A team of researchers led by the University of Birmingham (UK) and Harbin Institute of Technology (China) have developed a method of allowing composite materials used in aircraft and satellites to self-heal cracks at temperatures as low as -60 C.
“Fibre-reinforced composites are popular due to them being both strong and lightweight, ideal for aircraft or satellites, but the risk of internal micro-cracks can cause catastrophic failure,” said Yongjing Wang, a PhD student at the University of Birmingham. “These cracks are not only hard to detect, but also to repair, hence the need for the ability to self-heal.”
As the research team’s paper explains, the composites are embedded with 3-D hollow vessels, with the purpose of delivering and releasing the healing agents, and a porous conductive element, to provide internal heating and to defrost where needed. The composites draw inspiration from animals that maintain a constant body temperature to keep certain enzymes active. Just like animals, the self-healing composites maintains its core temperature.
“Both of the elements are essential. Without the heating element, the liquid would be frozen at -60°C and the chemical reaction cannot be triggered,” said Wang. “Without the vessels, the healing liquid cannot be automatically delivered to the cracks.”
As a result, the healed fibre-reinforced composite, or host material, has higher interlaminar properties – the bonding quality between layers. The higher those properties, the less likely it is that cracks will occur in the future.
The team conducted two tests: one using a copper foam sheet and another using a carbon nanotube sheet as the conductive layer. The carbon nanotube sheet was able to self-heal more effectively with an average recovery of 107.7 percent in fracture energy in a glass fiber reinforced laminate. However, the researchers say the technique could be applied across a majority of self-healing composites.