Session A5
Paper 202
Disclaimer—This paper partially fulfills a writing requirement for first year (freshman) engineering students at the University
of Pittsburgh Swanson School of Engineering. This paper is a student, not a professional, paper. This paper is based on publicly
available information and may not provide complete analyses of all relevant data. If this paper is used for any purpose other
than these authors’ partial fulfillment of a writing requirement for first year (freshman) engineering students at the University
of Pittsburgh Swanson School of Engineering, the user does so at his or her own risk.
SELF-HEALING CARBON FIBER POLYMER FOR USE IN THE AEROSPACE
INDUSTRY
Dillon Axarlis, [email protected], Mena 1:00, Emily Goss, [email protected], Vidic 2:00
Abstract— This paper will discuss the development of a selfhealing carbon fiber polymer for the use in the aerospace
industry. Self-healing polymers work by filling in damaged
regions of the matrix material and provide post-damage
support through the reaction between a healing agent and a
catalyst. The reaction between CYTEC Cycom 823, stored in
hollow glass fibers (HGF), with 913 Hexcel Composite
laminate in carbon fiber reinforced polymers (CFRP) bonds
to form a self-healing carbon fiber polymer. This technology
works by laminating the fibers of the CFRP with an epoxy
resin, 913 Hexcel Composite, and filling the HGF with a twopart healing resin, CYTEC Cycom 823. When a damaging
event occurs, the HGF crack open and the viscous healing
resin, CYTEC Cycom 823, flows out and bonds to the epoxy
resin. The healing resin then hardens to fill the crack.
A carbon fiber self-healing polymer offers a strong and
lightweight material that could heal intra-ply matrix cracks
and inter-ply delaminations that occur along the leading
edges of the wings and cockpit during flight. This type of
damage is difficult to detect visually and can lead to a major
reduction in strength and stability in the polymer. A selfhealing carbon-fiber used in the manufacturing of carbon
fiber aircraft would make these aircraft much safer to fly and
easier to maintain. In addition, the lighter material would
make the aircraft more fuel efficient. In addition to aerospace
applications, this paper will touch on other applications of
self-healing polymers and the future of polymeric research.
Key Words—Aviation, Carbon-fiber reinforced polymer,
Catalytic reaction, Hollow glass fibers, Self-healing polymer,
Vascular network
A CHEMICAL ENGINEER’S INFLUENCE
ON MECHANICAL ENGINEERING
An intense focus on the research of self-healing
polymers has cropped up in the past two decades. The need
for self-healing polymers stems from the desire to build
polymers that can repair sustained damage. Self-healing
polymers work by filling in damaged regions caused by
micro-cracks with a healing agent, and then curing the seal to
provide post-damage support through the reaction between
University of Pittsburgh Swanson School of Engineering 1
03.03.2017
the healing agent and a catalyst. The relevance of this
technology is applicable across many disciplines. The focus
in this paper is the use of self-healing polymers for the
detection and healing of micro-cracks that occur in carbonfiber aircraft. A carbon fiber self-healing polymer offers a
strong and lightweight material that could heal intra-ply
matrix cracks and inter-ply delaminations that occur along the
leading edges of the wings and cockpit during flight. This type
of damage is difficult to detect visually and can lead to a major
reduction in strength and stability in the polymer. A selfhealing carbon-fiber used in the manufacturing of carbon
fiber aircraft would make these aircraft much safer to fly and
easier to maintain. In addition, the lighter material would
make the aircraft more fuel efficient. To accomplish this goal,
CYTEC Cycom 823 and 913 Hexcel Composite will be used
to create a self-healing carbon fiber reinforced polymer
(CFRP), by the use of hollow glass fibers (HGF) in a vascular
network. This technology works by laminating the fibers of
the CFRP with an epoxy resin, 913 Hexcel Composite, and
filling the HGF with a two-part healing resin, CYTEC Cycom
823. When a damaging event occurs, the HGF crack open and
the viscous healing resin, CYTEC Cycom 823, flows out and
bonds to the epoxy resin. The healing resin then hardens to
fill the crack. However, this technology does not come
without its drawbacks. The effectiveness of incorporating
HGF into the carbon-fiber matrix structurally weakens the
material. Rods that are too thin may crack under non-damage
inducing pressure, though thicker rods weaken the base
strength of the material more and fail to break under sufficient
damaging pressure. This paper analyzes how HGF can be
effectively used to make a self-healing carbon-fiber aircraft
material, and the benefits of successfully utilizing this
technology.
CARBON FIBER AIRCRAFT: A NEW
GENERATION
The sustainability of a technology can be measured in its
usefulness to both the manufacture and the producer. A
material that is cheaper for the manufacture to produce or
allows a business to save money allows the consumer to save
money. Any technology that meet these criteria is sustainable
because it improves quality of life. Due to the benefit that
Dillon Axarlis
Emily Goss
carbon fiber polymers have on the fuel savings and weight
reductions when used in the aerospace industry, the
technology is considered sustainable.
Carbon Fiber Polymers are a rapidly developing field in
the aviation and aerospace industries. The allure of this
material comes from the many potential benefits of its
properties. Benefits include a 30% reduction in mass and a
40% reduction in cost compared to traditional aircraft [1]. In
addition, a carbon fiber aircraft would resist corrosion and
would offer added safety through increased structural strength
compared to aircraft constructed from aluminum alloy.
The development of this material for use in commercial
aviation started at the lowest level. Initially the material was
only used in small eight to twelve passenger aircraft such as
Beechcraft’s Starship and Raytheon Aircraft’s Premier. With
the use of a carbon fiber composite, Beechcraft’s Starship
achieved a weight savings of around 18% over traditional
aircraft and Raytheon Aircraft’s Premier achieved a weight
savings of about 20% [1]. The advantage of the lighter aircraft
is that it requires less thrust to propel it through the air
increasing fuel efficiency and speed. With the technology
proven in small aircraft, carbon fiber was brought to the large
scale commercial aviation industry with a prime example of
the Boeing’s 787 Dreamliner. The Dreamliner is made up of
50% composite material by mass and reaps many benefits.
Boeing claims that the new lighter aircraft sees a 20%
reduction in fuel usage than previous generations. In addition
to fuel savings, the composite structure of the aircraft reduces
maintenance cost by 30% [2]. The Boeing 787 Dreamliner has
been a great success for composite Aircraft with more than
1200 ordered as of January 2017 [3].
Carbon fiber polymers provide many advancements for
the aerospace industry in terms of sustainability. The reduced
use of fuel means less fossil fuels being burned and CO 2
emissions from aircraft. If the benefits of the Boeing 787 were
applied to all airlines, it would reduce the amount of fuel used
by airlines by 300 million barrels. It would also significantly
reduce the amount of CO2 emissions from airlines which
makes up for 12% of the CO2 produced by transportation [4].
While carbon fiber holds a promising future in the aerospace
industry, carbon fiber can have its downfalls. Repairing
carbon fiber is a complicated process and damage is hard to
detect. By introducing a self-healing aspect to the carbon
fiber, these problems can be rectified while simultaneously
decreasing its impact on the environment and the long-term
costs of the material when used in the aerospace industry.
With the introduction of a self-healing aspect to the
carbon fiber reinforced polymer, the benefits of the material
can be further enhanced. A self-healing polymer will allow
the carbon fiber polymer to repair intra-ply matrix cracks and
delamination of the layers of fibers that can lead to a
significant reduction in the strength of the material. This will
further reduce the cost of maintenance in aircraft as this kind
of damage is usually hard to detect and expensive to repair.
Another benefit of this ability to self-heal is further weight
savings. Due to the difficulty of repairing this type of damage,
components made of composites such as carbon fiber
reinforced polymers are made to be heavier and stronger than
needed. Because the self-healing component can repair this
type of damage, parts can be made lighter for additional
weight savings [5]. The self-healing component of the
composite material would come from the integration of a
vascular network of hollow glass fibers that will be explored
in the next two sections.
PERSONIFIYING MATERIALS: THE
SCIENCE OF SELF HEALING
The concept of self-healing polymers stemmed from the
idea of a polymer composite with a damage tolerance, or the
“ability for the material to sustain weakening defects under
loading without suffering reduction in residual strength, for
some stipulated period of service” [6]. Traditionally, once
damage had been detected within a composite structure,
cosmetic, temporary or structural repair ranging from
simplistic external patches to costly complex intrusive repairs
aimed at restoring some or all of the laminate's stiffness and
strength, had to be performed [7]. From this came the idea of
incorporating a self-healing system within the polymer
composite.
There are two main classifications of self-healing
polymers: Non-autonomic and Autonomic healing. Nonautonomic healing requires additional stimuli, such as heat or
UV-radiation for healing to occur [8]. The usefulness of nonautonomic self-healing is that those repairs are usually more
secure and return more of the initial strength back to the
material. While the healing components can be built into the
material, they require manual intervention to activate and
cure. These manual repairs can be just as time consuming as
replacing the damaged part, and can be equally costly. The
alternative to non-autonomic healing, is autonomic healing.
Autonomic self-healing materials are fully self-contained and
require no external intervention of any kind [8]. It utilizes
strategically placed catalysts within the material that interact
with the polymers to initiate a reaction when damage occurs.
Autonomic healing systems have lower success rates than
non-autonomic systems, but still return most of the material’s
initial strength without manual intervention. This is useful for
immediate fixes when micro-cracks appear in the material,
and can also prevent further structural weakening by stopping
the spread of micro-cracks from the weakened region. The
type of self-healing polymer of interest for this paper is
autonomic, focusing on healing and preventing further microcracks from forming on the leading edges of aircraft caused
by particles hit at high speeds during flight.
There are multiple types of autonomic healing, though
the focus of this paper is on the biological “bleeding”
approach to repair: microcapsules and hollow fibers [6]. The
biological “bleeding” approach to self-healing can be
compared to how the human body bleeds when it is cut. Once
the damage occurs, blood fills the damaged region, clots, and
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Emily Goss
starts to work on healing the area. The self-healing polymer
biological “bleeding” approach follows the same idea. When
damage occurs, the containers holding the healing agents
break, releasing the liquid to fill the gap, harden, and heal.
There are two methods of containing the healing agents:
microcapsules and hollow fibers.
One of the first and most extensively explored examples
of a fully self-contained self-healing system was the
microencapsulation of dicyclopentadiene (DCPD) and
Grubbs’ catalyst particles embedded in an epoxy matrix. In
this polymer, the micro-encapsulated DCPD is distributed
through the matrix of the material. When the material is
damaged, and cracks propagate through the matrix and
encounter the capsules. The force from the cracks break the
micro-capsules releasing DCPD which then flows into contact
with the Grubbs’ catalyst particles embedded in the epoxy
matrix. When the polymer and catalyst come into contact,
DCPD crosslinking occurs in the area via a ring-opening
polymerization (ROMPS), which bonds the two crack faces
together [9]. Figure 1 shows a simplified visual of how
crosslinking connects and bonds polymers together.
FIGURE 2 [7]
Variations of vascular systems using HGF
As in the micro-encapsulation approach, upon damageinduced cracking, the self-healing mechanism is triggered by
the rupture of the vascular network and subsequent release
and reaction of the healing agent in the affected region [10].
The reaction of interest for this paper follows the HGF
approach with the one-part resin CYTEC Cycom 823 in
carbon fiber reinforced polymer (CFRP) matrix laminated
with 913 Hexcel Composite.
SELF HEALING CARBON FIBER: CYTEC
CYCOM 823 AND 913 HEXCEL
COMPOSITE
FIGURE 1 [8]
Crosslink forming covalent bonds between
polymers
The other approach to the micro-encapsulation method
is the use of hollow glass fibers (HGF) in a vascular network.
This approach was inspired by biological vascular systems'
ability to supply fluid to an area from a point reservoir, giving
a branching network [6]. “A typical hollow fiber self-healing
approach used within composite laminates could take the
form of fibers containing a one-part resin system, a two-part
resin and hardener system, or a resin system with a catalyst or
hardener contained within the matrix material” [6]. Figure 2
illustrates the different types of HGF storage methods.
The foundation of the healing system, 913 Hexcel
Composite or HexPly 913, is a highly successful modified
epoxy matrix used extensively in the aerospace industry for
primary aircraft structures and helicopter blades. The reason
for its use in the aerospace industry is its “low temperature
cure cycle which exhibits outstanding environmental
resistance whilst retaining good hot/wet mechanical
performance [11].”
The matrix of the self-healing fiber polymer is made of
fiber composite layers laminated with the HexPly 913 to make
the 913 Hexcel composite material. Typically, the fiber is a
quasiisotropic stacking sequence of 16 plies [6]. The
quasiisotropic stacking sequence is shown in Figure 3.
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Dillon Axarlis
Emily Goss
however the inclusion of the HGF impacts an initial strength
reduction of 16%. Comparing the two samples, the 200 µm
exhibited little reduction in undamaged strength (2%), while
the 70 µm suffered an 8% reduction of baseline strength [6].
This was attributed to less disruption in the host laminate. The
tests concluded that the less intrusive HGF has higher baseline
strength, which could be correlated to larger diameter HGF
producing similar lower baseline values. However, when
comparing the baseline strength after initial damage, and
before healing, the 70 µm retained 76% of its baseline
strength while the 200 µm only retained 69% which was equal
to the control [6]. After the system had time to recover, the 70
µm sample went back to 89% its baseline strength, 9% higher
than the 200 µm, and 20% higher than the control [6]. After
examining the actual pieces of carbon-fiber, it was observed
that clusters of HGF can cause micro-cracks to deviate from
their path due to the weakened strength of the laminate. It was
also noted that the propagating crack did not pass directly
through the HGF matrix, but deviated around them causing
fiber rupture and release of healing agent [6]. What this
revealed about this healing system was that factors
contributing to strength and healing efficiently is how and
where the HGF are placed. The closer together the HGF are,
the weaker the strength of the composite material. The weaker
the composite material, the more susceptible it is for crack
spreading within the laminate. Studies investigating the width
of the fibers, which would correlate to how much healing
resin could be made available to cracked areas were
hypothesized for future research.
The reason why HGF were chosen for this technology
was that HGF allow for more liquid resin to be given to a
particular location. With the encapsulation method, the
capsules are built into the matrix and are stationary and
isolated, which can work for initial small cracks but not for
post-damage in the same location. HGF allow more resin to
flow to any given location in the matrix by its use of the
vascular system. This is shown in the experiments that the
healing of 200 µm separated fibers were comparable in
effectiveness to 70 µm separated fibers because the liquid
resin was allowed to flow continuously from the more
separated HGF. This is only possible due to the low viscosity
of CYCOM 823. HGF are also less disruptive of the original
matrix, which keep the baseline strength at a higher value than
that of multiple capsules.
FIGURE 3 [7]
Laminating carbon-fiber composite material
The HGF are then wound directly onto uncured CFRP plies
prior to lamination [6]. The embedded hollow glass fibers are
then later filled with CYCOM 823 to create the self-healing
system. CYCOM 823 is a liquid epoxy resin developed by
Cytec Engineered Materials. It has a low viscosity which
allows it to freely flow out the HGF when cracked.
Additionally, CYCOM 823 has high elongation values which
is indicative of a strong material [12]. When damagedinduced cracking occurs in the matrix, the HGF are broken
which releases the CYCOM 823. When CYCOM 823 comes
into contact with the HexPly 913, crosslinking occurs, curing
the liquid CYCOM 823 and partially repairing the damaged
region.
BUILDING AND TESTING A SELFHEALING CARBON FIBER
When making the HGF vascular system, there is a ratio
of the effectiveness of the repair that correlates to the
diameter, thickness, and spacing of the HGF. A study was
done in order to test how the spacing of the HGF affected the
strength and healing capabilities of the composite material.
The study investigated two fiber spacing options in a 16 ply
230 x 160 x 2.5 mm control material:70 µm and 200 µm. The
fibers
were
located
in
the
pattern
“−45◦/90◦/45◦/0◦/HGF/−45◦/90◦/45◦/0◦/0◦/45◦/90◦/−45◦/HG
F/0◦/45◦/ 90◦/ − 45◦” with their respective separation between
fibers [1]. To test the healing efficiency, each was hit, in
addition to the control, by a hardened steel 5 mm spherical
indenter, and hit with peak loads of 2000 N (~550 lbs) to
stimulate barely visible impact damage in the composite
laminate. In comparison to the control, the self-healing
samples recovered 82% of their original undamaged strength,
SELF-HEALING CARBON FIBER: WHY
IT’S WORTH IT
The addition of a self-healing aspect into a carbon fiber
reinforced polymer offers many benefits. One advantage of
self-healing CFRP is that they allow the material to repair
itself in case of an impact midflight. Due to the design of
carbon fiber polymers the fibers are oriented in such a way
that maximizes performance against in-plane forces such as
pressurization and depressurization of the cabin. The design
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Dillon Axarlis
Emily Goss
of these materials causes them to be more vulnerable to outof-plane forces such as impacts. An impact, such as a bird
strike, can cause the polymers that hold the layers of the fiber
to crack and delaminate [2]. Although 95% of bird strikes do
not cause major damage, by introducing a self-healing aspect
to the carbon fiber, the damage of a bird strike can be
minimized [13]. Prevention of this damage increases the
safety and reliability of the aircraft as well as the length
between maintenance cycles.
The main benefit of a self-healing aspect is the ability to
enhance structural integrity and increase the service life of an
aircraft. Structural integrity of an aircraft is of utmost
importance, illustrated by c-check maintenance checks every
eighteen months, or 6000 flight hours, and the more
comprehensive d-check occurring every couple of years. The
less intensive c-check is a check of all the aircrafts systems.
This includes a visual inspection of the aircraft’s structural
integrity and some nondestructive inspection methods. The ccheck can be very costly and time consuming for airlines as it
requires the aircraft to be out of service for 3 days to a week
[14]. The goal of the self-healing carbon fiber is to reduce the
necessity of these checks by increasing the life of the
structural components. The self-healing CFRP will also
contribute to a reduction in the length it takes to complete
maintenance checks. For example, the comprehensive dcheck, sometimes known as an overhaul, involves striping the
paint off the aircraft and removal of the outer panels of the
aircraft to expose the airframe. The airframe is then inspected
with advanced nondestructive methods. The problem with
this system is that it puts the aircraft out of commission for
months, an expensive downtime for the airline [14]. The
introduction of a detectable self-healing aspect to the
composite material provides a solution to alleviate this cost.
By introducing a UV dye into the hollow glass fibers, a
damage visual enhancement method can be created that
allows for easy detection of cracking and matrix
delamination. To accomplish this, the fluorescent dye Androx
985 would be integrated into the epoxy resin. When stress is
applied to the carbon fiber, the integrated fibers break and the
die flows with the resin. This dye will allow inspectors to
quickly inspect the airframe. When exposed to a black light,
the UV dye will fluoresce clearly, creating a bruise that
signals damage to the airframe. The addition of this dye will
cut down on the cost and duration of d-checks [8].
Composite materials in aircraft construction have
already illustrated decreased maintenance time over
traditional materials. According to Boeing, the Boeing 777
aircraft shows the decreased maintenance costs, citing the 777
composite tail, which is twenty-five percent larger than the
767 aluminum tail requires thirty-five percent fewer
scheduled maintenance labor hours. Boeing stated that the
composite material has a lower risk of corrosion and fatigue
compared with metal [15]. In addition, corrosion and fatigue
in a structure add significant non-routine maintenance, with
non-routine maintenance frequently doubling or tripling the
total labor hours expended during a maintenance check [15].
Boeing estimated that the non-routine labor costs would be
considerably lower than more conventional metallic
airframes, which provides a more cost sustainable solution for
repairs. Part of the reason for the sustainability of the 777
composite material, is its ability to be temporarily repaired,
which allows the airplane to fly despite minor damages that
might ground an aluminum airplane [15]. This repair uses the
non-autonomic healing polymers are capable of, which
requires maintenance after the detection of damage. By
utilizing the ability of a self-healing composite material,
preferably one capable of glowing under UV light, the time
needed to identity and manually repair minor damages can be
decreased even further. Furthermore, autonomic healing
polymers can help prevent crack propagation which would
increase the damage during the between impact in the air, and
repair on the ground. Repair time and labor are financial
burdens on the company which translate to higher costs for
the consumers, and with respect to planes, longer grounding
periods means less flights which also cost the company
money. The application of self-healing carbon fiber polymer
in the aerospace industry would provide a sustainable solution
both financially and for the life time of the material.
Reducing the cost of maintenance for airlines benefits
not only the airline, but the people that travel with the airline.
From a business perspective, an airline that can charge lower
rates can draw more passengers and make more money. The
consumer will also benefit from cheaper airline tickets,
making travelling around the world easier. Consumers will
also benefit from the lightweight material due to its fuel
savings. A fuel-efficient aircraft can go further on a single
tank of fuel meaning that there will be less layovers and more
direct flights. It also opens up the possibility to fly to airports
that normally are out of range or are not profitable enough to
warrant as many commercial flights [2].
In addition to the added reliability and reduced
maintenance, it is thought that a self-healing polymer will
reduce the weight of an aircraft. This is because that the small
micro-cracks and delamination that the self-healing carbon
fiber is meant to repair can cause up to a 50% reduction in
strength. Because of this, when using standard carbon fiber
reinforced polymers, manufactures will use additional
material to reduce the possibility of the stress that can
compromise the material structure [16]. With the self-healing
polymer, this abundant material reinforcing material can be
removed, and with it, the materials weight.
As self-healing carbon-fiber improves, the technology is
becoming more and more viable to move into production for
the industry. University of Bristol’s Professor Duncan Wass
stated that he expected self-healing products to reach
consumers in the very near future after testing an aircraft that
had been produced using self-healing carbon fiber to
construct its wings [17]. A self-healing CFRP has many uses
and benefits but it must be considered what will happen to the
polymer after it has been used.
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Dillon Axarlis
Emily Goss
healing tires, self-healing cabling, self-healing cell phones
[20].
In terms of sustainability, the advancement of selfhealing polymers can lead to a more environmentally friendly
outcome. If our phones could heal themselves, it would
decrease the amount of pollution from production and
electronic waste from component recycling. Instead of having
tires litter the highways, causing accidents for other cars and
polluting the roadsides, punctures and strain on the tires could
fix themselves which would extend their life and potentially
prevent tires from flying off trucks. The same is true for selfhealing carbon-fiber aircraft material. The costs of
maintaining a plane, the manual labor to scrutinize and repair
damages, add up. At this stage in development, a self-healing
aircraft with the ability to highlight where and to what extent
damages occurred could be cost changing: both financially
and with respect to human life. Small damages add up, and if
we can start small, we can make a big difference in the long
haul.
While composite materials in the structure of the aircraft
may decrease maintenance costs and increase flight time, the
cost of producing the material for the aircraft is not as cost
efficient. In May of 2016, Boeing opened its 777X Composite
Wing Center (CWC) which will make the composite wings
for the company's newest commercial jet, the 777X [22]. This
project cost over a billion dollars to make, but is expected to
provide jobs for thousands of people. The company has
received three hundred and twenty orders and commitments,
with the first delivery scheduled for 2020 [22]. This example
illustrates that the initial costs to transition into making carbon
fiber composite materials is not cheap, and the fruits of
Boeing's labor will not be fully realized until 2020, though its
787 which is 50% composite material is starting to show the
emergence of a new age in aeronautical technology. With
further research and testing, one step behind this change is the
self-healing ability of the composite, and a self-healing plane.
Each new discovery and advancement on self-healing
polymers generates new ideas for improvement and
advancement in the field. With more commercialization of
self-healing materials occurring each year, markets are
primed and applications nearly unlimited. It is now only a
matter of time.
RECYCLING CARBON FIBER
A self-healing carbon fiber would have many benefits to
the aerospace industry. However, the question arises, what
happens when the carbon fiber polymer has reached the end
of its lifespan? Fortunately, once the material has been used it
can be recycled. Old carbon fiber parts or scraps from the
manufacturing process that normally are sent to landfills can
be broken down and reclaimed for the use in other areas. The
downside of this material is that it loses some of the structural
strength that it originally had and does not meet the strict
requirements for use in an aircraft’s airframe [18]. Despite not
retaining its full strength, the recycled material has many
other uses. For example, Boeing has used this material for the
service hatches on the wings of the Boeing ecoDemonstrator
[19]. Other uses of this include the automotive industry. This
recycled material is used in the production of BMW i3’s and
i8s and would be viable for many other uses in the sporting
industry such as tennis racquets and kayak paddles [18]. As
well as being good for the environment and keeping waste
material out of landfills, the recycled composite has many
advantages. The material remains a lightweight alternative to
aluminum and other materials. The recycled material often
loses only 10 – 20% of its original strength being stronger
than many aluminum alloys. The material also has a cost
advantage. The recycled material is cheaper to manufacture
then the original material with projected savings is about 3040% [18]. The ability to recycle the carbon fiber polymer
makes the material much more viable in the future by creating
a material that can be reused multiple times.
THE FUTURE OF SELF-HEALING FIBERS
The goal of future self-healing polymer research is to
create a healing network capable of repairing all types of
composite failure modes and that can be replenished and
renewed during the life of the structure [2]. As of now, selfhealing technologies on the market include self-repairing
iPhone cases, self-repairing car paint, and self-healing
concrete [20]. N-Tech estimates the market for all self-healing
systems, including reversible polymers, and inorganic
capsules and vascular systems, will “grow to $2.7 billion by
2020,” with the automotive industry revenues up to $1.6
billion and revenue from consumers is estimated to reach
around $480 million in 2020 [21]. The market for inorganic
microencapsulation and vascular network healing systems is
currently negligible, but according to Research and Markets
report, “Markets for Self-Healing Material: 2017 – 2024,”
self-healing materials utilizing either microencapsulation or
vascular systems will generate revenues of $1.1 billion in
2022 [20]. Future and current applications for self-healing
materials are for “electronics, construction, automotive
energy, medical, military and aerospace” industries [20].
Some of the technologies within these markets include: self-
AUTHORS NOTE
The technology of self-healing polymers is growing at
an enormous rate, but there are still too few researchers
following specific technologies. In this paper, we compared
the distance of the fibers and how that affected the strength
and healing ability, and ideas were touched upon that led to
no fruitful information. One topic we hope to explore is how
the diameter of the HGF can be used to increase the retained
strength after healing while minimizing the initial structural
strength reduction. HGF with wider diameters can more
effectively transport the CYCOM 823, however the larger
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Dillon Axarlis
Emily Goss
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diameters reduce the initial composite strength of the
material. Another area we found lacking in information was
the costs of production of the self-healing polymers. The
concept of a safer aircraft promotes the socially oriented goal
of savings lives, though not enough information is out there
to find the financial drawbacks of this endeavor. Despite this
hurdle, the social and ethical reasons are enough to support
and attempt to utilize this technology.
CONNECTING THE DOTS: THE BIG
PICTURE
A self-healing carbon fiber is the next step in carbon
fiber reinforced polymers. In order to overcome the
drawbacks of carbon fiber polymers, such as low impact
resistance that results in matrix-ply delamination and intraply cracks, the self-healing polymer uses a vascular network
of hollow glass fibers in order to disperse an epoxy resin,
CYTEC Cycom 823 into the cracks when they form. When
the healing agent and catalyst come into contact, they form
covalent bonds which produce a strongly connected complex
that holds the two crack faces together. This self-healing
epoxy acts as preventative maintenance that will prevent the
micro-cracks from spreading and compromising the structural
integrity of the polymer. Integrating the self-healing vascular
network offers many benefits to the aerospace industry and
the construction of new aircraft. By producing aircraft that use
parts made of self-healing carbon fiber reinforced polymers,
aircraft can be made safer and they will last longer. The selfhealing polymer can repair damage that would be otherwise
hard to detect and repair manually from impacts such as bird
strikes. The added damage reduction will increase the length
between maintenance intervals, resulting in longer aircraft life
and decreased aircraft downtime. Maintenance time can be
further decreased with the inclusion of a UV dye in the epoxy.
Although it offers benefits to the aerospace industry, the
science of self-healing can be applied to more than just a selfhealing aircraft. With continued research, self-healing
technologies can be applied to consumer goods such as nail
varnish and iPhone screens [17].
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7
Dillon Axarlis
Emily Goss
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2f1dcf0079d98a894ee97ae841d55cac0d08
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ACKNOWLEDGEMENTS
We would like to thank Kathy Goss for assisting in
editing, and our friends for supporting us through this process.
In addition, we would like to thank Marade Bergen, Keely
Bowers, and the two anonymous peer reviewers who assisted
us in editing the paper.
8