Health Monitoring of Aligned Carbon Nanotube ( CNT ) Enhanced Composites

Hybrid advanced composites, termed nano-engineered composites, have been developed that are comprised of aligned carbon nanotubes (CNTs), epoxy resin, and advanced fibers. While the CNTs occupy ~1% of the volume of these bulk nanostructured composites, due to alignment and distribution, significant strength and toughness enhancement has been observed (e.g., more than 100% increase in interlaminar fracture toughness). A multifunctional aspect of these materials is electrical conductivity that is enhanced by many orders of magnitude (10 and 10, in-plane and through thickness, respectively). This multifunctional aspect is utilized to solve challenges in a resistive structural health monitoring (SHM) sensing concept. Composites present significant challenges for inspection due to their heterogeneity and anisotropy, the fact they fail by interacting modes, and since damage often occurs beneath their surface. Current effective laboratory non-destructive methods, such as X-ray and C-scan, are impractical for inspection of large integrated structures. Resistive SHM methods have been investigated previously, however they are hindered by small measurements (high resistivity giving rise to high noise-to-signal) and interconnection issues. Both issues may be addressed by the increased conductivity in the nanoengineered composites and by creating electrical break-out connections through 3-D leads brought to the surface. Undamaged and damaged 2-ply ‘fuzzy-fiber’ composites are inspected using a simple surface-mounted electrode configuration, and sensitivity to damage (both in-plane and through-thickness) is demonstrated. Future work includes improvements in sensor-structure integration and evaluation of strength-after-impact via impact and subsequent tensile testing.