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.