Conventional materials used in crumple zones have a drawback, however, according to a researcher at the University of Glasgow. “The current state of the art in the area of crashworthiness and impact protection is designed for a specific set of properties. If you want a different set of properties, to cater to a different impact situation or severity and type of impact, you need to design a different structure,” Professor Shanmugam Kumar tells Professional Engineering. “If you design for one situation, it will not be able to perform well in another situation.”
Kumar, along with colleagues from Marche Polytechnic University, the University of L’Aquila and the National Institute for Nuclear Physics in Italy, set out to provide an alternative that can effectively protect vehicle occupants from a range of impact types and severities. They call the result ‘adaptive twisting metamaterials’, outlined in a recent paper in Advanced Materials.
The focus of a new report by IMechE and the UK Metamaterials Network (UKMMN), metamaterials provide properties that are otherwise not found in their constituent material thanks to carefully designed internal structures.
In the recent work, the adaptive twisting metamaterials were made using additive manufacturing. Steel powder was 3D-printed, giving the team fine control over the material’s architecture and allowing them to weave a complex, highly porous shape known as a gyroid lattice throughout it. These twisting internal structures are designed to translate external forces into a corkscrew-like motion, absorbing the impact energy.
“By controlling the rotating motion of these twisting metamaterials, we’ll be able to tune the ability… to absorb less energy or more energy, become less stiff or more stiff, depending on the impact type and the severity of the impact,” explains Kumar.
In laboratory experiments, the team tested three versions of the material to assess their responses to two types of loading: rapid impacts and slower, steadily increasing strains.
When the metamaterial was constrained from twisting at all, it provided maximum stiffness and absorbed the most energy – 15.36 joules of energy per gram of the material. When the material was allowed to twist freely, its stiffness and energy absorption decreased by approximately 10%. In the material’s third configuration, it was forced to over-twist, reducing energy absorption by 33%.
“The results show that the material has the potential to provide a range of protection, from rigid shielding to softer energy absorption,” a University of Glasgow announcement said.
The different types of rotating motion are set by “boundary conditions”, Kumar says, which will be controlled by mechanical actuation when used in vehicles. The material “has to first sense the impact, and immediately it has to apply those boundary conditions,” he says. That link is missing, so the team is working on a parallel study to embed smart sensing.
In future, the team aims for the metamaterials to immediately sense their strain and damage state when a mechanical load is applied, without separate sensors or structural health monitoring.
“Although the material is adaptive, we still have to incorporate the sensing element into the system,” Kumar says. “We are switching to non-metals, primarily composite materials, in order to really demonstrate the idea of the structures which can quickly sense the type and severity of the impact.”
The researchers have not yet produced metal parts big enough for automotive structures, but Kumar believes it will be feasible in roughly five years’ time using industrial 3D printers.
“There are some challenges, in terms of the cost-effectiveness, speed and throughput – but there are ways to overcome them. [Other] people have been trying high-speed manufacturing processes and have also been utilising hybrid manufacturing, where they combine additive manufacturing with non-additive processes in order to speed up the process,” he says.
As well as crash protection in cars, the researchers hope the metamaterials could find use in rail and aerospace. The gyroid lattices could also reduce component weight while maintaining strength, Kumar says.
Published on Monday (1 December), IMechE and UKMMN’s Unlocking the Potential of Metamaterials report explores opportunities to maximise the social and economic benefits of the pioneering field. Recommendations include the need to strengthen research foundations, the development of a skilled workforce capable of realising metamaterials’ commercial potential and the scale-up of manufacturing processes.
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Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.