Engin C. Sengezer

Experimental Characterization of Damage Evolution in Carbon Nanotube-Polymer Nanocomposites

Dielectrophoresis under the application of AC electric fields is one of the primary fabrication techniques for obtaining aligned carbon nanotube (CNT)-polymer nanocomposites, and is used here to generate long range structure/architecture control at the structural level. As such, effort here is focused towards characterizing the piezoresistive response of this long range structure/architecture at a concentration below percolation threshold. In this study, the electrical conductivity and the piezoresistive behavior of randomly oriented, well dispersed and long range electric field aligned singlewall nanotubes (SWNTs) in a photopolymerizable monomer blend (urethane dimethacrylate (UDMA) and 1,6-hexanediol dimethacrylate (HDDMA)) are evaluated. The electrical resistance measurements demonstrated nanocomposite strain sensing potential and were able to provide continuous assessment of damage evolution. Gauge factors both in axial and transverse direction are measured for specimens with electric field aligned 0.03 wt% SWNTs and randomly oriented, well dispersed 0.030.1 wt% SWNTs under quasi static and cyclic tensile loading. Excellent piezoresistive capabilities having the potential to provide real-time structural health monitoring (SHM) and selfdiagnostic functionalities are obtained at a concentration below percolation threshold for aligned specimens with SWNTs loading as low as 0.03 wt%.

Structural Health Monitoring of Nanocomposite Bonded Energetic Materials Through Piezoresistive Response

The application of in situ structural health monitoring in polymer bonded explosive materials through the introduction of carbon nanotubes into the binder phase is investigated through piezoresisti...

Phenomenological Characterization of the Fabrication of Aligned Carbon Nanotube Nanocomposites via Dielectrophoresis Under AC Electric Field

Here we use dielectrophoresis under the application of AC electric fields as the primary fabrication technique (DEPFT) for aligned carbon nanotube (CNT)-polymer nanocomposites, and generate data sets in order to develop DEPFT fabrication models for the dispersion and orientation of CNTs. While the general understanding of how CNTs form aligned filaments under the influence of the dielectrophoretic forces and moments is well established, detailed multi-CNT-filament formation predictions of microstructure evolution from a random dispersion into this ordered structure remain intractable. As such we devote considerable efforts on characterizing the processstructure-property relationships towards the development of phenomenological fabrication models for controlling local CNT dispersion and orientation as a function of applied electric field magnitude, frequency, and exposure time. In this study, 0.03 wt % SWNTs were dispersed in a photopolymerizable monomer blend (urethane dimethacrylate (UDMA) and 1,6-hexanediol dimethacrylate (HDDMA)) using ultrasonication techniques to obtain the acrylate solution (0.03 % SWNTs / UDMA /HDDMA(9/1) solution). SWNT alignment was controlled and the SWNT-filament thickness formation was explored. In order to assess key morphological features of the asproduced SWNT-acrylate nanocomposite samples such as SWNT distribution and filament thicknesses, transmission optical microscopy has been used to observe the SWNT alignment and filament formation obtained in the resulting nanocomposite samples by digital mapping of individual overlapping images, respectively. Thus, sufficient amounts of data to construct statistically meaningful distribution functions for morphological features were obtained. The electrical properties of the as-produced SWNTs-acrylate nanocomposite samples were measured to connect the resulting structures to their respective properties.

Real time in-situ sensing of damage evolution in nanocomposite bonded surrogate energetic materials

The current work aims to explore the potential for in-situ structural health monitoring in polymer bonded energetic materials through the introduction of carbon nanotubes (CNTs) into the binder phase as a means to establish a significant piezoresistive response through the resulting nanocomposite binder. The experimental effort herein is focused towards electro-mechanical characterization of surrogate materials in place of actual energetic (explosive) materials in order to provide proof of concept for the strain and damage sensing. The electrical conductivity and the piezoresistive behavior of samples containing randomly oriented MWCNTs introduced into the epoxy (EPON 862) binder of 70 wt% ammonium perchlorate-epoxy hybrid composites are quantitatively and qualitatively evaluated. Brittle failure going through linear elastic behavior, formation of microcracks leading to reduction in composite load carrying capacity and finally macrocracks resulting in eventual failure are observed in the mechanical response of MWNT-ammonium perchlorateepoxy hybrid composites. Incorporating MWNTs into local polymer binder improves the effective stiffness about 40% compared to neat ammonium perchlorate-polymer samples. The real time in-situ relative change in resistance for MWNT hybrid composites was detected with the applied strains through piezoresistive response.

Computational modeling and experimental characterization of macroscale piezoresistivity in aligned carbon nanotube and fuzzy fiber nanocomposites

In this study, a multiscale computational micromechanics based approach is developed to study the effect of applied strains on the effective macroscale piezoresistivity of carbon nanotube (CNT)-polymer and fuzzy fiber-polymer nanocomposites. The computational models developed in this study allow for electron hopping and inherent CNT piezoresistivity at the nanoscale in addition to interfacial damage at the CNT-polymer interface. The CNT-polymer nanocomposite is studied at the nanoscale allowing for interfacial damage at the CNT-polymer interface using electromechanical cohesive zones. For fuzzy fiberpolymer nanocomposites, a 3-scale computational model is developed allowing for concurrent coupling of the microscale and nanoscale. The electromechanical boundary value problem is solved using finite elements at each of the scales and the effective electrostatic properties are obtained by using electrostatic energy equivalence. The effective electrostatic properties are used to evaluate the relative change in effective resistivity and the macroscale effective gauge factors for the nanocomposites. In addition, the piezoresistive response of aligned CNT-polymer and fuzzy fiber-polymer nanocomposites is investigated experimentally. The results obtained from the computational models are compared to the experimentally observed change in resistance with applied strains and associated gauge factors.

In-Situ Sensing of Deformation and Damage in Nanocomposite Bonded Surrogate Energetic Materials

The current work aims to explore the potential for in-situ structural health monitoring in polymer bonded energetic materials through the introduction of carbon nanotubes (CNTs) into the binder phase as a means to establish a significant piezoresistive response through the resulting nanocomposite binder. The experimental effort herein is focused towards electro-mechanical characterization of surrogate materials in place of actual energetic (explosive) materials in order to provide proof of concept for the strain and damage sensing. The electrical conductivity and the piezoresistive behavior of samples containing randomly oriented MWNTs introduced into the epoxy (EPON 862) binder of 70 wt% ammonium perchlorate-epoxy hybrid composites are quantitatively and qualitatively evaluated. Brittle failure going through linear elastic behavior, formation of microcracks leading to reduction in composite load carrying capacity and finally macrocracks resulting in eventual failure are observed in the mechanical response of MWNT-ammonium perchlorate-epoxy hybrid composites. Incorporating MWNTs into local polymer binder improves the effective stiffness about 42 % compared to neat ammonium perchlorate-polymer samples. The real time in-situ relative change in resistance for MWNT hybrid composites captured low values of applied strains less than 0.2 %.

Experimental Characterization of Strain and Damage Evolution in Carbon Nanotube-Polymer Nanocomposites

An experimental characterization of nanocomposite strain and damage sensing in support of development of CNT-polymer nanocomposites for structural health monitoring (SHM) applications was conducted. As such, effort here is focused towards examining the piezoresistive behavior of poly(dimethyl-siloxane) (PDMS) and epoxy filled with acid treated single walled carbon nanotubes (COOH-SWNTs) under quasi-static compression and tension. Precision LCR Meter with two terminal method, in conjunction with mechanical testing and data acquisition system were used to measure instantaneous resistance values. Given the emphasis on SHM applications which correlate changes in electrical resistivity to deformation and damage, 0.1 wt% COOH-SWCNTs concentration below the nanocomposite electrical percolation threshold was considered for PDMS and epoxy. Measurements confirmed the onset of damage prior to noticeable effects in the stress-strain response. The resistance measurements were able to both detect strain for PDMS nanocomposites and damage initiation and provide continuous assessment of the damage state between damage initiation events for epoxy nanocomposites. Digital Image Correlation (DIC) System was used to observe the crack propagation on notched epoxy nanocomposite compact tension samples.Copyright

Real Time In-Situ Sensing of Damage Evolution in Carbon Nanotube-Polymer Nanocomposites under Impact Loading

The current work aims to explore the potential for in-situ structural health monitoring in polymer bonded energetic materials through the introduction of carbon nanotubes (CNTs) into the binder phase as a means to establish a significant piezoresistive response through the resulting nanocomposite binder. The experimental effort herein is focused towards electro-mechanical characterization of surrogate materials in place of actual energetic (explosive) materials in order to provide proof of concept for the strain and damage sensing. The electrical conductivity and the piezoresistive behavior of samples containing randomly oriented, well dispersed MWNTs at concentrations of 0.09-0.6 wt% introduced into the epoxy binder of 70 wt% granulated sugar-epoxy hybrid composites are quantitatively and qualitatively evaluated. Ductile failure behavior going through the initial linear elastic behavior, formation of microcracks leading to reduction in composite stiffness and finally macrocracks result in eventual failure were observed in the mechanical response of MWNT-sugar-epoxy hybrid composites. The real time in-situ relative change in resistance captured the effect of microcracks and macrocracks earlier than the stress strain response resulting in gauge factors between 5-10 before significant macrocrack formation and over 50 at composite failure.