Polymer matrix composites offer high specific strength; however, their potential weight savings have been limited by the concern of damage tolerance. If microcracking and similar incurred damage could be autonomously sealed, composite structures could be built thinner and lighter while still addressing damage tolerance, thus achieving the weight savings they promise. Various self-healing mechanisms have been proposed to this end. Herein, a method of thermally reversible polymerization is investigated. To date, thermally activated repair of composites have been accomplished typically through resistive heating, which has certain inherent complexities. An alternate heating method, via microwave exposure of carbon nanotubes incorporated throughout a thermal reversible polymer matrix, is demonstrated. Carbon nanotube-doped composites exhibit enhanced microwave absorption over an undoped control sample. Furthermore, it is shown that these composites can be heated locally by a focused microwave source. The particular composite formulation and layup studied could be uniformly heated to the targeted healing temperature of 100°C in as little as 20 seconds, followed by a healing time on the scale of minutes with total time depending upon the extent of damage.
Advanced composite materials will continue to revolutionize a variety of commercial markets due to their many advantages. Foremost are their lightweight and high strength. Boeing’s Dreamliner is one significant achievement for composites’ contribution to the advancement of technology, with a composite airframe that makes it Boeing’s most energy-efficient commercial passenger aircraft in history [
Polymer matrix composites have an added benefit of cost reductions in energy, production, and maintenance associated with these structural materials. Their lightweight nature reduces energy expenditure not only for operation of the final structure, but also in the energy required for transportation, maintenance, and installation that occur along the path to the usable end product. The chemical resistance of many polymer systems makes them less susceptible to oxidative corrosion, thereby requiring less servicing over the component lifetime.
Unfortunately, polymer matrix composites have been limited by an integral design concern associated with damage tolerance. Any damage, however small, can result in a decrease of the structural integrity of the material and require costly repair. The concern of barely visible damage in the form of microcracks results in the design of thicker composite structures, which translates to an increase in overall weight. Challenges associated with damage tolerance in polymer matrix composites must be successfully addressed in order to ensure that high structural reliability is coupled with the significant weight savings. The introduction of manufacturing defects such as voids or delamination is another concern that must also be addressed. Self-healing materials provide a viable means to surmount damage tolerance and manufacturing concerns, thereby allowing for the realization of safe and reliable structures with reduced mass.
The natural healing capability of biological systems has been the inspiration for the development of smart materials that can undergo internal repair. Biological systems have the distinct advantage of self-repair owing to living organisms’ ability to adapt to changes in the environment. Replicating this healing property within material systems has been an enormous challenge in materials science. Many self-healing methods have been proposed and can be categorized as exhibiting either intrinsic or extrinsic healing mechanisms [
The Diels-Alder chemistry of maleimide and furan molecules allows for thermally activated healing through thermal reversible covalent bonding between the two chemical moieties. One particular system that has been studied extensively is the bismaleimide tetrafuran (2MEP4F), which undergoes thermally reversible polymerization above 90°C and has a Young’s Modulus above 4 GPa [
CNTs are known to absorb microwave radiation and readily convert it to heat with extremely high efficiency [
Composite panels were fabricated using Hexcel plain weave HexForce Carbon Fabric cut in 10 cm2 sheets such that a
Two sets of experiments were conducted to demonstrate heating and healing efficacy of composite panels subjected to microwaves. Heating effectiveness was measured by the ability to rapidly and uniformly heat the composite panels to the required temperature by microwave exposure. These experiments were designed to also show that a focused microwave source could be utilized to locally heat such composites. Healing effectiveness experiments were designed to show that CNT introduction into the composites could facilitate improved repair.
Composite panels were heated using both a custom-modified microwave oven and a custom-built compact focused microwave source. The microwave oven is a Microwave Research and Applications Inc BP-111 Microwave Processor equipped with the True-To-Power power control system. The oven operates at 2.45 GHz and has variable power up to 1000 W. The True-To-Power control system adjusts the power of the magnetron source in order to regulate the temperature of the sample. The oven was modified to allow continuous monitoring of the sample thermal profile. An opening was bored in the upper cavity of the oven and a wire mesh grid placed on the inside of that opening, while a germanium window was placed outside that opening to prevent microwaves from escaping. A FLIR A35SC infrared camera mounted over the germanium window allows for real-time thermal imaging of the sample under microwave exposure (Figure
Customized microwave oven that allows for controlled heating of samples with in situ thermal profile imaging. Microwave oven is manual or computer controlled and equipped with an optical pyrometer that acts as a feedback loop to provide regulation of the magnetron source and manage sample temperature. An IR camera is mounted onto a window bored into the top of the oven to allow thermal imaging of the sample under microwave exposure.
The compact microwave source shown in Figure
A compact microwave generator used to supply focused microwaves through an open end wave guide. The schematic shows the electrical configuration of the entire system used to supply directed microwaves.
For all microwave experiments, the sides of the composites are masked with 3 M metal tape to prevent electrical arching along the edges. Indentations were intentionally formed on several composites using a Future Tech Corp Model FM-7 Vicker’s Microhardness Tester with 1 kg force. Composite samples were impacted with an IMATEK impact tester using 1.13 kg (2.5 lbs) of weight and drop heights of 5 and 10 inches, corresponding to forces of 1.15 J (0.85 ft·lb) and 2.25 J (1.66 ft·lb), respectively. Composite damage was evaluated by optical microscopy using a Zeiss Axio upright microscope and a Zeiss Axiovert 200 M inverted microscope. Scanning electron images were captured with a Zeiss Supra 55 VP microscope. Indentations and microcracks were identified and copper tape markers were placed near the damage to allow for reidentification of these areas after healing.
Composites containing SWCNTs, MWCNTs, and spun CNT ribbons show rapid and uniform heating upon exposure to microwaves. Composite panels consisting of spun CNT nanoribbon tend to heat more rapidly and uniformly, which is attributed to higher concentration and better distribution of CNTs throughout the composite. To compare the heating efficacy between CNT-containing composites and composite without CNTs (control samples), two panels were simultaneously exposed to identical microwave conditions. The control panel and the panel containing spun CNT ribbon were irradiated side-by-side and their thermal profiles were captured with the FLIR camera. It was clearly evident that the CNT-containing panel heated more rapidly and uniformly to an elevated temperature of approximately 110°C (Figure
Microwave exposure of a composite panel without nanotubes (left) and a nanotube containing composite (right). Thermal imaging indicates more rapid and uniform heating as a result of nanotube incorporation.
The through-thickness heating of the panel is also of interest since repair of internal damage will depend on how well heating translates through the composite thickness. To determine this, a composite was heated and the temperature of the back side was monitored. After reaching a uniform 95°C on the front side of the sample, the microwave was powered off and the composite panel was quickly turned over to observe the temperature profile on the back side. The temperature of the back side of the panel was approximately the same and uniform. A piece of metal tape-covered cardboard was placed in the center of a CNT-containing composite to prevent microwave absorption in this area. After exposing the composite to microwave heating, the cardboard piece is removed and the thermal profile of the area underneath is inspected. The thermal image shows the same temperature profile as the surrounding area that was directly exposed to microwaves (Figure
Heating of composite panels with the center masked by a piece of metal covered cardboard in which removal of cardboard shows that the area underneath is the same temperature as exposed area. This is attributed to subsurface heating and thermal conductivity of CNTs which allows translation of heat to unexposed area.
Another premise of microwave heating is that use of a focused microwave source can lead to localized heating of the composites. To prove this, a wave-guided microwave generator was used to irradiate samples. Composites were placed approximately 0.8 mm beneath the open end of the waveguide, and the temperature was monitored in real time with the FLIR A35SC infrared camera. Figure
Localized heating of a composite panel subjected to 10 W focused microwaves from an open ended wave guide. Thermal image captured with IR camera displayed on laptop computer shows heating of area directly beneath wave guide. Composite can be heated to over 100°C in less than 20 seconds.
The first set of composite healing experiments was focused on the determination of the minimal temperature required to heal damage within the panel. Previous work on CNT-containing panels showed that the glass transition temperature of composites varied based on the method of CNT inclusion [
After establishing the minimal temperature for healing, experiments were conducted to establish the healing efficacy of composites. In this work the healing efficiency was assessed by visual inspection of the healing process. Evaluation of the extent of healing within composites subjected to the same damage conditions and duration of microwave heating was used as a qualitative measure of the healing efficiency. Therefore, a greater extent of damage repair occurring over the same period of microwave heating is interpreted as a higher healing efficiency. Additionally, a shorter period of microwave heating to remove the same extent of inflicted damage is interpreted as a greater healing efficiency. The first of these experiments was to show that CNT inclusion facilitates the healing process. A control panel and a panel containing CNT nanoribbons were intentionally indented, and copper tape markers were used to identify the location of these indentations. The two panels were set side-by-side in the oven and heated with the same microwave power (15%–40%) for a total of 10 minutes. The CNT-containing panel reached 95°C at about 5 minutes into the heating cycle while the control sample reached this temperature only in its corners (Figure
To further establish the healing efficacy of composites, heating experiments were conducted to determine the minimal time required for healing. From the previously described experiment, it was found that 5 minutes of heating at 95°C was sufficient to heal indents on a CNT panel. The same composite containing CNT nanoribbon was again indented and then heated for 1 and 3 minutes. The composite was indented prior to each heat cycle and inspected via optical microscopy. At 50% power, the composite could be heated to ~95°C in 1.5 minutes after which the power was regulated to keep the temperature constant for an additional 1 and 3 minutes. Partial indentation was still observed after 1 minute of heating while complete healing was observed after 3 minutes. Once minimal heating time was determined, composites containing different forms of CNTs were heated for 3 minutes to compare the healing process. Significant difference was observed in the healing of a composite panel that consisted of SWCNTs dispersed directly into the polymer resin prior to infiltration. In this panel, only partial healing was observed when heated for the same amount of time as compared to the panel containing only nanoribbon-coated fibers (Figure
Images of healing in composite panel contacting nanoribbons (a) and panel containing dispersed SWCNTs in polymer matrix (b). Panels were heated to 95°C for 3 minutes. SWCNT panel exhibits partial healing and complete healing is achieved by heating to higher temperature.
A composite panel containing MWCNT-coated carbon fibers was impacted and heated in order to heal damage. Prior to heating, both sides of the panel were inspected for damage by optical and scanning electron microscopy. Cracks were observed on the front and back sides of the composite panels, and copper tape markers were placed on both sides to enable reidentification of damaged areas. The composite was heated to 95°C for 5 minutes with the impacted side of facing up. It was also observed that opposite sides of impacted composite panels heal at different rates, with the underside healing more rapidly than the top side. Wider cracks showed partial healing, while thinner cracks exhibited nearly complete healing (Figure
SEM images of impacted composite before and after 5 minutes of microwave assisted healing. Near complete healing of crack on right is observed while partial healing of wider crack is observed. The smooth appearance of the coupon’s surface, induced by the mold during composite fabrication, is not retained posthealing.
Incorporation of carbon nanotubes into composites capable of thermal reversible polymerization can be effectively heated by microwave exposure. CNTs provide rapid and uniform heating of the composite to the temperature required for inducing thermally activated healing. Heating is believed to be subsurface since unexposed areas and the backside to the composite reach the same temperature. The enhanced thermal conductivity of CNT may also contribute to this observation. This effect is highly desirable since this could provide a means for microwave assisted curing of composites or repair of manufacturing induced defects. Additionally, the introduction of CNTs into composites allows localized heating when a focused microwave source is used. Localized heating may have many advantages since only the area that requires healing will be subjected to softening. When composite are exposed to a focused microwave source the exposed area can be heated to above 100°C in less than 20 seconds.
The healing efficacy of composites is significantly improved by the incorporation of CNTs. Intentionally damaged composites can be healed in as little as 3 minutes depending on the extent of damage and the means in which CNTs are introduced into composites. Dispersion of CNT directly into the polymer resin results in a longer healing time when heated to the same temperature compared to composites with only CNT carbon fiber coatings. This is believed to be attributed to either an increase in the viscosity of the polymer resin or direct bonding between CNTs and polymer resin. Composites with CNTs dispersed into the resin require slightly higher temperatures to achieve healing in the same period of time. It was also observed than different sides of impacted composites heal at dissimilar rates. The effect of gravity on polymer mobility is attributed to this observation. Future studies will investigate the effect of gravity and pressure on the healing ability of these composites. Composites will be placed under compressive and tensile loads to examine the influence of pressure on composite healing.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors would like to thanks Dr. Anvar Zhakidov, Dr. John Ferraris, and William Holmes at Solarno for the deposition of spun carbon nanotubes onto carbon fibers. Authors would also like to thank Dr. Dickey Arndt at the Johnson Space Center for use of the compact waveguided microwave source for localized heating experiments. This work was funded through the NASA Center Innovation Fund directed by the Johnson Space Center towards Independent Research and Development. This paper is based on work supported by the Air Force Office of Scientific Research Contract FA9550-11-C-0008. Appreciation is extended to Dr. B. Les Lee, Program Manager, for his encouragement and support.