Papers by Author: Tomas Trčka

Abstract: Composite materials such as concrete or polymer fiber composites contain a large number of inhomogeneities. It is possible to use the method of partial discharges to detect existing inhomogeneities. Tested materials covered with metal electrodes on both sides represent a parallel plate capacitor. Partial discharges have an impact on conductivity at a high electric field and they are important sources of current noise and acoustic emission signals. The concrete samples were measured before and after mechanical loading by the method of partial discharges and the correlation between them was studied. This method may be used for the diagnosis of inhomogeneities and cracks in composite materials.

Abstract: The results of many bending tests of fiber reinforced concrete specimens did not produce a clear answer to the question whether the fibers are activated before the macro-crack. Our diagnostic method is based on the measurement of acoustic emission signals during three-point bending tests of concrete specimens with stress concentrator. Acoustic emission activity measurement could help us to answer the question if the fibers are activated before the macro-crack is able to seen by human eyes. Several groups of concrete samples with various concrete composition formulas have been prepared for our experiment. This contribution describes the prepared concrete composition, measurement system and also includes the experimental results of one concrete samples set.

Abstract: This paper deals with excess noise sources in dielectric materials. We focus especially on the concrete samples that are frequently tested to ensure information about the reliability and level of degradation. Nevertheless, the testing methods are limited mainly by the proper contact creation, signal detection and noise defined sensitivity. Our efforts are directed to the noise properties assessment. It turns out that the Johnson-Nyquist noise and the 1/f (flicker) noise are generated in the different regions with the different response to the internal or external electric field. In addition the noise analysis is affected by the internal polarization phenomena and the material residual humidity. This issue in connection with the sample geometrical properties and the dielectric noise measurement methodology take part in this paper.

Abstract: In order to predict the mechanical behavior of a material during its service life, it is important to evaluate its mechanical response under different types of external stresses by studying the initiation and development of cracks and the effects induced by damage and degradation. The non-destructive technique of analysis of acoustic emission, especially with source location and full wave-form analysis, provides excellent results on detecting and identifying initiations sites, cracking propagation and fracture mechanisms of polymer matrix composite, ceramic materials and rocks. For this study, loading tests in two different configurations were provided on specimens of same material and geometry. The significant AE features were monitored during each test and also locations of each AE event were estimated. Results of AE analysis are compared with a finite element analysis of the stress distribution and crack propagation within the specimens.

Abstract: Conducted loading tests of plain concrete specimens are briefly introduced in the paper together with description of the performed numerical simulations of these tests in ATENA 3D software. The simulation results of concrete failure are analysed in detail and compared with the experimental results recording failure processes with the help of acoustic emission. The simulation results are in good agreement with the recorded data.

Abstract: Experiments were carried out for n-channel devices, processed in a 300 nm CMOS technology. The investigated devices have a gate oxide thickness of 6 nm and the effective interface area is AG = 1.5 m2. The RTS measurements were performed for constant gate voltage, where the drain current was changed by varying the drain voltage. The capture time constant increases with increasing drain current. The model explaining the experimentally observed capture time constant dependence on the lateral electric field and the trap position is given. From the dependence of the capture time constant c on the drain current we can calculate x-coordinate of the trap position. Electron concentration in the channel decreases linearly from the source to the drain contact. Diffusion current component is independent on the x-coordinate and it is equal to the drift current component for the low electric field. Lateral component of the electric field intensity is inhomogeneous in the channel and it has a minimum value near the source contact and increases with the distance from the source to the drain. It reaches maximum value near the drain electrode.

Abstract: This paper describes our new improved set-up for EME and AE signals measurement and includes some experimental results. This new measurement system is based on the PXI platform, which is one of the most extended modular platforms for measurement and automation systems. The complex program package in the LabVIEW graphical programming environment was developed for the continual multi-channel EME and AE signals measurement, real time processing and evaluation. It offers the possibility to observe the stressed materials response to applied mechanical load in real time.