The release of an Australian university’s study of how fire affects composite materials has caught the attention of the aerospace industry, given the growing use of composites as structural material, especially in large airplanes. The study is part of research started in 1998 by professor Adrian Mouritz, head of aerospace and aviation at Royal Melbourne Institute of Technology, and Zenka Mathys and Craig Gardiner at the Defence Science and Technology Organisation.
Australia’s Cooperative Research Center for Advanced Composite Structures and the University of Newcastle-upon-Tyne in the UK joined the research program in 2001.
This study examined how composites, specifically glass fiber or fiber polymer materials used in aircraft, ships and civil infrastructure, perform when stressed and heated. “Both single-skin laminates and sandwich composite materials have been evaluated,” Mouritz told AIN. “The goal of the research is to determine the safe operating life of fiber composite structures in fire.”
The study aims to provide models engineers can use to predict the time-to-failure when composite structures are involved in fires. The longer structural composites
can resist fire, the safer the airplane.
Using samples from aircraft structures, the team stressed the materials in a loading machine while exposing them to heating that closely replicates fire, according to Mouritz. “The time taken for the stressed composite to break or collapse when exposed to the fire is a direct measure of its survival life. We use this data to validate the models and ensure they are accurate.”
The results of the tests provided “reliable quantitative data on the survival life of fiber polymer composites under different load conditions (tension, compression) and different flame temperatures.” Different materials had varying results, and the study did find, according to Mouritz, “some flame-resistant phenolic composites can actually fail earlier in fire than more flammable composites. This is a surprising finding, although we have determined the weakening processes that explain why phenolic composites can be worse. We also found that composites generally perform better as structural materials in fire than aircraft-grade aluminum alloys.”
The team plans 18 months of additional testing to see how fire-hardened coatings affect the survival time of composite materials.
Justin Hale, deputy chief mechanic for Boeing’s 787 program, is not surprised at the study’s finding that composites handle fire better than aluminum alloys. “In the case of burn-through,” he said, “we’ve demonstrated that [Boeing’s material] is significantly superior to aluminum. Aluminum at 1,000 degrees gets pretty sloppy, and that’s when you get burn-through.” The BMS 8-276 carbon-fiber material used in the 787 and on the 777 horizontal stabilizer and floor beams, he added, “doesn’t even react that way. The fuel-fed flame can’t get hot enough.”
For composite airplanes certified under FAR Part 25, “the basic certification requirements are the same” Hale explained, regardless of whether the material is some form of composite or traditional aluminum.
During certification manufacturers must cover three fire-resistance categories– flammability of the material, burn-through in a fuel-fed fire and smoke toxicity.
The burn-through testing is designed to ensure passengers have enough time to egress during a survivable fire. Typically, Hale said, aluminum structure has to show at least five minutes of resistance before burn-through. “We have done the test
and exceeded 20 minutes with no burn-through with carbon-fiber.” While the resin that infuses the carbon-fiber will melt and vaporize when exposed to high heat, carbon-fiber is manufactured at temperatures as high as 5,000 degrees F, “which leaves it pretty impervious to high-temperature flames,” he said.
One problem with fire-resistant composites is that their ability to withstand fire for longer periods of time means that they can produce more fumes. “That’s an issue Boeing has worked through for more than a year, but we have demonstrated that we can meet the smoke and toxicity requirements for fire situations,” Hale said.
In any case, paint on metal structure has its own fire-related toxicity issues. “It’s not the first time we’ve put this material in,” he said, “and in terms of discussions with regulators, this is not a risk item in certification. All our testing for flammability is complete. In terms of the bulk of certification testing, we have the results we need to go to certification.”