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IAPETUS
Innovative Repair of Aerospace Structures with Curing Optimisation and Life-cycle Monitoring Abilities

Tags: Air

State of the Art - Background

The availability of efficient and cost-effective technologies to repair and extend the life of ageing airframes is becoming a critical requirement in most countries around the world. New aircraft incorporate new materials, processes and structural concepts which will shape the requirements for state-of-the-art repair techniques in the near future.

Bonded composite patches are ideal for aircraft structural repair as they offer enhanced specific properties, case-tailored performance and excellent corrosion resistance. Bonding also eliminates stress concentrations induced from mechanical fastening of metal sheets, seals the interface and reduces the risk of fretting fatigue between the patch and the component.

Adhesively bonded fibre composite patches still pose significant challenges, particularly when used to repair primary structures. With respect to the repair material system, existing composite material systems may be tailored to comply with the repair requirements. However, the introduction of novel material configurations is a challenge. Regarding the application of the composite patch repair, the aircraft industry is in dire need of reliable and cost-efficient in-the-field repair technologies that will facilitate patch application and will reduce depot service time for aircraft, contributing to a reduction of the overall operational cost.

Objectives

The main aim is to develop novel repair technologies and materials for metallic and composite aircrafts. This will be achieved with the use of novel hybrid composite systems, which offer the multi-functionality that will lead to the development of innovative repair technologies and life-cycle health monitoring capabilities, together with the enhancement of repair efficiency.

The scientific and technical objectives are to:

- address the problem of bonding composite patch repairs to ageing aluminium and new composite aero-structures by investigating new easy-to-apply heating-up concepts, either by direct resistance heating, using the conductive composite patch as a heating element, or by introducing induction heating technologies;

- develop innovative Carbon Nano Tubes (CNT)-doped conductive composite fibre patches which offer improved mechanical performance and provide built-in sensing capabilities;

- develop innovative conductive and non-conductive laminating resins and adhesives for achieving increased peeling and sheer strength of the patch/structure bond;

- develop a curing monitoring system compatible with the proposed processes and materials, which will be integrated within the composite patch and provide curing state data and thus a mean for optimal curing;

- demonstrate the developed materials and repair processes at coupon and component level.

Description of Work

The project has been organised into three technical sub-projects (SPs).

SP1: Developing the two novel curing methodologies (direct resistance and induction heating) and patch material improvement through the use of CNT additives. Solutions for galvanic corrosion will also be investigated and a curing monitoring system suitable for composite repairs and adapted to CNT-doped materials will be implemented. The proposed curing methodologies will be evaluated in terms of curing efficiency and patch bonding integrity.

SP2: Demonstrating the applicability of the smart patch concept based on the approach of electrical resistance measurements. The electrical resistance mapping performance for the detection of various types of damage in repaired aero-structures will be critically evaluated against typical ultrasonic inspection. A second Non Destructive Technique - flash thermography - will also be adopted.

SP3: Providing the framework to integrate the technologies of innovative curing, curing control and monitoring and the smart patch technologies. The technology integration will be made in small-scale components and validated through appropriate testing (quasi-static and dynamic/fatigue loading conditions). Then the integrated system will be validated in a suitable aeronautical structure. The envisaged construction will consist of thin (skin) and thick (web) components.

Expected Results

The major expected results of the project:

- Optimal CNT processing for the achievement of dispersion in the matrix, conductive properties of the adhesive layers and minimisation of galvanic effects in the case of aluminium substrate repair;

- The design and implementation of novel curing approaches based on the conductive properties of the materials using induction and resistance heating;

- The implementation and validation of novel offline and online sensing methodologies based on the two dimensional and through thickness mapping of the changes in the CNT conductive network that mirror the damaged system;

- The integration of the innovative curing system with the smart-patch sensing abilities and validation at coupon level and large-scale applications;

- A list of recommendations for the industrialisation of the technology and for securing the developed repair system within the existing repair standards.

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