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Harnessing next generation composite structures in the aerospace industry

Balancing the pros and cons: Composites enters the aerospace industry

Composite materials have been embraced by the aerospace industry primarily due to the weight reduction benefit they can realise in many areas of application. This is not purely because of the specific strength and stiffness properties they can achieve - they can also give other benefits such as improved fatigue and corrosion resistance, reduced part count and increased flexibility of form for designers.

Adoption has not been straight forward though. Composites may come with a series of exciting benefits, but they also come with concerns. For example, variability in mechanical performance within the same part, and inconsistency of materials and manufacturing processes make prediction of precise static, fatigue and damage tolerance characteristics difficult.

Then there is degradation over time, especially in the early years of composites, when there was limited longevity data to support decision making. Additionally, impact performance and crashworthiness need careful consideration as energy absorption characteristics are fundamentally different to the historic metallic structures they replace.

There are also concerns around quantifying regions of damage and ensuring the quality of repairs, especially in sectors where access to specialist equipment or suitable facilities may be restricted.

Understandable conservatism

Alongside this variability in material property tests comes statistics-based conservatism. Early adoption of composites was restricted by regulatory authorities, to protect safety records, particularly within the aerospace industry. Increased use of composites has been a slow and steady progression over the last 30 years, as understanding, manufacturing consistency and acceptance has grown.

For many years now, the industry has been focused on addressing basic issues, including:

  • Minimising variability in both materials and manufacturing methods
  • Accurately predicting performance
  • Ensuring consideration of through life maintenance and repair

This has resulted in innovations and improvements in available materials, automation of production methods and enhanced quality control processes. Alongside this, software capable of representing the ‘as manufactured’ parts, is also now commercially available. with attributes such as:

  • Draped fibre orientations
  • Simulated resin flow
  • Calculated mould springback
  • Residual stress modelling
  • Virtual testing and assembly

This helps to ensure Design Intent is met by Operations.

Adopting novel materials

The advancements in applied technologies for materials and processing have facilitated novel and innovative ways to use composite materials to further improve performance, including:

  1. Self healing

This can be achieved in many ways, and we’ve highlighted two of these below:

Solid-state, an example of which is single phase, where healing is achieved by activating dissociation of bonds to allow diffusion into the free volume of the defect.

Hollow fibres, where the core of some fibres is filled with either a resin or hardener such that a break locally causes the two chemicals to be released, mix and cure facilitating the repair.

  1. Morphing

If a structure can morph or change shape, this alleviates the need for rigid control surfaces being actuated by traditional mechanisms. These mechanisms can add weight, part count and complexity while limiting the configurations available to the pilot and requiring regular human interaction. Composites are ideal in this scenario as they can be used to make flexible parts with good fatigue properties. A great example of morphing is the SmartX-Alpha wing which 'senses' the environment and uses AI to determine its optimal shape, resulting in greater efficiency with minimal human input.

  1. Super lightweight composites

Some projects are so weight sensitive that they are only possible using a single ply open weave lamina or two laminate sparse uniaxial systems – effectively generating a sporadic, seemingly uncontrolled open lattice work.

Many composite material allowables are developed by testing coupons generated using the same manufacturing techniques as the actual component, but this assumes some level of commonality between disparate regions of the structure. This is almost impossible with these structures – how can we realistically qualify and certify this type of structure?  Clearly, these structures work, as the products a have flown in prototype form – the next important step is how to move from this to a certified airframe.

  1. Single ply composite flexibility

These single ply structures are too thin to be mechanically fastened together and, in any case, it would defeat their super lightweight nature, so they have to be bonded. Traditionally, bonded joint and interfaces are geometrically controlled, so no peel forces are induced that try to rip the components apart. However, when the two components (say a C-Section) are identical, for practical cost and manufacture reasons, and one is inserted into the other to be bonded, they are both forcing each other apart creating residual stresses and uneven bond line thickness – this is another difficult certification path.

Making composites commercially viable

Though these technologies offer much, they are step changes in the way composite materials are considered and as such often find themselves outside current regulatory control and therefore difficult to pursue in a commercially viable manner.

SMEs are known for bringing new ideas and innovation to the industry; however they are rarely able to take the financial risk of overcoming regulatory constraints. The reality is that the current regulatory system is preventing innovation and delaying time to market because of high costs and slow response times.

There is a popular perception that there are too many different stakeholder bodies involved. Equally, the nature of innovation means that these bodies often have limited understanding of the latest technologies. Some think that layers of bureaucracy go hand-in-hand with safety. But does speed vs safety have to be a taxing conundrum? Perhaps Value Stream Mapping would help identify wasted effort or needless delay that could be challenged and removed without compromising safety.

If you see the regulatory authority as a service provider to the industry – and they are certainly essential to holding the industry to account and keeping all in aviation safe – then we believe the industry is right to call for more rapid development of codes and standards to facilitate adoption of innovation. The question is: Do the regulators have the capacity, capability and appetite to answer this call?

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