Carbon fibre composites continue to proliferate throughout industry

Over the last decade, the civil aerospace industry has pioneered the manufacture of large composite components, taking the material from being considered exotic and destined for military and space bound applications, to the relative mainstream. And this has sparked interest from a number of different sectors including automotive and sporting equipment manufacturers looking to take out weight while improving strength.

However, the transition has not been straightforward with a string of composite related production problems reported by the major airliner OEMs. These varied from warpage caused by the misalignment of plies, to fibres being over or under infused by resin.

However, many of these production technicalities have been overcome with parts regularly manufactured to a high repeatable standard. Now, the major technical push revolves around processability and ramping up throughput. And again, it is the civil aerospace industry that is leading the way.

Since GKN Aerospace took on the production of the composite sections of the Airbus A380 wings, it has been actively developing the manufacturing technology behind it. Part of this has been to look at speeding up production, to match the expected ramp up rates of Airbus and Boeing, which forecast deliveries of between 29,000 and 35,000 new single aisle and wide-body aircraft by 2032.

Part of this know-how includes using automation to produce composite parts and structures. One technology that is already in use by the company, but continues to be developed, is what's broadly termed the 'rapid deposition' of fibres.

Two main types of machines are used here to put down pre-impregnated carbon fibre epoxy tapes. The first is known as Automated Tape Laying (ATL), which puts down unidirectional tape in fairly wide widths ranging from around 70mm up to 300mm. This means large composite parts can be laid-up much more quickly. The downside with ATL, however, is that it is almost impossible to use on complex geometry as 'steering' the tape, even around a relatively large radius, is impossible.

To compliment ATL, another automated machine is being developed known as Automated Fibre Placement (AFP). When doing lay-up, AFP uses much thinner unidirectional tapes, generally between 6mm and 8mm, to allow some 'steering' of the tape laying tool.

John Cornforth is head of airframe and special products for R&D at GKN Aerospace. He says: "The analogy is trying to put sellotape around a football. The edges of the tape crinkle and buckle as you try and bend it around the surface. But if you use a piece of string, you can cover over any surface."

Part of GKN Aerospace's development is seeing if these two technologies can be combined in to a single automated machine so that a flat and straight surface can be covered by the wider tapes, but on curved or more complex geometry, narrower tapes can be used.

The company is using AFP machines that use a bank of unidirectional tapes, each independently controlled to start, stop, and cut independently. On a curved surface only one or two tows might be used, and on the flatter surface much higher deposition would be possible by using all 12 tows.

"At the moment there are various research programs going on to evaluate if there is place for a machine that can combine really wide ATL with thinner AFP," says Cornforth. "However, the problem is the software hasn't caught up yet."

Rapid deposition of fibres, while an important part of composite manufacture, isn't a standalone breakthrough, however. It is only as important as keeping up with the next step of the process; curing.

Autoclaves have become a bottleneck for composite production and while speeding the lay-up process would improve quality, it could also exacerbate this shortcoming. GKN Aerospace, like the airliner OEMs, use autoclaves almost exclusively to harden the thermoset resin and secure the carbon fibre matrix in place.

Although many 'out of autoclave' (OOA) methods already exist, these tend to result in some drop off in mechanical performance and processability can be harder to control. Autoclaves can have pressure and temperature cure cycles precisely defined to drive out any volitiles in the resin system and give the low porosity quality of laminate needed.

However, GKN Aerospace wants to speed up this process and has developed a new method of curing the resin using microwave ovens. Despite murmurs of improvement in the technique for some time, the company says it is now in a position to take the technology forward. However, the parts that are most likely to benefit in terms of production time are not entirely across the board.

"It saves a reasonable amount of time, but that's only on the curing element of the process," says Cornforth. "We have done the business case analysis and the results were a little surprising, as we found for large components there is a high proportion of lay-up time, but the cure time is fixed.

"On a small component, even though it is only small, it can take almost the same amount of time to cure in an autoclave as a big part, but the lay-up time is a lot less. So microwave curing is actually more beneficial on small to medium sized components rather than large components. Everything has its sweet spot in terms of size, shape and volume. What we have been trying to do is find the family of components microwave curing is most applicable too."

GKN Aerospace is still developing production processes and continues to work closely with the National Composites Centre to improve manufacturing technologies. And while it has not traditionally looked to exploit its technologies by licensing or selling them, it hasn't ruled out the possibility in the future.

But, it is not working in isolation with other industries also actively developing composite manufacturing technologies. Indeed, Leicester based Formax has recently expanding it production facility with a purpose built factory to produce high volume glass and carbon fibres woven rolled sheets, destined for the automotive industry.

It uses a multi-axial machine made by textile machine builder Karl Mayer Malitronic, which has been customised to lay-up carbon and glass fibres. It is part of a £2 million investment programme by the company to allow specialty composite reinforcements to be used within industries that require higher volume production rates.

Formax hope the move will allow automotive OEMs and Tier 1 suppliers to design bespoke fabric or preform parts by providing a fast and cost effective way to produce them. Typical parts are likely to include Class A body panels, body-in-white structures and impact resistant long fibre reinforced parts.

The cut and lay machine can produce in variable widths between 1270mm and 1600mm, and will be dedicated to the production of tailored Non Crimp Fabrics (NCFs) specifically optimised for high volume automotive programmes.

The machine is equipped with 3-axes and is capable of laying ply angles from 20° through to 90° with both in-line and off-line spreading technology. This allows design engineers to get the maximum benefit of composite materials by specifying larger and lower cost fibre tows with ply weights as little as 50gsm.

Dan Norton, automotive sector manager at Formax says: "Formax has recognised the demand shown within the automotive industry to include carbon composite structures within multi-material platforms. With this machine and its cutting edge spreading technology, we are able to increase fibre throughput whilst maintaining and delivering high manufacturing efficiencies.

"Resin and tooling technology are all advancing at a rapid rate to help deliver the magic 60 second cycle time the automotive industry is pushing for, and we are confident that textile reinforcements can now be added to this list."

Property Improvement
As well as working on processability, GKN is looking to produce composite structures with enhanced properties.

Damage Detection: Spotting damage on composite structures is notoriously difficult, especially if the damage is to internal fibres as there could be no visible signs of an impact on the surface, however it could leave the internal structure significantly weakened.

By using a modification to the paint, engineers will be able to detect if a structure has suffered impact damage. The basic concept is making the material 'bruisable'. Upon impact, a dye will leak out. The dye is only likely to be visible under ultraviolet light so to not frighten passengers. And the weight of the dye is reported as being 'negligible'.

Through Thickness Reinforcement (TTR): This z-axis reinforcement aims to stop delamination between plies. While it can be achieved using pins, known as z-pinning, through thickness reinforcement can also be achieved using z-axis stitching or tufting. However TTR is an expensive process and would likely only be used in the most critical of locations.

Justin Cunningham

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