Roy M. Sullivan

Reinforcing polymer cross-linked aerogels with carbon nanofibers

We have previously reported cross-linking the mesoporous silica structure of aerogels with di-isocyanates, styrenes or epoxies reacted with amine decorated silica surfaces. These approaches have been shown to significantly increase the strength of aerogels with only a small effect on density or porosity. Herein, we examine the effect of including up to 5% (w/w) carbon nanofibers in the silica backbone before cross-linking. The addition of 5% carbon nanofibers to the lowest density aerogels studied triples the compressive modulus and the tensile stress at break is increased five-fold with no density penalty. The carbon fiber also improves the strength of the initial hydrogels before cross-linking, which may have implications in manufacturing.

Development of Design Analysis Methods for Carbon Silicon Carbide Composite Structures

The stress—strain behavior at room temperature and at 1100° C (2000°F) is measured for two carbon fiber-reinforced silicon carbide (C/SiC) composite materials: a two dimensional (2D) plain-weave quasi-isotropic laminate and a 3D angle interlock woven composite. Previously developed micromechanics-based material models are calibrated by correlating the predicted material property values with the measured values. Four-point beam-bending subelement specimens are fabricated with these two fiber architectures and four-point bending tests are performed at room temperature and at 1100°C. Displacements and strains are measured at the mid-span of the beam and recorded as a function of load magnitude. The calibrated material models are used in concert with a nonlinear finite-element solution using ABAQUS to simulate the structural response of the two materials in the four-point beam bending tests. The structural response predicted by the nonlinear analysis method compared favorably with the measured response for both materials and both test temperatures. Results show that the material models scale-up fairly well from coupons to subcomponent level.

Effect of Time at Temperature on the Ply-Normal Modulus of Carbon Phenolic

The effect of heating rate and time at temperature on the ply-normal tensile modulus of carbon cloth phenolic is investigated. The results of previous experimental studies reveal that slower heating rates and longer times at temperature result in a higher tensile modulus. It is speculated that the heating rate effect is due to a combination of moisture diffusion and the effect ofwater on the glass transition of thephenolic polymer.An equation is proposed that defines the value of the ply-normal modulus as a function of temperature andmoisture content. Numerical solutions for moisture diffusion in the ply-normal tensile specimen are performed, and the equation for the ply-normal modulus is applied to calculate themodulus as a function of temperature and heating rate. The numerical results are successful in simulating themeasured effect of heating rate and time at temperature on the ply-normalmodulus. The validity of the supposition that the heating rate and time at temperature effect is due to the combination of moisture diffusion and plasticization of the phenolic polymer is demonstrated.