There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Most of the signal processing that we will study in this course involves local operations on a signal, namely transforming the signal by applying linear combinations of values in the neighborhood of each sample point. You are familiar with such operations from Calculus, namely, taking derivatives and you are also familiar with this from optics namely blurring a signal. We will be looking at sampled signals only. Let's start with a few basic examples. Local difference Suppose we have a 1D image and we take the local difference of intensities, DI(x) = 1 2 (I(x + 1) − I(x − 1)) which give a discrete approximation to a partial derivative. (We compute this for each x in the image.) What is the effect of such a transformation? One key idea is that such a derivative would be useful for marking positions where the intensity changes. Such a change is called an edge. It is important to detect edges in images because they often mark locations at which object properties change. These can include changes in illumination along a surface due to a shadow boundary, or a material (pigment) change, or a change in depth as when one object ends and another begins. The computational problem of finding intensity edges in images is called edge detection. We could look for positions at which DI(x) has a large negative or positive value. Large positive values indicate an edge that goes from low to high intensity, and large negative values indicate an edge that goes from high to low intensity. Example Suppose the image consists of a single (slightly sloped) edge:

http://ieeexplore.ieee.org/iel5/5/31307/01456462.pdf

Linear systems

Field-assisted sintering technology/Spark plasma sintering is a low voltage, direct current (DC) pulsed current activated, pressure-assisted sintering, and synthesis technique, which has been widely applied for materials processing in the recent years. After a description of its working principles and historical background, mechanical, thermal, electrical effects in FAST/SPS are presented along with the role of atmosphere. A selection of successful materials development including refractory materials, nanocrystalline functional ceramics, graded, and non-equilibrium materials is then discussed. Finally, technological aspects (advanced tool concepts, temperature measurement, finite element simulations) are covered.

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Field-Assisted Sintering Technology/Spark Plasma Sintering: Mechanisms, Materials, and Technology Developments

Review on titanium and titanium based alloys as biomaterials for orthopaedic applications.

Shape-Memory Materials

A large quantitative fractographic study was carried out onan A508 C1.3 pressure vessel steel in the temperature range corresponding to the ductile-to-brittle transition. Fractographic analyses of fractured Charpy V-notch (CVN) and compact tension (CT) specimens revealed a certain proportion of ductile fracture preceding cleavage, even if the specimens were tested at temperatures below the DBTT. The influence of the ductile tearing on cleavage triggering was studied. In particular, the stress concentrations in CT and/or CVN specimens induced by the presence of an ellipsoidal defect representing the cluster of debonded MnS inclusions were calculated by finite element method and related to the fracture probability given by Beremin model.

Influence of ductile tearing on cleavage triggering in ductile-to-brittle transition of A508 steel

Microstructure and properties of extruded rods of a new machinable lead-free aluminum AA6023 (Al-Mg-Si-Sn-Bi) alloy having different levels of Mg content were characterized. In the structure of the alloy there are low-melting point particles containing Sn+Bi. During machining, the temperature generated in the cutting zone is high enough to melt these dispersed entities. This melting gives rise to a local loss of the material strength and ductility which in turn leads to the formation of short, discontinuous chips. In addition to tin and bismuth, some of the Sn+Bi particles contain also a high amount of magnesium and intermetallic compounds of Mg2Sn and Mg3Bi2 are thus formed. The presence of these stable phases increases the melting temperature of Sn+Bi containing particles and their originally positive impact on improving the machinability is therefore reduced. Hence increasing of Mg content in the Al-Mg-Si-Sn-Bi alloy reduces its machinability.

Influence of the Chemical Composition on the Structure and Properties of Lead-Free Machinable AA6023 (Al-Mg-Si-Sn-Bi) Alloy

A transmission electron microscopy study of a tempered bainitic nuclear reactor pressure vessel steel was carried out. Cross-section thin foils from nickel electroplated Charpy V-notch specimens fractured in impact and quasi static loading at −30 °C were prepared in the ductile tearing and cleavage regions of the fracture surface. In the ductile tearing zone, the microstructure was very heterogeneous. Dislocation cells, shear bands, and fine heavily deformed subgrains were found. The deformation in the impact specimen was localized only in the vicinity of the fracture surface, where long thin subgrains situated along the fracture surface were often observed. In the quasi static three-point bend specimen, the strain localization was found also in deeper areas under the fracture surface. There were shear bands (bundles of long thin subgrains) mostly aligned in parallel to the fracture surface. Numerous areas of the shear band intersections (at ∼45° to the main shear band direction) were also observed. In the vicinity of cleavage zones of both types of specimens, no significant differences in the microstructure with respect to the bulk material were observed. Only an increased dislocation density was found.

Microstructure of low alloyed steel close to the fracture surface

Pour la comprehension des proprietes des materiaux, l'analyse chimique locale en microscopie electronique (MET et STEM) est de plus en plus utilisee. Dans ce dossier, sont decrites, de maniere pratique, les deux techniques qui equipent les microscopes actuels, l'analyse des rayons X caracteristiques (EDXS - energy dispersive X-ray spectroscopy) et la spectrometrie des pertes d'energie (EELS - electron energy loss spectrometry). Dans la partie pertes d'energie, les transitions de faible energie (excitations de plasmons, transitions interbandes, effet Cerenkov...) ne sont pas traitees. Seule l'utilisation des seuils d'ionisation caracteristiques des atomes est abordee.

Étude des métaux par microscopie électronique en transmission (MET) - Analyse chimique locale

FeAl20Si20 (wt.%) powders prepared by mechanical alloying from different initial feedstock materials (Fe, Al, Si, FeAl27) were investigated in this study. Scanning electron microscopy, X-ray diffraction and nanoindentation techniques were used to analyze microstructure, phase composition and mechanical properties (hardness and Young’s modulus). Finite element model was developed to account for the decrease in measured values of mechanical properties of powder particles with increasing penetration depth caused by surrounding soft resin used for embedding powder particles. Progressive homogenization of the powders’ microstructure and an increase of hardness and Young’s modulus with milling time were observed and the time for complete homogenization was estimated.

Miroslav Karlík

Papers

Influence of ductile tearing on cleavage triggering in ductile-to-brittle transition of A508 steel

Influence of the Chemical Composition on the Structure and Properties of Lead-Free Machinable AA6023 (Al-Mg-Si-Sn-Bi) Alloy

Microstructure of low alloyed steel close to the fracture surface

Étude des métaux par microscopie électronique en transmission (MET) - Analyse chimique locale

Mechanical properties of FeAlSi powders prepared by mechanical alloying from different initial feedstock materials