Date of Award

2003

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science in Mechanical Engineering (MSME)

Department

Mechanical Engineering

Advisor

Michael L. Peterson

Second Committee Member

Donald A. Grant

Third Committee Member

Richard C. Hill

Abstract

Elastic properties are rarely sufficient in order to evaluate the condition of a composite material. Knowledge of the viscoelastic properties is very critical for design purposes, for they directly characterize damping. Damping related measurements in the material provides information about the degree of cross-linking and crystallinity of the polymer. For metals, damping may be related to dislocation motion among other characteristics. For composite materials, in general the interphase and the matrix dominate the damping of the material. Ultrasonic measurement of damping gives a non- destructive measure of strength of composite materials. This thesis considers damping characteristics of polymer matrix composites as well as reinforced carbon-carbon (RCC). These characteristics represent perhaps the best hope of developing a true index of state or damage tensor for composite materials. For polymer matrix composites the damping and elastic properties can be combined with either temperature or pressure to characterize the interatomic potentials in the matrix. This is the closest to a non- destructive measure of strength that is likely. By describing cross linkage and crystallinity this measure also provides insight into many of the most important degradation mechanisms. Part of this research looks into the recovery of stiffness tensor, material symmetry and principal axis. Damping is measured along this principal axis. It is also interesting to note that the damping is affected by oxidation, second part of this thesis looks into the oxidation effects of damping in RCC. Optimization routine is developed to solve Christoffel's equation to recover the material stiffness tensor. Method to determine the initial guesses for the optimization routine is also developed in this thesis. An ultrasonic microscope with immersion transducers is used to extract data from samples. Oxidation is carried out in a tube furnace run at 700°C. Stiffness tensors for the samples are recovered from the measured data using a routine applying the solution to the Christoffel's equation. Recovered stiffness tensors show satisfactory results. The orientation of the recovered stiffness tensors is very close to the material axis. Damping is measured with this variability. The variability in repeatability of damping measurements is less than 10%. Oxidation data obtained followed the initial assumptions and can be used for designing if damping accuracy can be improved.

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