SELF-HEALING OF A SINGLE FIBER-REINFORCED
POLYMER MATRIX COMPOSITE
Eyassu Woldesenbet and Rochelle Williams
Mechanical Engineering, Louisiana State University
2508 CEBA, Baton Rouge, LA 70803
ABSTRACT
This paper presents the experimental results of self-healing single fiber-reinforced polymer (FRP) matrix composites. The idea
for this work is inspired by natural living systems that initiate an autonomic healing process in response to damage. The initial
development of self-healing composites involves the placing of a healing agent within a hollow fiber. A catalyst is then placed
on the outer surface of the hollow fiber to avoid untimely polymerization. When a crack reaches the hollow fiber, the healing
agent fills and heals the crack after the filled hollow fibers partially or fully fracture allowing the healing agent to polymerize
when it comes in contact with the catalyst. Preliminary experiments show that self-healing is achieved in a single fiber test
with high degree of restoration of the original tensile strength. This work will be extended to include a combination of hollow
fibers, solid fibers, resin, a healing agent and a catalyst, that will be carefully assembled to develop functional self-healing
fiber-reinforced polymer matrix composites. Further development of autonomic/self-healing fiber reinforced composites and
associated advancement in the methods of self-healing mechanism will include the ability of localized healing and high healing
efficiency when large scale composite systems with multiple fibers are involved.
Introduction
Excellent mechanical properties such as high strength, low specific weight, and impact and corrosion resistance, as well as
advanced manufacturing methods and tailor ability of the lay-up make fiber-reinforced polymer matrix composites attractive
candidates for use in many performance oriented structures. However, their use is limited due to the difficulty of damage
detection and repairs, and lack of extended fatigue resistance. Healing of materials, such as glass, polymers, and concrete,
has been investigated [1-3]. In these investigations, the healing process involved human intervention and thus the materials
were not able to self-cure. There has been only a limited amount of research in self-healing of composite materials. Two of
these suggest the inclusion of tubes in a brittle matrix material for self-repair of cracks in polymers and corrosion damage in
concrete [4,5]. Other workers, such as Dry [6,7], adapted the self-healing concept for the use in concrete. Repair components
were stored inside vessels dispersed within the concrete and once damage occurred, the repair medium would be released. Li
et al. [8] also applied their concept of self-healing to concrete composites. Ethyl cyanoacrylate was used as the healing agent
and was placed in hollow glass tubes. This experiment introduced the ‘capillary effect’ as a means of filling hollow glass fibers
with healing agents. Motuku and Associates’ [5] concept was developed by considering different critical parameters such as a
method of storage and healing agents. In this experiment, they found that the release of a healing agent through glass was
the most suitable compared to copper and aluminum tubing.
Despite their usefulness, all approaches pose some type of problem in relation to the self-healing process. In the work by Dry
and McMillan [9], and Motuku et al. [5], a dye release was added to the healing agent, therefore, resulting in the inability to
cure. This combination also showed no improvement in mechanical properties. In the works of Kessler and White [10],
clusters of catalyst found in the matrix contribute to a decrease in virgin fracture toughness and unstable crack propagation. In
a recent study, Pang and Bond [11], developed self-healing unidirectional hollow fiber composites. Their composite systems
underwent flexural testing and they found that a significant fraction of lost flexural strength could be restored by a mixture of
repair agent MY750 Ciba-Geigy and acetone. Although flexural strength was restored, this method was not recommended as
a permanent means to repair damage. It was also reported that the ability of the self-repair deteriorated greatly over time as
the repair resin degraded.
A more recent development at University of Illinois, Urbana-Champagne, which attracted a considerable amount of attention,
was the incorporation of microcapsules that contained a polymer precursor into the matrix material of a composite [12]. The
polymer precursor was contained in microcapsules and embedded into the matrix. The matrix contained randomly dispersed
catalyst that was supposed to react with the precursor flowing through any crack formed due to damage and initiate
polymerization. The polymer was then supposed to bond the crack face closed. The investigators overcame several
challenges in developing microcapsules that were weak enough to be ruptured by a crack but strong enough not to break
during manufacture of the composite system. The researchers have shown that it was possible to recover up to 75% of the
maximum tensile strength of the virgin composites. Albeit conceptually interesting and a limited experimental success, there
are major weaknesses in this development. First, the inclusion of microcapsules in a brittle thermosetting polymer, regardless
of their size, cause stress concentrations and weakens the material. The authors have fabricated particulate composites made
from epoxy and microballoons and has demonstrated repeatedly that compressive strength of composites is reduced when
micron size spheres are included in the matrix [13,14]. Second, there is no guarantee that cracks could reach the filled
microcapsules unless there is a high concentration. Third, no method or statistical analysis is set that ensures contact
between the catalyst and the polymer precursor. Finally, the research does not address fiber-reinforced polymer matrix
composites.
The significance of developing self-healing technology or mechanisms can be illustrated by the following applications in
Aerospace, Medical, and Environmental fields. An example of an aerospace application is the repair of an antenna of the
space station. A repair now could involve a space walk, and the associated cost could be, by most standards, very high. If the
antenna were made of self-healing composites, no manual repair could be necessary and the life of the antenna would be
extended significantly. Another example could be damage on a passenger or a military aircraft. There are several instances
where cracks due to fatigue or impact by external objects cause catastrophic failure of aircraft with heavy human casualty.
Self-healing FRP composites could reduce the occurrence of such disasters and increase safety and reliability of aircrafts.
Examples of medical applications include several devices that are implanted in or used by humans, such as artificial hearts,
prosthetic limbs, bone fasteners, and pumps for portable lungs. These devices may break down for several reasons including
manufacturing errors, fatigue and degradation in the human body environment. It could be expensive to replace these devices
and could undoubtedly be highly traumatic for the patients to go through another surgery. Therefore the use of smart selfhealing composites could avoid the high cost of replacing the devices and provide relief to the patient.
Environmental benefits of this proposal are potentially tremendous. Many automobiles and other structures such as ship hulls
are utilizing composite materials more than any other time in history. This is mainly due to the inherent advantages of
composite materials, such as lightweight and corrosion resistance, and the demands for lighter vehicles to meet strict
environmental regulations and higher fuel cost. However, there has always been a concern in replacing structures made of
composite materials since scrap composites do not easily degrade. They add to the strain on the environment. Through the
development of self-healing technology, composite materials could have an extended useful life and therefore reduce the
existing effects on the environment. Other applications involve devices and structures that could be impossible to reach and
perform repairs on, and thus require a full replacement of the devices and structures. The self-healing FRP composites will
avert the need for repair or at least prolong the service duration. In other cases, damage detection may be difficult in the field
due to lack of instrumentation. The self-healing FRP composites could provide increased confidence in such cases until
proper facilities can be accessed.
This paper presents the experimental results of self-healing single fiber-reinforced polymer (FRP) matrix composites. The idea
for this work is inspired by natural living systems that initiate an autonomic healing process in response to damage. The initial
development of self-healing composites involves the placing of a healing agent within a hollow fiber. A catalyst is then placed
on the outer surface of the hollow fiber to avoid untimely polymerization. When a crack reaches the hollow fiber, Figure 1, the
healing agent fills and heals the crack after the filled hollow fibers partially or fully fracture allowing the healing agent to
polymerize when it comes in contact with the catalyst on the outer surface of the coated hollow fibers shown in Figure 2.
Preliminary experiments show that self-healing is achieved in a single fiber test with high degree of restoration of the original
tensile strength. This work will be extended to include a combination of hollow fibers, solid fibers, resin, a healing agent and a
catalyst, that will be carefully assembled to develop functional self-healing fiber-reinforced polymer matrix composites. Further
development of autonomic/self-healing fiber reinforced composites and associated advancement in the methods of self-healing
mechanism will include the ability of localized healing and high healing efficiency when large scale composite systems with
multiple fibers are involved.
Experimental
Experiments were performed to prove the feasibility of self healing in composites. In these experiments, polymer composites
that include hollow glass fibers (HGFs) were fabricated to test the healing efficiency and performance of mechanical
properties. The fibers have an inner diameter of 1.15m, outer diameter of 1.5mm, volume of 75µl and length of 75mm. Each
experiment involved only one hollow fiber and resin. The HGFs were filled with a liquid precursor, also referred to as the
‘healing agent,’ and then coated with a catalyst. The materials used for the healing agent and catalyst were a dimer,
Figure 1: A single fiber self-healing specimen.
Figure 2: Coated HGF filled with DCPD.
dicyclopentadiene (DCPD), and Grubbs’ catalyst. The fibers here were initially coated with the catalyst and then placed in a
mold filled with the resin material. After specimens’ curing took place, mechanical testing was performed on ‘virgin’ as well as
‘healed’ samples. The ‘virgin’ sample characterizes a specimen that has no initial crack on the fiber prior to testing. This
sample contained all the constituents of a self-healing composite material: healing agent, filled hollow fiber, catalysts, and resin
or matrix. The healed sample characterizes a specimen that is cracked, healed and tested after a short period of time.
A schematic diagram of the single fiber self-healing specimen is observed in Figure 1. In Figure 1, the self-healing system is
represented with a fiber embedded in the yellow area symbolizing the cured matrix. The purple area signifies the hollow glass
fiber (HGF) coated with Grubbs’ catalyst and the tan region denotes the liquid dimer enclosed within the hollow glass fiber.
The center, black line characterizes the initial surface crack on the hollow glass fiber. This small crack was used to direct the
crack when a flexural load was applied to the composite.
There were several barriers that had to be overcome to obtain the final self-healing specimens. These complications are
expected to have optimum solutions through further research. For example, at room temperature, DCPD was in a solid form,
and therefore, it had to be heated to 110°F for it to melt. However, in stead of keeping it at 110°F all the time, one milliliter of
chloroform was added to 25mL of DCPD to prevent it from solidifying at room temperature in the experiment. Additionally, the
Grubbs’ catalyst is a fine, purple powder and must be in a liquid form in order to coat the HGFs thoroughly. The challenge in
placing the catalyst on the outside of the fiber was the fact it was a solid. Poche and associates [15] found that mixing 10mg of
Grubbs’ catalyst and one milliliter of dichloromethane would put Grubbs’ catalyst while in liquid form. The benefit of the
dichloromethane (DCM) was that it too allowed the original properties of the Grubbs’ catalyst to remain intact and it evaporated
upon reaching room temperature. Therefore, 0.01g of catalyst was measured and one milliliter of dichloromethane (DCM) was
added in a dish to facilitate the coating of the HGFs. Then the hollow glass fibers were placed in the dish and coated, Figure
2. Figure 3 shows the close-up of the initial crack placed on the outside of the HGFs at midway between the ends. In addition
to directing the crack as desired, this crack would help in visualization. The specimens underwent tensile testing by a MTS
QTEST 150 machine in order to find the maximum load of each specimen.
Crack
Figure 3: Initial crack placed on hollow glass fibers.
Results and Discussion
When a bending force was applied to the composite and the initial crack grew larger, the DCPD healing agent was observed
seeping through the fiber in the area of the crack when the crack propagated through the thickness of the HGF, Figure 4.
Further observation after a few hours showed that the area adjacent to the healed cracked area still remained in a liquid form.
Figure 5 shows the area that remained in a liquid form adjacent to the healed area, demonstrating the attainment of localized
healing/polymerization. Localized healing is always preferred so that materials can be healed more than once in cases of
repeated or multiple cracks. Further research will allow to statistically examine the concentration of catalysts, healing agent,
chloroform, and DCM, required to attain the optimum amount of localized healing. The hollow glass fibers, with an inner and
outer diameter of 1.15mm and 1.5mm, respectively, were calculated to have a degree of hollowness, K, of 0.766. It has been
reported that a K of 0.85 is the most advantageous in composite materials, however, practical considerations such as the
ability of individual fibers to be handled and resistance to crushing limit K to 0.7 [16].
Healing efficiency is defined as the ratio of a material parameter of the healed and virgin materials as shown in Equation 1.
η = Ph P
o
[1]
where Ph represents the material property of the ‘healed’ composite material and P0 represents that of the ‘virgin’ composite
material. The material property could be the tensile or compressive ultimate strength, yield strength, shear strength, fracture
strength, deflection, or modulus. Values of the healing efficiency will be obtained from the experiments mentioned above or
similar experiments. The results of the single fiber and quasi-static tests were used to optimize design parameters in the
development of self-healing composites. In this experiment, the tensile strength of the ‘virgin’ and ‘healed’ specimens was the
material property investigated.
Discharge of
Healing Agent
Figure 4: Release of healing agent through crack in hollow glass fiber.
Solid/Cured Section
Liquid
Healing
Agent
Figure 5: Localized polymerization.
Table 1: Maximum load of virgin and healed specimens.
Maximum Load of Virgin Specimen
Maximum Load of Healed Specimen
σ = 1700 psi
σ = 1526 psi
The results for both the virgin and healed specimen are shown in Table 1. Average of 3 tests was conducted.
The results showed that the strength of the healed specimen was 90% of the strength of the virgin specimen. The preliminary
experiments showed that a significant portion (90%) of the tensile strength was restored. The results indicate that structures
can retain their significant portion of their strength after healing and thus allow for more efficient use.
Conclusions
Preliminary experiments show that self-healing is achieved in a single fiber test with high degree of restoration of the original
tensile strength. This paper presents the experimental results of self-healing single fiber-reinforced polymer (FRP) matrix
composites. The self-healing composite includes a healing agent, a catalyst, a hollow fiber, and resin. The catalyst was
placed on the outer surface of the hollow fiber to avoid untimely polymerization. When a crack went through the hollow fiber,
the healing agent filled and healed the crack after the filled hollow fiber fractured allowing the healing agent to polymerize
when it came in contact with the catalyst. Localized healing was achieved, allowing the remaining healing agent to remain in
a liquid form to be available for healing additional cracks the composite may go through.
Acknowledgments
Support from the National Science foundation is gratefully acknowledged. The authors would like to thank Southern University
undergraduate students, Aaron Davis and Jarvis Wagner, for their help during the fabrication and testing of these samples.
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