They could be remarkably close, however, thanks to a new ‘non-cuttable’ material developed by engineers at Durham University and the Fraunhofer Institute in Germany.
Researchers took inspiration from shells to create the strong and lightweight material, named Proteus after the shape-changing mythical god. Another unusual inspiration was grapefruit, which have very high impact resistance – when dropped from a height, for example – with very lightweight peel.
The material resists cutting by turning the force of a cutting tool back on itself. It is made of ceramic spheres encased in a cellular aluminium structure, similar to the organic tiles interlinked by biopolymers in abalone sea creatures.
Proteus has a dynamic ‘material system’, with an evolving internal structure that creates high-speed motion where it interacts with cutting tools. When cut with an angle grinder or drill, the vibrations created by the ceramic spheres inside the casing blunt the cutting disc or drill bit. The interaction between the disc and ceramic spheres creates an interlocking, vibrational connection that reportedly resists the cutting tool ‘indefinitely’.
Blades are gradually eroded and eventually rendered ineffective as the force and energy of the disc or the drill turns back on itself. “It is weakened and destroyed by its own attack,” the research announcement said.
Videos show angle grinders and drills easily making their way through a surface layer of automotive aluminium before being stopped dead by the Proteus underneath.
The material also resists cutting thanks to the ceramics fragmenting into fine particles, which fill the cellular structure of the material and harden as the speed of the cutting tool increases due to interatomic forces between the ceramic grains, resisting further attack.
Water jets were also found to be ineffective because the curved surfaces of the ceramic spheres widen the jet, substantially reducing its speed and weakening its cutting capacity.
Proteus reacted to different attacks in different ways – armour-piercing bullets fragmented thanks to shockwaves from the impact, said lead author Dr Stefan Szyniszewski, assistant professor of applied mechanics at Durham, to Professional Engineering.
Researchers said they were unaware of other manufactured ‘non-cuttable’ materials. They hope Proteus could be used in bike locks, lightweight armour and protective equipment for people working with cutting tools.
“We were intrigued by how the cellular structure of the grapefruit and the tiled structure of mollusc shells can prevent damage to the fruit or the creatures inside, despite being made of relatively weak organic building blocks,” said Dr Szyniszewski.
“These natural structures informed the working principle of our metallic-ceramic material, which is based on dynamic interaction with the applied load, in contrast to passive resistance.
“Essentially cutting our material is like cutting through a jelly filled with nuggets. If you get through the jelly you hit the nuggets and the material will vibrate in such a way that it destroys the cutting disc or drill bit.
“The ceramics embedded in this flexible material are also made of very fine particles which stiffen and resist the angle grinder or drill when you’re cutting at speed, in the same way that a sandbag would resist and stop a bullet at high speed.”
The researchers have a patent pending for the material technology and hope to work with industrial partners to develop products.
The findings were published in Scientific Reports.
The research was led by Durham University working alongside the Fraunhofer Institute for Machine Tools and Forming Technology IWU, Fraunhofer Institute for Wood Research, Wilhelm-Klauditz-Institut WKI, Hannover and Leibniz University, Hannover, Institute of Plastics and Circular Economy IKK, Germany, and the University of Surrey and University of Stirling in the UK.
The study was funded by the UK Home Office, the Engineering and Physical Sciences Research Council and a European Commission Career Integration Grant.
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Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.