Crash-resistant carbon fiber: Why your next frame might just be made of hybrid composites and thermoplastics
One of the typical ironclad rules of structural materials is that as stiffness goes up, ductility goes down. As far as carbon fiber frames are concerned, that’s essentially just a fancy way of saying what we all already know: whereas steel, aluminum, and titanium frames usually dent or bend upon a heavy impact, carbon fiber is more apt to crack or shatter. But what if it didn’t have to be that way? A new wave of composite materials and technology promises to keep the performance of carbon fiber, but considerably improve its durability.
The problem with carbon fiber
Carbon fiber has rightfully earned its place as the current king of bicycle frame materials. As compared to the usual alternatives, carbon fiber reinforced plastics (or CFRP, as they’re usually referred to in engineering circles) are fantastically stiff yet extremely light: a perfect combination for a human-powered activity where power is limited and weight is critical. Continual refinement has allowed engineers and designers to craft that material into wildly shaped structures that were the the stuff of dreams just a decade earlier.
Yet that stiffness doesn’t come without a cost. Carbon-fiber frames are lighter, stiffer, more aerodynamic, and more feature-laden than they’ve ever been, but yet they’re still unnervingly prone to impact damage.
Various solutions have been adopted over the years to combat the problem. Most commonly, engineers will substitute stronger (but less rigid) fibers in areas that are prone to crash damage, such as in the top tube where a handlebar might swing around. Alternatively, some frames will use patches made of aramid or other high-strength fibers as a sort of armor to guard underlying structural layers.
All of those solutions are invariably compromises from an engineering point of view, though, adding weight to otherwise hyper-efficient structures.
The lure of hybrid composites
Most carbon-fiber bicycle frames are multi-layer structures, built with hundreds of separate pieces, or plies, of material that are laid into a mold and held together with epoxy resin — like those art projects you did as a kid with paper mâché and a balloon.
Continuing on with that analogy, let’s say that the higher-modulus (stiffer) fiber plies that are ideal from a rigidity point of view are red pieces of paper, while lower-modulus fiber plies (stronger, but not as stiff) are blue. When engineers substitute a higher-strength/lower-stiffness material in one area of a frame, what they’re effectively doing is swapping out a red strip of paper for a blue one. Similarly, alternative-material armor is essentially extra bits of yet another color paper on top of what you’ve already made.
Hybrid composites, on the other hand, blend dissimilar materials together in the same ply of material, thus conferring some of the properties of both, but with minimal impact on the total weight of the structure — basically using smaller red and blue strands to form a single strip of purple paper. Woven layers, using a mix of aramid (Kevlar) and carbon fibers, have already long been in use on frames, forks, and other bicycle components, but companies shy away from them because they can be difficult to process —it’s difficult to get a smooth finish on an aramid layer, and the material also doesn’t always bond well with the surrounding epoxy resin.
However, an intriguing new crop of hybrid composites, recently presented at the 2nd annual Cyclitech International Conference on Bicycle Technology, shows promise.
Plastics, steel, and carbon fiber
Perhaps somewhat counterintuitively, two of the materials that are now being used in the cycling industry are essentially just plastic: Dyneema is an ultra-high molecular-weight polyethylene fiber, while Innegra is a high molecular-weight polypropylene fiber (both with similar base chemistry). By incorporating them into hybrid plies blended with carbon fiber, both companies claim impact durability improvements over using only carbon fiber in composite structures.
Dyneema’s approach to impact tolerance in hybridized carbon fiber structures is simple to understand. The fibers themselves are highly flexible yet have extremely high tensile strengths, so they’re more tolerant of the concentrated loading that damages carbon-only fiber composites. DSM Dyneema — the Dutch company that manufactures Dyneema fiber — claims the impact energy absorption of hybrid carbon structures “can be increased by up to 100%”, while also reducing weight (albeit presumably at the expense of stiffness) and adding some vibration damping characteristics.
“Dyneema has a higher elongation at break and is more durable than carbon fiber,” said Dr. Tim Kidd, new business development manager for DSM Dyneema. “So when you combine that together with the stiffness and strength of carbon fiber, you get the best of both combinations with the stiffness and strength of carbon fiber and the lightweight impact resistance, or energy absorption, of Dyneema.”
The mechanism by which Innegra operates is similar. Like Dyneema, Innegra can stretch much more than carbon fiber before breaking, but according to Innegra Technologies business development director Jen Hanna, the microstructure of the fibers themselves also impact some mechanical ability to lessen impact damage.
“Innegra fiber consists of micro-voids in the structure, which may lead to the energy dissipation qualities the fiber possess,” Hanna said. “The fiber also has a higher elongation than carbon, therefore providing more ductility and durability [and] allowing an impact to be absorbed or dissipated rather than the energy causing the Innegra fiber to fail as it does with carbon.”
One of the most interesting concepts was a carbon and steel fiber hybrid composite presented at Cyclitech from Dr. Michaël Callens, CEO and co-founder of REIN4CED, a Belgian engineering house that focuses on fiber-reinforced composites.
As with Dyneema and Innegra, Callens claims the added ductility of the steel fibers substantially improves the impact tolerance of composite structures using only carbon fibers, while also reducing the chances of a catastrophic failure. Unlike those flexible plastic fibers, however, the steel-and-carbon fiber hybrid composite doesn’t sacrifice as much stiffness and, theoretically, won’t require as much additional reinforcement to retain the desired rigidity characteristics.
In fact, Callens eventually foresees a “zero-zero weight gain” for frames, forks, and other components that use his company’s technology.
It’s important to note, however, that while all three of these technologies claim to improve the impact durability of carbon fiber structures, none of them will eliminate it completely. A suitably large impact force will still break the carbon fibers themselves; it’s just that there will now be a surrounding support structure to prevent the sort of spectacular catastrophic failures that can lead to rider injuries. In other words, your carbon frame will still be broken, but there will be less of a chance that it will separate into multiple pieces and you might even be able to ride it home.
“Dyneema increases the amount of energy that a carbon composite can absorb before it fails completely, before it breaks; it acts as a kind of cage to surround the carbon fibers,” said Dr. Kidd. “If there’s an impact, the carbon fibers are quite brittle. They’ll break, but the Dyneema ones won’t. They’ll break eventually, of course, but they’ll absorb the energy much more [than carbon fibers alone]. The part will not fail completely. The structural integrity is intact.”
The return of thermoplastics
The vast majority of carbon-fiber bicycle products currently on the market are made with thermoset epoxy resins, which start out as viscous and pliable but are permanently hardened once cured. Like thermosets, thermoplastics also serve to hold the carbon fibers together, but their different chemistry allows them to be reheated and reformed — sort of like repeatedly melting and refreezing ice. More importantly, in terms of impact resistance, they’re generally less prone to cracking than thermosets, which makes them another intriguing path toward building tougher carbon-fiber parts and frames that can handle abuse.
“Generally, I like to think of the comparison between thermosets and thermoplastics like this: thermosets have more of a 3-D cross-linking between molecules; thermoplastics are just long chains of polymers,” explained Jason Gabriel of TenCate Advanced Composites.
According to Gabriel, that less-ordered structure lends thermoplastics their added toughness relative to more commonly used thermosets.
“One of the main jobs of the resin, or matrix, is to transfer the forces on the actual part into the carbon fibers,” he said. “The fibers are quite strong compared to the resin. As long as the matrix and carbon fibers bond well, the part has very good strength and stiffness. But many times, the things you might associate with impact do not have much to do with the fiber breaking; rather it’s the resin between the fibers fracturing or the bond breaking between the resin and fiber. So if you have two resins that bond well to the fibers and transfer the loads well, the resin that is a bit more ductile and tough can handle impacts better and will provide a more durable composite material with similar stiffness and strength.”
In other words, thermoplastic’s more “rubbery” nature means that impact forces can potentially be spread out over a larger area, distributing that energy across a greater number of carbon fibers and decreasing the chance that any one of them will break.
As an added bonus, thermoplastic composites can be easily recycled with little additional energy investment — just grind them up, melt them, and mold them into short-fiber composite parts for less structurally intensive applications. And unlike thermoset pre-preg composites, thermoplastics don’t need to be stored at low temperatures, and the raw materials have a longer shelf life, too.
Early warning devices
One of the biggest issues surrounding carbon-fiber impact damage is that it often isn’t apparent on the surface. Even if the outermost layer looks pristine, one or more underlying layers may be cracked, hiding a weak spot that could unexpectedly fail later on from a seemingly minor impact. Non-destructive techniques such as ultrasound can be used to directly inspect parts for damage, but they’re not readily accessible to most riders, nor are they always practical. Regardless, the most intuitive indicator of damage is still visual, and all of these technologies may play a role here as well.
“There are certainly cases where if you hybridize — put Dyneema and carbon together — you can see the damage earlier than you would with a normal carbon-fiber composite,” said Dr. Kidd. “One of the problems in the past has been if you damage your frame, it’s not always visible; you don’t see it. And then the part is still damaged and you go out on your bike again, you push it a little hard, you go down a hill too fast, and that damaged part can fail quite a lot. Indeed, with Dyneema, with an impact, you do see the damage sooner.”
“For some thermoset epoxy composites, after an impact, you will not see a mark on the surface, but there can be fractures underneath, mainly between the layers,” added Gabriel. “And for some thermoplastic composites, they can mark easier on the surface, but with fewer fractures underneath. If the fractures don’t greatly affect part performance or safety, the mark in the thermoplastic could be seen as a disadvantage since there will be more marks on the surface of parts. However, if the internal fractures do affect safety or performance, the thermoplastic is more likely to show you there was an impact in the area.”
Challenges to implementation
Some of these technologies will actually be coming to market quite soon — or are already here — but for others, it may be quite some time before they’re actually incorporated into off-the-shelf products.
Of all the impact resistance technologies discussed at Cyclitech, though, none would be more difficult to incorporate into the bicycle industry than thermoplastics. Much as it might seem otherwise to the casual observer, replacing current thermoset resins would thermoplastic ones would be anything but a straight plug-and-play affair. Thermosets have been used for decades now, and they’re well understood and highly evolved. Although thermoplastics may offer some further improvements, turning those into reality would involve a wholesale redesign of how carbon fiber frames and components are manufactured.
“There are years and years of thermoset and epoxy experience in the aerospace industry, as well as the general composite industry,” said Gabriel, “so people have really learned how to process it well. They’ve learned how to improve the product. When you work with these epoxy materials, generally they’re very drapable, soft, and they’re very tacky — kind of like peeling off an adhesive sticker and wrapping it around a rubber tube or a rigid tool. It’s relatively easy to do by hand.
“High-performance thermoplastics are like working with a manila folder,” Gabriel continued. “It has no tack, and it’s stiffer and boardy. Generally, you want them to be hot in order to get any stickiness or drapability to them, and if they’re hot, you’re usually not using your hands to work with them; you’re usually using automated equipment. One of the best thermoplastics out there for carbon fiber composites is PEEK [polyether ether ketone], and they’re processing at 700-750°F.”
Older riders — especially those that spent time on mountain bikes in the 1980s and 1990s — will remember that the cycling industry dabbled with thermoplastics in the early days of fiber composites. Schwinn, Yeti, GT, and K2 all experimented with carbon-fiber reinforced thermoplastic frames, and Yeti and Scott both offered thermoplastic handlebars as well. None were ultimately successful.
“The commingled yarn we used from Cytec was, at the time, the best candidate for what we were trying to do, and certainly there may be better compositions now days,” said Forrest Yelverton, who was GT’s engineering manager at the time. “The fabrication design we used, which had great promise from a production standpoint, had less-than-optimal final product performance. The bottom line from my perspective at the time was that the material had great damping and impact characteristics, but was better utilized in the handle of a hammer. We probably would have been better off making seatposts out of it.”
Nevertheless, Gabriel says the landscape for thermoplastics is improving, in particular thanks to the aerospace and automotive industries where the technology is more conducive to their higher volumes and more sheet-like structures. That said, it may be some time before we see thermoplastics return to cycling in any appreciable scale, at least on complex shapes like frames and forks. Simpler geometries that are potentially easier to automate like handlebars and rims may come sooner, and their greater susceptibility to impact damage may make those applications more appealing, anyway.
Carbon fiber bicycle products augmented with Innegra and Dyneema, however, are just around the corner. Both already have a reasonably diverse range of products from which composite manufacturers can choose for immediate integration into existing designs, and implementing those technologies isn’t nearly as daunting a task as switching matrix chemistries.
In fact, Innegra is already being used in some carbon-fiber bicycle products. For example, PRO incorporates the fiber on its Vibe Aero carbon-fiber drop handlebar, just behind where the levers mount, to help hold the bar together after a crash. Slovenian company Berk Composites also uses Innegra in its custom carbon road frames, and new American company HIA Velo plans to incorporate the fibers into a new bike set for release early in 2017.
“We’re not really ready to disclose specific usage,” said HIA owner Tony Karklins, “but I can tell you that we are very impressed with the improved impact resistance that Innegra fibers provide to some of the more common areas of composite frame failures.”
Likewise, Dyneema is already used by Specialized as a reinforcement panel in some S-Works shoes, but the material will apparently soon find its way on to frames and other hard goods.
“We are in contact with several [bicycle brands],” said Dr. Kidd. “We are looking to develop already for next year at least one frame or bicycle component. I can’t say anything else at the moment on that because we are under development agreements, but it’s certainly in the cards for 2017.”