Date of Award

2016

Document Type

Open Access Thesis

Department

Mechanical Engineering

First Advisor

Prasun K. Majumdar

Abstract

Composites materials are often subjected to multi-physical conditions in different applications where, in addition to mechanical loads, they also need to sustain other types of loads such as electrical currents. The multi-physical behavior of composites needs to be understood and analyzed to facilitate new multi-functional material design. An essential first step towards this goal is to understand how multi-physics properties depend on local details (e.g. micro-structure). Composite materials have heterogeneous electrical properties (carbon/epoxy) at the local level that can be different at the global level. To conduct the multi-physics study, the electrical signal is employed to the composite sample for conducting coupled thermal-electrical-mechanical analysis. Anisotropic electrical behavior is measured experimentally and threshold of nonlinear behavior has been quantified. The electrical-thermal response is studied with thermography tests and finite element analysis. Their results are compared to understand the role of distributed microstructural damage.

The durability and damage tolerance of composite materials for both mechanical and electrical loads also need to be studied. Although the durability of composite materials under mechanical loading has been studied over several decades, their response to electrical currents is still not fully understood. On the one hand, the electrical response of the composite changes with the evolution of damage due to mechanical loads. On the other hand, the stages of damage evolution in composite laminates under mechanical loading can be clearly effected by electrical loading. This thesis investigates how existing damage due to prior mechanical loading history may grow when subjected to subsequent electrical currents. The behavior is multi-physical with interplay of mechanical damage and thermal behavior resulting from Joule heating by electrical current. Results show that anisotropy in electrical response heavily depends on material state consisting of evolving damage. A 3D X-ray tomography has been used to visualize damage and validate experimental observations.

A micromechanics model has been developed to further assist understanding of the anisotropic nature of composite materials at the micro scale. The effective anisotropic electrical conductivity of composites is strongly affected by many parameters including volume fractions, distributions, and orientations of constituents. Given the electrical properties of the constituents, one important goal of micromechanics of materials consists of predicting electrical response of the heterogeneous material on the basis of the geometries and properties of the individual constituents. An effective electrical conductivity estimation is performed by using classical micromechanics techniques (concentric cylinder method or CCM) that investigate the effect of the fiber/matrix electrical properties and their volume fractions on the micro scale composite response.

Rights

© 2016, Mohammad Faisal Haider

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