Author

Suma Ayyagari

Date of Award

8-2021

Document Type

Dissertation - Open Access

Degree Name

Doctor of Philosophy in Aerospace Engineering

Department

Aerospace Engineering

Committee Chair

Dr. Marwan Al-Haik

First Committee Member

Dr. Sirish Namilae

Second Committee Member

Dr. Ali Yeilaghi Tamijani

Third Committee Member

Dr. Daewon Kim

Abstract

In this dissertation, an effort was carried out to enhance the mechanical performance of fiber reinforced composites (FRPs) by modification of the fibers’ surface morphologies. The effects of various surface alterations of a plain-woven carbon fiber fabric surface, on the fiber/epoxy interface were investigated. The alterations were mostly achieved by growing different nanofillers like zinc oxide nanorods (ZnO NR), carbon nanotubes (CNTs), and metal organic frameworks (MOFs) on the fiber surface. While the growth techniques for ZnO NR and CNTs place restrictions on the size of the fabricated composites, MOFs route is uniform, affordable, and can be readily scaled up to any required size. This makes this method more feasible and versatile than any other nanofillers. This dissertation comprises three major investigations: Study of hybrid composites with zinc oxide nanorods based surface modifications; Study of hybrid composites with carbon nanotubes-based surface modifications; and Study of hybrid composites with metal organic framework-based surface modifications. The growth of zinc oxide nanorods on carbon fiber surface was performed with a combination of physical vapor deposition (PVD) and hydrothermal growth techniques. Various configurations of composites were fabricated to study the effects of altering the topology of the nanorods and functionalizing the fiber surface with polydopamine. Mechanical analysis showed a significant improvement in strength and stiffness in samples with patterning, when compared to samples with uniform nanofiller growth. Damping results show that polydopamine improved the adhesion between the carbon fiber and zinc oxide nanofillers, thus increasing the glass transition temperature of the composites. A molecular dynamics (MD) model was built to study this phenomenon in an atomic scale. The results showed a significant improvement in Young’s, bulk, and shear moduli as a consequence of the addition of zinc oxide nanorods.

Various topologies of CNTs were synthesized using physical vapor deposition and graphitic structures by design (GSD) techniques on the carbon fiber surface. Mechanical characterization of these hybrid composites was performed via tensile testing, dynamic mechanical analysis (DMA), and fracture analysis. A study of viscoplastic behavior of these composites using stress relaxation and creep tests was also performed for these different composites’ configurations. A phenomenological viscoplastic model was utilized for creep prediction over long periods of time utilizing shorter stress relaxation tests. The results showed that with proper geometrical patterning of CNTs, significant improvements in strength, stiffness, creep and relaxation resistance, and glass transition temperatures can be realized.

A comprehensive study on fracture analysis depicting delamination was performed using crack propagation experiments and ANSYS simulations for composites based on CNTs/carbon fibers hybrid reinforcements. ANSYS simulations employed both cohesive zone modeling (CZM) and virtual crack closure (VCCT) technique to model interlaminar delamination. The results in all the three setups showed that coarser patterns of carbon nanotubes on the carbon fiber surface perform better in resisting crack propagation due to more efficient energy dissipation mechanisms at the fiber/matrix interface.

Finally, an investigation on multiscale hybrid composites with nickel-based MOFs as an interface was conducted. Growing MOFs require both de-sizing and acid etching of the fibers. Acid activation made the carbon fiber surface chemically active, furnishing better adherence between the MOFs and the carbon fibers. These MOFs were also utilized as a catalyst to grow carbon nanotubes on the carbon fiber surface; hence, replacing the unscalable PVD. The hybrid composites based on MOFs as reinforcements at the fiber/matrix interface exhibited improvements in tensile and shear strength and enhanced the damping parameters for the composite.

In summary, all the investigations in this dissertation conclude that tailoring carbon fiber surface using various nanofillers, and functionalization play a crucial role in shaping the interlaminar strength of carbon fiber polymer composites among other mechanical enhancements.

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