Modeling and characterization of the mechanical and damping response of carbon nanotube nanocomposites

"Modeling and Characterization of the Mechanical and Damping Response of Carbon Nanotube Nanocomposites" ABSTRACT: Multifunctionality is a current trend in material design. The fast growing needs of industries are challenging the design of structures made of advanced lightweight composites that possess the capability of performing multiple functions. The superior mechanical properties of carbon nanotubes (CNTs) - besides the excellent electrical and thermal properties - make them ideal candidates to be used as reinforcement llers in composite materials. CNT nanocomposites, made of suitable polymeric matrices lled with carbon nanotubes, have shown enhanced mechanical and electrical features with an additional extraordinary feature, namely, a high structural damping capacity. The main objective of this work is to explore the mechanical and damping response of CNT nanocomposites, aiming at reaching a better understanding of the macroscopic behavior of the nanocomposite materials by taking into account their complex micro/nanostructural features. The practical goal is to effectively explore the potential exploitation of these nanostructured materials in demanding structural applications. In order to investigate and optimize not only the mechanical properties but also the damping capacity of CNT/polymer composites, a specic analytical model, based on the Eshelby and Mory-Tanaka approaches, is here presented. The proposed model is an effective tool for predicting nonlinear stress-strain curves, energy dissipation mechanisms and hysteresis of nanocomposite materials. A great deal of studies were conducted on the ability of CNT-reinforced materials to absorb vibrations and noise (damping capacity), analyzing the orientation, dispersion and aspect ratio of CNTs as main parameters that affect the mechanical and damping properties. As a step forward from the current state of the art, the present work suggests an innovative theoretical method to describe the macroscopic response of nanocomposites and explore energy dissipation mechanisms arising from the shear slippage of nanotubes within the hosting matrix. The major mechanism through which energy is dissipated, the stick-slip mechanism, can be properly treated by introducing a plastic eigenstrain in the CNT inclusions whose evolutive law is accordingly shaped after the physical phenomenology. A set of numerical tests are performed to estimate the elastic properties and the nonlinear response of nanocomposites, characterizing the hysteresis loops in the stress-strain curves. Parametric studies are conducted to investigate the in uence of the main constitutive parameters of the model on the mechanical response including the damping capacity. The numerical simulations revealed that the interfacial shear strength, the CNT volume fraction, the exponent of the evolution law for the plastic eigenstrain, as well as the strain amplitude, have a signicant effect on the hysteresis of CNT nanocomposites. Moreover, it is shown that an optimal combination of these micro-structural parameters can be achieved via differential evolutionary algorithms that allow to maximize the damping capacity, while preserving the high elastic properties of the nanostructured materials. Such approach further enables the calibration of the model and design the nanomaterial in order to provide an effective response according to the structural vibration control requirements and high mechanical performance goals. The validation of the effectiveness of the predictive computational tool, together with its theoretical framework, is also sought via an ad hoc experimental approach. The experimental campaign featuring mechanical tests on a variety of CNT nanocomposite materials was indeed a fundamental step towards the renement of the model and a reasonable tuning of the model parameters. In addition, a morphology investigation of the prepared CNT/polymer composites was a decisive step to dene the microstructural properties. The experimental activities highlighted and conrmed the relevance of several morphological aspects, such as the actual CNT aspect ratio variability within the nanocomposite, the CNT dispersion and agglomeration degree and the polymer matrix chemical structure, to mention but a few. Those results shed light to which nanocomposites constituents features can influence the macroscopic response of the material. The conducted experimental work aimed also at identifying and introducing parameters that can better enhance the nanocomposite mechanical and damping behavior, by investigating also aspects of the fabrication processes that can help improve the CNT dispersion or the CNT adhesion like, for instance, the CNT functionalization. These experimental findings allowed a final model update by overcoming the main limitations, generally present in the most common micromechanical theories for multi-phase materials, i.e., (i) the perfect nanoller dispersion and distribution in the surrounding matrix, and (ii) the perfect interfacial adhesion between the carbon nanotubes and polymer chains.