Development of Multifunctional Carbon Nanotube Nanocomposite Sensors for Structural Health Monitoring.

Abstract

The United States is currently facing a national crisis with many of its vital civil infrastructures deteriorating at an alarming rate. If left unaddressed, catastrophic structural failures like that witnessed from the Minneapolis’ I-35 bridge collapse (2007) will occur more frequently. Unfortunately, current structural health monitoring (SHM) technologies available are inadequate for detecting structural distress since (1) “point sensors” only measure localized structural response, and (2) damage is inferred and estimated using complex algorithms. Thus, this dissertation fundamentally solves this technology bottleneck by engineering a next-generation multifunctional material that serve as distributed sensors capable of sensing damage directly.

Specifically, single-walled carbon nanotubes (SWNT) and polyelectrolytes (PE) are molecularly manipulated to self-assemble nanocomposites with superior mechanical and electrical properties. Here, SWNTs are employed for intentionally tailoring thin film tensile strength and stiffness, bulk conductivity, and electromechanical/electrochemical responses. First, the mechanical properties of SWNT-PE composites are characterized through extensive load testing. Precisely controlling SWNT assembly yields nanocomposites of high tensile strength (i.e., ~250 MPa). Second, the nanocomposite’s piezoresistivity are explored via both time- and frequency-domain methods for deriving an equivalent parallel RC-circuit model that models thin film piezoresistivity and facilitates optimization of nanocomposites strain sensitivities (i.e., up to 7% change in conductivity per unit strain).

Then, two different applications of SWNT-PE nanocomposites for SHM are explored. First, the equivalent circuit derived is employed for designing miniaturized passive wireless strain and pH sensors. Upon patterning the nanocomposite to also serve as inductive coil antennas, wireless interrogation of sensor responses is accomplished. These passive wireless sensors do not require portable power supplies (e.g., batteries) and can operate indefinitely in the field. Second, a “sensing skin” is designed to provide two-dimensional maps of strain, crack damage, and corrosion. Mapping of composite electrical conductivity changes due to structural damage is performed using electrical impedance tomography. Unlike traditional SHM systems, these sensing skins can directly determine structural damage location and severity (i.e., due to strain, impact, pH, and corrosion) by monitoring the nanocomposite’s spatial conductivity changes due to damage. The ability to accurately detect damage location and severity facilitates efforts to prevent future catastrophic failures from occurring.