Advanced composite materials are increasingly replacing metals in the aerospace industry as they offer weight-saving improvements such as high specific strength and stiffness, while providing resistance to fatigue and corrosion. Traditional advanced composites, however, exhibit significantly reduced electrical and thermal conductivity relative to metals, and matrix-rich regions at ply interfaces result in relatively poor interlaminar properties. Additionally, composites that have sustained damage often have non-visible or barely visible damage, complicating damage assessment. Recent efforts to address the limitations of advanced composites include the incorporation of carbon nanotubes (CNTs) to take advantage of intrinsic and scale-dependent properties of these nanostructures.
Aligned carbon nanotubes (CNTs) are being investigated as a means for enhancing structural performance of composite structures. Inherent in introducing CNTs into existing polymer-matrix composites are new multifunctional attributes such as significantly enhanced electrical conductivity and piezoresistivity that may be used for damage sensing and inspection. Here, fiber-reinforced polymer-matrix laminates with aligned CNTs grown in-situ are coupled with a non-invasive sensing scheme utilizing the enhanced electrical conductivity of the laminates to infer damage based on resistance changes. The laminates contain long (~10 micron) aligned CNTs throughout the woven plies of the laminate, including at the ply interfaces. Electrodes are written onto the laminate surfaces using a direct-write process, and 3D damage inspection (in-plane and through-thickness) is demonstrated for impacted composite plates.
This sensing system can potentially be integrated into an overall integrated vehicle health management (IVHM) approach as in situ structural health monitoring (SHM). Such a concept is investigated in this work and the basic principles demonstrated, including both in-plane and through-thickness damage sensing, giving a non-destructive tomographic view of the three-dimensional damage state.
The present investigation built on prior work to instrument nano-engineered laminates with a non-invasive silver-ink electrode grid and multiplexing micro-switches connected to compact hardware for through-thickness resistance measurement. The painted electrode grid, inspired by flat-panel liquid crystal display (LCD) technology, uses an “active” layer of electrode columns on one surface of the laminate as positive electrode, and on the other surface, another layer of electrode rows will act as “passive” ground. Thus, by selecting a particular row and column, local through-thickness resistance measurements can be obtained for a grid of points over the structure. Furthermore, in-plane surface resistivity changes at a grid point can be obtained by probing resistance between two adjacent trace pairs closest to the point on either surface. As one would intuitively expect, damage in the form of delaminations or cracks will affect the CNT conductive network around the affected zone in the structure, and correspondingly, the local through-thickness (and in-plane) resistivity. Using the electrode grid and interpolating from the measurements at distinct grid points, high-resolution resistance maps over large structural areas can be obtained, allowing accurate localization of damage.
Both in-plane and through-thickness electrical resistance measurements were collected. Clear changes were observed in both sets of data for grid lines close to the impacted zone of the specimen, demonstrating that these parameters were sensitive to damage in the structure. The peak resistance changes were close to the center of the specimen near the impact site, and little to no change was observed in values at points away from the damage zone. Overall, the subsurface impact damage caused significant electrical resistance changes, allowing for a full-field representation of the damage locations by interpolating the collected data.
This work was done by Sunny S. Wicks, Roberto Guzman deVilloria, and Brian L. Wardle of the Massachusetts Institute of Technology; and Ajay Raghava and Seth Kessler of the Metis Design Corporation. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Materials category. MIT-0002
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Tomographic Electrical Resistance-Based Damage Sensing in Nano-Engineered Composite Structures
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