Airplanes of the future could be self-healing

Early tests promising
Although Bond’s team has yet to test its self-healing system on aircraft, the epoxy network has performed well in standard “drop-weight tests” designed to simulate the effect of a dropped tool or kicked up runway debris. After the impact, tests suggest the bleeding epoxy can restore between 80 percent to 90 percent of the damaged surface’s original compression strength, which measures the ability of a vertically positioned panel to withstand being squeezed.

Bond said the repair also reduces the risk that the aircraft’s composite layers will detach from each other, a structural issue that can lead to what’s known as low-failure mode.

“If you think of a pack of playing cards, if they’re all stuck together then it’s like a rigid block,” he said. “If they’re not stuck together, then they all behave independently.” The epoxy plug restores some of the block-like rigidity and helps prevent the individual movement of separate layers, he said.

Bond cautioned that any commercial applications would require rigorous validation and substantial industrial backing. Nonetheless, he said the self-healing system’s safety benefits could promote more widespread acceptance of fiber-reinforced polymer composites in aircraft, effectively reducing the weight of planes and thus both their fuel needs and carbon emissions. Both Airbus and Boeing have expressed preliminary interest, he said, as has the United Kingdom’s Ministry of Defence.

“Certainly within 5 to 10 years, we’d like to see these sorts of materials on structures,” Bond  said.

The renewable energy industry also might benefit from such self-healing surfaces. Wind turbines, Bond noted, are essentially wings, and European officials have clamored to locate more of them in offshore locations, increasing the expense of sending out crews for maintenance and repair. A turbine that could heal itself after wear and tear or impacts with, say, small birds, could dramatically lower costs.

Bond’s team also has explored how the system might be deployed in space, and he said the payback could be significant for high-risk expeditions, though he conceded extensive safety testing would be required and the technique’s finite nature would have to be addressed: “Once you use up the healing function, it’s gone.”

Current limitations
Two engineering experts said Bond’s idea struck them as innovative, but both expressed doubts about its practicality.

Steven Schneider, a professor of aeronautics and astronautics engineering at Purdue University, said he believed the system might work, though he questioned the useful shelf life of the capsule-enclosed epoxy and hardener and how much undesirable weight they would add to an aircraft.

Paul Fischbeck, a risk analyst and professor of engineering and public policy at Carnegie Mellon University in Pittsburgh, called the technology a “neat idea,” but questioned whether it would result in a significant risk reduction. He also expressed concern over whether the benefits would outweigh the extra cost and potential structural impacts of adding glass capsules to the existing composite fibers.

Even restoring the fiber material to 80 percent to 90 percent of its original strength may not be enough to pass muster, he said, potentially requiring engineers to increase the safety margins of the original design to offset the 10 percent to 20 percent loss.

For some military applications, “where the risks are much greater and getting back to friendly territory is critical,” Fischbeck said the self-healing technology might provide a better cost-benefit ratio.

Bond conceded that his team’s design needs to minimize the structural impact of adding the epoxy system, such as excessive weight, even as it seeks to combine the best features of nature’s original sources of inspiration. “Plants generally can take a lot of damage. You can take an axe to a tree and it will live,” he said. “You can’t do the same with many animals.”

If plants possess a more redundant vasculature that makes it more robust, however, most animals are more efficient. “You want robustness, but you also want efficiency,” Bond said.

A next-generation system now in development in Bond’s lab might provide some of both while overcoming current limitations. How? By incorporating a circulatory system that allows the epoxy and hardener to flow through the network and be replenished for repeated healing.

Reminiscent of blood pumped through veins and arteries, the system has been embedded in a foam sandwich set between two plastic-and-carbon layers. Initial impact tests suggest the network’s self-healing capabilities are working well, though as yet, it still lacks anything that could be likened to perhaps the best-known distributor of all: a beating heart.

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