The difficulty of qualifying exotic materials and processes for flight

The rise of non-standardised materials means the process of qualification has become a daunting and expensive task that’s in danger of suffocating further innovation. Chris Shaw asks, is there an answer?

Historically, designing and managing materials for aerospace or defence applications meant viewing the process from a particular perspective. Most materials - for example aerospace alloys - were manufactured in bulk and subsequently shaped. It was usually sufficient to qualify the material properties, publish them in an authoritative source, and then engineers could apply them in varied designs across the board.

However, while it's still vital that engineers get hold of the right 'design allowable' data, Dr Will Marsden, director of industry relations at Granta Design, has identified two major developments to this world view.

"Firstly," he observes, "qualification has become more complex, particularly for materials such as composites and areas like additive manufacturing. We need to qualify not only the material, but the material in combination with specific processes, geometries, and conditions, as well as qualifying the supply chain and even the operative making the material.

"Secondly, as materials become better understood, there is even more focus on extracting maximum performance from them for specific applications – so fine tuning material properties in every area of a component. For example a turbine blade that uses differential heat treatments with greater control over process parameters."

Light flight

The aerospace and defence sectors are plagued by weight and cost reduction issues.

Nevertheless, there are a number of materials available, with the greatly increased use of composites being the clearest trend. Most notably the Boeing 787 used more composite materials in its airframe and primary structure than any previous Boeing commercial aeroplane.

The airframe, which comprises nearly half carbon fibre reinforced plastic, achieved a 20% weight saving compared to conventional aluminium designs.

To determine the best materials, Boeing engineers analysed every aspect of the airframe, its operating environment and component loads over its lifespan. So, while composites aren't as efficient in dealing with compression loads as aluminium, they are excellent at handling tension and the material was therefore used extensively in the fuselage. This reduces fatigue based maintenance compared to an aluminium structure.
Similarly, titanium is used as a low maintenance design solution, as it can withstand comparable loads better than aluminium, has minimal fatigue issues and is corrosion-resistant. Around 14% of the Boeing 787's airframe comprises of titanium.

"Composites can make significant contributions to weight reduction," Marsden notes. "Whether they drive cost reduction depends, of course, on whether you are concerned with the materials and manufacturing costs or the full lifecycle costs of the aircraft. Since fuel burn costs are a major contributor to the latter, lower weight usually 'wins out' on this basis too.

"Where fuel costs matter, running engines at a higher temperature can also help, by making the engine more efficient. Material innovations can be required here, such as the use of ceramic matrix composites. And new manufacturing techniques like additive manufacturing hold out the prospect of further weight reductions by enabling engineers to realise designs for non-structural components such as brackets that minimise the amount of material used by shaping the part so that it only has material on the load-bearing paths."

However, there is still a reluctance to use composites, primarily because the qualification process has become increasingly expensive. Whereas a few years ago qualifying a material might have cost $3 million and taken six years, costs of up to $100 million are now quoted and, Marsden warns, with no real improvement in time-to-market.

"Composites are still a relatively new technology," he asserts. "They fail in ways that aerospace engineers are not familiar with and we still need better design tools to develop a deep understanding of their performance and application. All of this means aluminium alloys remain a common choice for commercial aircraft.

"Managing data about composites is an important factor here. Doing this effectively is essential to reducing qualification times, avoiding risk in the process, and delivering the right input data for design tools."

Composites, however, are a mature technology relative to additive manufacturing, which is still in its infancy. There are, as yet, no additively manufactured parts qualified to fly and much R&D is still needed to understand how to qualify them and lower production costs.

"Again, materials data management has an important role to play," says Marsden, "since capture and analysis of large quantities of data is integral to R&D programmes, such as the AMAZE project."

And, because an aerospace and defence design engineer's job is to balance functional requirements with constraints, it's easy to see why trade-offs are endemic in materials selection.

Composites and additive manufacturing technologies could offer a potential solution here, by enabling designers to arrive at an optimum choice of structural concept and material selection for a given weight. But, like everything, that will not come cheap.



The emerging discipline of Integrated Computational Materials Engineering (ICME)

As well as composite data management, Integrated Computational Materials Engineering (ICME) looks set to have a major impact on the aerospace/defence materials design process.

As an emerging discipline, it seeks to apply computational methods at multiple length scales, validating the results and applying them to understand and improve materials performance.

Different, complementary, simulation methods are applied in support of each other, to study and predict processes, structures, and properties - and their interactions with each other.

"The aims are to gain insight and design better materials, faster, while reducing reliance on expensive experimentation," says Dr Will Marsden, director of industry relations at Granta Design. "Again, effective management of materials information is vital to ICME projects to capture the results of one simulation (materials properties, structures, or processes) and apply them as inputs to another."



The AMAZE Project set to get metallic additive manufacturing airborne


The €20 million AMAZE project is coordinated by the European Space Agency for the rapid production of additive manufactured metallic components up to 2m in size, with zero waste. Targeting aerospace/defence industries, four Europe-based industrial centres will be built by 2016 with the aim to start production of high value parts, while halving the cost of traditionally processed finished parts.

The project will see an increase in the commercial use of adaptronics, an adaptive structure technology, based on functional integration and designed to optimise structure systems.

Combining conventional structures with these systems extends load bearing and form defining structure performance by including sensor and actuator functioning. When combined with adaptive controller systems, they can adapt to their respective operational environment. The aim of the adaptive structure technology is to influence structures and allow users to optimise products.

In order to turn additive manufacturing into a mainstream industrial process, the AMAZE project will focus on pre-normative work, standardisation and certification, in collaboration with ISO, ASTM and ECSS standards bodies.




Author
Justin Cunningham

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