Mechanical characteristics of continuous carbon nanotube and continuously reinforced carbon nanotube composite
Date
2016
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
Due to the outstanding mechanical, thermal and electronic properties of carbon
nanotubes (CNTs), CNT and CNT reinforced polymer composite are becoming more
and more pervasive in engineering applications, especially in energy absorbing and
damping materials. Therefore, the underlying mechanism of the intriguing mechanical
properties of CNT arrays and CNT reinforced composites is an essential and
fundamental science for the potential applications of CNT related materials.
It is fundamental and critical to investigate the mechanical properties of CNTs first,
since the intrinsic properties and collective behavior of CNTs play an important role in
the mechanical response of composite. The buckling behavior of vertically aligned
carbon nanotubes (VACNT) was investigated. By taking van der Waals interactions
into account, both experiments and modeling results confirm that VACNTs buckle in
the bottom region with a high mode buckling, following wave damping effect. Then,
the compressive behavior of VACNTs was quantified by strain energy density
function. The effects of CNT structure/morphology, including diameter, cross section
area, moment of inertia, defect degree and density, on mechanical properties were
statistically investigated and compared with cellular materials, showing significant
influence on determining the mechanical properties of VACNTs. The focus of CNT polymer composites is on the application-oriented viscoelastic
properties. The static viscoelastic characterization was conducted by creep and stress
relaxation tests with stress/strain variation and quantified by nonlinear power-law
model. The dynamic properties were characterized by dynamic mechanical analysis
(DMA) with frequency variation. And CNTs show significant enhancement in elastic
response and considerable influence on viscous response. In addition, the temperature
effects were investigated and composites show better thermal stability. By using timetemperature
superposition (TTS) and Williams–Landel–Ferry (WLF) fitting, the
prediction scale of viscoelastic behavior in time/frequency range can be significant
enlarged. The viscoelastic responses are complicated by the intrinsic anisotropy of CNTs, so it is
also essential to study their anisotropic properties. The compressive and viscoelastic
characterization were performed on longitudinal, transverse and random composites
and compared with PDMS. The results confirm the exceptional reinforcement of
CNTs in longitudinal composites, which have lateral support from polymer matrix.
And the increased damping effects of composites can be explained by the interfacial
sliding and the energy dissipation between nanotubes and polymer matrix.
Furthermore, the fatigue tests of CNT polymer composites were performed to
investigate mechanical robustness and long-term stability. From the stress-number of
cycles (S-N) data in cyclic DMA tests, CNTs improved the fatigue life of composites
considerably, especially in high-cycle fatigue strength, caused by the hindering of
crack propagation from CNTs, the interface debonding and the CNT reinforcement
effects. Also, the microscopy images of fracture surfaces indicate different fatigue
resistance and different fracture/crack mechanism between longitudinal and transverse
composites.