Aerospace & Defense Technology - June 2024 - 36

Tech Briefs
material in an autoclave to cure. To test
crack resistance, the researchers placed a
crack on the edge of the composite, right
at the start of the region between the two
middle layers.
" In fracture testing, we always start with
a crack because we want to test whether
and how far the crack will spread, " Furtado
explains.
The researchers then placed samples of
the nanotube-reinforced composite in an
experimental setup to test their resilience
to " delamination, " or the potential for
layers to separate.
" There's lots of ways you can get preThis
schematic shows an engineered material with composite layers. Layers of carbon fibers (the long silver
tubes) have microscopic forests of carbon nanotubes between them (the array of tiny brown objects).
These tiny, densely packed fibers grip and hold the layers together, like ultrastrong Velcro, preventing the
layers from peeling or shearing apart. (Image: MIT)
team used a process of chemical vapor
deposition to react various catalysts in
an oven, causing carbon to settle onto a
surface as tiny, hair-like supports. The
supports are eventually removed, leaving
behind a densely packed forest of
microscopic, vertical rolls of carbon.
The lab has previously shown that
the nanotube forests can be grown and
adhered to layers of composite material,
and that this fiber-reinforced compound
improves the material's overall
strength. The researchers had also seen
some signs that the fibers can improve
a composite's resistance to cracks
between layers.
In their new study, the engineers
took a more in-depth look at the
between-layer region in composites
to test and quantify how nanostitching
would improve the region's resistance
to cracks. In particular, the study
focused on an advanced composite
material known as thin-ply carbon
fiber laminates.
" This is an emerging composite technology,
where each layer, or ply, is about
50 microns thin, compared to standard
composite plies that are 150 microns,
which is about the diameter of a human
hair. There's evidence to suggest they
36
are better than standard-thickness composites.
And we wanted to see whether
there might be synergy between our
nanostitching and this thin-ply technology,
since it could lead to more resilient
aircraft, high-value aerospace structures,
and space and military vehicles, "
Wardle says.
Velcro Grip
The study's experiments were led by
Carolina Furtado, who joined the effort
as part of the MIT-Portugal program in
2016, continued the project as a postdoc,
and is now a professor at the University
of Porto in Portugal, where her research
focuses on modeling cracks and damage
in advanced composites.
In her tests, Furtado used the group's
techniques of chemical vapor deposition
to grow densely packed forests of vertically
aligned carbon nanotubes. She also
fabricated samples of thin-ply carbon
fiber laminates. The resulting advanced
composite was about 3 millimeters thick
and comprised 60 layers, each made
from stiff, horizontal fibers embedded in
a polymer sheet.
She transferred and adhered the nanotube
forest in between the two middle layers
of the composite, then cooked the
mobilityengineeringtech.com
cursors to delamination, such as from
impacts, like tool drop, bird strike,
runway kickup in aircraft, and there
could be almost no visible damage, but
internally it has a delaminat ion, "
Wardle says. " Just like a human, if
you've got a hairline fracture in a bone,
it's not good. Just because you can't see
it doesn't mean it's not impacting you.
And damage in composites is hard to
inspect. "
To examine nanostitching's potential
to prevent delamination, the team placed
their samples in a setup to test three
delamination modes, in which a crack
could spread through the between-layer
region and peel the layers apart or cause
them to slide against each other, or do a
combination of both. All three of these
modes are the most common ways in
which conventional composites can
internally flake and crumble.
The tests, in which the researchers
precisely measured the force required to
peel or shear the composite's layers,
revealed that the nanostitched held fast,
and the initial crack that the researchers
made was unable to spread further
between the layers. The nanostitched
samples were up to 62 percent tougher
and more resistant to cracks, compared
with the same advanced composite
material that was held together with
conventional polymers.
This research was performed by a team
of Brian Wardle and a team of engineers for
the Massachusetts Institute of Technology.
For more information, download the
Technical Support Package (free white
paper) at mobilityengineeringtech.
com/tsp under the Materials category.
Aerospace & Defense Technology, June 2024

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Aerospace & Defense Technology - June 2024

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