Seminars and Colloquia by Series

Friday, April 14, 2017 - 15:00 , Location: Skiles 006 , Patricio A. Vela , GT ECE , Organizer: Sung Ha Kang
Robotic locomotive mechanisms designed to mimic those of their biological counterparts differ from traditionally engineered systems. Though both require overcoming non-holonomic properties of the interaction dynamics, the nature of their non-holonomy differs.  Traditionally engineered systems have more direct actuation, in the sense that control signals directly lead to generated forces or torques, as in the case of rotors, wheels, motors, jets/ducted fans, etc. In contrast, the body/environment interactions that animals exploit induce forces  or torque that may not always align with their intended direction vector.Through periodic shape change animals are able to effect an overall force or torque in the desired direction. Deriving control equations for this class of robotic systems requires modelling the periodic interaction forces, then  applying averaging theory to arrive at autonomous nonlinear control models whose form and structure resembles that of traditionally engineered systems. Once obtained, classical nonlinear control methods may be applied, though some  attention is required since the control can no longer apply at arbitrary  time scales.The talk will cover the fundamentals of averaging theory and efforts to identify a generalized averaging strategy capable of recovering the desired control equations. Importantly, the strategy reverses the typical approach to averaged expansions, which significantly simplifies the procedure. Doing so  provides insights into feedback control strategies available for systems controlled through time-periodic signals.
Friday, February 17, 2017 - 15:00 , Location: Skiles 006 , Prof. Alper Erturk , GT Mechanical Engineering , Organizer: Sung Ha Kang
The first part of this talk will review our recent efforts on the electroelastodynamics of smart structures for various applications ranging from nonlinear energy harvesting, bio-inspired actuation, and acoustic power transfer to elastic wave guiding and vibration attenuation via metamaterials. We will discuss how to exploit nonlinear dynamic phenomena for frequency bandwidth enhancement to outperform narrowband linear-resonant devices in applications such as vibration energy harvesting for wireless electronic components. We will also cover inherent nonlinearities (material and internal/external dissipative), and their interactions with intentionally designed nonlinearities, as well as electrical circuit nonlinearities. Electromechanical modeling efforts will be presented, and approximate analysis results using the method of harmonic balance will be compared with experimental measurements. Our recent efforts on phononic crystal-enhanced elastic wave guiding and harvesting, wideband vibration attenuation via locally resonant metamaterials, contactless acoustic power transfer, bifurcation suppression using nonlinear circuits, and exploiting size effects via strain-gradient induced polarization (flexoelectricity) in centrosymmetric elastic dielectrics will be summarized. The second part of the talk, which will be given by Chris Sugino (Research Assistant and PhD Student),  will be centered on low-frequency vibration attenuation in finite structures by means of locally resonant elastic and electroelastic metamaterials. Locally resonant metamaterials are characterized by bandgaps at wavelengths that are much larger than the lattice size, enabling low-frequency vibration/sound attenuation. Typically, bandgap analyses and predictions rely on the assumption of waves traveling in an infinite medium, and do not take advantage of modal representations commonly used for the analysis of the dynamic behavior of finite structures. We will present a novel argument for estimating the locally resonant bandgap in metamaterial-based finite structures (i.e. meta-structures with prescribed boundary conditions) using modal analysis, yielding a simple closed-form expression for the bandgap frequency and size. A method for understanding the importance of the resonator locations and mass distribution will be discussed in the context of a Riemann sum approximation of an integral. Numerical and experimental results will be presented regarding the effects of mass ratio, non-uniform spacing of resonators, and parameter variations among the resonators. Electromechanical counterpart of the problem will also be summarized for piezoelectric structures.
Friday, January 27, 2017 - 15:00 , Location: Skiles 006 , Prof. Erik Verriest , GT ECE , Organizer: Sung Ha Kang
This talk contains two parts. First I will discuss our work related to causal modeling in hybrid systems. The idea is to model jump conditions as caused by impulsive inputs. While this is well defined for linear systems, the notion of impulsive inputs poses problems in the nonlinear case. We demonstrate a viable approach based on nonstandard analysis. The second part deals with dynamical systems with delays. First I will show an application of the maximum principle to a delayed resource allocation problem in population dynamics solving a problem in the model of a bee colony cycle. Next I discuss some problems regarding causality in systems with varying delays. These problems relate to the well-posedness (existence and uniqueness) and causality of the mathematical models for physical phenomena, and illustrate why one should consider the physics first and then the mathematics. Finally, I consider the post Newtonian problem as a problem with state dependent delay. Einstein’s field equations relate space time geometry to matter and energy distribution. These tensorial equations are so unwieldy that solutions are only known in some very specific cases. A semi-relativistic approximation is desirable: One where space-time may still be considered as flat, but where Newton’s equations (where gravity acts instantaneously) are replaced by a post-Newtonian theory, involving propagation of gravity at the speed of light. As this retardation depends on the geometry of the point masses, a dynamical system with state dependent delay results, where delay and state are implicitly related. We investigate several problems with the Lagrange-Bürman inversion technique and perturbation expansions. Interesting phenomena (entrainment, dynamic friction, fission and orbital speeds) not explainable by the Newtonian theory emerge. Further details on aspects of impulsive systems and delay systems will be elaborated on by Nak-seung (Patrick) Hyun and Aftab Ahmed respectively.  
Friday, December 2, 2016 - 15:00 , Location: Skiles 006 , Prof. David McDowell and Shouzhi Xu , GT ME and MSE , Organizer: Sung Ha Kang

Talk by Shuozhi Xu,

Title: Algorithms and Implementation for the Concurrent Atomistic-Continuum Method.

Abstract: Unlikemany other multiscale methods, the concurrent atomistic-continuum
(CAC) method admits the migration of dislocations and intrinsic
stacking faults through a lattice while employing an underlying
interatomic potential as the only constitutive relation. Here, we
build algorithms and develop a new CAC code which runs in parallel
using MPI with a domain decomposition algorithm. New features of the
code include, but are not limited to: (i) both dynamic and
quasistatic CAC simulations are available, (ii) mesh refinement
schemes for both dynamic fracture and curved dislocation migration
are implemented, and (iii) integration points in individual finite
elements are shared among multiple processors to minimize the amount
of data communication. The CAC program is then employed to study a
series of metal plasticity problems in which both dislocation core
effects at the nanoscale and the long range stress field of
dislocations at the submicron scales are preserved. Applications
using the new code include dislocation multiplication from Frank-Read
sources, dislocation/void interactions, and dislocation/grain
boundary interactions.

Crystal plasticity modeling is useful for considering the influence of anisotropy of elastic and plastic deformation on local and global responses in crystals and polycrystals. Modern crystal plasticity has numerous manifestations, including bottom-up models based on adaptive quasi-continuum and concurrent atomistic-continuum methods in addition to discrete dislocation dynamics and continuum crystal plasticity. Some key gaps in mesoscale crystal plasticity models will be discussed, including interface slip transfer, grain subdivision in large deformation, shock wave propagation in heterogeneous polycrystals, and dislocation dynamics with explicit treatment of waves. Given the mesoscopic character of these phenomena, contrasts are drawn between bottom-up (e.g., atomistic and discrete dislocation simulations and in situ experimental observations) and top-down (e.g., experimental) information in assembling mesoscale constitutive relations and informing their parameters.
Friday, November 18, 2016 - 12:00 , Location: Skiles 006 , Luca Dieci and Sung Ha Kang , GT Math , Organizer: Sung Ha Kang
This is an information session about research opportunities related to GT MAP activities.  If you are a math graduate student, please join for free pizza as well.
Friday, October 21, 2016 - 15:00 , Location: Skiles 006 , Prof. Julian Rimoli , GT AE , Organizer: Sung Ha Kang
Most available techniques for the design of tensegrity structures can be  grouped in two categories. On the one hand, methods that rely on the  systematic application of topological and geometric rules to regular polyhedrons have been applied to the generation of tensegrity elementary  cells. On the other hand, efforts have been made to either combine  elementary cells or apply rules of self-similarity in order to generate  complex structures of engineering interest, for example, columns, beams and  plates. However, perhaps due to the lack of adequate symmetries on  traditional tensegrity elementary cells, the design of three-dimensional  tensegrity lattices has remained an elusive goal. In this work, we first  develop a method to construct three-dimensional tensegrity lattices from  truncated octahedron elementary cells. The required space-tiling  translational symmetry is achieved by performing recursive reflection  operations on the elementary cells. We then analyze the mechanical response  of the resulting lattices in the fully nonlinear regime via two distinctive approaches: we first adopt a discrete reduced-order model that explicitly accounts for the deformation of individual tensegrity members, and we then utilize this model as the basis for the development of a continuum approximation for the tensegrity lattices. Using this homogenization method, we study tensegrity lattices under a wide range of loading conditions and prestressed configurations. We present Ashby charts for yield strength to density ratio to illustrate how our tensegrity lattices can potentially achieve superior performance when compared to other lattices available in the literature. Finally, using the discrete model, we analyze wave propagation on a finite tensegrity lattice impacting a rigid wall. 
Friday, September 30, 2016 - 15:00 , Location: Skiles 257 , Tomas Zegard , GT CE , Organizer: Sung Ha Kang

Bio:  Tomas Zegard is a postdoctoral fellow in the School of Civil and Environmental Engineering at Georgia Tech. He received a PhD in Structural Engineering from the University of Illinois at Urbana-Champaign in 2014. Afterwards, he took a position at SOM LLP in Chicago, an Architecture + Engineering firm specializing in skyscrapers. He has made significant contributions to the field of topology optimization through research papers and free open-source tools.   Xiaojia Zhang is a doctoral candidate in the School of Civil and Environmental Engineering at Georgia Tech. She received her bachelor’s and master’s degrees in structural engineering from the University of Illinois at Urbana-Champaign. Her major research interests are structural topology optimization with material and geometric nonlinearity, stochastic programming, and additive manufacturing.    

Topology optimization, an agnostic design method, proposes new and innovative solutions to structural problems. The previously established methodology of sizing a defined geometry and connectivity is not sufficient; in these lie the potential for big improvements. However, topology optimization is not without its problems, some of which can be controlled or mitigated. The seminar will introduce two topology optimization techniques: one targeted at continuum, and one targeted at discrete (lattice-like) solutions. Both will be presented using state-of-the-art formulations and implementations. The stress singularity problem (vanishing constraints), the ill-posedness of the problem, the large number of variables involved, and others, continue to challenge researchers and practitioners. The presented concepts find potential applications in super-tall building designs, aircrafts, and the human body. The issue of multiple load cases in a structure, a deterministic problem, will be addressed using probabilistic methodologies. The proposed solution is built around a suitable damping scheme based on simulated annealing. A randomized approach with stochastic sampling is proposed, which requires a fraction of the computational cost compared to the standard methodologies.
Wednesday, August 17, 2016 - 09:30 , Location: Skiles 249 , Various speakers , Georgia Tech , Organizer: Sung Ha Kang
The workshop will launch the themetic semester on Material for GT-MAP activities. This is a three day workshop: The first two days (Wed, Thurs) focusing on the theme of Material, and third day includes broad research topics, open to introducing your research. See the complete Schedule.
Friday, April 22, 2016 - 15:00 , Location: Skiles 006 , Prof. Ting Zhu , Mechanical Engineering, Georgia Tech , Organizer: Sung Ha Kang
Multiscale and multiphysics materials modeling addresses the challenging materials problems that involve multiple physical phenomena at multiple spatial and temporal scales. In this talk, I will present the multiscale and mulphysics models developed in my research group with a recent focus on energy storage materials and advanced structure materials. Our study of rechargeable lithium ion batteries for energy storage applications reveals a rich spectrum of electrochemically-induced mechanical degradation phenomena. The work involves a tight coupling between multiscale chemomechanical modeling and in situ nanobattery testing. Our study of nanostructured metals and alloys elucidates the effects of nanostructures on the size-dependent ultrahigh strengths and surface/interface mediated deformation mechanisms. Finally, I will present my perspectives on the multiscale and multiphysics modeling that requires a synergistic integration of engineering physics and applied mathematics, in order to design the advanced structural and functional materials to realize their potential to the full.
Friday, April 15, 2016 - 15:00 , Location: Skiles 006 , Prof. Massimo Ruzzene , Aerospace Engineering and Mechanical Engineering, Georgia Tech , , Organizer: Sung Ha Kang
Recent breakthroughs in condensed matter physics are opening new directions in band engineering and wave manipulation. Specifically, challenging the notions of reciprocity, time-reversal symmetry and sensitivity to defects in wave propagation may disrupt ways in which mechanical and acoustic metamaterials are designed and employed, and may enable totally new functionalities. Non-reciprocity and topologically protected wave propagation will have profound implications on how stimuli and information are transmitted within materials, or how energy can be guided and steered so that its effects may be controlled or mitigated. The seminar will briefly introduce the state-of-the-art in this emerging field, and will present initial investigations on concepts exploiting electro-mechanical coupling and chiral and non-local interactions in mechanical lattices. Shunted piezo-electric patches are exploited to achieve time-modulated mechanical properties which lead to one-directional wave propagation in one-dimensional mechanical waveguides. A framework to realize helical edge states in two identical lattices with interlayer coupling is also presented. The methodology systematically leads to mechanical lattices that exhibit one-way, edge-bound, defect-immune, non-reciprocal wave motion. The presented concepts find potential application in vibration reduction, noise control or stress wave mitigation systems, and as part of surface acoustic wave devices capable of isolator, gyrator and circulator-like functions on compact acoustic platforms.