School of Mechanical Engineering Seminar
Monday, May 22, 2017 at 14:00
Wolfson Building of Mechanical Engineering, Room 206

 Critical aspects of crack dynamics in single crystals

Merna Shaheen-Mualim

PhD. Student of Prof, Dov Sherman

1Dept. of Materials Science and Engineering, Technion, Haifa, Israel

2School of Mechanical Engineering, Tel-Aviv University, Tel-Aviv, Israel

 We investigated two critical aspects of the physics of fast crack propagation in ideal brittle crystals when loaded by time independent (quasi-static) loading. The first is a rate dependent cleavage energy, the second is the effect of reflected stress wave on crack speed with this type of materials and loading. Silicon crystal was used as a model material in this investigation.

It was found that the cleavage energy is crack extension dependent of the driving force, G0, namely, dG0/da. The dynamic cleavage energies at high  of cracks propagating on two low energy cleavage systems of silicon crystal, (110)[1 0] and (111)[11 ], were evaluated using our Coefficient of Thermal Expansion Method (CTEM). Previous studies [1,2] showed that when  is low, the cracks are subjected to stress corrosion cracking mechanisms. At high , however, these mechanisms are vanished, and higher energy than the theoretical Griffith barrier is required to initiate and propagate the crack. We suggest that this complex behavior is due kinking mechanisms along the propagating crack front, which are governed by various energies. 1 2

We further show that crack speed under time independent loading is only slightly affected by stress wave reflecting from the specimen boundary. The maximal speed reduction was estimated by comparing the experimental energy-speed relationship to that of Freund equation of motion. It was found to be less than 8.5% and occurred at speed above 2,500 m/sec in our specimens.

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1. A. Gleizer, M. Shaheen-Mualim, D. Sherman. " cracks dynamics in silicon crystal at the low energy and speed: from macroscopic energy flow to atomistic kinks", Submitted (2017).

2. A. Gleizer, M. Shaheen-Mualim, D. Sherman. "Complex and divers dynamic stress corrosion cracking in silicon crystal", Submitted (2017).

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Speaker: Ori Cohen

M.Sc. student under the supervision of Prof. Hagit Messer-Yaron

Wednesday, April 26^th 2017 at 15:30

Room 011, Kitot Bldg., Faculty of Engineering

Fog mapping using measurements from commercial microwave networks

Abstract

Fog may affect human in many ways, but existing technologies for fog monitoring are limited. 

In this work I present a method for generation a two-dimensional fog map from a near-ground sensors network of Commercial Microwave Links (CMLs). Treating the CMLs as multi-sensor and adopting methods from the data fusion domain, I demonstrate large spatial observations of fog, which were acquired based on received signal level measurements from hundreds of CMLs that were deployed over a country wide area. The suggested algorithm combines data from a standard cellular communication networks with topographic prior information about the area where the network is deployed. 

In addition, it enhances the spatial coverage and reduces the false alarm detection by using side information from other types of sensors as relative humidity measurement provided by the Israeli Meteorological Service.

The performance of the suggested procedure is tested and validated using real measurements from CML, satellite and humidity gauges. 

Speaker: Lital Yodla

M.Sc. student under the supervision of Prof. Meir Feder

Wednesday, June 7th, 2017 at 15:00

Room 011, Kitot Bldg., Faculty of Engineering

Sampling Multiple Output Channel to Maximize the Capacity

Standard analysis of Single Input Multiple Output (SIMO) assumes sampling each of the output signals at the full Nyquist rate. In the noiseless case, Papoulis’ Generalized Sampling Expansion (GSE) shows that for a SIMO channel a unique reconstruction of the input signal is possible when sampling each output signal at 1/N the Nyquist rate.

In this work we consider the problem of maximizing the capacity of a single input, two outputs additive Gaussian noise channel at various output sampling regimes. The goal is to find which sampling rates and relative delay between the output channels attain the maximal capacity.

School of Mechanical Engineering Seminar
Wednesday, May 24, 2017 at 14:00

Tokhna Building, Room 101

Theoretical Study of the 3-branches Explosion Limits of a Flammable System

Alon Lidor

Faculty of Aerospace Engineering

Technion – Israel Institute of Technology

The phenomenon of explosion (or self-ignition, not to be confused with detonation) of fuel mixtures is well known, both as a desired phenomenon, such as the ignition of the fuel in Diesel engines, and as a phenomenon to be avoided, like knocking in SI engines or the risk of fires on oil and gas platforms. The explosion limits are defined as the curve on a pressure-temperature diagram which describes the threshold between the explosive and non-explosive regions of a fuel mixture. These limits are uniquely defined for a specific fuel-oxidizer pair, under a specific ratio and are also partially dependent on the vessel wall surface. For hydrogen and many hydrocarbon fuels distinctive 3-branched limits exist. Although the explosion limits have been studied extensively (experimentally and theoretically) for many years, there exists no model to date which can accurately predict the explosion limits over the complete range, capturing this unique branching behavior.

This research is composed of theoretical investigation of the explosion limits of an H₂-O₂ mixture, with the objective of understanding fundamental governing physics of the explosion limits, especially in regards to the uniqueness of the branching limits. We also aim to develop a universal explosion criterion for H₂-O₂ mixtures, and gain insights into the explosion process for other fuel types. By using a novel approach, combining elements of chain ignition theory and linking between the explosion limits and the ignition delay, we present a unified model capable of accurately predicting the explosion limits of H₂-O₂ mixture. We also investigate the effects of fluctuations at the molecular level, to evaluate the effect of different impurities in the mixture on the explosion initiation and the thermodynamic stability of the system. Our results from the different models developed and the conclusions and insights gained about the explosion phenomenon will be presented in the talk.

Biography

BSc in Mechanical Engineering from Ben-Gurion University in the Negev in 2011

MSc in Aerospace Engineering from the Technion in 2013

Currently studying towards a PhD in the Faculty of Aerospace Engineering at the Technion