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Completed TRL 1 (started at 1, targeting 3)
The use of composite structures in aerospace and space applications is growing each year. Since composites are inherently brittle materials it is important to monitor the structural health of these structures. Presently strain gauges and fiber optic strain sensors are used. Both types of sensors are brittle and can themselves fail. Neither can be used during flight to continuously monitor structural health. It is proposed to produce a piezoelectric carbon nanotube based structural health monitoring (SHM) system. Basically this will be carbon nanotubes and lead zirconium titanate (PZT) nanopowders dispersed in a PVDF-TrFE polymer matrix. These sensors will be flexible and can be integrated between plies as well as adhered to the structure surfaces. It is planned to 3-D print these sensors using an nScrypt 3-D printer. Once printed, heating to remove any solvent and transform the PVDF-TrFE to the piezoelectric beta phase will treat the sensor. Samples will be poled using corona poling and hysteresis curves generated. Samples will also be embedded within plies of composite bend specimens and curves of voltage response versus strain generated. Preliminary wdrk on a wireless monitoring system for these sensors will be initiated.
The objective of the proposed research is to 3-D print SHM sensors for use in composite structures. Presently, SHM sensors are either strain gauges or fiber optic strain sensors are utilized to determine structural health of composite structures. These sensors are brittle in nature and unwieldy to use for actual flight. Our plan to develop piezoelectric sensors consisting of carbon nanotubes and lead zirconium titanate nanopowders dispersed in PVDF-TrFE polymer will lead to a flexible sensor which can not only be attached to external surfaces, but also between composite plies. Ultimately, wireless antennae can be printed on the sensor so that the SHM sensor can be monitored externally without the need of wiring. This would allow the SHM of a composite structure from construction through flight. Other applications include surface habitats, commercial airlines, DOD aircraft, bridges and buildings.
It is proposed to 3-D print SHM sensors for use in composite structures. The first step will be to disperse carbon nanotubes and PVDF-TrFE polymer in dimethyl formamide (DMF) solvent. Dispersion will be aided by the appropriate dispersant. This solution will be thoroughly mixed at 60C until the carbon nanotubes are totally dispersed. Next lead zirconium titanate nanopowders will be added and mixed until a uniform mixture is obtained. Lead zirconium titanate had the highest piezoelectric response of any material. When cured PVDF-TrFE forms a piezoelectric beta phase. In this composite sensor, the PVDF-TrFE and lead zirconium titanate yield the piezoelectric properties while the carbon nanotubes add structural strength. The random distribution of carbon nanotubes and lead zirconium titanate will ensure a flexible structure. On a previous CIF the Pl found adding barium titanate nanopowders to PVDF-TrFE increased the piezoelectric response of the composite material. Once a suitable viscosity is obtained for the solution{~ 20,000 cP) sensors will be 3-D printed using our nScrypt 3-D printer.
The sensors will be printed on glass plates, then cured in a vacuum furnace at 150C. We will print thicknesses from 10 microns up to 500 microns. Our printer had the capability to print up to an area of 1 square foot, so we have latitude in the size of sensors we will be able to print. The piezoelectric effect will be tested at AMRDEC's Weapon's Laboratory. They will first be corona poled in order to align the dipoles in the PVDF-TrFE, then hysteresis curves will be generated from which piezoelectric constants can be obtained. Then a formulation, which yields a consistent piezoelectric response, sensors, will be produced which can be attached to the surface of a composite and also between plies. In both cases, composite bend specimens will be produced, the sensor applied, the testing in bending mode. While the bending test is ongoing, voltage response will be measured in order to generate a response curve of voltage versus stress. Finally, a design for wireless antennae will be produced. This antenna can be directly printed on the sensor yielding a SHM sensor, which can be used from manufacture through flight. As a preliminary to the proposed research we have 3-D printed a sensor of 2 weight percent carbon nanotubes in PVDF-TrFE polymer.
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