However, impact damage to these composite structures can be difficult to detect, and can result in a drastic reduction in their mechanical properties and even their complete failure.
What if these materials could repair themselves autonomously in response to such impacts, much like how human skin heals itself after being cut? To the uninitiated, the idea might sound far-fetched, but researchers around the world are working to make self-healing composites a reality.
One such researcher is professor Duc Truong Pham, the head of the School of Mechanical Engineering at the University of Birmingham. He says: "Self-healing composites belong to the class of smart materials that can provide continuous service without requiring maintenance. This potentially reduces overall maintenance costs as well as preventing catastrophic failure.
"Initial uses envisaged for self-healing composites are in safety-critical applications – for example, in aerospace systems – or in applications where access for maintenance is difficult, such as offshore systems. The importance of having materials that can repair themselves to allow uninterrupted operation in such situations is obvious."
Self-healing materials are already being used in industrial applications. For example, drilling technology specialist Schlumberger sells its Futur self-healing cement. Designed to mitigate the risk of oil or gas leaks from wells, Futur's self-healing mechanism is activated in response to exposure to hydrocarbons.
Pham continues: “Different types of self-healing concrete are being trialled. It will be a matter of time before we see self-healing concrete in real use for constructing roads and bridges. The techniques being tested – involving the addition of bacteria or other healing agents – do not involve anything particularly expensive and thus we do not expect material cost will be a major issue compared to the savings that will accrue through the reduction or elimination of the need for maintenance.”
However, self-healing fibre-reinforced plastics are still at a relatively early stage of development, according to Pham. He explains: “Work is needed to make the recovery of mechanical properties more complete, especially when the damaged area is large, and the healing process quicker and more reliable.”
For example, many self-healing composites require a suitably high ambient temperature or an appropriate radiation treatment for them to work, which is often impossible to ensure in practical application. Take the composites used on an aircraft; these must endure temperatures as low as −60°C, at which almost all the healing liquids employed in these materials would be frozen and unable to be activated.
Pham and his colleagues at the University of Birmingham, working with researchers at the Harbin Institute of Technology in China, claim to have developed composites that can autonomously self-heal in these sub-zero temperatures.
The Birmingham–Harbin material features wave-like hollow vessels that break when delamination occurs, so releasing a healing agent. This technique was pioneered by University of Illinois Professors Nancy Sottos, Scott White and Jeff Moore in 2011.
The process developed by the Illinois team begins with the mechanised weaving of polylactic acid (PLA) sacrificial fibres into an eight-ply stack of satin-weave E-glass fabrics. This stack is then infiltrated with a thermosetting resin and cured.
After curing, the sample is trimmed to expose the ends of the sacrificial fibres, which are then removed by heating the sample to above 200°C in order to vaporise the PLA, yielding empty channels and a 3D vascular network throughout the composite.
Conventional PLA spontaneously depolymerizes into gaseous lactide monomers at temperatures above 280°C. The researchers lowered this depolymerization temperature by adding metal catalysts.
In 2014, the Illinois researchers used this method for what they claimed to be the first demonstration of repeated healing in a fibre-reinforced composite system, and it enabled the self-healing of large holes in thermoplastics and thermosets.
Indeed, by controlling the reaction kinetics of a shape-conforming dynamic gel delivered through the vascular networks, and the rate of this delivery, the researchers were able to fill holes that exceeded 35 mm in diameter within 20 min, and restore mechanical function within 3 hours.
The Birmingham/Harbin team use an almost identical method to produce their vascular networks. The healing agent, a pre-mixed two-part epoxy, is injected into the resulting vessels using a controllable liquid dispenser.
Where the composite differs from other self-healing materials is in the incorporation of an electrically and thermally conductive porous sheet of carbon nanotubes (CNTs). When electricity is supplied to this sheet, it generates heat—melting any frozen healing agent and accelerating its cure cycle.
University of Birmingham PhD student Yongjing Wang, says: “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. Without the vessels, the healing liquid cannot be automatically delivered to the cracks.”
The laminate is stacked in the following sequence (from bottom to top): four layers of normal glass fibres; eight layers of glass fibres with the sacrificial components; two layers of normal glass fibres; the conductive sheet; two layers of normal glass fibres. It is then infused with epoxy resin.
The conductive layer could also be a sheet of porous copper foam, but the researchers have found that composites with the CNT film are able to self-heal more effectively and stably, with an average recovery of 107.7% in fracture energy and 96.22% in peak load.
Wang and his colleagues have observed a healing efficiency of over 100% at these low temperatures in their glass fibre-reinforced laminates, but the technique could be applied to a variety composites.
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, however, is to develop new healing mechanisms for more composites that can recover effectively regardless the size of faults in any condition.