An international team of researchers has introduced a self-healing fiber-reinforced composite that allows structures to repair themselves in place, without having to be removed from service. The technology relies on a mendable thermoplastic 3D-printed onto woven glass/carbon fiber reinforcement. This could significantly extend the lifespan of structural components such as wind-turbine blades and aircraft wings, as reported in a recent issue of Nature Communications (https://doi.org/10.1038/s41467-022-33936-z).

“Researchers have developed a variety of self-healing materials, but previous strategies for self-healing composites have faced two practical challenges,” says Jason Patrick, an assistant professor of civil, construction and environmental engineering at North Carolina State University.

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(a) Overhead images of 3D printed serpentine patterns of poly(ethylene-co-methacrylic acid) (EMAA) thermoplastic (blue overlay) directly onto woven carbon fiber reinforcement demonstrating both scalable (top) and precise (bottom) placement. (b) Three-dimensional x-ray computed microtomography (μCT) reconstruction of self-healing glass fiber-reinforced composite. (c) Cross section of laminated composite revealing EMAA melt-bonded to microfiber bundles (tows) and resistive heater interlayers containing a percolating network of electrically conductive carbon whiskers (inset). Image credit: Nature Communications.

“First, the materials often need to be removed from service in order to heal. For instance, some require heating in an oven, which can’t be done for large components or while a given part is in use. Second, the self-healing only works for a limited period. For example, the material might be able to heal a few times, after which its self-repairing properties would significantly diminish. We’ve come up with an approach that addresses both of those challenges in a meaningful way, while retaining the strength and other performance characteristics of structural fiber composites,” Patrick says.

The laminated composites are made from layers of fibrous reinforcement, such as glass and carbon fiber, bonded together. Damage most often occurs when the “glue” that binds these layers together begins to peel away from the reinforcement, or delaminate. The research team addressed this problem by 3D printing serpentine micropatterns (300 μm × 500 μm) of poly(ethylene-co-methacrylic acid) (EMAA)—a commodity copolymer thermoplastic—healing agent onto the reinforcement material. The researchers also embedded thin “heater” layers (conductive carbon whiskers) in the composite. When an electrical current is applied, the heater layers warm up. This, in turn, melts the healing agent, which flows into any cracks or microfractures within the composite and repairs them.

“We’ve found that this process can be repeated at least 100 times while maintaining the effectiveness of the self-healing,” Patrick says. “We don’t know what the upper limit is, if there is one.”

The printed thermoplastic also enhances inherent resistance to fracture by up to 500 percent.

Another advantage of this technology is that, if incorporated into aircraft wings, the internal heating elements would allow airlines to stop using chemical agents to remove ice from wings when aircraft are on the ground, and also to de-ice in flight, according to the researchers.

Source: North Carolina State University