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Department of Aerospace Engineering

Hong Group Mechanics of Materials and Structures

Open Positions

Undergraduate and graduate students are encouraged to contact Prof. Hong for ongoing research projects.

Experimental and Model Developments of Interfacial Mechanical Integrity of Layered Stacks

Supported by SRC (2009-2012)
In collaboration with Ashraf Bastawros (PI)
This research seeks to develop simplified experimental protocols and procedures, which enable quantitative characterization and understanding of the mechanical robustness of layered structures and failure detection at a scale comparable to individual layer thickness. The measurements would provide the critically needed quantities for mechanism-based modeling. A modular design tool, "a figure of merit", would be developed for the assessment of mechanical robustness of stacks with multiple interconnects layers.
The proposed approach encompasses four main elements. (a) Uniform loading system for relatively large samples (mm-scale), with geometries that provide stable and well calibrated stress fields. The specimen would be tested under a primary deformation field (e.g. bending) in a stiffness-controlled micro-loading frame to impose well characterized macroscopic deformation as well as deformation rate (stored elastic strain energy in the frame) to prevent unstable failure11-3. (b) A local probing system with the needed high resolution measurements. Such system could be a nano-scratch/nano-indentation apparatus, equipped with nano-scale acoustic emission tip. (c) Modeling framework for non-standard test with complex geometry to extract the fracture energy (cohesive or interfacial) at the site of failure. The loading ratio of uniform to localized loading at the onset of failure would provide the boundary loading for simulation to evaluate local stresses, the loading phase angle and the selectivity range to promote interfacial vs. cohesive failure. The corresponding fracture energies at the onset of experimentally monitored failure can then be evaluated. (d) The combined measurements and modeling would be used to develop a figure of merit for simplified system level design.

Magneto-Active Polymers: Characterization and Modeling for Engineered Microstructures

Supported by NSF through CMMI-0900342 (2009-2012)
The objective of this research is to gain understanding of the basic science governing the behavior and tunability of Magneto-Active Polymers (MAPs). These polymer-based composites with a filler of magnetic particles have the ability to change their size, stiffness, water retention and other magneto-mechanical properties when exposed to a magnetic field. The specific changes desired can be engineered into the microstructure of the materials by controlling the local elastic and magnetic ordering during the cure process as well as manipulating a variety of other synthesis conditions. This is only possible with a thorough understanding of the behavior of MAPs and a predictive model of this behavior. The specific aims of this project are: (1) the experimental characterization of MAP magneto-mechanical property dependencies, (2) micromechanics-based analytical models explaining the behavior of MAPs, (3) quantitative prediction of the performance of MAP-based devices, and (4) synthesizing protocols for MAPs with an engineered microstructure.
The successful completion of this research will enable applications that will transform not only how MAPs are used but also the capability of technology in fields ranging from virtual and augmented reality to bio-medicine, noise and vibration control, and high resolution positioning.

Modeling the Formation of Self-Ordered Nanoporous Anodic Oxides

Supported by NSF through CMMI-1000748 (2010-2013)
In collaboration with Kurt Hebert (CBE, PI) and Pranav Shrotriya (ME)
The objective of this project is to develop new methods for the rational design of highly ordered porous anodic oxide (PAO) films. These films are formed electrochemically by applying voltages to metals such as aluminum and titanium, in electrochemical cells. The films contain self-ordered hexagonal arrays of nanoscale pores, covering macroscopic areas. While much attention has focused on the use of PAO as templates for functional devices, these structures have been developed empirically, in the absence of robust understanding of processes controlling film growth. The goal of this research is to develop a fully predictive model of pore formation and self-ordering. The work will be guided by the concept that oxide material is transported by the combined influence of the electric field and mechanical stress. Experimental stress measurements will determine the operative balances of viscous, electrostatic and oxidation-induced stress governing interface motion in PAO films. Using this knowledge, descriptions of transport processes and driving forces will be formulated as a predictive simulation PAO growth, revealing the relations between electrochemical polarization, bath chemistry, and the dynamics of the self-ordering.
If successful, the chemical-mechanical model of PAO formation will provide a fundamental basis for model-based design of self-organized porous anodic films. Thus, the model may permit rational manipulation of process conditions for high-rate fabrication of defect-free films, thus enhancing the commercial potential of PAO-based functional devices for solar energy conversion, catalysis, and biomedical applications.

Mechanics of multi-responsive ceramics for electrical capactors with high power/energy density

Supported by NSF through CMMI-1027873 (2010-2013)
In collaboration with Xiaoli Tan (MSE, PI)
Compared to most dielectric materials used in capacitors for electrical energy storage, PbZrO3-based antiferroelectric ceramics display a much higher energy density due to the reversible antiferroelectric ? ferroelectric phase transition, which is manifested by an abrupt development of large electrical charge and volume change. As such, the phase transition makes these ceramics responsive to both electric fields and mechanical stresses and forms the fundamental basis for energy-storage applications. The associated volume change will induce complicated internal stress states in the ceramic and thus influence the reliability of capacitors. This project is focused on the critical mechanics issues in antiferroelectric ceramics subjected to electrical and/or mechanical loads. Through an integrated experimental and theoretical approach, this project aims to rigorously establish the novel phase-transition-toughening mechanism in antiferroelectric ceramics, which will lead to next-generation energy-storage devices with a much improved reliability.
The success of this project will pave the way to the large-scale usage of antiferroelectric capacitors with high power/energy density. Specific applications including the storage of electricity generated from renewable sources, such as wind and solar, whose intermittent nature requires efficient energy-storage technologies to ensure around-the-clock delivery.