Designing composites into industry

In the second part of our Designing in Composites articles, we talk to experts from both the design and manufacture world about the opportunities for innovation. Justin Cunningham reports.

Design for manufacture is a well-known phrase, but is not always applied to composites. That is not necessarily because it is overlooked, but more because composites require a certain amount of practical knowledge and experience to get right.

For newcomers, composites can seem temperamental and well-established OEMs such as Boeing and McLaren have had more than a far share of pitfalls and struggles getting the materials to really work.

Simply swapping one material for another to achieve a result is far from ideal. Yet engineers can fall in to the trap of trying to make a like-for-like part and expect the lighter material to automatically give a part significant performance gain.

"I call it the black metal syndrome," says Dan Fleetcroft, design director at Performance Engineered Solutions (PES), a multi-disciplinary team of design and performance engineers working across multiple sectors and based on the Advanced Manufacturing Park in Sheffield.

"If you have had no experience or exposure to composite materials, then there is a great potential for not getting the most out of it and you can actually cause yourself a lot of trouble trying to make a black metal part."

First experiences with composites, particularly if they are unguided, can be disastrous. There are a number of common pitfalls that tend to catch engineers out. If you have limited experience with the materials, it is a good idea to talk to someone and get advice in either a formal or informal capacity.

"They don't have to be involved in the final design, but can go through the things to consider when designing a composite part," says Fleetcroft.

"Things like tooling and moulding, fibres, resins; there is a vast amount to think about. You don't want to invest a lot of time designing a part which is so difficult to manufacture it makes it uneconomical or impractical to produce."

Composite parts often need specific tooling made for them and, though an actual part might be well thought out, the tooling specified may be ineffective or off the mark. Additionally, experience counts for a lot when producing composite parts and knowing how to lay up the different sheets of fibre to optimise strength and stiffness for a specific application involves a real understanding of composites to unlock the full performance benefits of the material. While some engineers think there is some black magic involved, it is the same as any material, it is knowledge and experience that will deliver the best results.

"You can tailor properties within a composite component that you can't achieve in any other material," says Fleetcroft. "But there is also a high degree of understanding required to make those parts optimised for cost, manufacture and performance. Joining steel parts together is obvious, but you might have a large, one-piece, composite part and, if the fibres are laminated in the wrong orientation, it could cause a catastrophic failure. Imagine if that is an aircraft wing.

"So it also depends on the criticality of what you are doing. Most parts will actually perform OK with a generic lay up, probably showing some improvement, but are they really optimised and exploiting what a composite can really do?"

Much of the time, composite parts are outsourced to specialist manufacturing shops to be made. This can be helpful in 'sanity checking' a part, as it is in their interest to make sure they can deliver what the customer expects. For this reason, it is not unusual for a specialist composite manufacturer to recommend changes to lay up, orientation of fibres or even combine components together to make a single part.

In some cases, manufacturing facilities ask for the designs and do the tooling themselves. They may even go so far as to specify a laminate or recommend ways of optimising the design further from a cost or structural basis.

Ian Handscombe, general manager at Prodrive Composites, says: "Designing something 'right' with composites will make an enormous difference to costs. If you don't get the design right up front, the costs in tooling and manufacture will be hugely different. You don't want to make a sub-optimal part that ends up being more expensive.

"We have been asked by previous customers to work with them at the very early stages of future projects to ensure we don't have the same challenges that we might have had on previous programmes. So our customers are starting to recognise that it is so important for us to be involved at the early stages of the design."

Prodrive has a wealth of experience both designing and manufacturing composite parts and has opened a dedicated composites facility to serve customers wishing to take advantage of the material. It worked with a disabled designer setting up a company to produce wheelchairs. The designer was frustrated with off-the-shelf wheelchairs so wanted to make one from a disabled person's perspective and he was keen to use carbon fibre because of its inherent light weight.

"This is a good illustration of how Prodrive collaborates on the manufacture of parts," says Gary White, composites engineering manager, Prodrive. "He came up with a basic design of what he wanted it to look like, which was actually a really difficult thing to manufacture. In conjunction with an external design consultant, we went through all the tooling and manufacture of components helping with the design for manufacture."

In general, White and Handscombe, describe the two main methods for the structural design of a part. One way is through trial and error, whereby a structure is over- or under-engineered at the start and then iterated down until it reaches an optimum. This method is both time consuming and expensive, while the reality is often that time and money may force an output before an optimum is ever reached.

The other method is using specialist software packages such as CAD, CFD and FEA. There are some excellent tools on the market, but again these are limited by the user's ability to fully exploit the software and their knowledge of composite behaviour.

"What works in that perfect CAD world doesn't always work in reality," says White. "Ultimately, you need a balance. If you rely on software entirely, then you find problems, much like we saw with Virgin Racing [the unsuccessful F1 car designed entirely using computer simulation technology]. It is a combination of theory, software and practical experience that, when put together, offers a really good solution."

It is important not to assume that if you have tried unsuccessfully to apply composites in the past that they do not work. Generally the failure is through lack of knowledge and poor application. Advancements in composite materials and manufacturing techniques are continually developing and could very easily meet a performance criteria that may not have even been a consideration a few years ago.

Top tips: designing in composites

• Avoid sharp corners or radii: soften radii as much as possible. If you have complicated features within the mould, it makes the part much more technically difficult to get right.

• Use balance and symmetry: balance laminates, plies and material orientation. This helps to minimise any distortion after the part is removed from the mould tool and to minimise bending, warping or twisting in the structure.

• Avoid stacking too many unidirectional plies at the same angle: this can lead to matrix micro cracking as the composite components become resin-dominated in one direction.

• Look at the loading directions and consider some off-axis capability: in most applications components will see some form of load in a secondary direction. If you are not careful with composites you could have a truly matrix-governed property in a particular direction that may not be capable of withstanding the loading.

• Take advice on fibres and resins: the materials recommended will depend on your application as well as your budget.

• Design single-piece parts: assembling parts adds complexity and cost. Also, single-piece parts avoid bonding and glue lines which is aesthetically better.

• Multiple curvatures within a component: as fibres are laminated over curved surfaces and corners, their orientation changes. This needs consideration in terms of the properties of the laminate that you calculate and the actual properties that are achieved.

• Don't be put off: Don't assume that, because you have tried unsuccessfully to apply composites in the past, that they do not work. Generally failure is caused by lack of knowledge and poor application.

Autoclave still yields best results for complex parts: There are advantages in using out-of-autoclave materials, but if you want a very good visual part the autoclave is still the preferred manufacturing method. Press moulding is also very popular, but for complex geometries it can be difficult to do.

Ian Handscombe (right), general manager at Prodrive Composites, says: "If you have complex geometry and you are laying it up by hand, most of the customers you are dealing with will want it to look quite good, so not only are we trying to make it structurally superior, but also cosmetically good as well."

But it is not the future: There needs to be changes in the materials themselves to be able to manufacture in volume. Ultimately for very high volume – possible on a standard family car – it is not going to be composites as they are known today. It is possible it could be a thermoplastic rubber rather than a thermoset material. And it could likely be some kind of press moulding resin transfer process.

"I would imagine that will enable vehicle manufacturers to get the sort of volumes they need," says Handscombe. "But that will be for lightweight structural and body components not for today's cosmetic applications. The new process is initially unlikely to give top-quality visual finish you might expect today."

Author
Justin Cunningham

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