There’s something satisfyingly solid about metal, especially steel. When you pick up an old wrench, you feel the cold handle slowly warm. It feels reassuringly heavy, dependable, when you heft it. The Man of Steel. Steel is real. It’s why it’s used in bike frames, and pickup truck beds.
That’s why Ford’s introduction of the aluminum-bodied F-150 was such a paradigm shift in the pickup world. And General Motors pounced on the advertisement opportunity.
We’re all too familiar with the ‘real people’ in GM’s ads. But there’s another reason to cringe when you watch them. And with the launch of the updated 2019 GMC Sierra and it’s carbon fiber pickup bed, GM’s marketing department might be cringing along with you.
Ford, as Land Rover later did, switched to aluminum to cut massive weight out of the F-150, in the interest of fuel savings and lower emissions. Several Chevrolet ads directly attacked this, showing the F-150's bed being punctured and scratched up, while the Silverado’s steel bed remained mostly unharmed.
David Tracy correctly pointed out that the marketing strategy seemed squarely pointed at poo-pooing aluminum as a manufacturing material—even though GM claimed it was a dig at the way aluminum was put to use—and that it was going to bite them in the ass one day. Well, the Sierra’s new carbon-fiber CarbonPro bed is here to haul stuff and bite a—OK, I’m not going to finish that sentence.
Point is, GM has now introduced a material which is not only significantly lighter than aluminum into one of its (admittedly upscale) pickup beds—saving 62 lbs over the equivalent steel bed—but is also not usually associated with the word “durable”. Carbon fiber is incredibly strong, especially for its weight, but it isn’t the first thing that comes to mind when I think “impact resistance.” There’s a reason I chose steel for my long-distance adventure bike’s frame.
Chevy’s going to have to shove a lot of ad dollars into convincing the general public that a carbon-fiber pickup bed will withstand things like rocks and bear attacks. But that raises a big question: can it actually do that?
In past Supporting Cast articles, I’ve discussed the various terms engineers use when discussing material properties. A material’s ‘strength’ is based on how it withstands to stress: being pulled, bent, compressed, and so on. In the case of someone throwing something into the back of the pickup, we’d be most interested in the Young’s modulus, yield strength, hardness, impact strength—aka toughness—and fracture toughness.
- Young’s modulus describes stiffness, the ability to snap back into shape after a load’s been applied, and is dependent on both geometry and the material. Pickup beds are ribbed (for your pleasure) to increase stiffness
- Yield strength is how much stress you can put a material under before it can’t snap back into shape anymore. Think of it as the ding threshold
- Hardness is resistance to permanent deformation due to constant pressure from a sharp object, like gardening shears or a pickaxe
- Impact strength measures how easy or difficult it is to crack a material; picture the baseball you accidentally hit into Mrs. McLeary’s window
- Fracture toughness gets involved once a crack forms: the higher this value, the more energy needed to make the crack grow
Straight away, we have a problem. Metals like steel or aluminum are isotropic, meaning their properties don’t change based on the angle you’re coming from. Doesn’t matter if you’re stamping it, cutting it, or pulling it, steel’s yield strength won’t change. But carbon fibers are anisotropic: strong in one direction, weak in another. That’s why the fibers are woven together, like the strands of a rope.
But that isn’t the biggest issue. Because carbon-fiber composites are, well, composites, their exact properties depend on the quality and kind of fibers being used, how they’re arranged, the exact plastic resin used for the matrix, and the interaction between the fibers and resin. We’d need exact specs from the supplier to make the best comparison.
Luckily, Automotive News released a report that details GMC’s supplier, Japan-based Teijin Limited. GM partnered with Teijin back in 2011 to develop novel composite technology, under Teijin’s Toho Tenax brand.
Based on images of the Sierra’s CarbonPro bed, details from Autoblog’s coverage, as well as information provided by Engineering Manager Mark Voss during the reveal, it appears that the specific CFRP used is similar to the Forged Composite technology used in the Lamborghini Huracan Performante: the fibers are short—only 1" long—rather than woven, to rapidly decrease production time, and are based on nylon 6 in a reusable thermoplastic resin. Toho Tenax lists this on their website as “Part via Preform”, or PvP, composite.
Sadly, because the CarbonPro bed is an industry-first, Toho Tenax doesn’t provide any specs on the PvP. But I can offer the next best thing, and compare the forged composite technology to what aluminum and steel offer.
First, the raw numbers.
In a paper detailing the design of the Sesto Elemento’s suspension arms, Lamborghini lists a tensile strength for their Forged Composite as 35.8 ksi (246 MPa), or 35800 psi, and a Young’s modulus of 4.9 Msi (33.8 GPa), or 4900000 psi. While that sounds like a lot—because it is—Lamborghini also compared it to the 6xxx-series aluminum alloy they normally use in the suspension arms. The alloy was reported to have a minimum tensile strength of 310 MPa, or 45 ksi, with a Young’s modulus of 10 Msi, or 70 GPa and a minimum yield strength of 260 MPa, or 37.7 ksi.
Not only was the Forged Composite not quite as strong as the aluminum, it also wasn’t as strong as the more ‘normal’ CFRPs available. The strongest of the two composites had a tensile strength of 745 MPa or 108 ksi, with a Young’s modulus of 41.4 GPa, or 6 Msi. This is most likely due to those chopped, short fibers: they make assembly much faster, but you lose out on some of the ultimate strength.
It gets worse when we bring steel into the mix, with high-strength low-alloy (HSLA) automotive steels having yield strengths of at least 276 MPa (40 ksi) and tensile strengths of at least 414 MPa (60 ksi), with a Young’s modulus of roughly 200 GPa (29 MSi). If you’re confused as to why the steel’s stiffer than the composite, as I said before, the latter’s properties are dependent on the fibers/resin ratio and the fiber orientation.
TL;DR, a composite similar to the one making up the CarbonPro bed is less stiff and not as strong as aluminum or steel. OK, not looking good so far. But what about impact and fracture toughness, or hardness?
For whatever reason, I had a hard time finding impact toughness data on these materials. So instead of taking these results as definitive, think of them more as points of relative comparison.
Quantum Composites’ lists their AMC 8593 forged composite as having an impact strength of 1068 J/m, or 20 ft-lb/in, using the notched Izod impact test. I unfortunately wasn’t able to find impact toughness data for the 6xxx-series alloy, but MatWeb lists Aluminum 7039-T64 as having an impact toughness in a notched Charpy impact test of 183 J/m or 3.43 ft-lb/in (on average). Finally, using data from the US Air Force Research Lab, and assuming a standard Charpy specimen thickness of 10 mm, a low alloy steel was found to have an impact strength of 4067 J/m, or 76.9 ft-lb/in.
Fracture toughness is determined by the stress intensity factor K, which has units of MPa · m1/2. Steel alloys can have K values as high as 50, and aluminum alloys in the neighborhood of 24. Because forged composites are a relatively new development, I couldn’t find any definitive data. The best I could find was a 2008 study in the International Journal of Aerospace and Mechanical Engineering, which gave a K value as high as 1.15.
Finally, hardness. One of the most widely-known scales is the Brinell hardness scale. Carbon-fiber composites have a Brinell hardness of about 120, which compares favorable with hardened aluminum’s 75, though some high-strength steels can have a Brinell hardness of over 300.
On the face of it, the CarbonPro material doesn’t appear to be as strong as aluminum or steel, but it would appear to resist impacts and indentations from pointy things a lot better than aluminum would. If a crack does form, though, the owner would probably have to get it checked quickly.
But should future owners actually worry about all this? Not really.
The composite parts are just one part of the bed itself. Any force that gets exerted on it will be supported by the steel and aluminum that’s also framing the bed. GM has plenty of qualified engineers who have sweated for the better part of a decade to make sure the material is as durable as possible. The reveal even featured footage of people throwing hammers, dumbbells, and even cinder blocks at the CarbonPro bed, and nothing happened to it. It’s entirely plausible that in the 7 years GM and Teiji Limited spent developing this composite technology, they found some way of improving its material properties enough to make it stronger and more resilient. Remember, the way a material is molded can have a huge effect on how forces get distributed within it. Aluminum itself isn’t as stiff as steel, but aluminum bike frames get their stiffness from their tubes geometry, not entirely from the material itself. That low Charpy test result? That isn’t uncommon with aluminum alloys—it’s because of their crystal structure. The numbers aren’t the whole story.
Additionally, based on what I’ve read on carbon-fiber failures, it seems as if it’s the resin that’s cracking, not the fibers themselves. Choosing a thermoplastic resin, as GM did, makes a lot of sense, because it’s easy to reuse and recycle. Mark Voss even mentioned during the reveal that scrap resin is used to make two other components in the truck.
It’s also worth noting that if carbon-fiber composites were really as weak as some may fear, there’s no way they’d be used in all the different motorsports and cycling applications we see today. If composites weren’t up to the task of handling cornering forces, why would Ford go through all the expense of making the GT350R’s wheels out of the stuff? And although packaging issues prompted BMW to change the M3 & M4's driveshafts back to steel, it was quite clear the carbon ones could handle themselves with aplomb. It’s not like rust’s an issue.
Ultimately, absent any additional info from the supplier, or a public info-dump from an automotive lab, we’re just going to have to wait and see how this new bed plays out. But honestly? As much as give GM grief for their ad work at times, seeing this is really cool.
In providing this kind of technology, even as an option, GM has begun the process of making carbon-fiber composites more accessible to the general public. Jeremy Clarkson predicted it with the Alfa Romeo 4C, BMW teased it with the BMW i-series, but I think this GMC Sierra’s pickup bed is going to be the real first step into affordable, ubiquitous composite cars. That’s a real opinion, not an act.