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Seminars

Fri 26 Apr 14:00: Bio-electromechanical interfaces and sensors based on functional nanomaterials

From Engineering Department Bio- and Micromechanics Seminars. Published on Apr 16, 2019.

Bio-electromechanical interfaces and sensors based on functional nanomaterials

Abstract not available

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Fri 03 May 14:00: Self-similarly expanding regions of phase change yield cavitational instabilities and model deep earthquakes Tea and coffee will be available in the foyer after the seminar

From Engineering Department Bio- and Micromechanics Seminars. Published on Mar 01, 2019.

Self-similarly expanding regions of phase change yield cavitational instabilities and model deep earthquakes

The dynamical fields that emanate from self-similarly expanding ellipsoidal regions undergoing phase change (change in density, i.e., volume collapse, and change in moduli) under pre-stress, constitute the dynamic generalization of the seminal Eshelby inhomogeneity problem (as an equivalent inclusion problem), and they consist of pressure, shear, and M waves emitted by the surface of the expanding ellipsoid and yielding Rayleigh waves in the crack limit. They may constitute the model of Deep Focus Earthquakes (DFEs) occurring under very high pressures and due to phase change. Two fundamental theorems of physics govern the phenomenon, the Cauchy-Kowalewskaya theorem, which based on dimensional analysis and analytic properties alone, dictates that there is zero particle velocity in the interior, and Noether’s theorem that extremizes (minimizes for stability) the energy spent to move the boundary so that it does not become a sink (or source) of energy, and determines the self-similar shape (axes expansion speeds). The expression from Noether’s theorem indicates that the expanding region can be planar, thus breaking the symmetry of the input and the phenomenon manifests itself as a newly discovered one of a “dynamic collapse/ cavitation instability”, where very large strain energy condensed in the very thin region can escape out. In the presence of shear, the flattened very thin ellipsoid (or band) will be oriented in space so that the energy due to phase change under pre-stress is able to escape out at minimum loss condensed in the core of dislocations gliding out on the planes where the maximum configurational force (Peach-Koehler) is applied on them. Phase change occurring planarly produces in a flattened expanding ellipdoid a new defect present in the DFEs. The radiation patterns are obtained in terms of the equivalent to the phase change six eigenstrain components, which also contain effects due to planarity through the Dynamic Eshelby Tensor for the flattened ellipsoid. Some models in the literature of DFEs are evaluated and excluded on the basis of not having the energy to move the boundary of phase discontinuity. Noether’s theorem is valid in anisotropy and nonlinear elasticity, and the phenomenon is independent of scales, valid from the nano to the very large ones, and applicable in general to other dynamic phenomena of stress induced martensitic transformations, shear banding, and amorphization.

Tea and coffee will be available in the foyer after the seminar

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Fri 10 May 14:00: Title to be confirmed

From Engineering Department Bio- and Micromechanics Seminars. Published on Apr 16, 2019.

Title to be confirmed

Abstract not available

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Fri 24 May 14:00: Strain gradient plasticity: Numerical modeling of size-effects in porous metals

From Engineering Department Bio- and Micromechanics Seminars. Published on Mar 22, 2019.

Strain gradient plasticity: Numerical modeling of size-effects in porous metals

Micron scale size-effects in metals have been confirmed in numerous experiments, exhibiting the general trend that smaller is stronger. For torsion of micron scale wires and bending of thin films, it has been found that normalized twisting and bending resistance in the plastic deformation range increase with decreasing specimen size. In micro- and nano-indentation testing increased hardness for smaller indentations are measured. It is generally accepted these size-effects in metals are due to so-called geometrically necessary dislocations that arise in the presence of very large plastic strain gradients. Such gradients may be imposed due to inhomogeneous overall deformation as in the examples above, but they can also arise under homogeneous deformation when boundaries are passivated, such that plastic deformation is hindered due to blockage of dislocations.

The first part of this presentation presents a basis for numerical modeling of strain gradient plasticity, both in the context visco-plastic materials and time-independent plasticity.

The second part treats size-effects in void growth, with emphasis on extensions of conventional porous metal yield surfaces to include micron scale size-effects.

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Fri 31 May 14:00: Atomistically inspired origami Tea and coffee will be available in LR5 after the seminar

From Engineering Department Bio- and Micromechanics Seminars. Published on Mar 29, 2019.

Atomistically inspired origami

In the simplest case “Objective Structures” are structures like carbon nanotubes, graphene and phosphorene in which each atom sees the same environment. We comment on their striking prevalence in nanoscience, materials science and biology and also explain why they arise in a natural way as distinguished structures in quantum mechanics, molecular dynamics and continuum mechanics. The underlying mathematical idea is that the isometry group that generates the structure matches the invariance group of the differential equations. Their characteristic features in molecular science imply desirable features for macroscopic structures. We illustrate the latter by constructing some “objective origami” structures.

Tea and coffee will be available in LR5 after the seminar

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Fri 07 Jun 14:00: Title to be confirmed

From Engineering Department Bio- and Micromechanics Seminars. Published on Apr 16, 2019.

Title to be confirmed

Abstract not available

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Wed 12 Jun 14:00: Title to be confirmed

From Engineering Department Bio- and Micromechanics Seminars. Published on Mar 22, 2019.

Title to be confirmed

Abstract not available

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