fibers, electrical sensors, piezoelectric sensors) are being referred and how they can be used for crack detection.

• Structural Health Monitoring (SHM) of CFRP laminates cab be achieved either by applying an electrical potential change method with an artificial neural network or a delamination monitoring analysis of CFRP with a multi-probe electrical method measuring the electrical resistance change.
• Electric Resistance change methods for cure/strain damage monitoring of CFRP laminates involve cure monitoring with capacitance change, smart cure monitoring using capacitance change with AC frequency, degree of cure monitoring during curing, electrical resistance change with integrated surface probes, assymetrical dual charge EPCM for delamination monitoring, orthotropic conductance measurement and analysis of the effect on the delamination monitoring with an electric resistance change method.

The introduction of the proposed sensing technologies in the composite patch repair process will directly affect the basic two elements of repair cost, which are the active composite repair application phase and the inspection for quality assurance, reducing the active repair cost and time needed. It will also provide an on line damage monitoring set up of the structural integrity of the repaired site, throughout the service life of the repaired structure.

Finally, the advantages of the adhesively bonded patches versus the mechanically bolted patches are proved to be significant as with bonded patch repair better efficiency and superior behavior to fatigue loading can be achieved while edge effect can be minimized.

D3.3.3 reports the necessary steps that are required for the industrialization of the entire integrated system that was developed within the framework of ―IAPETUS project

The novel curing methodologies that were investigated along with the smart patch technologies that were implemented consist a promising approach for the development of the repair techniques for both metallic and composite structures. Even though the first results of the tests that were performed, derived fruitful and efficient conclusions there is a lot of work to be done in order these technologies to be assessed from the end users and be adapted into their maintenance and repair manuals

Potential Impact:

IAPETUS is expected to have a major impact in the field of aircraft repair as it proposes repair technologies that are applicable on both older and new generation aircrafts, readily addresses issues that arise from the current repair technologies, such as on aircraft repair, galvanic corrosion or increased damage tolerance, and is introducing novel health monitoring concepts.

IAPETUS aims at revolutionizing aircraft repair processes with composite materials, (mainly the hot bond repair approach used in field repair) by using novel hybrid composites. These materials are expected to fully exploit the unique properties of nano-scaled fillers in the matrix and adhesive systems in order to step change the hot bond field repair by:

• Introducing an innovative curing methodology (either via direct resistance heating or by induction heating) of the patch/adhesive system, with the benefits of homogenous heating and curing and the minimization of the developed thermal stresses. In addition the introduction of an on line curing monitoring system will allow for a precise control of curing concluded to enhanced quality of the resulted repair.
• Providing direct inspection of the bonding/repair integrity and continuous health monitoring of the repaired site in service.
• Offering increased mechanical and bonding performance in the repair itself

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