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 WAVELET REPRESENTATION OF HVSR TECHNIQUE

In order to evaluate eathquake site amplification characteristics, horizontal to vertical Fourier spectrum of microtremor has been widely used. As long as the Fourier transform (FT) cannot distinguish between stationary and non-stationary coefficients we cannot eliminate the contamination of microtremors signals from short period transients. The wavelet transform (WT), using the property of localization of wavelet bases has been widely used in signal processing. Unlike the Short Time Fourier Transform (STFT) in which the width of window is fixed, the WT localizes signal in a variable window using the dilation parameter. This property, which derived directly from multiresolution analysis provide us the ability to decompose a signal in a well localized set of coefficients and identify the non-stationary portions of it. In the present study we use the WT in order to eliminate the non-stationahties in microtremor signals before we calculate the spectrum of each one using conventional FFT algorithm

A WAVELET REPRESENTATION OF HVSR TECHNIQUE

ABSTRACT In order to evaluate eathquake site amplification characteristics, horizontal to vertical Fourier spectrum of microtremor has been widely used. As long as the Fourier transform (FT) cannot distinguish between stationary and non-stationary coefficients we cannot eliminate the contamination of microtremors signals from short period transients. The wavelet transform (WT), using the property of localization of wavelet bases has been widely used in signal processing. Unlike the Short Time Fourier Transform (STFT) in which the width of window is fixed, the WT localizes signal in a variable window using the dilation parameter. This property, which derived directly from multiresolution analysis provide us the ability to decompose a signal in a well localized set of coefficients and identify the non-stationary portions of it. In the present study we use the WT in order to eliminate the non-stationarities in microtremor signals before we calculate the spectrum of each one using conventional FFT algorithm

Crack identification in plates using 1-D discrete wavelet transform

In the present work, the defect detection while using Discrete Wavelet Transform in rectangular plate structures is investigated. The plate bending is described by using the Boundary Element Method with boundary integral equations formulated in a modified simplified approach. The boundary elements of a constant type in a non-singular approach are implemented. Defects are introduced by additional edges forming slots or holes in relation to the basic plate domain. Estimation of the defect position is performed while using wavelet coefficients of curvature and deformation signals as well as a newly proposed moving variance estimator

Crack identification in plates using 1-D Discrete Wavelet Transform

In the present work, the defect detection while using Discrete Wavelet Transform in rectangular plate structures is investigated. The plate bending is described by using the Boundary Element Method with boundary integral equations formulated in a modified simplified approach. The boundary elements of a constant type in a non-singular approach are implemented. Defects are introduced by additional edges forming slots or holes in relation to the basic plate domain. Estimation of the defect position is performed while using wavelet coefficients of curvature and deformation signals as well as a newly proposed moving variance estimator

The Equal Areas Pulse Width Modulation (EAPWM) Method: An alternative approach to programmed PWM schemes

This paper investigates a direct approach to programmed PWM modelling directed to simplification of implementation. Analytical equations of Equal Areas Pulse Width Modulation (EAPWM) method are presented along with harmonic analysis results for odd number of pulses in half period of a 3-level single phase VSI. Simulation and practical work is being presented also. Estimation of the marginal modulation index has been carried out, inside the linear region of operation. The proposed method provides a rigid framework since it lies on a well-defined mathematical formulation without the need of large computational effort.http://www.journal.esrgroups.org/jes/papers/12_1_12.pd

The South Aegean seismological network – HSNC

In the present work, the installation and the technology applied for the operation of the Hellenic Seismological Network of Crete (HSNC), located in the front of the Hellenic Arc, are presented. The topology, the communication modes (wire and satellite) along with data collection and processing methodologies applied leads to the operation of a new seismological infrastructure in South Aegean, one of the most seismically active regions in Europe

The electrical activity of Saharan dust as perceived from surface electric field observations

We report on the electric field variations during Saharan dust advection over two atmospheric remote stations in Greece, using synergistic observations of the vertical atmospheric electric field strength (Ez) at ground level and the lidar-derived particle backscatter coefficient profiles. Both parameters were monitored for the first time with the simultaneous deployment of a ground-based field mill electrometer and a multi-wavelength polarization lidar. The field mill time series are processed to extract the diurnal variations of the global electric circuit and remove fast field perturbations due to peak lightning activity. In order to identify the influence of the elevated dust layers on the ground Ez, we extract a localized reference electric field from the time series that reflects the local fair-weather activity. Then, we compare it with the reconstructed daily average behaviour of the electric field and the Saharan dust layers' evolution, as depicted by the lidar. The observed enhancement of the vertical electric field (up to g≈100 V m-1), for detached pure dust layers, suggests the presence of in-layer electric charges. Although higher dust loads are expected to result in such an electric field enhancement, episodic cases that reduce the electric field are also observed (up to g≈60 V m-1). To quantitatively approach our results, we examine the dependency of Ez against theoretical assumptions for the distribution of separated charges within the electrified dust layer. Electrically neutral dust is approximated by atmospheric conductivity reduction, while charge separation areas within electrically active dust layers are approximated as finite-extent cylinders. This physical approximation constitutes a more realistic description of the distribution of charges, as opposed to infinite-extent geometries, and allows for analytical solutions of the electric field strength so that observed variations during the monitored dust outbreaks can be explained. © 2021 Copernicus GmbH. All rights reserved