materials testing
Scientists witness metal healing itself Green marks the spot where a fissure formed, then fused back together in this artistic rendering of nanoscale self-healing in metal, discovered at Sandia National Laboratories. Red arrows indicate the direction of the pulling force that unexpectedly triggered the phenomenon. Image credit: Dan Thompson
Scientists from Sandia National Laboratories and Texas A&M University have witnessed pieces of metal crack and then fuse back together without any human intervention, overturning fundamental scientific theories in the process.
I
f this newly discovered phenomenon could
be harnessed, it could usher in an engineering revolution — one in which self-healing engines, bridges and airplanes could reverse damage caused by wear and tear. Fatigue damage is one way machines wear out and eventually break. Repeated stress or motion causes microscopic cracks to form; over time, these cracks grow and spread until the whole device fails. “From solder joints in our electronic devices to our vehicle’s engines to the bridges that we drive
From theory to reality
injury. Over time, the crack regrew along a
In 2013, Michael Demkowicz — then an
different direction.
assistant professor at Massachusetts Institute of Technology, now a full professor at Texas A&M
“This was absolutely stunning to watch firsthand,” Boyce said.
— published a theory in Physical Review Letters,
“What we have confirmed is that metals
based on findings in computer simulations,
have their own intrinsic, natural ability to heal
that under certain conditions metal should be
themselves, at least in the case of fatigue damage
able to weld shut cracks formed by wear and
at the nanoscale.”
tear. The discovery that his theory was true
Boyce, who was aware of Demkowicz’s
came inadvertently at the Center for Integrated
theory, contacted the professor to share the
Nanotechnologies, a Department of Energy user
team’s findings. Demkowicz then recreated the
facility jointly operated by Sandia and Los Alamos
experiment on a computer model, substantiating
national laboratories.
that the phenomenon witnessed at Sandia was the
Khalid Hattar, now an associate professor at
same one he had theorised years earlier.
over, these structures often fail unpredictably due
the University of Tennessee, and Chris Barr, who
A lot remains unknown about the self-healing
to cyclic loading that leads to crack initiation and
now works for the Department of Energy’s Office
process, including whether it will become a practical
eventual fracture,” said Sandia materials scientist
of Nuclear Energy, were running an experiment at
tool in a manufacturing setting. As noted by Boyce,
Brad Boyce. “When they do fail, we have to contend
Sandia when the discovery was made. They only
“We show this happening in nanocrystalline metals
with replacement costs, lost time and, in some cases,
meant to evaluate how cracks formed and spread
in vacuum, but we don’t know if this can also be
even injuries or loss of life.”
through a nanoscale piece of platinum using a
induced in conventional metals in air.”
Although scientists have created some self-healing
specialised electron microscope technique they
Yet for all the unknowns, the discovery —
materials — mostly plastics — the notion of a self-
had developed to repeatedly pull on the ends of
which has been published in the journal Nature
healing metal has largely been the domain of science
the metal 200 times per second.
— provides a leap forward for the field of materials
fiction. As noted by Boyce, “Cracks in metals were only
Surprisingly, about 40 minutes into the
science. Demkowicz concluded, “My hope is that
ever expected to get bigger, not smaller. Even some of
experiment, the damage reversed course. One
this finding will encourage materials researchers
the basic equations we use to describe crack growth
end of the crack fused back together as if it was
to consider that, under the right circumstances,
preclude the possibility of such healing processes.”
retracing its steps, leaving no trace of the former
materials can do things we never expected.”
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