Nanocomposites are prominent candidates to extend the capabilities in areas of established fiber reinforced composites. Carbon nanotubes (CNT) with their outstanding mechanical, electrical, and thermal properties are of particular interest. Especially polymers profit from the addition of CNTs, and can be rendered significantly stiffer, stronger, and even electrically conductive. The resulting electrical conductance is deformation-sensitive, known as piezoresistivity, and is utilized in strain sensing applications. However, the polymer matrix introduces time- and temperature-dependency into the mechanical behavior, known as viscoelasticity, and thus affects the relationship between deformation and electrical conductivity over time. Although piezoresistivity and polymer viscoelasticity have been studied separately, the interaction of both phenomena is not well understood. This thesis presents a combination of numerical, experimental, and analytical investigations of the behavior of viscoelastic, piezoresistive nanocomposites. The major goal of this research is to elucidate the underlying mechanism of viscoelasticity on strain sensing via piezoresistivity, without relying on the ill-defined viscoelastic Poisson's ratio. In the study of piezoresistive nanocomposites, a statistical, three-dimensional representative volume element is created first via the finite element method, and validated through fundamental quantities, such as total conductance and elastic piezoresistivity, against experimental data from literature. A novel electron tunneling model is proposed, incorporating the chirality of individual CNTs with regard to the local alignment between CNTs. The change of tunneling resistance via mere reorientation of the CNTs is identified as another source of bulk resistance change under deformation and lead to an increase in the previously underestimated numerical strain-sensitivity. The multiaxial viscoelasticity is characterized via uniaxial creep tests at elevated temperatures, simultaneous measurement of axial and transverse strain, and the time-temperature superposition principle. A non-constant electrical resistance is observed numerically during stress relaxation at constant axial strain. The loss of sensing repeatability is shown in a cyclic numerical simulation.
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Doctor of Philosophy (Ph.D.)
College of Engineering and Computer Science
Mechanical and Aerospace Engineering
Length of Campus-only Access
Doctoral Dissertation (Open Access)
Klimm, Wolfgang, "Influence of Nonlinear Multiaxial Matrix Viscoelasticity on Piezoresistivity of Carbon Nanotube Polymer Composites" (2022). Electronic Theses and Dissertations, 2020-. 1234.