Papers by Keyword: Acoustic Emission (AE)

Abstract: Acoustic Emission (AE) testing in concrete structures shows great potential for monitoring and assessing the health condition of structures. Source location is normally based on the arrival times of transient signals, the simplest method is known as the Time of Arrival (TOA) method, where the location of the damage can be determined from the arrival time of the event at two or more sensors. When using this method, the wave velocity of the signals that propagate through the material needs to be determined. Homogenous materials, such as steel, have welldefined velocities, but in non-homogeneous materials such as concrete the wave velocity is more difficult to predict. This makes the use of a single wave velocity as required in the TOA method very difficult due to the variety of wave velocities obtained, especially for large structures. This paper explores wave propagation in concrete structures over a variety of source to sensor distances. Experiments were performed on a reinforced concrete beam and a reinforced concrete slab, using an Hsu-Nelsen (H-N) Source. It is found that, in general, as the source to sensor distance increases, the wave velocity decreases. The presence of longitudinal and transverse waves is demonstrated and the influence of the part of the waveform used for temporal measurement is explored. In order to provide a practical approach to velocity determination, different thresholds are investigated and the results are discussed in relation to the wave modes present.

Abstract: Acoustic Emission (AE) monitoring was performed during Pseudo-Dynamic Testing of a torsionally unbalanced, two-storey, one-by-one bay reinforced concrete frame structure. The structure represented a 0.7-scale model of a real-size frame structure designed and detailed according to the standards prevailing in Greece in 60's, without engineered earthquake resistance. Real time monitoring of AE activity versus the complex applied load resulted in semi quantitative damage characterization as well as comparative evaluation of the damage evolution of the different size columns. Evolution of the AE energy rate per channel, as revealed from zonal location, and the energy rate of linearly located sources enabled the identification of damage areas and the forecast of crack locations before cracks were visible with naked eye. In addition to that, the results of post processing evaluation allowed for the verification of the witnessed damaged areas and formed the basis for quantitative assessment of damage criticality.

Abstract: Acoustic emission (AE) techniques have been extensively applied to concrete structures. Up-dated results are currently standardized to inspect and evaluate the structures in service. Consequently, test results based on these standards are reviewed and discussed. In order to assess the damage levels of the structures, the recommended practice is proposed by the Japanese Society for Non-Destructive Inspection. Two AE parameters of load ratio and calm ratio are defined for qualification of the damages. It is reported that damages qualified are in good agreement with actual damages of reinforced concrete members under bending. AE behavior of concrete under compression could be analyzed, applying the rate process analysis. By quantifying intact Young’s moduli of concrete from the database, relative damages of concrete in existing structures are successfully estimated. The committee on nondestructive inspection of steel-reinforced concrete structures in the Federation of Construction Materials Industries, Japan, has recently published new standards on estimation of concrete properties by elastic-wave methods. One of them is a monitoring method for active cracks in concrete by AE, which is applied to study corrosion cracking in concrete. Moment tensor analysis of AE can identify cracking kinematics of location, crack-type and crack orientation, which is implemented as SiGMA procedure. Nucleation of micro-cracking due to corrosion is identified. It is found that the mechanisms iof corrosion cracking in concrete are nearly equally of mode I, mixed-mode and mode II.

Abstract: Deteriorated concrete structures are repaired by means of grouting. In order to evaluate the repair effectiveness two NDT techniques namely seismic tomography and acoustic emission are applied. Using traveling time of elastic waves, structural velocity is estimated. AE activity is monitored along with water pressurization of the permeability test. As a result, contrary to the common expectation that the velocity increase after filling with grouting agent, enormous amount of velocity drop was obtained after repair due to the incomplete developing process of grout material, whereas AE activity showed dramatic decrease after repair. Furthermore after comparison of those results to the quantity of grouting material, it became clear that damage indices based on AE activity exhibited well the actual damage of concrete structures.

Abstract: A new measuring method for the detection of fine spectra of electromagnetic and acoustic emission (EME and AE) signals from small cracks is described. It requires wide band ultra-low noise amplifiers, analogue filters, the optimization of signal to noise ratio of sensors and the application of noise elimination methods. Analyses of noise sources in sensors and preamplifiers are given. They are thermal noise, polarization noise and low frequency 1/f noise. Measuring set-up background noise suppression involving also the electromagnetic shielding allows us to detect signals in the range of 1 to 100 nV. This measuring set-up was used to observe crack initiation in granite samples. AE and EME signals show different behaviour in the first interval of about 20 μs just after crack initiation. For the first stage of crack initiation the frequency spectrum of EME signal is given by the eigen vibrations of crack walls, by the internal friction and the sample electrical conductivity. We observed that the crack opening and crack wall vibration create the high frequency signal in the frequency band up to 10 MHz. These signals were observed in the first time interval of about 20 μs. After that the frequency spectrum is given by the sample eigen vibration or the sample boundary conditions, and we have observed the spectra in the frequency range 100 kHz to 2 MHz.

Abstract: This work presents the results of field measurements and laboratory studies carried out with a view to developing ways to monitor rail-wheel interaction using Acoustic Emission. It is known that impact, wear and cracking generate AE and it is therefore expected that axle loads, wheel out-of-roundness, speed and traction will influence the AE generated by an interaction. It is hoped that the extent of the effect might be sufficient to permit a measure of “interaction intensity” that could be used to quantify cumulative damage by wear and contact fatigue. In the field measurements, AE was acquired as a train with 20 moving sources of AE (20 wheels) passed a single sensor position and a laboratory rig has been devised which uses a single wheel whose condition, speed and loading can conveniently be modified. Simulated source tests have indicated that the AE wave characteristics on real rails are similar to those in the laboratory rig. A simplified analytical model, devised for AE waves propagating from a moving source(s), based on a ‘vehicle’ speed and wave damping coefficients, has been compared to measured results. As a wheel rolls towards a sensor and then away from the sensor the measured AE generally rises and falls in a predictable way. The effects of wheel and rail surface features appear to complicate the results by introducing sharp spikes in the signals. The numerical model for AE wave propagation from the moving sources (wheels) shows good agreement with the more slowly changing envelope of the signals.

Abstract: The article describes the application of AE technique in control and evaluation of the pipe overstraining process. The trials of overstraining were conducted according to a programme of pipe loading with internal pressure, developed and adjusted to the examined pipe material. In the studies two systems of acoustic emission were used, viz. PAC Mistras 2001 and Vallen AMSY. The procedure adopted in the development of evaluation criteria based on an analysis of the recorded acoustic events was described. Next, the criteria of evaluation were presented. The effectiveness of the applied method, the validity of the adopted criteria, and the consistency of the obtained quantitative results were checked during tests performed by other NDT methods. A comparative analysis was made to serve as a complementary tool in final evaluation of the pipe overstraining process done by AE technique.

Abstract: It is known that the basic mechanism of fatigue crack growth in ductile pressure vessel steels, which is repetitive blunting and re-sharpening of the crack tip, is a weak acoustic emission (AE) source. On the other hand, a large number of AE events occur during cyclic crack growth. Most AE events are caused by repetitive friction at the fracture surfaces, but these AE events show relatively weak correlation with the crack growth velocity. In the paper it is shown, based on examples – fatigue crack growth starting from an artificial weld defect, cycling of a natural crack defect, crack initiation and growth in gas cylinders - which information can be gained by relatively simple analyses of the AE data from cyclic pressure tests.

Abstract: Contribution contains an application of acoustic emission method during instability and growth of fatigue crack in reactor pressure vessel specimen loaded by means of bending. Material of specimen was typical material of pressure water reactor vessel covered by austenitic cladding as a protection layer against corrosion. Specimen had shape of prism with defect in the middle where fatigue crack was created very close to austenitic cladding. Loading of specimen was done in special loading machine with defined speed of loading resp. increased force. Investigation of instability and propagation of inertial crack was observed by acoustic emission technique, potential drop method, by using of straingauges and by high-speed camera. Acoustic emission sensors were located from both sides of specimen. By this experimental technique was possible to observe "pop-in" of crack during loading. Different types of specimen with different mode of aging were investigated. Results of acoustic emission activity are described graphical curves in relation to individual stages of loading.

Abstract: The main obstacle which prevents the rapid spread of Acoustic Emission (AE) testing on refinery structures in Russia is the complexity of this NDT tool. The lack of well- defined methods of data analysis and the huge amount of AE data collected within each test are the major problems. The possibility of mistakes in a period of testing and data analysis leading to erroneous conclusions related to the condition of the structure sometimes negatively affects the image of the AE method. However, the correct application of AE can bring unique opportunities in the inspection of structures in the industry. Some examples of successful applications and the unique possibilities of AE testing are presented in this paper.