Tech Briefs Magazine - August 2024 - 22

How Tough Is Your Material? 3D Images Will Tell You
Understanding how cracks propagate in brittle solids is essential for developing and testing safe
and cost-effective composite materials for use in construction, sports, and aerospace engineering.
EPFL, Écublens, Vaud, Switzerland
T
he last time you dropped a mug, you
may have been too preoccupied to
take much notice of the intricate pattern
of cracks that appeared in the broken object.
But capturing the formation of such
patterns is the specialty of John Kolinski
and his team at the Laboratory of Engineering
Mechanics of Soft Interfaces
(EMSI) in EPFL's School of Engineering.
They aim to understand how cracks
propagate in brittle solids, which is essential
for developing and testing safe
and cost-effective composite materials
for use in construction, sports, and aerospace
engineering.
But traditional mechanics approaches
to analyzing crack formation assume
that cracks are planar. In fact, simple
planar cracks are just the tip of the iceberg:
most cracks - like those in everyday
brittle solids like glass - propagate
into 3D networks of ridges and other
complex features.
Due to material opacity and the speed
with which cracks form, observing this
complexity in real time is extremely difficult.
But now, armed with a Swiss Army
knife and a confocal microscope, Kolinski
and his team have managed to do just
that - and they have discovered a positive
correlation between crack complexity
and material toughness in the process.
" The energy required to drive cracks
has traditionally been considered a material
property, but our work yields unique
insights into the key role of geometry:
namely, that by increasing the complexity
of geometric features at the crack tip, a
material can be made effectively tougher,
because more strain energy is required to
advance a complex crack than a simple
one, " Kolinski said. " This highlights an
important gap in the current theory for
3D cracks. "
The researchers' method involved
creating very thin slices of four different
hydrogels and an elastomer. Transparent
and brittle, but easy to deform
and measure without shattering, the
hydrogels served as a proxy for understanding
how cracks form in glass and
22
The scientists induced cracks in hydrogel samples with a standard Swiss Army knife. (Image:
EMSI EPFL CC BY SA)
brittle plastics. The elastomer was likewise
a proxy for materials like rubber
and silicone polymers.
While the experimental cracks were
observed with a state-of-the-art confocal
microscope, they were induced using
a standard Swiss Army knife: the
shearing action of the device's scissors
naturally produced geometrically complex
cracks in the hydrogel samples.
Using a custom apparatus developed
by the EMSI team to control sample
alignment and loading, a series of fluorescent
images was generated with the
confocal microscope, and then stacked
to assemble a unique, 3D map of each
fracture surface.
" People have long known that cracks
can become complex by looking at fracture
surfaces after the fact, but what is lost
is the understanding of the loading conditions
when the crack emerged, or what
forces the sample was exposed to, " Kolinski
said. " Our innovative imaging method
has made it possible to characterize this
relationship rigorously in-situ. "
In a nutshell, these experiments revealed
that the strain energy required to
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drive the sample cracks was directly proportional
to the lengths of the crack tips.
This suggests that the increased geometric
complexity of a 3D crack generates
more fracture surface as the crack advances,
thus requiring additional strain
energy to drive it.
In another experiment, the researchers
showed how, as a smoother crack approached
a rigid obstacle embedded in
the sample, the crack's planar symmetry
was broken, increasing both the crack tip
length and the energy required to drive
the crack forward.
" The fact that we can isolate how geometric
complexity emerges with such an
inhomogeneity in the material could motivate
new design approaches, " Kolinski
said. " Our work also highlights the importance
of care in carrying out materials
testing, as we now know that any geometric
deviation from a planar crack front
may lead to a mis-measurement - and
potentially dangerous over-estimation -
of material toughness. "
For more information, contact Celia
Luterbacher at celia.luterbacher@epfl.
ch; +41-216-938-759.
Tech Briefs, August 2024

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Tech Briefs Magazine - August 2024

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