Lightweight sandwich structures for EV chassis

  • 04-Sep-2012 02:13 EDT
Connected sandwich panel.jpg

This replacement chassis structure for a Porsche 356 Speedster uses lightweight Inrekor sandwich panels.

Designers of today’s electric vehicles (EVs) know that every kilogram that they can shave from a vehicle’s overall mass gets them about 3 km (1.8 mi) in additional driving range per charge. Given that no one expects that battery technology will lighten up anytime soon, it’s little wonder that EV makers are searching for new lightweighting technologies.

One promising weight-savings approach uses as its fundamental structure polymer foam-core/metal-skin sandwich members developed by a Dorset, U.K.-based product-design firm, Inrekor, in collaboration with JSP Corp., the Tokyo-based global supplier of impact-absorbing car bumper materials. The cores of the high-strength, low-mass composite panels are made of JSP’s Arpro expanded polypropylene (EPP) foam.

The Inrekor sandwich components comprise the lightweight chassis of the QBEAK, an award-winning concept EV that was produced by EcoMove, a Danish design group. Without batteries, the prototype’s structure weighs only about 400 kg (880 lb), which is almost a third less mass than a conventional unit. If fitted with standard batteries, the boxy, minivan-like QBEAK would have a range of about 300 km (185 mi), almost double that of other EVs. The design also makes wide use of Arpro foam elsewhere in the vehicle, both inside and out.

Even though the small but roomy urban passenger car/delivery vehicle is only 3 m (10 ft) long, it can accommodate up to six passengers in certain configurations. The Horsens, Denmark-based firm, which aims to bring the QBEAK into mid-series production in early 2013, is considering several potential powertrains, including a hybrid battery/bio-methanol fuel-cell power plant.

EcoMove approached Inrekor to create a low-cost lightweighting solution for the QBEAK design several years ago, according to Stewart Morley, Technical Director for Inrekor. “Despite some negative associations with sandwich technologies that had crippled its usage in the past,” he said, “I was familiar with the successful use of high-performance honeycomb paneling in military applications, which led me to think a bit differently about how to apply it to an electric vehicle.”

The most obvious configuration, Morley continued, was “a metal skin over a polymer core, but we had to investigate a variety of different material combinations and different manufacturing methods before selecting one.” Inrekor’s engineering team eventually focused on JSP’s well-established Arpro EPP technology as the most effective choice for the core. Expanded polypropylene, he said, has several specific characteristics that are important to the performance of the sandwich technology, citing Arpro’s isotropic—omnidirectional—crush behavior as well as its easy, low-toxicity processing (molding), recyclability, and affordability as key.

“Arpro is a closed-cell EPP material that is resistant to chemicals, insulates both thermally and acoustically, and has a wide operational temperature range [+130 to -40°C],” said Bert Suffis, Development and Applications Sales Manager at JSP. In addition, it absorbs impact energy extremely well and can withstand multiple impacts. “You can compress Arpro to 4% of its original volume and it will recover to 98% of initial size,” he noted.

“For the QBEAK sandwich application, using tensile skins of aluminum hit the sweet spot in terms of weight and cost,” Morley said. “In this case, aluminum was the most achievable and deliverable choice.” He added that Inrekor can use other sheet materials as the outer panels, including other metals such as steel, fiber-reinforced polymers, and natural-fiber fabric panels including flax or hemp.

“We can also vary the panel thickness depending on the application,” he continued. “For example, we can make the bottom of the floor panel thicker to better resist external strike or even swap it out for a low-gauge stainless-steel skin.”

The Inrekor components used in the QBEAK’s chassis feature tensile skins of 1.0- to 1.2-mm (0.039- to 0.047-in) 5251A aluminum alloy sheet book-ending 20- to 30-mm (0.79- to 1.18-in) core thicknesses of 90-g/L Arpro. The panels are heat- and pressure-bonded to the thermoplastic foam cores and then welded or bolted together at interlocking tongue-and-groove joints. The skins and cores can also be glued together with epoxy-based adhesives. The strong joint configurations are designed to resist peeling and tearing.

Sample chassis built from Inrekor components have passed independent structural tests conducted by the Warwick Manufacturing Group at the University of Warwick as well as crash-testing at MIRA, an automotive consultancy company that is headquartered in Warwickshire.

As with other sandwich panels on the market, the Inrekor components can rather easily incorporate internal channels of air ducts, wires, and cables within the insulating foam cores. The thermal insulation capabilities of Inrekor are useful for helping to maintain the temperature of the sensitive battery packs as well.

Morley noted that Inrekor panels could also find widespread use in recreational vehicles to help meet European Union RV size/weight regulations and integral thermal insulation needs.

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