Self-repairing carbon-composite structures trialled

A doctoral student at Switzerland’s Ecole Polytechnique Fédérale de Lausanne (EPFL) is developing a way for carbon-composite structures, such as yachts, to heal themselves.

One weakness of these superstrong materials is a tendency to delaminate when big impacts cause cracks parallel to the surface of the material.

To counter this, a self-healing composite material is impregnated with hundreds of tiny, evenly dispersed bubbles filled with healing liquid-­monomer molecules and smaller particles of catalyst. When the material sustains a sufficient impact, the delamination cracks cause some bubbles to rupture and release their monomer and catalyst. The monomer fills the crack and hardens, and the damage is repaired.

But the problem with this approach is that for maximum material strength, you want to keep the concentration of bubbles and their size to a minimum. But when the bubbles are too small, their payload doesn’t completely fill the cracks.

Student Eva Kirkby has found a way to close the gap, by incorporating wires of what’s known as a shape-memory alloy (SMA) into the composite. When electricity flows through an SMA wire that has been bent, it snaps back into shape. That forces the delamination crack shut, constraining it to a size that can be effectively filled by fewer, tinier monomer bubbles. In initial tests, Kirkby’s electrically activated material closed crack openings from 120 micrometers to about 17 µm.

Kirkby says that among the key issues is how to get this material to autonomously pulse the current to the SMA wires. To do that, the composite must first ­work out where it has been damaged and then send current only to the SMA wires at the damage site.

To solve this one, Kirkby embedded optical fibers in the composite. When a shock pulse from an impact compressed or tensed the fibers, the light that passed through them dimmed or intensified in response. By tracking the pulse as it transected the fibers, Kirkby was able to pinpoint the impact site to within a few centimeters.

Kirkby says one obvious application for her new material is high-­performance sports equipment like ­racing bikes and yachts. The aerospace sector would benefit from autonomously healing aircraft wings or meteor-proof satellites, and for injured soldiers returning to duty wearing ­prosthetics, self-repair could be essential.

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