YSM Issue 91.3

materials science
FEATURE
tigue. Auxetic and stress-resistant structures
found in nature, like the muscular
grip of eagles and the locking joints in
fleas’ feet, showed a specific hinge structure
similar to a tessellation of heart patterns,
which inspired the researchers to
develop their own s-hinge.
The s-hinge’s main new properties are
increased elastic deformation, or a structure’s
ability to return to its original shape
after being put under stress, and tunable
Poisson ratios, a measure of how much a
material expands perpendicular to an applied
force. Auxetic materials are defined
by having negative Poisson ratios, which
is why they become thicker when they are
stretched and thinner when compressed.
The s-hinge’s tunable Poisson ratios
allows it to switch between a negative
and a positive Poisson value. Essentially,
the s-hinge structure can go from being
a regular material to being an auxetic
material, which makes it especially flexible
and resistant to damage from stress.
This property is particularly important
for future practical applications, where
a material’s performance depends on its
resistance to repeated stress.
To make their new design, the researchers
created a computerized simulation on
ABAQUS software and modeled different
aspects of auxetic materials’ behavior under
applied stresses. Designing accurate
simulations is important to materials science;
if successful, they allow future researchers
to build off the original design
idea. This study’s simulation compared
the properties of the common honeycomb
design to those of the new s-hinge
design. After running simulations, the
researchers 3D printed both an s-hinge
and a honeycomb auxetic structure and
experimentally tested them according to
the simulation. During 3D printing, another
advantage of the s-hinge design was
made apparent: structures like the honeycomb
are vulnerable to defects in the 3D
printing process that the s-hinge design
avoids, such as rounded corners or badly
connected edges.
Following examples of smooth hinge
geometry in nature, such as the Venus
flytrap, the researchers improved that
s-hinge’s stress distribution by designing
the hinge to bear stress throughout its entire
length, not disproportionately in the
corners. The s-hinge structure distributes
stress by replacing the straight edges of
the honeycomb with carefully designed
arcs that are both flexible and capable of
being made on a 3D printer.
Next, the researchers compared their
predictions from the simulation to their
experimental results from the physical 3D
printed models. They found that the simulation-predicted
results matched their
experimental data. This suggested not
only that the s-hinge design was durable
and successful, but also that the modeling
simulation was reliable and could be used
to design new auxetic material structures.
In the experimental conditions, the researchers
proved that the s-hinge was far
better at distributing stress and recovering
from damage compared to the honeycomb
THE NEW DESIGN, NAMED
THE S-HINGE, MAKES
MATERIALS BETTER ABLE
TO OVERCOME THE STRESS
FROM APPLIED FORCES.
model. The s-hinge’s increased flexibility
will allow materials that were formerly
too weak or fragile to be used for auxetic
structures, such as glass and ceramic,
which will ultimately expand auxetics’
range. In particular, the s-hinge design
outperformed the honeycomb in a repeated
cyclability test in which forces were
applied periodically. This cyclical stress
imitates what the structures would have to
withstand in practical applications.
The honeycomb and s-hinge’s different
responses to stress stem from the two
categories of how a material can react
to applied force: plastic deformation or
elastic deformation. Elastic deformation,
as previously described, is the amount of
force a material can withstand before irreparable
damage is created. Plastic deformation,
on the other hand, happens
when the force applied surpasses the
threshold for irreparable damage. When
the force is removed, the structure will
have permanent damage and be unable
to recover completely. Compared to the
honeycomb, the s-hinge was more durable
because it has a higher range of elastic
deformation.
To exhibit further the s-hinge’s ability
to recover from strain, the researchers
altered a 3D printed Batman logo.
They adjusted the angles of the arcs to
turn the previously positive Poisson ratio
into a negative one, which changed
the amount of stress the material could
withstand from that of a normal material
to that of a stronger, auxetic material.
The researchers envision this property
extending new possibilities to materials
science, such as designing structures to
change shape when they are compressed.
Inspired by the s-hinge’s resistance to
stress, the researchers also designed a
latching mechanism based off the general
s-hinge design. The latch works to
gather and save the structure’s elastic
energy, or the potential energy created
when the material is under stress. Similar
ways of latching or storing elastic
energy are very common in nature, and
they are used in a variety of ways. The
Venus flytrap is a common example, as
are muscle structures of birds of prey.
Their tendons can pull their talons shut
with extreme force, and because of their
latching properties, the birds do not
have to repeatedly contract their muscles
in order to hold their grip.
The talons of birds of prey catch our
interest because they can exert force
without having to continually reapply
it. But most natural structures function
by reapplying force cyclically, and the
researchers designed the materials with
this in mind. Like an eagle’s talons, the
s-hinge was modeled to distribute stress
more efficiently, and its tunable Poisson
ratio allowed the hinge’s structure
to change, becoming stronger as force
is applied. Additionally, the accuracy
of the researchers’ computer modeling
simulation will make designing future
auxetic materials far easier. Ultimately,
the researchers envision designing a new
class of smart materials, capable of reacting
with a Venus flytrap’s adaptability,
that could lead to a new field of material
robotics. Robots built with ‘smart materials’
could store energy without a continual
application of force, and release
that energy in reaction to many different
kinds of stimuli. Perhaps a material like
that used in T’Challa’s suit isn’t as far out
of reach as we think.
www.yalescientific.org
October 2018
Yale Scientific Magazine
31