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21 October 2016

Research Team Develops Sub-Zero Self-Healing Composites

Researchers in a joint team from the University of Birmingham (UK) and the Harbin Institute of Technology (China) last month released a paper on the Royal Society’s Open Science platform detailing technology they developed that allows structural composite materials to self-heal in sub-zero temperatures. The team’s research was necessary as conventional self-healing methods in composite materials do not function correctly at such low temperatures.

Getting machine components to work properly and efficiently at sub-zero temperatures presents a whole new set of challenges. Maintenance and repair processes have to take into account these challenges, based around whether or not equipment can be removed from the cold environment. Either it can, and it undergoes large temperature variations, or it cannot, and the difficulties of performing a repair at sub-zero temperatures become apparent.

This can become even more of an issue when problems arise in components operating in sub-zero surroundings that are difficult or impossible to reach, and whose continuous function is critical, such as those on offshore wind turbines, aeroplanes, and satellites. Increasingly, these parts are made from fibre-reinforced composites, which provide the requisite balance between strength and lightness to make them perfect for the job. However, the problem with these composites is that small micro-cracks can form within them, presenting a risk of instability or breakup. The paper’s introduction describes how these micro-cracks ‘may cause catastrophic failures and that are hard to detect and repair. Hence, enabling them to self-heal has been proposed as a potential method to improve the reliability of FRCs [Fibre-Reinforced Composites] and increase their service life as well as to decrease repair costs.’

There have already been concerted efforts to design conventional self-healing materials, those that repair their own damage just like biological organisms, and these efforts have borne fruit in the form of what the researchers describe as healing efficiencies above 100%, ‘indicating that the function or performance of the healed material can be better than that prior to damage.’ However, problems arise when these healing processes are subjected to sub-zero temperatures, leading to inefficient or ceasing function. To circumvent this, the Birmingham and Harbin team opted to continue emulating biological mechanisms by regulating the temperature of the material just like an animal regulates its own body temperature.

To achieve this, the team fitted the composite with two key structural features: a network of vessels and a layer of thin conductive material. The vessels were 3D and hollow, and embedded within the material in order to deliver and release healing agents to damaged areas. The thin conductive layer, made out of metal foam, transfers heat from within the composite to external areas, de-icing the surface and allowing the chemical reactions of the healing agent to work properly. As Birmingham PhD student Yongjing Wang explained, “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.”

Img source: Royal Society Open Science
The team had great success with their design, achieving a 100% healing performance rate at a temperature of -60°C. One minor concern was whether the modifications they made would have a significant effect on the structural performance of the material. After much testing, it was revealed that the changes actually increased the tensile strength of the material, but had negative effects on its interlaminar strength (the strength of the bond between its layers). A proposed counterbalance to this would be to put patterns on the sheets that would increase interlaminar tension.

Overall, the team’s work here represents an exciting development, marking as it does the first time self-healing tech has been made to function efficiently at low sub-zero temperatures. This kind of technology could be invaluable, even life-saving, when employed in sub-zero environments on aeroplanes or satellites. It will be fascinating to watch how it continues to develop.

Sam Franklin

With a master’s in Literature, Sam inhales books and anything readable, spending his working hours reformulating the info he gathers into digestible articles. When not reading or writing, he likes to put his camera to work around the world, snapping street photography from Stockholm to Tokyo. Too much of this time spent in Japan teaching English has nurtured a weakness for sashimi, Japanese whisky, and robot cafés.