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

Nonlinear analysis of biomagnetic signals recorded from uterine myomas

OBJECTIVE: To determine if there is any non-linearity in the biomagnetic recordings of uterine myomas and to find any differences that may be present in the mechanisms underlying their signal dynamics. METHODS: Twenty-four women were included in the study. Sixteen of them were characterised with large myomas and 8 with small ones. Uterine artery waveform measurements were evaluated by use of Pulsatility Index (PI) (normal value PI<1.45). RESULTS: Applying nonlinear analysis to the biomagnetic signals of the uterine myomas, we observed a clear saturation value for the group of large ones (mean = 11.35 ± 1.49) and no saturation for the small ones. CONCLUSION: The comparison of the saturation values in the biomagnetic recordings of large and small myomas may be a valuable tool in the evaluation of functional changes in their dynamic behavior

Numerical analysis of stress concentration factors on tensile armour wires inside the end-fitting of an axially tensed unbonded flexible pipe

The present paper examines the case of a metal based unbonded flexible pipe under a number of axial tensile operational loads of varying intensity, expressly focusing on the response of the segments of the pipe’s tensile armour wires that lie inside the pipe’s End-Fitting (EF), with intent to track the development of high stress concentration areas there and assess the stress concentration factors (SCF) involved. Additionally, a parametric study is performed, to identify potential factors that might affect the maximum SCF values measured on these wire segments inside the EF. Based on the parametric study’s findings, an approximate relationship for evaluating the expected maximum SCF on the tensile wire inside the EF is developed. The paper adopts a numerical approach to the problem, based on the development of a two-dimensional finite element model for evaluating the wire’s response to axial tensile operational loads. The model considers both the effects the EF’s assembling procedure and the Factory Acceptance Test (FAT) have on the behavior of wire segments lying inside the EF. An analytical approach is employed as a benchmarking tool of the numerical model’s performance

Stress state of unbonded flexible pipe’s tensile armour wires inside end-fittings

The present paper addresses the stresses developing, inside the End-Fitting (EF), on the tensile armour wires of a metal based unbonded flexible pipe under axial tensile operational loads. To this end, a bi-dimensional finite element model that estimates the stress state of tensile armour wires inside the EF is developed. The stresses induced during the EF assembling procedure as well as the Factory Acceptance Test (FAT) are considered in the developed numerical model. An analytical approach to this problem, suggested by Campello (2014) and Campello et al. (2016), is utilized as a benchmarking tool in order to evaluate the developed numerical model’s performance. The results obtained show good agreement between the developed model and the analytical approach. A high concentration of stresses on the tensile armour wires inside the EF, at a key point near the EF’s entrance, is observed under all operational load cases. Moreover, the results suggest that the EF assembly procedure and the FAT test need to be indeed considered, as they yield significant residual stresses on the tensile armour wires inside the EF

GARCH Modelling of High-Capitalization Cryptocurrencies’ Impacts During Bearish Markets

This study investigates how twelve cryptocurrencies with large capitalization get influenced by the three cryptocurrencies with the largest market capitalization (Bitcoin, Ethereum, and Ripple). Twenty alternative specifications of ARCH, GARCH as well as DCC-GARCH are employed. Daily data covers the period from 1 January 1 2018 to 16 September 2018, representing the intense bearish cryptocurrency market. Empirical outcomes reveal that volatility among digital currencies is not best described by the same specification but varies according to the currency. It is evident that most cryptocurrencies have a positive relationship with Bitcoin, Ethereum and Ripple, therefore, there is no great possibility of hedging for crypto-currency portfolio managers and investors in distressed times

Performance-Risk Nexus of Global Low-Rated ETFs During the QE-Tapering Period

This study investigates the performance of 50 global, one star (based on Morningstar rankings), ETFs during the US QE-tapering period starting in October 2014 up to September 2018, using the S&P500 as the market index. The methodology employed is based on the CAPM model. We adopt the Jensen’s Alpha, Beta, a / b, Sharpe and Treynor ratios measures in order to examine whether those ETFs have achieved abnormal returns. We conclude that managers of most ETFs do not exhibit selectivity skills and only six of these ETFs achieve higher returns than the market by showing bullish behavior. At the same time, most ETFs have positive Sharpe and Treynor ratios due to high expected returns during the period under scrutiny