Ilias Stavrakas

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.

Nonlinear control of a DC-motor based on radial basis function neural networks

This paper presents a nonlinear controller based on an inverse neural network model of the system under control. The neural controller is implemented as a Radial Basis Function (RBF) network trained with the powerful fuzzy means algorithm. The resulting controller is tested on a nonlinear DC motor control problem and the results illustrate the advantages of the proposed approach.

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.

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.

Towards Air Quality Estimation Using Collected Multimodal Environmental Data

This paper presents an open platform, which collects multimodal environmental data related to air quality from several sources including official open sources, social media and citizens. Collecting and fusing different sources of air quality data into a unified air quality indicator is a highly challenging problem, leveraging recent advances in image analysis, open hardware, machine learning and data fusion. The collection of data from multiple sources aims at having complementary information, which is expected to result in increased geographical coverage and temporal granularity of air quality data. This diversity of sources constitutes also the main novelty of the platform presented compared with the existing applications.

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.

A preliminary study for prefailure indicators in acoustic emissions using wavelets and natural time analysis

Acoustic emission tests were conducted on prismatic Dionysos’ marble specimens subjected to compressive mechanical loading cycles. Two groups of acoustic emission time series were analyzed: one group comprised the number of counts per acoustic emission hit and a second group comprised the RA quantity (RA=rise time/amplitude). Both time series were studied during the load increase period. The investigation of prefailure indicators was carried out using an interdisciplinary approach: using two solid methods from different scientific areas (i.e. cardiology and seismology) we were able to detect failure indicators along before the final collapse. More specific, by means of multiresolution wavelet analysis for the study of temporal variation of the wavelet-coefficients’ standard deviation and natural time analysis using the variance (κ1) of natural-time transformed time-series, a common pattern for both domains was discovered, clearly showing that it is feasible to estimate if the applied mechanical load has led the specimen in the nonlinear region (σ* > 70% σf) regarding the stress–strain behavior.

Acoustic Emissions versus Pressure Stimulated Currents during bending of restored marble epistyles: Preliminary results

The efficiency of two modern sensing techniques, namely the “Acoustic Emissions” and the “Pressure Stimulated Currents” ones, when they are used as Continuous Structural Health Monitoring tools, is assessed experimentally. The protocol includes multi-point bending of an accurate copy of a fractured marble epistyle of the Parthenon’s Temple on the Acropolis of Athens, under a scale of 1:3. The integrity of the epistyle is restored with three pairs of bolted titanium bars, according to the pioneering technique developed by the scientists of the “Committee for the Conservation of the Acropolis Monuments”. The data provided by the above techniques are considered in juxtaposition to each other and also in comparison to data provided by the “Digital Image Correlation” technique. It is concluded that, at least from a qualitative point of view, the data of all three techniques are in good mutual agreement. Combined exploitation of the various sets of experimental data enlightens interesting aspects concerning the succession of failure mechanisms activated during the loading procedure, revealing the critical role of the internal interfaces characterizing the restored epistyle. Moreover it is definitely indicated that both the “Acoustic Emissions” and the “Pressure Stimulated Currents” techniques provide clear signs of upcoming failure well before macroscopically visible damages are detected at the external surface of the specimen.

Probing the electrical properties of the Si nitride/Si interface

The present publication employs Dielectric Relaxation Spectroscopy for the examination of the relaxation mechanisms in silicon nitride MIS structures. These results are combined with capacitance and conductance measurements in order to give a more complete picture of the dielectric behaviour of silicon nitride. The analysis concludes with a method based on Dielectric Relaxation Spectroscopy for the calculation of the depletion layer width.