The conditions governing crack propagation in prismatic, concrete specimens, are explored, taking advantage of the Acoustic Emissions recorded during an experimental protocol comprising of standardized bending tests. The time series of the Acoustic Emissions are modelled adopting the concepts of Non-Extensive Statistical Mechanics, as formulated by means of Tsallis entropy and entropic index. The innovation of the present approach lies in the fact that the acoustic data are analyzed using a parameter exhibiting transient characteristics and strong fluctuations, namely, the “Variable Returns” of the “counts” of the acoustic hits. Moreover, the evolution of this parameter is modelled using q-Gaussian- rather than Gaussian-distributions, since the processes that take place while brittle materials are loaded up to fracture are characterized by memory effects and, also, long-range interactions, rendering the traditional Boltzmann-Gibbs Statistical Thermodynamics inadequate for proper modelling. The analysis reveals that Tsallis formalism for the Non-Extensive Statistical Mechanics, models quite satisfactorily the mechanical response of the loaded system and, also, that the q-Gaussian distribution fits almost perfectly the respective Probability Distribution Functions. Deviations are observed only for the tails of the experimental distributions, which correspond to an extremely small portion of the overall number of the acoustic hits that were recorded. In addition, it is revealed that the temporal variation of the entropic index is characterized by a global maximum, which designates entrance into the stage of impending macroscopic fracture and provides a Structural Health Monitoring tool that, under certain conditions, could be interesting from the engineering point of view.

The Acoustic Emission technique is one of the most commonly used ones when the damage and fracture of a variety of materials need to be studied (either at the laboratory scale or in the field). The technique is especially useful when materials such as rocks are mechanically loaded since their brittle nature makes the study of their fracture process extremely difficult. In this context, taking advantage of the distances between successive acoustic events, i.e., the interevent distances, a new function is proposed, the D*n one, which is calculated in two steps: (a) the D*-function is calculated as the moving average of n successive interevent distances, the coordinates of which had been previously normalized with respect to their maximum and minimum values, and (b) the D*n-function is obtained by normalizing with respect to its maximum and minimum values the D*-function. The D*n-function actually provides quantitative information about the local concentration of the sources of the acoustic events or their dispersal. Its temporal evolution was studied in three experimental protocols with different materials. In all cases, a region, during which the lowest values of D*n are observed, is detected (or in other words, during which the sources of the acoustic events are localized in a specific area of the specimen) and it is correlated to the specific area of the specimen where the macrocrack is expected to be formed and start propagating.

The rate of change of the normalized Cumulative Counts of the acoustic hits, recorded while marble specimens are compressed uniaxially, and is analyzed in the Natural Time Domain. The analysis reveals the systematic presence of a characteristic plateau that could potentially serve as a precursor to fracture. Starting initially well below unity, the specific parameter increases towards a limit equal to one and is stabilized around this value with minor fluctuations. The starting “instant” of this plateau is linearly related to the loading rate applied. This “instant” and the respective load level are in very good agreement with the abrupt change of the average rate of generation of acoustic signals. These findings are juxtaposed to the respective ones drawn by analyzing data from previously published experimental protocols involving marble specimens of varying geometries subjected to various loading schemes and are found highly consistent with each other. The same holds true for the agreement between the data of the present study and recently published ones dealing with both plain and fiber-reinforced concrete beams. This consistency suggests that the conclusions drawn exhibit a kind of universality, highlighting the potential of this plateau’s onset to serve as a reliable index for Structural Health Monitoring purposes.

A novel approach for detecting preliminary signals designating upcoming entrance of a loaded system to the critical stage of impending fracture is assessed. The approach is based on the analysis of a time series of the cumulative number of acoustic events, the amplitude of which exceeds the respective average value of all the events recorded during loading. Using the "sliding window" technique, the average slope of the evolution of this time series is quantified, either against conventional or natural time (the latter provides a more detailed view of the stage before macroscopic fracture, during which the "information" gathered is very densely packed in a short interval). For the needs of this study, data from a previously published experimental protocol are exploited. The protocol comprised notched, beam-shaped specimens, made of either plain or fiber-reinforced concrete, under three-point bending. It is concluded that the slope of the evolution of the above time series systematically attains a value equal to unity slightly before the applied load attains its peak value. The results of the present analysis are in qualitative agreement with the respective ones based on either the instantaneous frequency of generation of acoustic events or the Euclidean distance between the sources of acoustic signals.

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 analysis of the acoustic activity is implemented in terms of the energy content of the acoustic signals rather than of their interevent time or their interevent distance. The Energy Distribution Functions were properly fitted using the expression proposed by Shcherbakov, Kuksenko and Chmelet. This study reveals that the loading and fracture processes of the specific materials are definitely non-additive and non-extensive. It is concluded that the presence of notches is crucial since it assigns non-additivity and non-extensivity from relatively low loading levels due to the early formation of the fracture process zone around the crown of the notch. The values of the Tsallis entropic index, q, that were determined are in very good agreement with the respective ones obtained in previous studies by means of different analysis tools. Finally, a clear correlation between the index q and the average energy content of the acoustic signals is highlighted for the whole range of values of the energy content of the acoustic signals.

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 quantified in terms of the normalized number of acoustic hits, their average rate of production and their cumulative energy, and, the cumulative counts and their average rate of change. The electric activity is studied in terms of the pressure stimulated currents and the electric charge released. The analysis revealed that the acoustic and electric activities are linearly correlated to each other, suggesting that they are different manifestations of the same damage mechanisms. In addition, Kaiser’s effect, governing the acoustic activity, is found to govern, also, the electric activity. Moreover, it is concluded that entrance into the critical stage is safely predicted by means of a simple criterion, based on the evolution of the average rate of change of the normalized cumulative counts in the natural time domain. These predictions are almost identical with those of the criterion based on the “variance” and the “entropies” of the time series of acoustic events in this domain.

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 the average frequency of generation of acoustic signals or of their Cumulative Energy content. Two classes of specimens were tested, differing with respect to the loading rate imposed. It is concluded that the electric activity is very weak, or even negligible, until the critical instant designated by the entrance into the stage of thermodynamically irreversible response of the material. Beyond this instant the electric activity starts increasing very rapidly almost until the instant at which the load attains its peak value. A few seconds before fracture, the electric signal exhibits an abrupt drop. The temporal evolution of the electric activity and that of the acoustic one are in excellent agreement, independently of the parameter used for their quantification. The study revealed that both activities provide clear pre-failure indices, early warning about upcoming disastrous fracture. Moreover, it was highlighted that the loading rate diversifies the results only from a quantitative point of view, “translating” the stress interval within which the pre-failure indices are located: The higher the loading rate the lower the stress level at which the pre-failure indices are detected.

Considering that the phenomenon of fracture is a manifestation of the series of non-linear processes, which characterize complex dynamical systems, it is reasonable to approach the specific phenomenon using advanced statistical tools rather than traditional Statistical Mechanics, taking into account that the response of such systems (at non-equilibrium stages) is not compatible to the thermodynamic principle of additivity.

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 exhibits a clear plateau, which is terminated just a few tenths of a second before fracture. The specific conclusion is verified by considering the temporal variation of the average energy of the acoustic events against the respective variation of the Pressure Stimulated Currents produced while a restored marble epistyle of the Parthenon Temple is loaded under multi-point bending. It is thus safely stated that the onset of the specific plateau can be considered as an interesting pre-failure signal warning about entrance of the system into its critical stage.

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 device suggested by International Society for Rock Mechanics (ISRM) and also the three-dimensional digital image correlation (3D-DIC) technique. Design/methodology/approach The analytical solution highlighted the role of geometrical factors, like, for example, the width of the notch, which are usually disregarded. The data of the experimental protocol were comparatively considered with those concerning the response of the specific material under uniaxial tensile load. Findings This combined study provided interesting data concerning some open issues, as it is the exact crack initiation point and the level of the critical load causing crack initiation. It was definitely indicated that the crack initiation point is not a priori known (even for notched specimens) and, also, that the maximum recorded load does not correspond by default to the critical load responsible for the onset of catastrophic macroscopic fracture. Originality/value It was suggested that the load considered critical one for the determination of mode-I fracture toughness K-IC is erroneous. At a load equal to about 70% of the maximum one, a process zone is formed (zone of non-reversible phenomena) around the notch's crown, designating termination of the validity of any linear elastic solution used to determine the normalized stress intensity factors (SIFs). Moreover, at a load level equal to about 95% of the macroscopically observed fracture load, crack propagation has already begun. Therefore, the experimental procedure must be monitored with additional equipment, providing an overview of the displacement field developed during loading.

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 either plain or fiber-reinforced concrete. The parameter adopted for the analysis was the average rate of change in the Cumulative Counts. Τhe evolution of the specific parameter was considered in the Natural Time Domain, rather than in the conventional time domain. Experimental data from already published three-point bending protocols were used. It was revealed that the specific parameter attains, systematically, a limiting value equal to unity exactly at the instant at which the load reaches its maximum value, which is not identical to the load recorded at the instant of fracture. Similar observations were made for a complementary protocol with uniaxially compressed mortar specimens. The conclusions drawn were supported by the b-values analysis of the respective acoustic data, again in terms of Natural Time. It is, thus, indicated that the evolution of the average rate of change in the Cumulative Counts in the Natural Time Domain provides an index about the proximity of the applied load to a value beyond which the specimen enters into the critical state of impending fracture.

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 Statistical Mechanics, a discipline founded on a class of entropies violating the additivity principle, which is the cornerstone of the Boltzmann-Gibbs Statistical Mechanics. The specimens used for the experiments were marble blocks simulating either fragmented and restored epistyles or mutually interconnected intact epistyles of the Parthenon Temple on the Acropolis of Athens. The specimens consisted of three mate­rials (marble, metallic connectors and cementitious pastes) exhibiting, thus, a strongly non-homogeneous nature. The entropic index, i.e., the parameter quantifying the degree of non-additivity, was used for the analysis of the ex­pe­rimental data. The results were considered in juxtaposition to the re­spective ones from experimental protocols with specimens of macroscopically homo­geneous nature. It was concluded that the temporal evolution of the entropic index provides very good insight into the level of damage ac­cumu­lated in the loaded structure, independently of whether the structure is homogeneous or not, providing an interesting pre-failure indicator.

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 the rapid development of low-cost sensors. Given that citizens are concerned about the air quality of the environment in which they live, they are turning to the supply of low-cost sensors, as they are affordable. The question of the reliability of measurements from low-cost sensors remains an area of research. In this research work, the optimization of ozone (O3) and nitrogen dioxide (NO2) measurements of low-cost electrochemical air quality sensors is investigated by applying nonlinear regression, using a second-order polynomial equation as a correction factor. The proposed correction method is implementable in IoT devices, as it does not require high computational resources. The results show that the measurements are susceptible to correction, with the effect that the corrected values are close to the actual values obtained by the reference instruments of the Department of Environmental Pollution Control Project of Athens (PERPA), a service of the Greek Ministry of the Environment and Energy.

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 loading were analyzed in terms of their interevent time intervals and the respective cumulative probability. The experimental data were then properly fitted by means of the q-exponential function providing the temporal evolution of the entropic index q throughout the duration of each experiment. The evolution of q was studied in juxtaposition to that of the load applied as well as to that of some additional parameters used to describe the acoustic activity, namely the b-values and the F-, P- and D-functions. The analysis indicated that at load levels approaching the load-carrying capacity of the specimens, the response of both the unreinforced and the fiber reinforced beams corresponds to q-values approaching 2. It is thus highlighted that, according to NESM principles, the damage processes at the specific load levels are characterized by the existence of a number of nonindependent subsystems with long-range mutual interactions and memory effects, which cannot be properly described by the concepts of the traditional Boltzmann-Gibbs Statistical Mechanics. On the contrary, description in terms of NESM provided useful information about the damage evolution during the very last loading stages before macroscopic fracture. In addition, the analysis provided indications that the onset of decrease of the qvalues, towards the critical limit of 1, could be considered as a potential pre-failure index, warning about upcoming entrance of the system into its critical stage, namely that of impending fracture.

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 for studying fracture mechanisms and damage evolution processes during fracture experiments in specimens made of brittle materials. In the current work, the time intervals of the recorded AE data are analyzed in terms of NESM. For each examined specimen, the corresponding q entropic indices and the βq parameters were calculated, and their variability in terms of the stress applied were studied. Furthermore, a possible linear relationship between the entropic index q and the parameter βq was examined, and it was investigated whether the observed deviation from monotonicity between q and βq may signal increased accumulation of damage, eventually leading to the final fracture of the specimens. Through this work, the emergence of an additional pre-failure indicator (i.e., the deviation from monotonicity between q and βq) alongside well-established ones can provide further insight regarding the underlying crack development mechanisms and damage accumulation processes during the fracture of rock materials.

The target of this study is to detect the existence of criticality indices, namely, indices which may be considered as in-time warning signals about the entrance of a mechanically loaded system into its critical stage, i.e., the stage of impending fracture. To achieve this target advantage is taken of the experimental data provided by two sensing techniques, namely the Acoustic Emissions and the Pressure Stimulated Currents ones. The innovation of the present study is that the temporal evolution of the acoustic and electrical activities is implemented in terms of the Natural Time rather than in terms of the conventional time. The experimental protocol included three-point bending tests with centrally notched beam-shaped specimens made of either plain concrete or concrete reinforced with either plastic or metallic short fibers. The comparative analysis of the acoustic and electric activities in the Natural Time Domain revealed that both techniques provide clearly distinguishable criticality indices. Moreover, it was proven that the overall evolution of the acoustic activity is quite compatible with the respective one of the electric activity, concerning both the instant at which the pre-failure indices are detected, and, also, the duration of the respective regions of criticality.

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 vicinity of fracture. The axial compressive loading protocol incorporated a constantly increasing stress, from early stress values up to the vicinity of fracture and a sequential stress stabilization until the time the specimen collapses, due to severe growing internal damages. Concurrent recordings of the electrical and acoustic emissions were performed. The AE recordings were analyzed, by incorporating the recently introduced F- and P-functions, and the well-known b-value. The experimental results highlight strong similarities regarding the variations of the PSC signal, the AE hits occurrence rate (F-function), and the AE hits energy release rate (P-function). The above was also confirmed with another similar experiment in an identical specimen. It is noteworthy that, during the stay of the specimens under a constant load regime near their strength levels, a peak appears in the above quantities, which is directly related to an increased rate of axial deformation. The temporal evolution of the b-values is also presented. Results show that the local minima appearing at values close to b ≈ 1.0 correspond to the local maxima of the PSC signal. It is straightforwardly concluded that when both the PSC signal and the AE data are combined, they provide clear pre-failure indicators.

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 beams, made of cementitious materials, subjected to three-point bending are analyzed. It is concluded that in case the acoustic activity is described with the aid of the F-function in the Natural Time domain, its evolution is governed by a power law, independently of geometrical details and the type of the loading scheme. It appears that the onset of validity of this law provides an interesting pre-failure indicator.

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. The target of the study is to investigate the existence of indices that could be considered as pre-failure indicators of the upcoming fracture. In addition, an attempt is undertaken to classify the damage mechanisms activated to tensile or shear nature. Considering comparatively the spatio-temporal evolution of the position of the acoustic sources and the respective temporal evolution of the frequency of generation of acoustic events, it was concluded that for relatively low load levels the acoustic sources are rather randomly distributed all over the volume of the specimens. As the load increases toward its maximum value, the acoustic sources tend to accumulate in the immediate vicinity of the crown of the notch and the average distance between them approaches a minimum value. When this minimum value is attained, the load is maximized and the generation frequency of the acoustic events increases rapidly. The simultaneous fulfillment of these three conditions is observed a few seconds before the onset of propagation of the catastrophic macrocrack for all classes of specimens tested, providing a kind of warning signal about the upcoming fracture. Moreover, the classification of the damage mechanisms to tensile and shear ones revealed a crucial difference between the plain and the reinforced specimens after the maximization of the load applied. Indeed, while for the plain specimens, the prevailing damage mechanism is tensile microcracking, for the reinforced specimens a balance between tensile and shear damage mechanisms is observed after the load applied has attained its peak and starts decreasing.

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 designed to assess the students' proficiency to solve electronics problems. Data were collected using a 15-item questionnaire which included a free text question. Overall the students expressed a preference for the objective-type examination format. The students who self-reported to face learning difficulties (LD) were equally divided between the two examination formats. Their examination format preference was determined by the details of their learning difficulties, indicating that none of the two assessment formats effectively solves the assessment question for these students. For the rest of the respondents, examination format preference was accompanied by opposing views regarding answering by guessing, having the opportunity to express their views, selecting instead of constructing an answer, having the opportunity to demonstrate their knowledge, and having control of the exam answers.