Introduction and Motivation

• The overall rating for safety of the bridges in the United States is C+ and one in nine of the nation’s bridges are rated as structurally deficient (2013 ASCE Report Card).
• Structural health monitoring (SHM) and Nondestructive Evaluation (NDE) are becoming viable tools to collect quantitative data from in-service structures concerning condition and performance.
• When carbon nanotubes are properly dispersed into the polymer matrix of a composite they form electrically percolating networks that can act as in situ strain and damage sensors.
• We developed a novel non-destructive evaluation (NDE) methodology for imaging of damage in composites using the electrical impedance tomography (EIT) imaging technique applied to a distributed carbon nanotube (CNT)-based composite sensor.

Carbon Nanotube-Based Sensing

Why Carbon Nanotube (CNT)-Based Sensing?
• Extremely high sensitivity to strain
• Reliable performance (redundancy through network)
• Real-time and distributed sensing capability
• SHM and NDT (non-destructive testing) applications
Mechanism of CNT-based sensing:
• Electrically percolating networks
• Piezoresistive composite sensors
• In situ electrical resistance measurements

Selection of Carriers for CNT-Based Sensors

Why the carrier for CNTs is important?
• Electrical tunneling and tip-contact between neighboring CNTs
• Geometrical change of CNT conductive network
Nonwoven fabrics: plane sheets consisting of randomly distributed and oriented short fibers:
• Mechanically isotropic all in-plane directions
• Large void ratio, high resin intake

Two-Step Self-Assembly Technique: The "Wetting"

1st Step --
The “Wetting” Process: depositing CNTs on carrier fabric
1. Carrier fabric trimming
2. CNT-aqueous solution preparation
3. Full bath impregnation

2nd Step --
The “Drying” Process: finalizing composite sensing layers using Vacuum Assisted Resin Transfer Molding) VARTM Manufacturing

Electrical Impedence Tomography

The 2D CNT-Based Sensor

CNT-based nonwoven fabric composite
32-electrode EIT sensing scheme
3.2 by 3.2 mm silver painted electrode pads
Two lead wires at each electrode: one for current injection and the other for measurements