GMC CarbonPro for Pickups

Industry-first carbon fiber composite box

America is in love—with the pickup truck.

The first one sold 209 units in 1917, the initial year of its production. Compare that to the first half of 2018 when 1.4 million pickups were sold.

In the 1950s, the pickup went from a work vehicle meant for hauling to a passenger conveyance that also hauled. As it evolved from its rudimentary beginnings, consumers demanded more—more payload, better towing capacity and luxury items found in a sedan. These add to the weight, which in turn, lowers fuel economy.


With government regulations calling for better gas mileage, truck manufacturers have to find ways to oblige. That’s what GM was facing when it decided to develop a pickup box with unprecedented strength and durability that would, at the same time, weigh less. Their efforts would lead to the revolutionary CarbonPro pickup box, on the 2019 Sierra AT4 1500 and Denali 1500. (See Figure 1.) Mark Voss, engineering group manager at General Motors, presented the effort at the Lightweighting World Expo 2018, and here is the story we heard.

In the 1950s, the pickup went from a work vehicle meant for hauling to a passenger conveyance that also hauled.

Work began in December 2011 with Teijin, which specializes in high-performance fibers such as aramid, carbon fibers and composites, coming on board. The early discussions led to some conclusions.
• The final product must leap-frog the industry, get ahead of what competitors were offering.
• It had to forge a non-linear, revolutionary development path.
• The team had to find the best materials, design, molding and joining processes.
• They needed to accept there would be multiple learning loops.
• Once a process had been established, they were charged with putting it to use as soon as possible.

MATERIAL
It was decided thermoplastics would be the material of choice because of several characteristics.

Its scrap can be reused without losing significant mechanical performance or incurring costly, labor-intensive separation of fiber from resin. The short cycle time enables high-volume production at a lower cost. Additional joining methods, such as welding and staking, are feasible. Added to that, thermoplastics are tougher than thermoset composites.

The specific thermoplastic, Teijin’s SEREEBO I (Save the Earth, Revolutionary & Evolutionary Carbon), was chosen. Launched in 2013, it is a 35-percent volume fraction carbon-fiber reinforced Nylon 6 for a one-inch long, randomly oriented mat with isotropic properties. The scrap produced by the process is of structural grade.

MATERIAL CHARACTERIZATION
The next step was to thoroughly exam the material. (See Figures 2 and 3.) This entailed extensive testing of its behavior, which included:
• Environmental conditions and loads
• Mechanical properties—tensile, compression, flex, impact, fatigue, and creep at different temperatures and humidity
• UV resistance
• Galvanic corrosion
• Resistance to chemicals

For thermoplastic composites, the issue is warping, or warpage. Warpage is driven by an angle change at each corner when the material shrinks.

IMPACT-RESISTANT BED DESIGN
Computer-assisted engineering (CAE) methodologies were developed after the extensive material characterization and component-level testing. To develop an impact-resistant bed design, strain-based damage initiation criteria were imperative and needed. Next came a component-level test to ensure that there was an excellent correlation to impact performance of the full box assembly. A simulated point striker that correlates to the edge of a barrel or a block of concrete striking the floor of the bed was used.

All of this led to optimized corrugation geometry with up to a six-times improvement in impact performance when compared with the baseline metallic material-substitution design. (See Figure 4.)

BONDED JOINT FATIGUE
Still in the CAE development phase, the CarbonPro team needed to ascertain the fatigue of bonded joints. So, a fatigue envelope was created from coupon testing. Then, CAE results were reduced to resultant forces/moments. The fatigue envelope could then be used to identify at-risk joints.

WARPAGE PREDICTION
As in many products produced in a thermal process, careful consideration is taken to manage distortion when the component cools. For thermoplastic composites, the issue is warping, or warpage. Warpage is driven by an angle change at each corner when the material shrinks. Significant warpage in early mule parts caused large bond gaps and residual stresses in the parts. However, lessons learned from mule builds effectively eliminated part warpage. This was made possible with CAE tools that could precisely predict angle changes.

MOLDING PROCESSES
The cycle time is less than a minute. (See Figure 5.)

MOLDING CHALLENGES—ROBOTIC MOUNTED PERFORM DEVICE (RMPD)
The RMPD picks up material from the oven and transports it into the press where it is transformed from 2-D to 3-D by a series of concert motions. The material is then carried out by servo drives and needle grippers. Placement into a mold is followed by press closure/ compression. The oven-to-compression cycle time is ≤ 20 seconds. The material cools at 10 degrees Celsius/second.

MOLDING CHALLENGES— COMPRESSION MOLDING
Compression molding can reproduce a high volume of parts with accuracy, speed and repeatability. The CarbonPro team chose a 3600T Press with a narrow process window for quenching nylon. And very importantly, a quick cycle team—because EVERY SECOND COUNTS. (See Figure 6.) This machine has unique press parameters and precise charge placement.

JOINING METHODS Adhesive bonding is the primary joining method to the substructure. Forty-eight meters of Ashland 8500 2-part urethane-based adhesive were used on each vehicle. Supplementary joints have mechanical fasteners and peel stoppers (Avdelok).

There are 128 fasteners including 78 for outer panel attachment on each pickup and 12 stainless-steel Avdeloks. Metallic sub-assemblies are either spot welded or mechanically fastened. In all, there are 72 spot welds and 73 projection welds.

SECONDARY OPERATIONS AND ASSEMBLY
After compression molding, the composite mat is processed as follows:
• Trimming—Wet Router, Waterjet
• Bonding Sub-Assemblies
• Bonding Main Assembly
• Hot Impinged-Air Cure
• Coherix Vision system for adhesive-dispense monitoring

FINAL ASSEMBLY STATION
In the final assembly station, the fasteners, heat shield, cargo lamp, wiring harnesses and labels are installed.



VALIDATION—HOW WELL DOES THE CARBONPRO DESIGN HOLD UP?

Now that there is an aesthetically pleasing composite bed lying flat, buyers need to be sure that the pickup box can withstand all manner of impacts—loads of bricks, snowmobile track studs and shifting cargo. (See Figure 7.)

Standard validations for steel beds do not properly test all the failure modes of a carbon fiber reinforced thermoplastic subsystem. That meant validations had to be developed for this unique solution. One modification was testing in -30 degree Celsius to 80 degrees Celsius and a high humidity environment. The presentation included videos of the impact loads—the sledgehammers, the cinder blocks. To us, the surprise was the bounce. We think of composites as stiff material. Yet this bed has bounce, a resilient nature that correlates to toughness.

Ad-hoc testing was performed on composite boxes to see how they would stand up to real-world usage. And they did—withstanding all types of punishment—firewood from a loader (see Figure 8), to sharp snowmobile track studs to a hot generator running for long periods of time.

RESULTS
The end product is an industry-first carbon fiber composite box. It adds unprecedented strength and durability to the truck. Its UV-resistant material means no paint or bed liner is needed. And it weighs 62 pounds fewer than a steel bed. It’s approximately 100 pounds lighter if you count the bed liner.

The CarbonPro pickup box is best in class for cargo volume, one cubic foot more than the leading steel box. It’s best-in-class in terms of resistance to dent and cargo damage. And to top it off, it’s eco-friendly with minimal material waste because it can be recycled.

It took a team and more than eight years to design, engineer, test and manufacture the cutting-edge CarbonPro pickup box. The result was well worth it. Contributions came from Teijin, GM Body Engineering, Continental Structural Plastics, Global Manufacturing Engineering, Validation Engineering, GM Quality, GM Global Corrosion Engineering and GM Research & Development.

Once an application like this one as a carbon fiber thermoplastic composite gets a foothold in the market, and if it proves to be durable, the applications will grow. Lightweighting is always top of mind. When the risk is gone, then it boils down to the business case, which ought to get easier as the manufacturing process matures and the design safety factors are better understood.

Authored by Andrew Halonen

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