MDC has developed an impressive portfolio of practical applications that make use of multifunctional materials. The core intellectual property stems from the concept of a flexible nano-engineered assembly that can be introduced as a conformal applique on the surface of a structure, between two physical components, or even co-cured with a composite material. The majority of these innovations make use of carbon nanotubes (CNT) to provide tailored resistive paths. Various forms of ultra-thin conductors are also used to spread and direct currents within these assemblies, and the outer layers provide electrical isolation and protection against wear. Multifunctional assemblies have been produced and demonstrated that can reliably deice leading edges, accurately measure fatigue crack lengths in metallic skins and efficiently cure composites outside of an autoclave. Each assembly can be engineered to serve multiple roles, while adding as little as 20 gsm mass and 50 micron thickness.
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Carbon Nanotubes
Carbon nanotubes (CNT) have been increasing investigated as a means for enhancing traditional composite laminates. They have demonstrated the ability to improve impact resistance, fracture toughness, and can serve as inter-ply reinforcements to prevent the onset of delamination. Beyond their mechanical properties, CNT have sparked interest due to their electrical and thermal properties as possible lightweight replacements for traditional copper wire, ESD/EMI/lightning-strike protection and heat sinks. Most of the work at MDC has focused on their tailorable resistivity. By controlling CNT height, orientation and forest density, their sheet resistance can be varied between 0.05 and 500 Ohm/sq, providing an efficient means for generating heat. Similarly, CNT are piezoresistive, meaning their resistivity changes as a function of strain, thus they can be used as an embedded sensor network for monitoring structural health & usage. While MDC does not manufacture CNT, we have worked with leading CNT vendors, and have established reliable modeling and fabrication processes to incorporate their materials into robust nano-engineers components.
Anti-Icing System
Reliable Ice Protection Systems (IPS) are flight-critical for aircraft, from fixed-wing to rotorcraft and unmanned vehicles (UAV/RPA). They provide anti-icing capabilities to prevent the formation of ice and/or de-icing to remove ice build-up from aerosurfaces. Traditional de-icing fluid used in commercial applications is not environmentally friendly, and is impractical for most military platforms. Current embedded solutions have proven unreliable, lacked durability, caused manufacturing issues or reduce vehicle efficiency. MDC has demonstrated that CNT-based multifunctional appliques can be integrated into both composite laminates and metallic assemblies to be used as an effective means for ice protection. They can provide equivalent performance at a fraction of weight (as little as 1% compared to metal-based heaters), and have the potential to operate with significant power savings. MDC has performed several successful ice-tunnel tests using the FAA recommended icing envelope, and has plans for flight validation in 2019.
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Fatigue Crack Gauge
MDC has developed an extremely simple “fuse-style” sensor for monitoring fatigue crack growth. A fuse-style sensors could just be a single conductive trace with a binary response; either a crack has grown long enough to break electrical continuity or not. Multiple traces can be patterned to produce a pseudo-digital response. MDC has developed a continuum crack gauge fabricated using commercial carbon nanotube (CNT) sheets embedded into a conformal sensor comprising of film adhesive and electrode layers. Any crack growth below the sensor disrupts the CNT electrical network, therefore increasing the network resistance. Using orthogonal parallel pairs of electrodes, one would be able to not only measure extent of a crack, but also deduce the orientation based on relative changes seen by each pair. Thus far, experimental data has matched analytical predictions, and presently a statistical model assisted probability of detection (MAPOD) curve is being generated according to MIL-HDBK-1823A. This CNT sensor will be coupled with RFID technology to provide a passive (RF power harvesting) wireless fatigue crack monitoring capability.
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Out-of-Oven Curing
Studies have shown that the prevalent cost element for manufacturing composite components is the cost of operating an autoclave. Traditional laminates are cured under vacuum, pressure and high-temperatures in an autoclave per manufacturer recommendations to flow, consolidate and cure the resin. However there are many drawbacks to using an autoclave; most notably, they are very inefficient in delivering heat to composite parts, thereby consuming excessive power. Autoclaves heat convectively, meaning they resistively heat air through a blower, which in-turn heats the surface of the composite. Since autoclaves contain a fixed volume of air, the cost to produce the heat remains fixed regardless of the size of composite part being cured, yielding the potential for much waste. MDC has demonstrated the use of CNT appliques to conductively cure composite components not only without an autoclave, but out-of-oven (O3) entirely. O3 conductive curing costs scales with part surface area rather than autoclave volume, thus providing a path for reducing composite acquisition costs by up to 50%, while providing more uniform properties and a means for degree-of-cure feedback.