The idea of an indestructible killer robot might seem like something straight out of a Terminator movie.
But they may soon become a reality, as scientists have seen metals heal for the first time without any human intervention.
A US-based study overturned what we thought we knew about metals and revealed that cracks from wear and tear can actually repair themselves under certain conditions.
It’s a discovery that has the potential to revolutionize engineering, with the possibility of self-healing engines, planes and even robots now on the horizon.
“It was absolutely stunning to see first hand,” said Sandia National Laboratories scientist Brad Boyce, who led the study with Texas A&M University.
The idea of an indestructible killer robot might sound like something straight out of a Terminator movie
The scientists were 40 minutes into the experiment when the damage reversed as a ‘T-junction’ crack rejoined as if it had never been there in the first place.
‘What we have confirmed is that metals have their own inherent, natural ability to heal itself, at least in the case of fatigue damage at the nanoscale.’
How does metal heal itself?
Although much remains unknown about the self-healing process, scientists believe that a process known as cold welding plays a role.
This can occur at the tip of metal fatigue, allowing bare metals to ‘join’ when compressed under contact.
However, it was initially tested in vacuum conditions, so it is not yet clear whether lack of oxygen also plays a role.
Metals used today to build critical infrastructure such as bridges and planes are subjected to repeated stresses and motions that develop microscopic cracks over time.
While this fatigue damage would normally cause machines to break, Mr Boyce and his team saw nano-sized fractures shrinking by 18nm.
This was a completely unexpected discovery because the scientists only wanted to assess how cracks would propagate through a 40-nm-thick piece of platinum when pressure was applied.
They were 40 minutes into the test when the damage reversed, a ‘T-junction’ crack reattached as if it had never been there in the first place.
Then, when more pressure is applied, the crack snaps back in the other direction, as the amazed scientists watched through the microscope.
‘From the solder joints of our electronic devices to the engines of our cars to the bridges we drive over, these structures often fail unexpectedly due to cyclic loading that initiates cracks and ultimately leads to fracture,’ continued Mr Boyce.
‘When they fail, we have to contend with replacement costs, lost time and in some cases even injury or loss of life. The economic impact of this failure is measured in the hundreds of billions of dollars per year for the United States
When more pressure is applied, the crack snaps back the other way, as amazed scientists observed through a microscope (pictured).
‘The cracks in the metal were only expected to get bigger, not smaller. Even some of the basic equations we use to describe crack growth exclude the possibility of such a healing process.’
Although much remains unknown about self-healing, scientists believe a process known as cold welding may play a role.
This can occur at the tip of metal fatigue, allowing bare metals to ‘join’ when compressed under contact.
Their research comes 10 years after Michael Demkowicz of the Massachusetts Institute of Technology presented a similar theory about self-healing metals.
He promoted the idea that nanocracks depend on the generation of crystal defects known as ‘disclinations’ to heal in metals.
In this study, the researchers recognized that different crystalline materials with different grain sizes and textures can be affected by this in different ways.
Their nano-sized platinum was initially tested in oxygen-free conditions so they believe there is scope to explore more non-zero conditions.
Mr Boyce added: ‘The extent to which these findings are generalizable is likely to be the subject of extensive research.
‘We show this happening in nanocrystalline metals in vacuum. But we don’t know if it can also induce metals in the air.’
Ten years after his own study, Mr. Demkovich praised the latest research and expressed his hopes for the future.
He said: ‘My hope is that this discovery will encourage materials researchers to consider that, under the right conditions, materials can do things we never expected.’