Dimos Triantis

Non-extensivity of the isothermal depolarization relaxation currents in uniaxial compressed rocks

The isothermal depolarization current relaxation in uniaxial compressed rocks follows a generalized exponential function which explicitly introduces hierarchically constrained dynamics and macroscopic interactions. The interactions are associated with the non-extensive entropy parameter q and exhibits a behavior indicating a scaling with normalized uniaxial stress Sigma = sigma/sigma(Y), where sigma(Y) is the yield stress where deviation from the elastic region starts in a stress-strain curve. Combining ideas of Levy and Tsallis statistics we argue the remarkable result that a Levy-walk-type mechanism can organize the geometry of the heterogeneous system to criticality. The stress-dependent q-estimation leads to the conclusion that fracturing is a subextensive process with strong interaction. Copyright (C) EPLA, 2011

Large Earthquake Occurrence Estimation Based on Radial Basis Function Neural Networks

This paper presents a novel scheme for the estimation of large earthquake event occurrence based on radial basis function (RBF) neural network (NN) models. The input vector to the network is composed of different seismicity rates between main events, which are easy to calculate in a reliable manner. Training of the NNs is performed using the powerful fuzzy means training algorithm, which, in this case, is modified to incorporate a leave-one-out training procedure. This helps the algorithm to account for the limited number of training data, which is a common problem when trying to model earthquakes with data-driven techniques. Additionally, the proposed training algorithm is combined with the Reasenberg clustering technique, which is used to remove aftershock events from the catalog prior to processing the data with the NN. In order to evaluate the performance of the resulting framework, the method is applied on the California earthquake catalog. The results show that the produced RBF model can successfully estimate interevent times between significant seismic events, thus resulting to a predictive tool for earthquake occurrence. A comparison with a different NN architecture, namely, multilayer perceptron networks, highlights the superiority of the proposed approach.

Is pressure stimulated current relaxation in amphibolite a case of non-extensivity?

The application of uniaxial stress on rocks is accompanied by the production of an electric signal which is described by the term pressure stimulated current (PSC). In this work the high rate step stress (HRSS) technique is applied, and the PSC relaxation in amphibolite samples from KTB drilling, is studied in the frame of non-extensive statistical mechanics. After the application of an abrupt stress step, a PSC spike is recorded up to its maximum value PSCmax, decreasing gradually, right after the stress stabilization in a value σH. In the present work we study PSCs relaxation in uniaxial compressed rocks using a generalized exponential function called q-exponential, which explicitly introduces hierarchically constrained dynamics and interactions. The interactions are associated with the non-extensive entropy parameter q which exhibits a behavior indicating a dependence with the applied uniaxial stress as we approach fracture. The stress-dependent q-estimation leads to the conclusion that fracturing is a subextensive process with hierarchically constrained dynamics. Furthermore, assuming an assembly of relaxed subdomains created due to microfracture, a non-extensive behavior of the observed macroscopically PSC relaxation is discussed in the frame of a superstatistical approach.

Pressure stimulated electrical emissions from cement mortar used as failure predictors

The electrical signals emitted during the application of uniaxial compressive mechanical stress upon cement mortar specimens are observed and discussed in this paper. This work discusses the electrical signals that are detected when the specimens are excited by a stepwise uniaxial stress increase from a low level (σL) to a higher level (σH) at a fast or slow rate and consequently remain at a high pressure regime for a long time. When maintaining constant mechanical stress for a long time, creep phenomena are evident in the specimen and the corresponding electrical emissions are recorded and analyzed. The characteristics of the electrical signal give clear information regarding the breaking stress (σF) of the material. The electrical emission recordings are of great interest when the applied σH is located in the vicinity of the failure stress; the emitted electrical current increases greatly due to the sequential formation and propagation of cracks that occurs in this stress region. Thus, by correlating the strain rate variations to the electrical emissions this methodology can be used to predict failure due to compressive stress in cement mortars.

Temperature-dependent visible to near-infrared optical properties of 8 mol% Mg-doped lithium tantalate

We report the experimental determination of the ordinary and extraordinary refractive index of 8 mol% Mg-doped congruent lithium tantalate (MCLT). Refractive index measurements cover a spectral range from 450nm to 1550nm and temperatures varying from 22°C to 200°C. Experimental data are fitted to a temperature-dependent dispersion relation that has not been previously used with this material family. Based on this relation, various optical properties of MCLT are calculated, including thermo-optic coefficient, group velocity dispersion, phase matching curve and temporal walk-off. In an additional quasi-phase-matching second-harmonic-generation experiment it is shown that the proposed dispersion relation may be used to predict grating period with remarkable nanometer-scale accuracy.

Low Temperature Dielectric Relaxations in ZnO Varistor

The main purpose of this work is to study the behaviour of ZnO varistor by means of dielectric relaxation spectroscopy at the low temperature range. The complex frequency spectrum of dielectric losses has been studied in detail and analysed carefully by fitting of a sum of Havriliak–Negami expression. Three relaxation processes are studied here, exhibiting a very strong Cole–Davidson behaviour. The faster relaxation process shows an unusual thermal behaviour. Initially, its relaxation time increases as the temperature rises but later it becomes thermally activated. The transition temperature of thermal behaviour of this process was found very close to the characteristic value of 0.4θD=160 K, where θD is the Debye temperature of ZnO. The de-activation energy of this process has been estimated by using the Arrhenius type relation to be 78 meV, a value very close to the higher longitudinal optical (LO) phonon energy in ZnO, 72 meV. This is a strong indication that the predicted Holstein transition between large to small-polaron motion should be observed in ZnO. The two remaining relaxations are electronic processes, the slower one is associated with the doubly ionised zinc interstitial Zni••, while the other should be related to defects induced by the dopants, or of complexes of intrinsic native defect with dopant impurities.

A non-extensive view of the Pressure Stimulated Current relaxation during repeated abrupt uniaxial load-unload in rock samples

Laboratory experiments have confirmed that the application of uniaxial stress on rock samples is accompanied by the production of weak electric currents, named Pressure Stimulated Currents (PSCs). In the present work the PSC emissions in marble and amphibolite (from the German Continental Deep drilling) samples are studied in the frame of non-extensive statistical mechanics, after having applied sequential loading-unloading cycles. It was ascertained that by increasing the sequential order of the loading-unloading cycle, the emitted PSC exhibits q-exponential behavior with a non-additive entropy parameter q which decreased in each applied uniaxial load-unload cycle. The behavior of the Tsallis q-parameter leads to the conclusion that fracturing is a subadditive process with hierarchically constrained dynamics. Furthermore, assuming an assembly of relaxed subdomains created due to microfracture, a non-extensive behavior of the observed macroscopically PSC relaxation is discussed in the frame of a superstatistical approach.

Modelling of electric signals stimulated by bending of rock beams

Experimental laboratory work was conducted to study the electric signals of marble beams that have been subjected to three-point bend tests. The specimens suffered 12 abrupt loading steps up to failure and the corresponding Bending Stimulated Current (BSC) was recorded by means of copper electrodes placed on the lower surface of the sample. Alongside the application of abrupt loading to the samples, BSC peaks were observed and the current followed an exponentially decreasing relaxation process. In this paper the BSC peaks, the total charge and the characteristic relaxation time variables are correlated with each other and are analysed towards the applied loading. The ultimate purpose of this work is the evaluation of the fatigue of the sample using simple criteria.

Using New Technologies for Teaching Power Electronics and Assessing Students

In the Technological Educational Institution (T.E.I.) of Athens, new technologies are used for teaching electronic engineering modules for some time. This work presents the teaching of the module of power electronics by introducing a number of new methods. The educational portal, software simulations and electronic examinations support the teaching process and finally evaluate knowledge assimilation. Features and advantages of their implementation are presented and discussed

A Physical Access Control System that utilizes existing networking and computer infrastructure

Physical access control systems are installed in areas that need to be supervised and user access control is necessary. Existing commercial applications require a dedicated communication infrastructure and special hardware requirements comprising a compact system not flexible to user customization. This paper presents a physical access control system that utilizes existing networking and computer infrastructure for its operation allowing increased reliability and scalability.