Tomas Trcka

Noise diagnostics of advanced composite materials for structural applications

Our study is focused on noise diagnostics of an advanced composite material for structural applications called extren, which consists of a combination of a fibre glass reinforcement and a resin binder. Electromagnetic and acoustic emission signals appear during mechanical loading of the material under investigation. They carry information about micro-cracks generated in tested materials. We made long-time simultaneous multi-channel measurements of both signals for various loading conditions, carried out time development of noise spectrum, evaluated parameters of EME and AE events, studied their time development, correlations between them and made statistical analyses of signals and their parameters.

Electromagnetic and Acoustic Emission Signals Real-Time Measurement, Processing and Data Evaluation

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.

Acoustic Emission from Quasi-Brittle Failure of Cementitious Composites - Experimental Measurements and Cohesive Crack Model Simulations

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.

Detailed Determination of Mechanical Fracture Parameters of Concrete after Fire Experiments

The aim of this paper is to describe the procedure of determining the mechanical fracture parameters of selected concrete specimens taken from panels after the fire experiments. The records (in form load vs displacement diagrams) of three-point bending fracture tests of these specimens with initial stress concentrators was first advanced corrected and subsequently evaluated using the Effective Crack Model and the work-of-fracture method. The increasing temperatures during the fire experiments ranging between 550 to 1000 °C led to a decrease of modulus of elasticity and fracture toughness values and to the increase of fracture energy value. The 2D laser profile scanner was used to estimate the degree of complexity of fracture surfaces; its statistical dependence on the mechanical fracture parameters proved to be moderate – the absolute value of the correlation coefficient was about 0.5°[–].

Determination of Surface Roughness Parameters by Optical Profilometry and Sand Patch Test

In cases when two concrete parts are cast against in different times are not connected by dowels, main contributors to the resistance are cohesion and friction. Shear resistance of the interface is highly dependent on surface treatment and its roughness. In this paper, besides the review of available methods of surface roughness determination, the optical profilometry will be introduced and described. Optical profilometry represents non-contact and non-destructive method for characterizing surface topography. Furthermore, results obtained by abovementioned method will be compared with Sand Patch Test, in order to determine its usability and limitations.

Field Measurement of Natural Electromagnetic Emissions near the Active Tectonic and Mass-Movement Fractures in Caves

Laboratory tests on a wide range of solid materials shoved that the electromagnetic emission (EME) signals are generated during the samples mechanical stress. EME anomalies have been observed also under natural conditions in association to fracture processes, tectonic loading, stress redistribution and crack propagation prior to earthquake or in relation to deep-seated gravitational mass movements. This paper describes a first prototype of the Emission data logger, which was specially developed for the continual EME monitoring in field conditions. Our equipment has been installed and tested in Obir Caves (Austria) at an active tectonic fault. The pilot long-term EME measurement results from this location are also presented in this paper.

Analysis of excess noise sources in concrete composite materials and impact on electromagnetic emission measurement

This paper deals with an excess noise sources in concrete materials. The theoretical description is put forward as well as supporting experiments of real samples. Physical variables under consideration are spectral densities of voltage and/or current fluctuation for different sample dimension and electrical excitation. It managed to get information about Johnson-Nyquist noise, 1/f noise and Polarization noise contributions. These individual components define detection limit or ability to recognize electromagnetic emission signal when mechanical load is applied and micro-cracks are created. Described investigations also pointed out real requirement on noise sensing equipment and sample preparation methodology.

Noise diagnostics of composite materials by using spectral characteristics of electromagnetic emission

Our study is focused on the use of spectral characteristics of electromagnetic emission signals for damage assessment of mechanical loaded composite materials. We have developed a methodology for evaluation of the power spectral density of the crack electric dipole moment waveform based on the measured voltage at the capacitance sensor measurement circuit output. The obtained characteristics were studied in dependence on the applied force for the various loading conditions.

Experimental Studies of Excess Noise Sources in Concrete Based Materials as a Limiting Factor for Electromagnetic Emission Measurement

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.

Comparing the Properties of Polypropylene Fibers Contained in Ordinary and Lightweight Concrete Using Three-Point Bending Tests with Stress Concentrator and Acoustic Emission Method

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.