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Seminar on Mechanical Science and Bioengineering
122th | February 2, 2017 14:00-15:00 Room A145 (1st floor) |
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| Atomistic Aspects of Nanoscale Fracture | |
| Nanoscale metallic objects like nanowires, thin films, or nanoparticles are usually nearly void of dislocations and can consequently sustain large stresses. Cracks or crack nuclei in nanoscale objects are furthermore inherently small. Their propagation is therefore controlled by the stress to break the atomic bonds at the crack tip rather than by the release of elastically stored energy. At the high applied stresses required to propagate such short cracks, effects like tension-shear coupling can no longer be neglected. Crack tip plasticity in dislocation-starved nano objects furthermore becomes dislocation nucleation controlled and individual crack-microstructure interactions have a more pronounced effect on the fracture behavior than in bulk materials. Cracks in nano objects and nanoscale cracks in general are also typically characterized by small radii of curvature. All these aspects can only be studied to a certain extend with the conventional continuum mechanics approaches and many require explicit atomistic modeling.
Here we present the results of recent atomistic simulations of cracks in five different simulation setups of various sizes. Using EAM-type potentials for various bcc metals we studied the influence of crack length, crack front curvature and boundary conditions on crack tip plasticity. For small cracks, crack tip plasticity was facilitated by the presence of T-stresses and tension-shear coupling. Fully-3D simulations of penny-shaped cracks revealed an increased tendency for crack tip plasticity compared to straight cracks due to the availability of more slip systems and the resulting dislocation ? crack interactions. Simulations of cracks interacting with individual pre-existing lattice dislocations showed stimulated dislocation nucleation and new crack tip blunting mechanisms. The results are discussed in the context of fracture of nanoscale objects as well as crack nuclei in bulk metals. | |
| Prof. Erik Bitzek Department of Materials Science and Engineering, Friedrich-Alexander-Universitat Erlangen-Nurnberg | |
121th | January 27, 2017 13:00-14:00 Room A145 (1st floor) |
| Mechanics of disordered materials: (Unrelated) cases of oxide glasses and fibrous entanglements | |
| In this talk, I will address two aspects of mechanical behavior in disordered solids at very different length scales. First, I will present an atomic-scale study of the mechanics of oxide glasses with a special emphasis on how the mechanical energy is dissipated at high frequencies, in the THz regime of the so-called Boson peak. In particular, I will show that in this regime, energy dissipation can be expressed analytically allowing to analyze on an atom basis the source of energy dissipation. On a rather unrelated topic, I will move to the macroscopic scale to discuss the unusual mechanics of a particular type of fibrous material, called single-wire entangled material, because it is made of a single, self-entangled fiber. I will compare experiments and discrete element method calculation to highlight the remarkable mechanical properties of this architected entanglement, in particular concerning its very large deformability and its Poisson’s function, which is beyond the usual bounds. | |
| Prof. David Rodney Institut Lumiere Matiere, University of Lyon, France | |
120th | December 22, 2016 17:30-18:30 Seminar Room C |
| In Japanese | |
119th | December 21, 2016 16:30-17:30 Room B401 (4th floor) |
| Rehabilitation Robotics: Translating Neuroscience Motor Control into Practice | |
| The demand for rehabilitation services is growing apace with the graying of the population. This situation creates both a need and an opportunity to deploy technologies such as rehabilitation robotics, and in the last decade and half several research groups have deployed variations of this technology. Results so far are mixed with the available evidence demonstrating unequivocally that some forms of robotic therapy can be highly effective,even for patients many years post-stroke, while other forms of robotic therapy have been singularly ineffective. The contrast is starkest when we contrast upper-extremity and lower-extremity therapy. In fact, the 2010 and 2016 Stroke Care Guidelines of the American Heart Association (AHA) and of the Veterans Administration/Department of Defense (VA/DoD) endorsed the use of the rehabilitation robotics for upper extremity post-stroke care, but concluded that lower extremity robotic therapy is much less effective as compared to usual care practices in the US. We submit that the contrasting effectiveness of upper- and lower-extremity therapies arises from neural factors, not technological factors. Though, no doubt, it might be improved, the technology deployed to date for locomotor therapy is elegant and sophisticated. Unfortunately, it may be misguided, providing highly repeatable control of rhythmic movement but ultimately doing the wrong thing. The technology we have deployed to date for upper-extremity therapy is firmly based on an understanding of how upper extremity behavior is neurally controlled and derived from decades of neuroscience research. The limitations of lower-extremity robotic therapy lie not in the robotic technology but in its incompatibility with human motor neuroscience. Here I will briefly review the evidence supporting such negative views, and based on our experience with upper extremity robotic therapy, we describe what we are presently investigating to revert and work towards a future endorsement of the AHA and VA/DoD for rehabilitation robotics for lower extremity post-stroke care. | |
| Hermano Igo Krebs Department of Mechanical Engineering, Massachusetts Institute of Technology | |
118th | November 16, 2016 (1)15:00-15:45 (2)15:45-16:30 Sigma Hall Seminar Room (1st floor) |
| (1)Airflow in relation to human speech sound production (2)Mechanical replicas for studying the physics of the vocal tract | |
| (1) Despite the obvious crucial role of airflow to drive human speech sound production, traditionally, researchers studying the mechanisms underlying speech soundproduction considered extremely simple flow models in order to describe flow through the human upper airways. Whereas such a crude description might be sufficient to understand basic mechanisms for normal vowel sound production, it might be questioned for instance in the case of pathologies or when considering the production of non-vowel speech sounds. Indeed, moderate Reynolds numbers are associated with upper airway flow so that it is reasonable to assume that complex flow phenomena associated with the transition of laminar to turbulent flow can occur. In addition, geometrical asymmetries are likely to affect the flow during sound production. In this talk, experimental evidence will be presented illustrating the occurrence of complex flow phenomena related to the upper airway geometry. The shown studies aim to increase our understanding of the described phenomena as well as to comprehend their role in normal or pathological human speech sound production. (2) Making measurements on the human voice organ in vivo is invasive and difficult. Reproducibility and accuracy is usually hard to achieve with living or excised tissue. For studying fundamental phenomena of the physics of voice production, it is often more convenient to use mechanical replicas of the various parts of the voice organ. In comparison to computer simulations, the physicality of replicas can impart a more immediate and intuitive appreciation of the structural dimensions and the mechanical and/or aeroacoustical aspects of voice production. In some cases, a highly simplified geometry for the vocal folds or the vocal tract is useful; it facilitates the specification of parameters, and the validation by measurement of numerical simulations of the corresponding geometries. In other cases, a more realistic 3-D geometry is needed to answer the research questions; such as for the study of the influence of particular pathologies or the cause of particular voice features. Modern 3-D printers have opened up the possibility to recreate complex shapes from MRI data, recently even in soft materials. In this talk, we first present a brief historical review of past or present work dealing with mechanical replicas and then illustrate some applications of these replicas in voice research. | |
| (1)Annemie Van Hirtum (CNRS researcher, GIPSA-lab; and Specially Appointed Professor, Osaka Univ.) (2)Prof. Xavier Pelorson (CNRS Director of Research, Gipsa-lab, France) | |
117th | November 4, 2016 15:00-16:30 Room B205 (2nd floor) |
| In Japanese | |
116th | November 1, 2016 13:00-13:40 Seminar Room D (4th floor) |
| In Japanese | |
115th | October 24, 2016 13:00-13:40 Sigma Hall Seminar Room (1st floor) |
| Changes in the mechanical properties of the cell nucleus with implications in cardiovascular disease and aging | |
| The sequencing of the human genome has provided a wealth of scientific information, but this information is limited by the poor understanding of the mechanisms which control gene expression. In addition to containing the code for the cell and beyond, the genome within the nucleus is a self-assembled polymeric structure with unique mechanical properties. Using spectrocopy, imaging, micromanipulation and computational techniques, we measure the mechanics of the nucleus at various length scales. We are particularly interested in the role that force and cytokine treatment play in altering nuclear mechanics and gene expression in primary human endothelial cells that line blood vessels. We examine the role of fluid shear stress and endothelial-specific growth factors on the reorganization of the nucleus and changes in nuclear mechanics. We find both a shear-dependent and time-dependent influence on nuclear mechanics. However, expression of proteins related to premature aging reduce or eliminate the ability of the nucleus to respond appropriately to applied force. Thus, there appears to be a balanced stiffness maintained throughout the cell including in the nucleus that can be modulated by external force; if this balance is altered by the accumulation of aging-specific proteins and the nucleus is stiffened pathology arises. | |
| Prof. Kris Noel Dahl Chemical Engineering and BioMedical Engineering, Carnegie Mellon University | |
114th | September 16, 2016 16:00-17:30 Seminar Room C |
| In Japanese | |
113th | August 3, 2016 14:40-16:10 Seminar Room D (4th floor) |
| EXTREME VORTEX STATES AND THE HYDRODYNAMIC BLOW-UP PROBLEM | |
| In the presentation we will discuss our research program concerning the study of extreme vortex events in viscous incompressible flows. These vortex states arise as the flows saturating certain fundamental mathematical estimates, such as the bounds on the maximum enstrophy growth in 3D (Lu & Doering, 2008). They are therefore intimately related to the question of singularity formation in the 3D Navier-Stokes system, known as the hydrodynamic blow-up problem. We demonstrate how new insights concerning such questions can be obtained by formulating them as variational PDE optimization problems which can be solved computationally using suitable discrete gradient flows. In offering a systematic approach to finding flow solutions which may saturate known estimates, the proposed paradigm provides a bridge between mathematical analysis and scientific computation. In particular, it allows one to determine whether or not certain mathematical estimates are "sharp", in the sense that they can be realized by actual vector fields, or if these estimates may still be improved. In the
presentation we will review a number of new results concerning 2D and 3D flows characterized by the maximum possible growth of, respectively, palinstrophy and enstrophy. It will be shown that certain types of initial data, such as the Taylor-Green vortex, which have been used in numerous computational studies of the blow-up problem are in fact a particular instance (corresponding to an asymptotic limit) of our family of extreme vortex states. We will present results comparing the growth of relevant quantities in high-resolution direct numerical simulations of the Navier-Stokes system obtained using our extreme vortex states and different initial data employed in earlier studies. [Joint work with Diego Ayala] | |
| Prof. Bartosz Protas Department of Mathematics and Statistics, McMaster University | |
112th | July 29, 2016 13:00-15:00 Room A145 (1st floor) |
| (1)Deviation from High-Entropy Configurations in the Al1.3CoCrCuFeNi Alloy (2)Study of Serrated Flows in Bulk Metallic Glasses and High Entropy Alloys | |
| (1)The alloy-design strategy of combining multiple elements in near-equimolar ratios has shown great potential for producing exceptional engineering materials, often known as “high-entropy alloys”. Understanding the elemental distributions, and, thus, the evolution of the configurational entropy during solidification, are the goal of the present research. The case of the Al1.3CoCrCuFeNi model alloy is examined, using integrated theoretical and experimental techniques, such as ab initio molecular-dynamics simulations, neutron scattering, synchrotron X-ray diffraction, high-resolution electron microscopy, and atom-probe tomography. It is shown that even when the material undergoes elemental segregation, precipitation, chemical ordering, and spinodal decomposition, a significant amount of disorder remains, due to the distributions of multiple elements in the major phases. The results suggest that the high-entropy-alloy-design strategy may be used to develop a wide range of complex materials, which are not limited to single-phase solid solutions. The integrated experimental and theoretical techniques, discussed here, are particularly well-suited to studying partially-ordered materials, produced using the high-entropy-alloy design strategy. (2)Bulk metallic glasses (BMGs) and high entropy alloys (HEAs) attract more and more attention for their unique mechanical properties. Recent work suggests that BMGs and HEAs show serrated flows at certain temperatures and strain rates, which is similar to the Portevin-Le Chatelier effect (PLC) in traditional alloys. Therefore, the study of serration behavior could provide a unique way to investigate the deformation dynamics of BMGs and HEAs, and, consequently, to endow us with the fundamental understanding of deformation mechanisms for BMGs and HEAs. In this study, compressive behavior of BMGs and HEAs are characterized statistically, and a new model developed from the mean-field theory is utilized to describe the serrated flows in BMGs and HEAs. | |
| Prof. Peter K. Liaw Department of Materials Science and Engineering, University of Tennessee | |
111th | July 8, 2016 11:00-12:00 Seminar Room D |
| In Japanese | |
110th | June 10, 2016 16:20-17:20 Seminar Room C |
| In Japanese | |
109th | April 15, 2015 13:00-14:00 B300 |
| Haptics for Augmented Reality and Teleoperated Robots | |
| The word haptics means “related to the sense of touch”. In the psychology and neuroscience fields, haptics is the study of human touch sensing, specifically via kinesthetic (force) and cutaneous (tactile) receptors, associated with perception and manipulation. In the robotics and virtual reality fields, haptics is broadly defined as real and simulated touch interactions between robots, humans and real, remote, or simulated environments, in various combinations. This talk focuses on the use of specialized robotic devices and their corresponding control, known as haptic interfaces, that allow human operators to experience the sense of touch in simulated (virtual) and remote (teleoperated) environments. We will discuss the design, development, and experimental validation of haptic interfaces that can be used to enhance human performance in applications ranging from physics education to robot-assisted surgery. | |
| Prof. Allison M. Okamura Dept. of Mechanical Engineering, Stanford University |
