Pressure stimulated currents in rocks and their correlation with mechanical properties

The spontaneous electrification of marble samples was studied while they were subjected to uniaxial stress. The Pressure Stimulated Current (PSC) technique was applied to measure the charge released from compressed Dionysos marble samples, while they were subjected to cyclic loading. The experimental results demonstrate that, in the linear elastic region of the sample, no PSC is recorded, while beyond the stress limit (s>0.60), observable variations appear, which increase considerably in the vicinity of sample failure, reaching a maximum value just before the failure. The emitted current is reduced on each loading cycle and it has a reciprocal dependence to the normalized Young modulus. The MCD model, applied out of the vicinity of sample failure explains successfully the above findings. The existence of a "memory-like" behavior of the sample, could justify the weakness or absence of electrical earthquake precursors, during an aftershock sequence

Piezo stimulated currents in marble samples: precursory and concurrent-with-failure signals

International audienceThe Earth?s electric field transient variations are promising candidates of earthquake precursors. In order to study the physical mechanisms of such precursory signals, laboratory experiments of uniaxial compression were carried out. More specifically the behaviour of stressed marble samples from Penteli Mountain was investigated. The samples were subjected to a time-varying uniaxial compression at both variable and constant stress rates. During the first set of experiments weak electric currents were detected during pressure variations. Such Piezo Stimulated Currents (PSC) were detected while stress steps, both positive and negative were applied, the maximum stress never being greater than the elasticity limit. During the second set of experiments stress was applied at a constant rate starting from zero-stress and ending in fracture. In the region beyond the elastic limit a PSC was detected which after reaching a peak suffered a reversal in its polarity just before fracture. In a third set of experiments the same procedure was applied to previously structurally damaged samples taking care not to fracture them. In all cases the PSC followed the variation of stress and moreover it was observed that a linear relationship existed between the PSC maxima and the corresponding stress-rate maxima. The mechanism responsible for the described phenomena can be ascribed to the Moving Charged Dislocations model

Correlation of pressure stimulated currents in rocks with the damage parameter

Pressure Stimulated Current (PSC) experiments were conducted on marble samples to correlate PSC with the damage parameter, D. The phenomena and procedures taking place in the vicinity of the fracture limit were observed and analytically described. PSC recordings were conducted by application of uniaxial compressional stress, both at a constant stress rate and at a constant deformation rate. A linear relationship was shown to exist between the emitted PSC and the damage parameter which quantifies the deviation from linear elasticity and the concentration of microcracks

Pressure Stimulated Currents (PSC)in marble samples

The electrical behaviour of marble samples from Penteli Mountain was studied while they were subjected to uniaxial stress. The application of consecutive impulsive variations of uniaxial stress to thirty connatural samples produced Pressure Stimulated Currents (PSC). The linear relationship between the recorded PSC and the applied variation rate was investigated. The main results are the following: as far as the samples were under pressure corresponding to their elastic region, the maximum PSC value obeyed a linear law with respect to pressure variation. In the plastic region deviations were observed which were due to variations of Young s modulus. Furthermore, a special burst form of PSC recordings during failure is presented. The latter is emitted when irregular longitudinal splitting is observed during failure

Innovative Experimental Techniques in the Service of Restoration of Stone Monuments - Part I: the Experimental Set up

AbstractThe mechanical response of marble epistyles interconnected to each other by means of metallic connectors when subjected to shear loading is studied experimentally. The study is motivated by the need of the scientific group of the Athenian Acropolis to substitute damaged ancient connections by new ones. In this direction “I”-shaped titanium connectors are placed in the grooves sculptured by ancient stonemasons in the marble blocks and the empty space is then filled by a suitable cement-based material. Guided by the experience gathered from the inspection of failed connections, which clearly indicates that failure starts at the interior of the “titanium-mortar-marble” complex, along the material interfaces, an experimental protocol was improvised, aiming at pumping data from the interior of the interconnected epistyles. For this to be accomplished innovative sensing techniques like pressure stimulated currents, digital image correlation and acoustic emission were used in conjunction with traditional ones. In the first part of this short two-paper series the experimental set-up, the materials and the specimens’ geometry are described

A strategy to incorporate prior knowledge into correlation network cutoff selection

Correlation networks are frequently used to statistically extract biological interactions between omics markers. Network edge selection is typically based on the statistical significance of the correlation coefficients. This procedure, however, is not guaranteed to capture biological mechanisms. We here propose an alternative approach for network reconstruction: a cutoff selection algorithm that maximizes the overlap of the inferred network with available prior knowledge. We first evaluate the approach on IgG glycomics data, for which the biochemical pathway is known and well-characterized. Importantly, even in the case of incomplete or incorrect prior knowledge, the optimal network is close to the true optimum. We then demonstrate the generalizability of the approach with applications to untargeted metabolomics and transcriptomics data. For the transcriptomics case, we demonstrate that the optimized network is superior to statistical networks in systematically retrieving interactions that were not included in the biological reference used for optimization

Pull-out of threaded reinforcing bars from marble blocks

AbstractRejoining 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 to enlighten the mechanisms activated before and during the phenomenon. The study is implemented experimentally. The main problem hard to overcome was the fact that the weak link of the marble-cement-titanium complex, which is the marble-cement interface, is inaccessible for traditional sensing techniques. In this context innovative techniques were employed (Acoustic Emission and Pressure Stimulated Currents), which can detect failure and damages at the interior of the complex. Traditional sensing techniques were used in parallel mainly for calibration / validation reasons. The specimens, prepared by experienced personnel of the Parthenon’s worksite, were prisms made of Dionysos marble. Threaded titanium bars were inserted into through holes filled with a liquid cement paste. The quantities recorded during the tests were the load, the displacement, the bar’s axial strain and relative slip with respect to the marble, the electrical signals emitted and the acoustic emissions produced. Conclusions are drawn concerning correlations between the above quantities. The data gathered were then used to validate numerical models which will be used for parametric analyses

Pressure Stimulated Currents (PSC)in marble samples

The electrical behaviour of marble samples from Penteli Mountain was studied while they were subjected to uniaxial stress. The application of consecutive impulsive variations of uniaxial stress to thirty connatural samples produced Pressure Stimulated Currents (PSC). The linear relationship between the recorded PSC and the applied variation rate was investigated. The main results are the following: as far as the samples were under pressure corresponding to their elastic region, the maximum PSC value obeyed a linear law with respect to pressure variation. In the plastic region deviations were observed which were due to variations of Young s modulus. Furthermore,
 a special burst form of PSC recordings during failure is presented. The latter is emitted when irregular longitudinal
 splitting is observed during failure

Problem-Based Multiple Response Exams for Students with and Without Learning Difficulties

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. © 2022, Springer Nature Switzerland AG