Date of Award

12-14-2015

Document Type

Open Access Thesis

Department

Mechanical Engineering

First Advisor

Prasun Majumdar

Abstract

Heterogeneous materials are increasingly used in a wide range of applications such as aerospace, civil infrastructure, fuel cells and many others. The ability to take properties from two or more materials to create a material with properties engineered to specific needs is always very attractive. Hence engineered materials are evolving into more complex formulations or heterogeneities in multiple disciplines. Design of microstructure at multiple scales controls the global functional properties of these materials and their structures. However, local microstructural changes do not directly cause a proportional change to the global properties (such as strength and stiffness). Instead, local changes follow a latent evolution process including significant interactions for the most of the life and only shows significant bulk property change prior to failure. Therefore, in order to understand property evolution of engineered materials and predict potential catastrophic failure, microstructural changes need to be effectively captured. Characterizing these changes and representing them by material variables will enable us to further improve our material level understanding. In this work, we will demonstrate how microstructural features of heterogeneous materials can be described quantitatively using broadband dielectric spectroscopy (BbDS). The frequency dependent dielectric properties can capture the change in material microstructure and represent these changes in terms of material variables, such as complex permittivity. These changes in terms of material properties can then be linked to a number of different conditions, such as increasing damage due to impact or fatigue. Two different broadband dielectric spectroscopy modes are presented: i) standard BbDS for measurements of bulk properties, and ii) continuous scanning mode (Scanning BbDS) to measure dielectric property change as a function of position across the specimen. In this study, we will focus on both ceramic materials and fiber reinforced polymer matrix composites as test bed material systems. In the first part of the thesis, we will present how different micro-structural design of porous ceramic materials can be captured quantitatively using BbDS. These materials are typically used in solid oxide fuel cells (SOFC) as anode materials. Results show significant effect of microstructural design on material properties at multiple temperatures (up to 800 C). In the later part of the thesis, we will focus on microstructural changes of fiber reinforced composite materials due to impact and static loading. The changes in dielectric response can then be related to the bulk mechanical properties of the material and various damage modes. Observing trends in dielectric response enables us to further determine local mechanisms and distribution of properties throughout the damaged specimens. A 3D X-ray microscope and a digital microscope have been used to visualize these changes in material microstructure and validate experimental observations. The increase in damage observed in the material microstructure has been captured by the changes in BbDS characteristics. Results show that BbDS is an extremely useful tool for identifying microstructural changes within a heterogeneous material and particularly useful in relating remaining properties. For example, the remaining property (such as modulus) after low velocity impact shows significantly higher sensitivity to dielectric properties and hence provided a more accurate representation of material state change. This sensitivity holds for both predominantly dielectric glass fiber to mixed conductor carbon fiber reinforced composite. The great advantage of using material variables is that these variable can be defined at multiple scales, and hence used directly in property degradation laws to help develop a framework for future predictive modeling methodologies.

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