Engineers claim to have created the first manufactured material that can’t be cut because it turns the force of a cutting tool back onto itself. That sounds a bit like an advanced martial art – and the material is named Proteus after the shape-changing mythical god – but it’s actually inspired, they say, by the tough cellular skin of the grapefruit and the fracture resistant shells of molluscs.

Credit: René Vogel, Fraunhofer Institute for Machine Tools and Forming Technology

In the lightweight material, ceramic spheres are encased in a cellular aluminium structure, which in tests could not be cut by angle grinders, drills or high-pressure water jets.

The idea was developed by an international team led by Durham University, UK, and Fraunhofer Institute for Machine Tools and Forming Technology IWU in Germany.

And it works, they write in a paper in the journal Scientific Reports, because of a dynamic response that is more akin to living structures: the material’s evolving internal structure creates high-speed motion where it interacts with the cutting tools.

The interaction between the disc and ceramic sphere creates an interlocking, vibrational connection that resists the cutting tool indefinitely. The blade is gradually eroded, and eventually rendered ineffective as the force and energy of the disc or the drill is turned back on itself, and it is weakened and destroyed by its own attack.

In addition, the ceramics fragment into fine particles, which fill the cellular structure of the material and harden as the speed of the cutting tool is increased due to interatomic forces between the ceramic grains.

Water jets are ineffective because the curved surfaces of the ceramic spheres widen the jet, which substantially reduces its speed and weakens its cutting capacity.

“Essentially cutting our material is like cutting through a jelly-filled with nuggets,” says Durham’s Stefan Szyniszewski, the lead author. “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.”

And their inspiration? “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.

“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.”

A CT scan of Proteus

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