Electric-field-induced Alignment of Carbon Nanofillers in Multifunctional Glass-epoxy Composites

Open Access
Author:
Gungor, Sila
Graduate Program:
Engineering Science and Mechanics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 14, 2013
Committee Members:
  • Charles E Bakis, Dissertation Advisor
  • Charles E Bakis, Committee Chair
  • Clifford Jesse Lissenden Iii, Committee Member
  • Kevin L Koudela, Committee Member
  • James Hansell Adair, Committee Member
Keywords:
  • composites
  • carbon nanotubes
  • carbon black
  • electrical anisotropy
  • damage detection
  • glass fiber reinforced plastics
Abstract:
Advancing the methods for structural health monitoring (SHM) of composites and controlling the material properties for adapting to existent SHM methods is important for continuous examination of damage in composites. This investigation aims to address these problems by tailoring the electrical properties of glass-epoxy composites by the addition and electric-field-induced alignment of carbon nanofillers in order to develop a new type of material, which is a suitable candidate for damage detection using electrical resistance method. As carbon nanofillers, short unfunctionalized carbon nanotubes (CNTs) and carbon black (CB) nanoparticles were used due to their low aspect ratio, which is believed to ease their motion between the glass fibers. Short CNTs and CB were dispersed in a bisphenol-A based epoxide diluted with alkyl glycidyl ether by using bath sonication and stirring methods. Short CNT-filled epoxy and CB-filled epoxy composites were manufactured with and without electric-field-induced alignment of nanofillers due to dielectrophoresis. In addition to nanofilled-epoxy composites, random and aligned short CNT-filled and CB-filled glass-epoxy composites were also manufactured by using hand lay-up and vacuum bagging techniques. Significant increases were obtained in the electrical conductivity through the thickness of the composites filled with CB by aligning the CB nanoparticles through the thickness. It was further observed that, for a given concentration of nanofiller, higher electrical conductivities are obtained in aligned CB-filled glass-epoxy composites than in aligned short CNT-filled glass-epoxy composites, which turned the focus of this investigation on CBs rather than CNTs for damage detection studies. A parametric experimental investigation was carried out for CB-filled glass-epoxy composites by using different CB concentrations, applied AC field strengths and frequencies. Studies showed that increasing CB concentration and AC field strength up to a point increases the electrical conductivity through the thickness while imposing minor effects on the in-plane conductivities, whereas frequency does not have a significant effect on the conductivities. With the use of a proper CB concentration, AC field strength and frequency, a material with an anisotropy ratio, defined as the ratio of through-thickness conductivity to in-plane transverse conductivity, equal to 3.3 was achieved for CB-filled unidirectional glass-epoxy composites. This is the first investigation which shows that through thickness conductivity can exceed in-plane transverse conductivity. Finite element analyses were conducted to map the place and the size of damage for four different cases. It was shown that perpendicular electrode configuration is better compared to the parallel electrode configuration and it is possible to detect multiple damage and distinguish different sized damage from each other. A graphic user interface code was created to graphically illustrate the place and the size of the damage by a 3D animation, which permits easier understanding of the nature of the damage. Damage detection capability of random and aligned CB-filled glass-epoxy composites by electrical resistance method was investigated using carbon fiber electrodes. It was shown that electrical resistance method is more sensitive for detecting damage in aligned CB-filled glass-epoxy composites compared to random ones. It was further shown that continuous carbon fibers placed on parallel surfaces of composites allow the detection of damage and perpendicular configuration locates damage more precisely than parallel configuration.