Ilias Stavrakas

• B.Sc., M.Sc., Ph.D.
• Professor at University of West Attica

The temporal evolution of the electric activity generated in marble specimens under uniaxial compression is analyzed and quantified in terms of the Pressure Stimulated Voltage (Electric Potential) developed. The evolution of the electric activity is considered in juxtaposition to that of the respective acoustic one, quantified either in terms of th...

The attenuation of the acoustic activity in marble specimens under uniaxial compressive loading-unloading loops is quantified in juxtaposition to that of the electric activity. In parallel, the existence of ‘‘pre-failure indices” warning about entrance into a critical stage, that of impending fracture, is explored. The acoustic activity is quantifi...

The fracture process of heterogeneous materials is studied here in the framework of the discipline of Non-Extensive Statistical Mechanics. Acoustic emission data provided by an experimental protocol with concrete specimens, plain or fiber-reinforced, under bending are taken advantage of. This innovation of the study lies in the fact that the analys...

Air quality directly affects the health of humans. The health implications of poor air quality are recognized by professionals and the public alike and these concerns have driven both the proliferation of formal sensor networks, but also low-cost sensors which can be used in the home. The advancement of technology in recent years has also led to th...

Taking advantage of the interevent time intervals, namely the time intervals between two successive acoustic events recorded during mechanical loading of structural elements, it is attempted to detect indices warning about upcoming failure. The innovative aspect of the study is that the analysis is implemented in the frame of Non-Extensive Statisti...

The acoustic activity generated in marble specimens under elementary loading schemes (direct tension and uniaxial compression) is studied in terms of the average energy of the acoustic signals recorded. It is highlighted that several seconds before the macroscopic disintegration of the specimens the temporal variation of the specific quantity exhib...

Purpose The determination of mode-I fracture toughness of brittle structural materials by means of the notched Brazilian disc configuration is studied. Advantage is taken of a recently introduced analytical solution and, also, of data provided by an experimental protocol with notched marble specimens under diametral compression using the loading de...

The present study is the continuation of a previously published one (Part I), in which the D- (three-dimensional distance between the sources of any two successive acoustic events) and P-functions (the rate of the energy of the acoustic signals) were introduced for the analysis of the acoustic activity developed in marble specimens under uniaxial t...

This study aims to explore the possibility of detecting indices that could potentially provide warning about the proximity of internal damage to critical levels, beyond which catastrophic fracture is impending. In this direction, advantage was taken of the Cumulative Counts that were recorded during the mechanical loading of specimens made of eithe...

Air quality is a subject of study, particularly in densely populated areas, as it has been shown to affect human health and the local ecosystem. In recent years, with the rapid development of technology, low-cost sensors have emerged, with many people interested in the quality of the air in their area turning to the procurement of such sensors as t...

Nowadays, the study of air quality has become an increasingly prominent field of research, particularly in large urban centers, given its significant impact on human health. In many countries, government departments and research centers use official high-cost scientific instruments to monitor air quality in their regions. Meanwhile, concerned citiz...

Air pollution, which is mainly caused by industrialization, intensive transportation, and the heating of buildings, is one of the most important problems in large cities because it seriously harms the health and the quality of life of their citizens. This is why air quality is monitored not only by governmental organizations and official research i...

The acoustic activity, generated in notched, beam-shaped concrete specimens, loaded under three-point bending, is studied in terms of the position of the sources of acoustic events, and the frequency of their generation. Both plain specimens (without any internal reinforcement) and specimens reinforced with various types of short fibers were tested...

A novel approach for describing the acoustic activity in brittle structural materials while they are loaded mechanically at levels close to those causing macroscopic fracture is proposed. It is based on the analysis of the rate of acoustic emissions in terms of the Natural Time concept. Experimental data from protocols with either intact or notched...

The continuously growing human activity in large and densely populated cities pollutes air and consequently puts public health in danger. This is why air quality monitoring is necessary in all urban environments. However, the creation of dense air monitoring networks is extremely costly because it requires the usage of a great number of air monitor...

Acoustic emission (AE), recorded during uniaxial compressive loading with constantly increasing stress and stepped stress increments until the fracture of prismatic marble specimens, were analyzed in terms of non-extensive statistical mechanics (NESM). Initially introduced by Tsallis, NESM has proven to be an autonomous robust theoretical framework...

Acoustic emissions (AEs) and weak electrical signals, also known as pressure stimulated currents (PSCs), were concurrently recorded in order to investigate their behavior and detect precursory indicators when cement mortar specimens were subjected to mechanical compressive loading, emphasizing the behavior of the AEs and the PSC signal in the vicin...

The massive transition from in-person to remote teaching increased the impact of technology on the everyday life of the universities. Without the face-to-face component, learning and teaching became a completely different experience for students and teachers. Recording the attitudes and perceptions of the undergraduate students on the new situation...

Concentration of particulate matter constitutes a major, environmental problem affecting human health especially in urban areas. Although research centers and government agencies have air quality monitoring systems installed, their limited number prevents the spatiotemporal analysis in urban areas. EU funds research projects to engage citizens for...

The main goal of the present study is to explore the potentialities of Non-Extensive Statistical Mechanics (NESM) to provide signals that could be considered as pre-failure indicators. In this direction concrete beams, either unreinforced or reinforced with short fibers, were submitted to three-point bending. The acoustic signals detected during lo...

Objective computer-assisted examinations (CAA) are considered a preferable option compared to constructed response (CR) ones because marking is done automatically without the intervention of the examiner. This publication compares the attitudes and perceptions of a sample of engineering students towards a specific objective examination format desig...

The acoustic activity in beam-shaped specimens made of cement is studied, assuming that the beams are loaded in three-point bending under a step-wise loading scheme. Attention is focused to the attenuation of the acoustic activity during the constant-load stage of each specific loading step. The experimental data are analyzed in terms of the intere...

The temporal variation in terms of the “time-to-failure” parameter of the recordings of the electrical resistance and the acoustic emissions from concurrent measurements in three cement mortar specimens of prismatic shape that were subjected to a three-point bending test until fracture are studied. The novelty of the work at hand lies in the demons...

Broadband dielectric spectroscopy in a broad temperature range was employed to study ionic conductivity and dynamics in tetraalkylammonium- and tetraalkylphosphonium-based ionic liquids (ILs) having levulinate as a common anion. Combining data for ionic conductivity with data obtained for viscosity in a Walden plot, we show that ionic conductivity...

In the context of bio-based ionic liquids (ILs), in recent years levulinate ILs have been explored in various applications with appreciable performances. To expand their scope of use, seven selected levulinate ILs featuring different cationic moieties have been prepared and their physico-chemical properties compared. While all the ILs generated dis...

An attempt is described in this study to monitor the damage level within mechanically loaded structural members made of cement-based materials in terms of the change of their electrical resistance R. The experimental protocol included three-point bending (3PB) tests with rod-shaped specimens made of cement. During the experiments the load applied a...

All restoration projects carried out all over the world are based on principles mentioned mainly in the "Venice Charter". Respecting and protecting the ancient material is perhaps the principle of paramount importance. This fact is usually the guide for any kind of intervention, among which is the substitution of the ancient connectors with new one...

The investigation of the mechanical status of materials subjected to load levels approaching these causing macroscopic fracture seriously concerns the structural engineering community. Several experimental works that involve a wide variety of materials are available in the literature, demonstrating a series of sensing techniques which are capable o...

The acoustic activity in brittle specimens submitted to mechanical loading is explored in terms of two alternative approaches. The first one is based on the Euclidean three-dimensional distance between the sources of successive acoustic events while the second is based on the power of the acoustic signals recorded. In the direction of quantifying t...

The quantitative determination of the current load carrying capability of already loaded structural elements and the possibility to detect proper indices that could be considered as signals for timely warning that the load carrying capacity is exhausted is the subject of this study. More specifically, it aims to explore the possibility of detecting...

The time evolution of the electric activity in brittle materials, mechanically loaded at levels approaching the ones causing catastrophic fracture, is studied and modeled in terms of the time-to-failure parameter. The electric activity is considered in juxtaposition to the respective acoustic one. The latter was analytically described in a recently...

The investigation of the mechanical status of materials subjected to load levels approaching these causing macroscopic fracture seriously concerns the structural engineering community. There are several experimental works demonstrating a series of sensing techniques capable of providing indications warning about the upcoming catastrophic failure of...

The acoustic activity developed in marble specimens under various loading schemes is explored in terms of the recently introduced F-function. The novelty of the study is that instead of describing the temporal evolution of the F-function in terms of conventional time, the Natural Time concept is employed. Although completely different geometries an...

Knowledge of the mechanical response of structures at load levels close to those causing failure, as well as their remaining load carrying capacity, is crucial for structural engineers, independently of whether the structures are intact or damaged. In this direction, intensive technological research is carried out worldwide aiming to the developmen...

The air quality, especially in cities, is an important factor that affects life quality and health. Despite the continuously growing trend to monitor the air quality the challenge to collect reliable data is still on the table. Particulate matter, Nitrogen Dioxide and Ozone concentrations constitute some of the most widely measured quantities in or...

Nowadays, air quality is an important factor that affect life quality, especially in big cities. Concern about the air citizens breathe is of increasing interest since it has impact on their health. For this reason, there is a movement of citizens who want to actively participate in air quality measurements. An affordable solution is the use of low...

A simple method is presented for the calculation of the activation energies of first order peaks of Thermally Stimulated Depolarization Current (TSDC). Three equations are derived, which relate the activation energy of the relaxation process, to three characteristic temperatures of the TSDC thermographs, which correspond to the peak temperature TM,...

The risk of theft of goods is certainly an important source of negative influence in human psychology. This article focuses on the development of a scheme that, despite its low cost, acts as a smart antitheft system that achieves small property detection. Specifically, an Internet of Things (IoT)-based participatory platform was developed in order...

The mechanisms activated while natural building stones are loaded mechanically at levels tending to exhaust their load-carrying capacity are intensively studied during the last years by means of sensing techniques providing data from the interior of the specimens. Among these techniques the Acoustic Emissions one is considered as the most mature an...

Problem solving is a fundamental part of engineering education. The aim of this study is to compare engineering students’ perceptions and attitudes towards problem-based multiple-choice and constructed response exams. Data were collected from 105 students, 18 of them reported to face some learning difficulty. All the students had an experience of f...

Non-extensive statistical mechanics (NESM), introduced by Tsallis based on the principle of non-additive entropy, is a generalisation of the Boltzmann–Gibbs statistics. NESM has been shown to provide the necessary theoretical and analytical implementation for studying complex systems such as the fracture mechanisms and crack evolution processes tha...

The acoustic activity detected during mechanical loading of notched marble plates under mode-I conditions is studied. Attention is paid at the very last loading steps, i.e., when fracture is impending. The specimens were made of Dionysos marble. In each specimen two edge notches were mechanically machined normally to the loading axis. Four classes...

Non-extensive statistical mechanics (NESM), which is a generalization of the traditional Boltzmann-Gibbs statistics, constitutes a theoretical and analytical tool for investigating the irreversible damage evolution processes and fracture mechanisms occurring when materials are subjected to mechanical loading. In this study, NESM is used for the ana...

This study presents the results of Pressure Stimulated Currents (PSC) analysis using non-extensive statistical physics (NESP) approach when marble and amphibolite specimens were subjected to mechanical stress up to fracture. The specimens were subjected to a constant uniaxial stress, interrupted by an abrupt step-wise stress increase (Step-Stress T...

The increasing number of students per class and the limited teaching resources are important factors for increasing the popularity of multiple-choice questions based (MCQ) exams amongst the academic tutors. Weighted grading of MCQ items is compatible with a wide range of options, therefore it can reward those students who have successfully answered...

Specular reflectance of monochromatic linearly polarized light, reflected on the interface of a prism coupled to a transparent medium at the critical angle of the transition to total internal reflection, has been long utilized to accurately determine optical constants, such as the refractive index and the dielectric constant, by the so-called ‘deri...

The aim of this study is to present the development-and field evaluation-of an IT platform for the monitoring and recording of atmospheric conditions in urban environments, mainly through a wireless network of sensor-based air quality monitoring systems that can supplement existing infrastructures.

Acoustic Emissions data recorded while marble specimens were subjected to various types of mechanical loading are analysed in the direction of detecting possible common characteristics that could be considered as fracture precursors. The tests carried out were either elementary (three‐point bending and uniaxial tension) or structural ones (pure she...

An experimental study is presented, aiming to comparatively consider the electric and acoustic activity in fiber-reinforced concrete beams subjected to three-point bending. The study is part of an ongoing project attempting to detect phenomena considered as warning signals of upcoming catastrophic fracture, in case brittle materials are loaded mech...

An alternative way to represent data regarding internal damage processes, specifically the data provided by the acoustic emission (AE) technique and the pressure stimulated currents (PSCs), is attempted in this study, in terms of log‐periodic power‐law models. Both monitoring techniques have been already successfully considered widely, either indep...

The time evolution of the electric signals emitted during uniaxial compression of prismatic cement mortar specimens is investigated in this study. The electric signals are recorded in the form of very weak electric currents, and their analysis is achieved in terms of the Pressure Stimulated Currents technique. The study is focused on the quantifica...

The spatiotemporal evolution of damage in marble speci¬mens under uniaxial compression is monitored using Pressure Stimulated Currents (PSCs) and Acoustic Emissions (AEs). The novelty of the study is the use of an integrated grid of sensors (instead of a single pair of electrodes) to detect the weak electrical signals, emitted during loading. The u...

The acoustic activity monitored during stepwise compression of prismatic marble specimens is analysed in terms of an alternative, recently introduced approach (based on the inter‐event time intervals) denoted as the F‐function. Attention is paid to the temporal evolution of this function during the constant‐stress phase of the loading scheme, in an...

The present study describes the results after the analysis of the Acoustic Emissions (AE) activity that was recorded when marble and cement mortar specimens were subjected to mechanical load until failure. The conducted experiments involved compressive axial loading and three point bending loading on prismatic marble and cement mortar specimens. Th...

Non-extensive statistical physics (NESP) provides the theoretical tool to investigate complex systems that appear in different length-scales and violate the classical Boltzmann–Gibbs statistics. In the present study, we apply the concept of NESP to analyze the Acoustic Emissions (AE) series recorded during uniaxial compression of brittle rocks, suc...

The mechanical response of mutually interconnected epistyles is studied experimentally. The specimens are made of two marble blocks connected to each other with an “I”-shaped titanium connector placed in grooves sculptured on the blocks and covered with cementitious material. The specific way of connecting epistyles simulates the one used by scient...

Among the various non-destructive geophysical methods applied for detecting subsurface imperfections and for determining the composition of the Earth’s crust, the electromagnetic (utilized via Ground Penetrating Radar - GPR) is, perhaps, the one with the highest potential for tracking structures of limited dimensions [1, 2]. The present study attem...

Low-level electric signals, generated during mechanical loading of specimens made of quasi-brittle materials, provide valuable information about internal damage processes. These signals are the result of formation and growth of micro-cracks, which are the origin of electric charges, constituting either electric dipoles or even more complicated elec...

Abstract The use of electrical and acoustic signals emission as fracture precursors when brittle materials like rocks are subjected to mechanical stress has been evaluated and found to be applicable by several researchers. A detailed review and description within the framework of the complexity approach will be performed on laboratory experiments...

Although air pollution is one of the most significant environmental factors posing a threat to human health worldwide, air quality data are scarce or not easily accessible in most European countries. The current work aims to develop a centralized air quality data hub that enables citizens to contribute to air quality monitoring. In this work, data...

Monitoring damage in the interior of mechanically loaded elements is, nowadays, implemented with the aid of various innovative experimental techniques like, for example, the Acoustic Emissions (AE) and the Pressure Stimulated Currents (PSC) [1] ones. In the present study, both the AE and PSC techniques are employed in a combined manner in order to...

The deformation of solid materials is a procedure which is accompanied by numerous effects like the emission of acoustic and electromagnetic signals or the generation of Pressure Stimulated Currents (PSCs) [1-3]. In the present paper, attention is focused on the study of acoustic emissions and PSCs, detected during a recent bending experiment on an...

An experimental protocol, including the combined application of both innovative and traditional sensing techniques, is described aiming to explore the mechanical response of marble and also to check the possibilities of detecting precursor phenomena designating upcoming catastrophic fracture. The protocol consisted of three-point bending tests with...

The mechanical response of notched marble plates under direct tension is studied experimentally. The motive of the study emerges from the needs of scientists working for restoration of marble monuments, the structural elements of which are often damaged and cracked. The specimens are made of Dionysos marble, used for the restoration of the Athenian...

Pressure Stimulated Currents (PSCs) are emitted while brittle materials of high electrical resistivity are subjected to mechanical loading. The PSCs are related to the generation of cracks and the consequent evolution of cracks’ network in the bulk of the specimen. In the experimental protocol described here cement-mortar beams of rectangular cross...

The mechanical response and fracture of Double Edged Notched Tensile (DENT) specimens, made of Dionysos marble, is studied in an effort to enlighten the mechanisms activated before and during catastrophic crack initiation and propagation in pre-cracked structures. The innovation of the study is that four modern sensing techniques, both contact and...

An experimental protocol was implemented to investigate possible correlations between the data provided by the Acoustic Emissions and the Pressure Stimulated Currents techniques and the role of the loading rate induced. Prismatic cement-mortar specimens were subjected to three-point bending (3PB) under various loading rates. It was concluded that o...

The potentialities of the pressure stimulated currents technique to be used as a continuous structural health monitoring tool in applications related to stone monuments are assessed experimentally. The data gathered by this technique from experiments with notched marble plates submitted to direct tension are considered in juxtaposition to the respe...

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 A...

The mechanical response of a restored marble epistyle subjected to multipoint bending is explored experimentally. Attention is focused to the role of the inclination angle of the fracture plane with respect to the loading axis. Moreover the capability of some innovative sensing techniques to act as convenient Structural Health Monitoring (SHM) tool...

The damage development due to externally applied mechanical stress is a hot topic of interest involving several applications of everyday life, like civil engineering, monument restoration, construction evaluation and health monitoring. Repetitive loadings of brittle materials cause internal damages that gradually extend, leading to inevitable failu...

Metallic “I”-shaped connectors, properly placed in sculptured grooves, are used nowadays for restoring damaged connections of the Parthenon Temple. The grooves are then filled with a suitable cement mortar. In this study, the mortar was reinforced with multi-walled carbon nanotubes, in an effort to develop a smart material which could monitor the s...

Rejoining fragmented marble structural members of the Acropolis of Athens monuments is achieved by inserting threaded titanium bars into holes pre-drilled in the body of the members. Then the holes are filled with liquid cementitious material. This study aims to investigate the extraction of the bars from the marble volume (pull-out) in an effort t...

This work discusses the experimental results of Acoustic Emission (AE) recordings during repetitive loading/unloading loops of cement mortar beams subjected to three point bending. Six repetitive loading cycles were conducted at a gradually higher load level until the failure of the specimens. The experimental results clearly show the existence and...

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 ima...

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 ima...

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...