Materials used in aircraft and satellites are subject to intense stress and temperatures well below freezing. Now a team has developed self-healing materials that are able to patch up cracks at very low temperatures.
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. These cracks are hard to detect and repair, hence the need for the ability to self-heal.
The multinational team led by the University of Birmingham (UK) and the Harbin Institute of Technology (China) showed that self-healing materials can be manipulated to operate at temperatures down to -60°C.
The applications of such self-healing composites include situations where repair or replacement is challenging such as offshore wind turbines, or even ‘impossible’, such as aircraft and satellites during flight.
Self-healing composites are able to restore their properties automatically, when needing repair. Previous research has yielded composites capable of impressive healing efficiencies in favourable conditions. Some materials even heal at efficiencies above 100 per cent, indicating that the function or performance of the healed material can be better than that prior to damage.
However, under adverse conditions such as very low temperatures, these materials previously were unable to heal sufficiently.
The new structural composite addresses the issue of low temperatures by maintaining its core temperature. Embedded within the composite are a porous conductive element to provide internal heating and to defrost where needed, as well as three-dimensional hollow vessels which deliver and release the healing agents.
The elements work in tandem, with the heating element ensuring the liquid remains fluid at low temperatures in order to trigger the chemical reaction. The vessels allow the healing liquid to be delivered to areas where cracks have developed.
The team attained a healing efficiency of over 100 per cent at temperatures of -60°C in a glass fibre-reinforced laminate, but the technique could be applied across a majority of self-healing composites.
They also ran tests using a copper foam sheet or a carbon nanotube sheet as the conductive layer. The latter of the two was able to self-heal more effectively with an average recovery of 107.7 per cent in fracture energy and 96.22 per cent in peak load.
The healed fibre-reinforced composite therefore had better bonding quality between layers, making it less likely that cracks will occur in the future.