Nathaniel Chisholm, Ph.D. (2006)

Dissertation : 
Synthesis, Manufacturing and Characterization of Nanophased Carbon/Epoxy Composites

Major Professor:  Dr. Hassan Mahfuz, Professor of Mechanical Engineering

M.S.:  Mechanical Engineering, Georgia Tech, Atlanta, GA

B.S.: Mechanical Engineering, Tuskegee University

Employment:  Dekalb County Public Works Department, Macon, GA

Dissertation Abstract:

The fundamentals of the development of structural nanocomposites lie in the interaction between the particle and the polymer. When the size of these particles is in the nanometer range, the number of atoms at the surface is significantly higher, which facilitates interaction between the polymer chain and the particles. Moreover, the polymer chain dimensions and the coil radii are all in the nanometer scale, which allows nanoparticles to be comfortably accommodated within a polymer, if they can be dispersed uniformly. Besides interaction, the close proximity of the particles to the polymer chain provides a sort of anchoring mechanism during the uncoiling of the chain in the event of application of load. Together, these two mechanisms attribute a significant improvement in the mechanical and thermal properties of the polymer. While the technique of particle infusion into polymers has been investigated quite extensively by many researchers, the actual application of these modified polymers to develop structural composites remains relatively unexplored. In this dissertation, an attempt was made to fabricate nanocomposites by using reinforcing carbon fibers in a resin, which was modified by infusing nanoparticles. The intent was to see whether the benefits of nanopartic1e infusion were carried over when the polymer was in structural form in conjunction with fibers. This necessitated a set of studies that formed the body of this dissertation.

A series of basic studies were conducted to investigate the morphological changes in the polymer due to nanopartic1e infusion. It was observed that incorporation of nanoparticles into SC-15 epoxy affected the rate of cure at the initial stage of gelation, controlled the crystallinity of the polymer, and significantly improved the thermal properties. Specific tests were then conducted to examine whether nanoparticle infusion would have any adverse effect on the fiber/matrix interface when the resin was reinforced with fibers. In the next step, since the results were quite positive from the initial tests, SC-15 was utilized in a Vacuum Assisted Resin Transfer Molding (V AR TM) process with plain weave carbon fibers to fabricate laminated composites. Mechanical tests on these nanocomposites have revealed that at both quasi-static and cyclic loading conditions the improvement in properties was consistently higher by 20-30% over the neat composites. The enhancement in properties was, however, very similar to what was observed earlier with the modified resins. The investigation then continued to study the high strain response. It was found that the nanophased composites retained their improved properties over the entire range of high strain rate loading except when the specimens were loaded in the fiber direction. Since delamination was the primary mode of failure when loading was in the fiber direction, the strength of the nanophased composites decreased sharply. This prompted an investigation of the delamination fracture toughness (G1) and stress intensity factor (K1) of the composite.  
Fracture toughness studies revealed that indeed there was a 25% reduction in the value of Gr. On the other hand, there was an increase in Kr by 30%.

The study demonstrated that nanophased structural composites made from nanoparticle modified resins are superior to their neat counterparts except when delamination is the predominant failure mode.