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A picture of health: Clean Sky's SHERLOC leaves no clue unturned

Clean Sky’s SHERLOC (Structural Health Monitoring, Manufacturing and Repair Technologies for Life Management Of Composite Fuselage) team embarked on a research journey to investigate the possibilities of an emerging technique known as Structural Health Monitoring (SHM) for monitoring, remote inspection and damage assessment based on integrated sensors. 

Concept of Smart aircraft
Concept of Smart aircraft

SHM has piqued the interest of the aeronautical community because it presents an alternative way of non-destructive inspection (NDI) that could significantly improve damage detection in composite structures. 

As well as adding safety improvements for modern commercial aircraft, the industrial uptake of SHM will lead to weight savings and reduced fuel consumption as the design of composites becomes less conservative, in line with Clean Sky’s ambitions and the vision of the European Green Deal. 

The technique will also eliminate the need for harmful chemical agents that have been used up until now for maintenance reasons, since SHM does not require them.  

The SHERLOC project — which supports Clean Sky’s Airframe ITD and REG IADP — is investigating the viability of combining advanced SHM and smart repair technologies with novel and low cost composite manufacturing. 

SHERLOC has developed a novel probabilistic design philosophy, which was designed specifically to be applied in the major sub-components of a regional aircraft composite fuselage, each of which are manufactured with different technologies and designed with different SHM systems. 

In SHERLOC, SHM monitoring has been uniquely tailored to target the type of defects and damages that are likely to occur for each composite manufacturing method and addresses different geometrical complexities. By incorporating SHM from the design stage to the manufacturing stage and through the service life of the structure, it will contribute to significant savings on operational costs, reduce manufacturing waste and cut down on resource use, resulting in a more sustainable aircraft manufacturing process. 

SHERLOC’s consortium comprises of experts around Europe from both from academia and industry: Imperial College London (Coordinator), Hellenic Aerospace Industry (HAI), Element Materials Technology Seville (ELEMENT), Barcelona Supercomputing Centre (BSC), University of Sheffield (US), Universidad Politécnica de Madrid (UPM), Vrije Universiteit Brussel (VUB), FIDAMC. 

To ensure the development of the most reliable, cost-effective and sustainable system, the SHERLOC consortium undertook the most comprehensive assessment of existing SHM technologies and methodologies to date for composite airframes under the operational and environmental conditions of a regional fleet. 

The advantages of structural health monitoring

According to SHERLOC’s Topic Manager Ferri Aliabadi, Zaharoff Professor of Aviation and the former Head of Aeronautical at Imperial College London, there are several benefits to SHM.  

‘You can monitor the health of the structure remotely; without the need to ground the plane, and send maintenance engineers inside the aircraft, which incidentally, during maintenance, presents the danger of causing accidental damage to the airframe,’ he said. 

The remote real-time monitoring potential of SHM makes diagnosis, prognosis, life extension and predictive maintenance of components viable, thereby bringing safe, economic and ecological benefits to aviation. 

What’s more, SHM reduces downtime and makes it possible to monitor the health of the structure in generally inaccessible locations. SHM also makes it possible to roster maintenance interventions in timeframes that dovetail with the aircraft operator's flight schedules. The technique can also provide valuable information regarding the manufacturing process and integration of different composite parts. 

To date, the SHERLOC consortium has already designed, developed, fabricated and tested five structural demonstrators (5m curved fuselage panels, window frames, floor beams, aft pressure bulkhead, and fittings) which all incorporate various SHM technologies. 

A clear advantage of the SHERLOC system is that it is capable of providing an early warning of occurrence of damage and guiding the ground crew to the location of the damage with high level of certainty. The developed system is modular so it can be applicable to large structures and extended to an entire fleet. The SHERLOC teams have conducted thousands of tests to select appropriate sensor technologies to go along with the design of their Smart Fuselage concept.  

The development and integration of these technologies feed into one of the deliverables of the project, SHERLOC’s novel Bayesian-based Dynamic Data Driven Application System (DDDAS). The DDAS facilitates the characterisation of uncertainty and conditional probabilities in terms of what is known about the structure from the model, and what is measured during inspection.

You can monitor the health of the structure remotely; without the need to ground the plane, and send maintenance engineers inside the aircraft, which incidentally, during maintenance, presents the danger of causing accidental damage to the airframe

Climbing the pyramid

The Deputy Coordinator of SHERLOC, Zahra Sharif Khodaei, associate professor in the Department of Aeronautics, Imperial College London, says that many questions have to be answered surrounding the suitability and configuration of different materials and manufacturing processes for different purposes within the five different SHERLOC demonstrators, and that SHM can aid with that.

‘When you aim to design and manufacture different structural items from composite, such as a window frame or a floor beam as we have done in SHERLOC, you have to ask whether this novel design is cost-effective and what the benefits of it are. Composites are environmentally beneficial, but what would the implication be in terms of their structural integrity and life cycle assessment for each component?’ she says. 

Three different manufacturing technologies and materials have been considered in SHERLOC: 1) hand layup and automated layup of thermoset prepreg; 2) Liquid Resin Infusion (LRI) and Resin transfer moulding (RTM) of thermoset material; 3) Thermo-forming for thermoplastic woven fabric. And each manufacturing technology or material brings with it different challenges.  

‘The question that we had to ask in terms of integrating SHM systems is: what is the typical damage scenario specific to each manufacturing technology? The type of manufacturing defect that you get from LRI, for example, would be very different if it was made using a hand layup with prepreg with unidirectional tapes,’ says Sharif Khodaei. 

The question that we had to ask in terms of integrating SHM systems is: what is the typical damage scenario specific to each manufacturing technology?

She explains that in order to optimise the SHM technology selection, a different SHM system has been designed for each individual structural item, in line with selected materials and manufacturing technology. 

‘The diagnostic methodology was then developed and optimised based on the reliability of damage detection for the type and severity of the defects that are probable to each specific material and manufacturing procedure,’ Sharif Khodaei concludes. 

Concept of Smart aircraft
Concept of Smart aircraft

One of the contributing projects, SHERLOC-QSP, focused on the design, manufacture and integration of SHM technology into a thermoplastic/composite window frame using 'Quilted Stratum Processes' (QSP). 

The frame is designed and sensorised by Imperial College London and manufactured by the company CETIM (French Technical Centre for Mechanics), from thermoplastic carbon composite ‘PEEK’, a material that is compliant with aerospace requirements and has good impact resistance compared to the thermoset material currently used in aircraft parts. QSP makes it possible to obtain 'Net Shape' parts right from the thermoforming phase, which means that they do not require any additional finishing operations, they used the fabric cut to shape thereby saving material, the curing time is also in order of seconds (60 to 90 seconds), thus saving time and energy significantly. 

‘QSP is a green process with little wastage and has a very short curing time, so it was attractive for the window frame, and we wanted to assess it against the more traditional manufacturing ways,’ says Aliabadi. ‘QSP was initially developed for the car industry, and we wanted to assess it for aircraft.’

QSP was initially developed for the car industry, and we wanted to assess it for aircraft

The QSP 'Net Shape' technique saves time by using an 'out-of-autoclave' process whereby the window frame is bonded to the thermoset panel in such a way that the SHM elements can be embedded. But it’s also the use of carbon PEEK that is key to ensuring the necessary high standards of precision involved in producing the window frame.

‘PEEK is an expensive, but recyclable, material. It was down to us to demonstrate the cost benefit of manufacturing the window frame,’ says Aliabadi. ‘All through the SHERLOC project, everything was assessed, compared and down-selected. This is the philosophy of the pyramid approach.’

Overall, the combination of PEEK and the QSP is highly beneficial when it comes to time-saving, according to Thomas Jollivet, project manager at CETIM and Coordinator of SHERLOC-QSP. 'It's important to keep in mind that the process currently used to manufacture thermoset composite window frames is RTM (Resin Transfer Moulding). The amount of time typically taken to manufacture one part is around one hour. The QSP system used in SHERLOC QSP for lay-up, forming and consolidation of the window frame can be done in one automatised process in under 5 minutes per part,’ says Jollivet. ‘For recurrent parts like a window frame, when you have a lot of planes, you need to manufacture a lot of parts quickly.’ 

The most comprehensive SHM assessment for composite airframes

SHERLOC represents the most comprehensive assessment of a complete SHM system to date for composite airframes, covering integrity, durability, and longevity of every piece of equipment and installation on board the aircraft as well as diagnostic and prognostic assessments. 

For the fuselage panel demo, sensors were integrated into three 5m long curved composite panels using a specially developed large tri-axial testing machine. Large-scale mechanical and SHM testing is complemented with virtual simulations using a massively parallel finite element code and high performance computing. 

In the short term, the plan is to implement the SHM system in field applications for regional aircraft. And though the SHERLOC technologies have been developed for composite airframes they can also be tailored to other industrial sectors such as bridges, space, offshore, oil and gas. Academic partners are already exploiting the results in training and research. Additionally, the consortium reports that funding from the EU’s Fast Track to Innovation (FTI) programme is being considered for possible commercialisation of certain aspects. 

Roadmap to industrialization
Roadmap to industrialization

Counting the cost of SHM integration 

Consolidating the achievements of SHERLOC and its associated sub-projects, Clean Sky's ongoing MASCOT (Modular multilevel cost Analysis Software for COmposite smarT fuselage) project is developing an open-source multi-disciplinary cost optimisation software package, which integrates the cost estimation of real manufacture and production with the cost of maintenance and repair of SHM-enabled composite structures. 

The software estimates costs related to production, assembly, maintenance, repair, as well as the costs associated with the integration of SHM equipment. Furthermore, utilities are being developed within the software to make it compatible with the aerospace industry standard Computer Aided Design (CAD) and Finite Element Method (FEM) analysis tools. 

The resulting software will provide industry with a powerful tool to make economic comparisons of different solutions across all phases of design, development and manufacture, resulting in a cost and quality-optimised approach for designing and manufacturing aircraft composite fuselages.

The partners behind MASCOT are the Università degli Studi di Ferrara, bringing experience and knowledge of cost modelling and open source modular software development to complement Imperial College’s expertise in multi-disciplinary optimization and of nonlinear finite element analysis. Università degli Studi della Campania Luigi Vanvitelli, which is versed in developing cost models for composite parts; and Plyform Composites Srl, which brings knowledge of aero-industry composite manufacturing, repair and maintenance processes and costs. These are complemented by the experience of Skytechnology SRL, developers of open-source modular software platforms.

Although there are various pre-existing cost estimation models available to the aeronautical industry, what makes MASCOT unique is the integration of their composite manufacturing cost models with SHERLOC’s cost/benefit analysis of structural health monitoring systems within the overall model.

Digital Clone concept, supporting experiments with simulation
Digital Clone concept, supporting experiments with simulation

While the cost and operational advantages of SHM are already understood, if not fully quantified, what is less known is the additional cost of integrating the SHM technology into composite structures.

‘You're adding sensors to the composite structure, so you're adding weight, which means either less payload or more fuel. You have to balance that over the life of the aircraft against the reduction in the direct operating and maintenance costs,’ says Ferri Aliabadi.

‘MASCOT helps demonstrate the cost benefit to aircraft operators because you can't justify adding extra weight and cost unless you are able to demonstrate the benefits. With maintenance accounting for as much as 25% of operating costs, you can imagine how attractive it is to reduce these costs, as well as improving safety.’

From Clean Sky’s perspective, the SHERLOC project has shown real value to date, and project officer Elena Pedone advises future projects to learn from its example. 

‘The all-encompassing and comprehensive nature of the SHERLOC project, with its application across multiple aspects of the aircraft, could certainly inspire future projects to take a holistic approach as SHERLOC has done, for example, because then it's easier to assess and validate the results at the end because you really have the 100% view of the technologies,’ she says.