Automated dry-fibre placement drives aerospace efficiency

Materials World magazine
30 May 2019

Robotic laying of carbon-fibre strips is pushing forward manufacturing efficiency.

Weight reduction is a constant challenge in aerospace. Whether it’s a Boeing 787, an Airbus A350 or a fighter jet, the aim is to make the craft as light as possible so it can either fly longer on the same amount of fuel, take an increased payload in terms of passengers and freight, reduce emissions, or a combination of these factors. Composite materials offer the ability to reduce weight and maximise a craft’s working lifespan compared with metals, particularly by reducing the need for fasteners to produce complex shapes, avoid fatigue monitoring and corrosion. While advances in carbon-fibre may be gradual, the biggest area of opportunity is the speed at which the material can be laid down to create the part.

Structural composites

The F-35 miliatry plane introduced in 2006 is likely to have a lifespan into the year 2050 and sustainment out to 2070, so the materials used need to help achieve that. Solvay supplies structural composite materials to the team producing the craft. These components make up about 30% by weight and 50% by volume on the plane.

The aircraft is made up of base epoxy and bismaleimide (BMI), a high-temperature material that fills the gap from a cost and performance perspective between epoxy and polyimides. It is a material that has a glass transition temperature – Tg, the point where the resin softens from a rigid material to a softer, more rubbery state – between 230°C–300°C.

Some high-temperature BMIs go up to 340°C. Polyimides generally have a much higher Tg than that, but they’re very expensive. Epoxies are relatively cheap but have a much lower Tg, so where there are hot spots, for example around the engine, there is generally a transition of materials from an epoxy to a BMI. Normally, the skins and some of the structural spars underneath will all be made of composite.

Both military and commercial aircraft typically have composites as part of their subskeleton, while other parts might be aluminium or titanium. Polyimides are often used to replace metal in engines, but these materials cost more than aluminium so there has to be another added advantage to make it justifiable. For example, can the same piece be made with fewer parts and rivets, or will the alternate material bring a weight and emission saving?

Composites lack the corrosion issues of aluminium in salt water and they do not have the same fatigue, so the aircraft is not taken out of service for an ultrasonic sweep to see if there are any fatigue cracks developing. This way, the aircraft stays out of maintenance and flies more during its life.

On some of the more modern aircraft, there might be an incorporation of other functionality such as strain gauges or other health and usage monitoring systems (HUMS) into a composite part over and above the traditional metal aircraft part.

Limitations of labour

Automation and taking the cost of labour out of the building process is a current challenge. Getting a robot to do something from an engineering set of data is much cleaner and less prone to faults. Labour is a big expense when a robot could run constantly.

For the F-35, Solvay has been supplying materials for advanced fibre placements. But now, the move is almost to divorce the resin and fibre combination to a dry-fiber and then use a resin transfer moulding (RTM) or resin infusion (RI) process after that.

You can drive or steer a dry-fibre around certain geometries, which a traditional wet prepreg fibre placement method cannot do. With steering fibres, not only is the right amount of material put on the airframe, they are also steered in the direction of the load, so that less material and weight is required on the aircraft compared with classical automated dry-fibre placement (AFP).

Resin infusion for aircraft wings has been a technique used since Bombardier used it for the C Series – now the Airbus A220 – about 10 years ago. But looking at dry-fibre placement using a robot, it is possible to be clever with geometry and weight savings. I think the issue for the parts builder would be how to qualify a product where there is no traditional zero or 90 degree or quasi isotropic orientation.

Lighter and faster

AFP starts by taking a dry-fibre tow, a strip, and using the robot to help keep the fibres together and maintained at a very tight tolerance width, for example, a width of 6.35mm +/-0.1mm. The dry fibres are placed on what looks like a series of tape dispensers on the AFP machines, of which there are numerous manufacturers including MTorres, Coriolis, Electroimpact and Fives.

The robots can lay the material down to make a part either as a gantry, like painting on the floor, or on an angle where you effectively put the tool up at 90 degrees to the floor when you lay up the part. These machines put down between eight–32 fibre tows at any one time.

Each individual tow, whether it is an 0.3175cm (eighth of an inch) or 0.0635cm (a quarter inch) can be cut individually on the fly. The robot puts down the right amount of material in exactly the right place, so there is very little parasitic excess weight during the part-making process.

In terms of getting the fibres to stick together, traditionally this might happen by applying heat to stick the next layer down. AFP machines normally have an infrared gun, so as the tows are being laid, the gun is applying heat to stick it to the layer below.

On a normal hand lay manual prepreg process, the whole area would be covered with the same thickness and material, which results in some parasitic weight. This is avoided by the design of the part, but there is also a counter-argument that the thinner the the material, the more time it is going to take to put multiple layers down. There are opposing commercial forces of ‘let’s make it as fast as possible’ in as many kilos an hour versus ‘as light as possible’.

From a manufacturing-engineering point of view, there is always a cost trade-off with the two, but with AFP systems, you can put exactly the right material down, so the parts are going to weigh less.

The finished part would still have the temperature capability of traditional materials, and steering makes it simpler to change the weight of the end component.

Some parts are too large for a human being to lay materials in the middle of, for example, an Airbus A380’s tail. So when robots come from above, you haven’t got two people from the side trying to get into the middle and put material down on to a tool. The tape layers on the AFP machines give you that ability to make very wide parts and very long parts much more efficiently.

Getting the right balance

Choosing the right materials and production methods comes down to the shape of the finished product. We have been able to achieve highly complex shapes with composites as we try to integrate more and more parts into one finished article without bonding or riveting. In some cases, robots are very good for relatively flat parts. In addition, additive manufacturing may be a preferred production method.

Some complex shapes require workers to have a very good level of manual dexterity, or require sophisticated stereolithography or additive manufacturing.

For instance, robots can put a lot of material in the right place in a very quick time, but they are not always the best machine for the application. As these robots run on a gantry at high-speed, they need a long distance to accelerate and decelerate, to turn around and come back down the tool. Therefore, ideal parts for robot to work on have to be relatively big – at least 1.5m. Otherwise, hand lay operatives might put the material down much faster.

Composites are also harder to repair than aluminium following a crash, which can further delay getting a craft back in the air if it has sustained damage.

Life in the air

Composite materials are an effective and ever-increasing part of aircraft material structurally for their light weight, sustainability and ability to perform, particularly in high temperatures. Choosing which one to use can be a balancing act between the cost of the material and its ability to be fabricated and perform a particular function. While automation in laying carbon-fibre strips down can increase efficiency, the technology’s usefulness needs to be considered against what type of application is being performed and if manual labour could potentially offer a more efficient result.

Aircraft have a long life, decades long. Any material advancements which can help maximise lifespan and time in the air, at the lowest fuel usage and emissions, is welcomed and strived for.

Robotic laying of carbon-fibre strips is pushing forward manufacturing efficiency. Andi Clements is the Solvay F-35 Programme Manager.