Renee' Rodgers, Ph.D. (2007) 

Dissertation Topic:  Processing and Characterization of Nano Particle-Modified SiC/SC-15 Epoxy Composites:  Studies of Mechanical Properties at Room and Elevated Temperatures

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

B.S.:  Aerospace Science Engineering, Tuskegee University

Employment:  Senior Engineer, Raytheon Missile Systems, Tucson, AZ

Dissertation Abstract:

The idea behind this dissertation was to examine a particle-polymer system, namely, SiC/epoxy, and examine its mechanical response under various mechanical loading and temperatures up to the glass transition temperature (T g) of the polymer. As mentioned, the system of interest was silicon carbide (SiC) nanoparticles dispersed into SC-lS epoxy, a thermoset polymer. SiC was chosen for the excellent mechanical and thermal properties it exhibits. The first important step in modifying a matrix with nanoparticles is achieving a uniform dispersion of particles. Ultrasonic cavitations was used to disperse the 30 nm size SiC nanoparticles into part-A of the SC-15 epoxy. The study revealed that a particle loading of 1 wt.%, elevated Tg by 12°C while the thermal decomposition temperature increased 30°C. The flexural tests results revealed showing about 36% and 21 % increase in modulus and strength, respectively. The enhancement of the tensile performance was quite impressive. The fracture strain of the nanocomposites during tension was increased. by about 56%, while the strength increased by about 20%. The increase in the toughness of the nanocomposites is attributed to prolific crack growth which served as energy absorbing mechanisms during failure. The compressive tests did not reveal any improvements in modulus and strength at slow or high strain rates. Both flexure and tensile tests have showed that improvements 10 mechanical properties of nanocomposites are well maintained up to about 40-SSoC range. However, at temperatures approaching the Tg, of the resin, improvements in properties gradually faded. Micromechanics analysis of Young's modulus of nanocomposites was conducted.

The measured moduli of the nanocomposites were close to lower bound predictions. With nanoparticle infusion, we expected nanocomposite moduli to be close or even exceed the upper bound due to the enhanced interfacial interactions but the results suggest that such particle/matrix interactions did not greatly enhance stiffness, although, more importantly, the toughness was greatly elevated.

The component in which the engineer has the least knowledge. This lack of knowledge makes it difficult to develop finite element models for foam core sandwich composites. There are many issues that must be taken into consideration when modeling closed-cell foams. First, one needs to know the effects of various parameters, which may affect the compressive behavior, such as temperature, frequency, and perhaps cell-gas compression. Secondly, the model must include the deformation mechanisms that occur in practice. Thirdly, the model must consider the polymer mechanical properties.