Design News for Mechanical and Design Engineers

This hardened box, which won a design award during the recent Composite Fabricators Association Show in Atlanta, consists of flexible composite panels attached to an aluminum (6061-T6) frame. During an explosion, the panels expand like a balloon to contain the blast forces. "The displacement absorbs the energy from the blast," explains Rick Fingerhut, a Telair research engineer who helped develop the box.

And the box, or Hardened Unit Load Device in industry parlance, doesn't just contain explosions in theory. It passed some chillingly realistic blast tests administered by the FAA last year. These tests, which are today governed by the Transportation Security Administration, involve setting off explosions not just in freestanding containers but also in ones placed inside the fuselage of a decommissioned Boeing 747. Citing security concerns, Fingerhut declined to disclose the magnitude of the blast forces. But videos of the tests show that the Telair container does keeps explosions in check while ordinary, unhardened containers failed catastrophically, allowing some blasts to penetrate fuselage. "Simulations are fine," Fingerhut says. "But blast tests are what separates the men from the boys."

The composite panel material, a layered structure of Kevlar and a thermoplastic, may play the most obvious role in containing a blast. But the joints between the panels are where Telair engineers put much of their technology muscle. To secure the edges of the composite panel to the rigid frame members, they assemble the edges of the composite fabric around mounting strips, which can be aluminum or other matrials. Depending on the number of layers in a given panel, the engineers then bolt these wrapped structures together, using high-strength fasteners inserted through corresponding holes in the fabric layers and mounting strips. Flanges built into the mounting strips allow the fabric panels to be attached to the container's structural frame. Meanwhile, the composite fabric's thermoplastic layers—currently a DuPont ionomer—act as an adhesive to help hold the Kevlar layers in place.

This arrangement may sound simple enough, but the joints had to strike a delicate balance between strength and flexibility—strong enough not to fail but flexible enough not to prevent the composite panels from expanding. Fingerhut likens this design challenge to "finding a way to pick up a convertible by its roof." He managed to strike the right balance by carefully matching the strength of the joint to the tensile strength of the composite fabric. "The resulting joint flexes with the panels rather than constraining the panels," he says.

Further design challenges came from less explosive concerns. To appeal to airlines, a hardened cargo box has to fight explosions without growing too big or heavy. On both scores, Telair did well. Fingerhut notes that the 79- x 60.4- x 64-inch blast-resistant container is built on the same frame as many of the company's non-hardened models. And at 325 lbs, the hardened container weighs about 130 lbs more than a standard container, but airlines can offset the extra weight by pairing hardened units with newly available ultra light containers. This way, they can put bomb protection where needed without taking on additional weight overall, Fingerhut notes. "Anyway, an all-metal structure capable of containing the same blast would have been far heavier," he adds.

While at least one other container has passed the blast test, only the Telair model has so far received a Technical Standard Order, the FAA's stamp of approval from an airworthiness perspective.