A method of allowing composites to self-heal cracks in extremely low temperatures could enable repair of materials in challenging situations, such as offshore wind turbines—or even aircraft and satellites during flight.

Scientists from the University of Birmingham, UK, and Harbin Institute of Technology, in China, have now developed a way to fabricate fiber-reinforced materials that can self-heal at temperatures as low as -60° Celsius. To do so, they embedded in the composite material three-dimensional hollow vessels for the purpose of delivering and releasing the healing agents, together with a porous conductive element to provide internal heating and to defrost as needed.

Composites that self-heal cracks in ultra-low-temperature environments could potentially enable repair of spacecraft during flight. Image credit: NASA.“Both of the elements are essential," says Yongjing Wang, PhD student at the University of Birmingham. "Without the heating element, the liquid would be frozen at -60° Celsius, and the chemical reaction cannot be triggered. Without the vessels, the healing liquid cannot be automatically delivered to the cracks.”

The researchers obtained a healing efficiency of over 100% in a glass fiber-reinforced laminate at temperatures of -60° Celsius, but they say the technique could be applied across the majority of self-healing composites. According to the researchers, tests run using a carbon nanotube sheet as the conductive layer demonstrated the ability to allow the composite material to self-heal with an average recovery of 107.7% in fracture energy and 96.22% in peak load.

The fiber-reinforced composite, or host material, would thus have higher inter-laminar properties—the bonding quality between layers—after healing than when in its original state. The higher those properties, the less likely it is that cracks will occur in the future.

“Fiber-reinforced composites are popular due to [their] being both strong and lightweight—ideal for aircraft or satellites—but the risk of internal micro-cracks can cause catastrophic failure," Wang says. "These cracks are not only hard to detect, but also to repair; hence the need for the ability to self-heal.”

The group will now look to eliminate the negative effects that heating elements have on peak load by using a more advanced heating layer. Their ultimate goal is to develop new healing mechanisms for composites that can recover effectively regardless of the size of fault and under any conditions.