Analysis and processing of mechanically stimulated electrical signals for the identification of deformation in brittle materials

The fracture of brittle materials is of utmost importance for civil engineering and seismology applications. A different approach towards the aim of early identification of fracture and the prediction of failure before it occurs is attempted in this work. Laboratory experiments were conducted in a variety of rock and cement based material specimens of various shapes and sizes. The applied loading schemes were cyclic or increasing and the specimens were tested to compression and bending type loading of various levels. The techniques of Pressure Stimulated Current and Bending Stimulated Current were used for the detection of electric signal emissions during the various deformation stages of the specimens. The detected signals were analysed macroscopically and microscopically so as to find suitable criteria for fracture prediction and correlation between the electrical and mechanical parameters. The macroscopic proportionality of the mechanically stimulated electric signal and the strain was experimentally verified, the macroscopic trends of the PSC and BSC electric signals were modelled and the effects of material memory to the electric signals were examined. The current of a time-varying RLC electric circuit was tested against experimental data with satisfactory results and it was proposed as an electrical equivalent model. Wavelet based analysis of the signal revealed the correlation between the frequency components of the electric signal and the deformation stages of the material samples. Especially the increase of the high frequency component of the electric signal seems to be a good precursor of macrocracking initiation point. The additional electric stimulus of a dc voltage application seems to boost the frequency content of the signal and reveals better the stages of cracking process. The microscopic analysis method is scale-free and thus it can confront with the problems of size effects and material properties effects. The AC conductivity time series of fractured and pristine specimens were also analysed by means of wavelet transform and the spectral analysis was used to differentiate between the specimens. A non-destructive technique may be based on these results. Analysis has shown that the electric signal perturbation is an indicator of the forthcoming fracture, as well as of the fracture that has already occurred in specimens.EThOS - Electronic Theses Online ServiceNational Foundation of Scholarships (IKY) GreeceGBUnited Kingdo

Reform of the Royal College of Surgeons.

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The role of propagating stress waves on a geophysical scale: Evidence in terms of nonextensivity

Laboratory experiments have shown that, during a fracture, the breaking of a bond launches a propagating stress wave which may trigger the breaking of other bonds. We examine here the possibility that the same holds on a geophysical scale. Based on a nonextensive approach, we examine whether the transient stresses of seismic waves from a major earthquake (EQ) can trigger a considerably distant significant EQ. We use three different analytical approaches: (i) a recently introduced fragment-asperity interaction model for EQ dynamics based on nonextensive Tsallis statistics; (ii) the Hurst exponent; (iii) organization in terms of Fisher information. We find that the triggered seismicity displays higher nonextensivity, increased persistent behavior, and a higher level of organization. Using the same approaches, we further elucidate the link between the associated precursory kHz electromagnetic (EM) activity and the last stage of the impending EQ generation. We examine whether the statistics of regional seismicity could be a macroscopic reflection of physical processes in the EQ source, as would be expected by the fractal nature of fracture and faulting. © 2012 Elsevier B.V. All rights reserved

Fractal evolution of MHz electromagnetic signals prior to earthquakes: results collected in Greece during 2009

This paper addressed a fractal evolution of 11 one-month lasting MHz electromagnetic disturbances, recorded in Greece prior to nine significant earthquakes of 2009. Time-space wavelet-based power spectral techniques were employed in the analysis. All investigated signals evolved naturally to epochs of fractal organization in space and time. Continuous organization was detected in seven signals. Significant number of successive (Formula presented.) power-law (Formula presented.) -values were observed lying between 1.5 and 3.0 or above. The majority of fractal segments exhibited anti-persistent (Formula presented.) or persistent (Formula presented.) behaviour. Switching between persistency and anti-persistency was also found. Locality and sensitivity were traced. Findings were considered indicative of self-organized critical states of the last stages of preparation of the investigated earthquakes. Results implied fractional Brownian modelling. Explanations were proposed in view of the asperity model. Persistent–anti-persistent MHz anomalies were due to self-organized micro-cracking of the heterogeneous medium of the earth's crust which may have led the system's evolution towards global failure. The precursory value of the signals was discussed. © 2014 Taylor & Francis

Traces of self-organisation and long-range memory in variations of environmental radon in soil: Comparative results from monitoring in Lesvos Island and Ileia (Greece)

This paper addresses issues of self-affinity, long-memory and self-organisation in variations of radon in soil recorded in Lesvos Island, Greece. Several techniques were employed, namely (a) power-law wavelet spectral fractal analysis, (b) estimation of Hurst exponents through (b1) rescaled-range, (b2) roughness-length, (b3) variogram and (a), (c) detrended fluctuation analysis, (d) investigation of fractal dimensions and (e) analysis of five block entropies: (e1) Shannon entropy, (e2) Shannon entropy per letter, (e3) conditional entropy, (e4) Tsallis entropy, and (e5) normalised Tsallis entropy. Long-lasting antipersistency was identified during a period of anomalous radon variations following fractional Brownian modelling. Remaining variations did not exhibit analogous behaviour and followed fractional Gaussian modelling. Antipersistent power-law-beta-exponent-values between 1.5 and 2.0 were detected during anomalies. Persistent values were also found. Hurst exponents were mainly within 0 < H < 0.5. Some persistent exponents (0.5 < H < 1) were also observed. Fractal dimensions were within 1.5 < D < 2. Radon anomalies presented lower fractal dimensions. Shannon entropy ranged between 0.77 ≤ H(n) ≤ 2.38, Shannon entropy per letter, between 0.19 ≤ h (n) ≤ 0.59, conditional entropy, between 0.01 ≤ h (n) ≤ 0.58, Tsallis entropy, between 0.55 ≤ S q ≤ 1.01 and normalised Tsallis entropy between, 0.98 ≤ {S} S ≤ 5.42 (block-size n = 4). Entropies were lower during anomalies, indicating strong self-organisation. Persistency-antipersistency switching was observed, consistent with long-memory dynamics. Potential geological sources were discussed. The asperity-model was proposed. Findings were compared to results obtained under analogous methodologies in Ileia, Greece. © 2013 Akadémiai Kiadó, Budapest, Hungary