George Hloupis

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.

Wavelet-Based Rapid Estimation of Earthquake Magnitude Oriented to Early Warning

The main goal of an earthquake early warning system (EEWS) is to estimate the magnitude of an underway rupture from the first few seconds in order to allow hazard assessment and mitigation before destructive events occur. This letter investigates the application of a wavelet-based algorithm for local magnitude estimation in the South Aegean Sea (focusing on Crete Island) which is covered by a sparse seismological network. A relation between the first few seconds of the first-arriving energy at the surface, the P wave, and the local magnitude of the earthquake has been developed for the area of interest. Results show that the errors produced by the proposed method present less scattering than relevant magnitude rapid estimation methods. It is the first time that such a method is applied in a sparse seismological network since all the previous studies took place in high-density networks. This fact expands the applicability of EEWS and also provides an alternative magnitude estimator for the currently developed EEWS.

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.

HVSR Technique Improvement Using Redundant Wavelet Transform

In this study we demonstrate the use of Wavelet Transform (WT) as all improvement tool for the horizontal to vertical spectral ratio (HVSR) technique. Since the use of non-stationary transients in microtremor signals remains an open question we investigate the effect of long duration undetectable (by amplitude thresholding methods) transients in HVSR estimation. For the urban areas, where the existence of these types of transients is intense, we used the WT in order to isolate undetectable transients. A number of examples with HVSR improvement are also presented.

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.

The Use of PSC Technique to Estimate the Damage Extension During Three Point Bending Test

It is already known that when a mechanical loading is applied to cement-based specimens weak electrical currents are generated. Their existence is attributed to the creation of cracks and the eventual evolution of the cracks’ network in the bulk of the specimen. This work introduces the simultaneous recording of electrical signal emissions at both the tension and the compression region of cement mortar beams of rectangular cross-section that were subjected to mechanical loading using the Three-Point-Bending technique. During the experiments the behavior of the electrical signal was studied during four sequential load stages: (1) Abrupt load increase up to the vicinity of 3 PB strength, (2) maintaining the high load level for relatively long time, (3) abrupt load decrease to a low load level (i.e. 50 % of the 3 PB strength approximately), (4) maintaining the low load level for relatively long time. The electrical signal analysis was conducted using non extensive statistical physics (NESP) and specifically the Tsallis entropy model studying the values of its q-parameter. The aim of this work was to study the electrical signal relaxation process that follows the change of the mechanical load and the law that describes this relaxation with respect to the mechanical status of the specimen using statistical physics analysis.

Comparing Multiple-Choice and Constructed Response Questions Applied to Engineering Courses

A study was performed on the comparison of multiple-choice questions (MCQs) and constructed-response questions (CRQs) in terms of objective grading. A CRQ exam is graded by a tutor though an MCQ is automatically graded. An MCQs drawback is the infiltration of the “guessing” factor during selection thus producing unreliable results that do not reflect students’ knowledge. To eliminate this problem MCQs pairs were composed that addressed the same topic. A student who did not possess adequate knowledge could not tell this similarity. A group of undergraduate students enrolled to the “project management” module were examined. A software for electronic examinations was used during the process. A statistical evaluation was performed in order to check whether there was statistically significant difference between the scores provided using CRQs and MCQs. By applying a suitable scoring rule to the MCQs, very similar results are obtained when compared to those of the CRQs examination.

Electrical characterization of polymer matrix — TiO2 filler composites through isothermal polarization / depolarization currents and I–V tests

Specimens of polymer matrix — ceramic TiO2 filler composites were prepared. The contribution of the filler content on the electrical conductivity and energy storage properties of the samples was examined. I–V and Isothermal Polarization/Depolarization Current (IPC/IDC) measurements were conducted. Dc conductivity values directly calculated from the I–V curves exhibited excellent agreement with corresponding values derived from the IPC/IDC recordings. Standard models were employed for fitting the IPC/IDC data. In specific, the short and the very long depolarization times were fitted by use of power laws of different slopes, while the intermediate depolarization times were fitted as a sum of three exponential decays. The present study reveals a strong dependence of the depolarization and polarization processes, as well as of the dc conductivity, on the filler concentration.

An adaptive soft-sensor for non-destructive cement-based material testing, through the use of RBF networks

This paper presents the development of a soft-sensor receiving as inputs Pressure Stimulated Current (PSC) characteristics in order to predict a critical mechanical property of cement-based materials, in a non-destructive manner. The soft-sensor is based on a Radial Basis Function (RBF) network that starts with an empty hidden layer and evolves its structure and synaptic weights as new data become available. Results have shown that the proposed approach can be used successfully to evolve a predictive tool based on input-output data, whereas it is superior compared to other adaptive modeling techniques.