Center for Composite Materials - University of Delaware

Research Summary

Carbon Nanotube Integrated Multifunctional Composites for In Situ Structural Health Monitoring Application

Authors: Hao Liu (PhDME), Dirk Heider, and Erik T. Thostenson

Introduction

Carbon nanotubes (CNTs) have gained extensive research due to high specific stiffness, strength and conductivity, combined with the exceptionally large aspect ratio.
Integrated CNTs offer the potential to detect strain due to the change in the electrical properties of CNTs under applied stress.
CNT sheets offer the potential to integrate damage sensing technology in layup prepreg-based composites.

Objectives

Integration of carbon nanotube (CNT) in situ strain sensors between layers in a fiber composite and establish their electrical-mechanical response.
Examination of the negative electrical piezoresistivity of laminated carbon nanotube sheet / glass Fiber Composites.
Characterization of the CNT sheet.
Exploration of the effects of thermal processing and manufacturing on the sensing behavior.

Materials

- Glass/Epoxy prepregs (CYCOM 7668)
- Conductive Epoxy adhesive (40-3900RC/RS101)
- FR-4 glass/epoxy phenolic
- CNT sheets from Nanocomp Technologies, Inc.

Manufacturing

Mechanical and Electrical Characterization

1. Four-point bending tests were conducted to evaluate the sensor response.
2. Instron 4434 was employed to apply the sensors in both tension and compression.
3. The region between the center load supports has constant moment and by flipping the specimen over the full tensile and compressive response of the sensor.
4. Resistive strain gages (Vishay Micro-Measurements®) were used to measure the strain during tests.
5. The electrical resistance of the CNT sheet in the longitudinal direction was measured during loading by an in-house developed LabView® based data acquisition system that was designed for recording strain, load, voltage, and electrical resistance data every 0.1 s.

Loading and Sensing Behavior

Three distinct locations: linear negative piezoresistivity under compression, nonlinear transition from negative to positive piezoresistivity under tension with a local minimum occurring, and linear positive piezoresistivity under tension.

After post-cure there is a much stronger negative piezoresistive response in tension. The crossover between the negative and the positive piezoresistivity occurs at a much higher strain and the local minimum of the resistance change is increased.

Acknowledgements

This research is supported by the National Science Foundation NUE program Grant No. (1138182), Dr. Mary Poats, Program Director.

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