Tomoki Shiotani

Detection and evaluation of AE waves due to rock deformation

Abstract In Japan, rock failure is one of the most frequent disasters as well as earthquakes. Because rock materials have brittle natures, deformation due to micro to macro fracture is normally of transient phenomenon. Acoustic emission (AE), detected as a premonitory phenomenon could be useful for prediction of eventual rock failure. In the paper, therefore, a prospective method to install AE sensors into the rock slope (WEAD) is developed. A series of studies on the WEAD are conducted to evaluate AE signals in the laboratory. Based on the experimental results, the criteria for classifying fracture states are proposed. Then, applying the criteria to the field data, fracture states estimated are compared with internal slope behavior evaluated from the displacement, borehole-strain, temperature and so forth. Thus, the applicability of the WEAD to evaluate rock stability is clearly demonstrated.

Acoustic Emission and Ultrasound for Damage Characterization of Concrete Elements

The acoustic emission (AE) technique is widely used for real time damage detection in concrete. It uses stress waves emerging from nucleation and propagation of cracks recorded on the surface of the material by suitable sensors. In the present study, AE is used to monitor the deterioration progress of reinforced concrete beams subjected to four-point bending. The specimens consist of two layers of plain and fiber concrete. At different loading steps, ultrasonic pulse velocity measurements were also conducted to obtain the transient three-dimensional tomographic reconstruction of the internal structure. The AE source location is in good agreement with the velocity structure visualization and the results are confirmed by visual observation of the actual cracks developed. The results show that the AE technique and velocity tomography are useful tools to study the failure progress of concrete.

Experimental study of surface wave propagation in strongly heterogeneous media

In the present paper, the propagation of Rayleigh waves in a strongly heterogeneous medium is discussed. Scattering of stress waves is a difficult scientific problem. Specifically, the interaction of surface waves with distributed inhomogeneity seems highly complicated due to the existence of two displacement components. Rayleigh waves undergo significant attenuation and velocity change depending on the frequency and the inhomogeneity content. The aim of this study is to highlight the dispersive behavior of concrete, especially when damaged, and increase the experimental data in an area where the work is limited.

The influence of propagation path on elastic waves as measured by acoustic emission parameters

Apart from the quantitative parameters of acoustic emission testing, such as the total activity or the location of the sources, much more information can be exploited by qualitative characteristics of the signals. The shape of the waveform strongly depends on the source, supplying information on the type of cracks. Shear cracks which normally follow tensile during fracture, emit signals with longer rise time as well as lower average frequency. However, due to the inherent inhomogeneity of the media, which is enhanced by the nucleation of cracks, each pulse suffers strong dispersion which results in serious alteration of the waveform shape. Therefore, classification of cracks based on acoustic emission parameters would be probably misleading in case the separation distance of the sensors is long or the material contains many cracks. In the present study, numerical simulations were conducted in order to examine the influence of distance on the shape distortion of an excited wave inside concrete. Results are compared with actual experiments on steel fiber reinforced concrete, showing that the distance between the source crack and the acquisition point should not exceed a threshold value in order to lead to reliable crack classification.

Acoustic emission characteristics of concrete-piles

Abstract Acoustic Emission (AE) characteristics due to microcracking are studied in full-scale prestressed concrete-piles. By applying AE techniques, fundamental study on the fracture mechanism of the piles under both cyclic and monotonic loads is made. The prestressed concrete-piles are subjected to bending and shear loads. Crack growth is monitored, and three-dimensional (3-D) AE source locations are conducted along with crack classification and crack orientation by SiGMA analysis. The results obtained are compared with those of the pile integrity test (PIT) and visual observation. To investigate AE characteristics of damaged piles, uni-axial load is applied to the damaged RC piles installed in an experimental pit of sand. AE is monitored directly in the RC piles. In addition, indirect monitoring is carried out by using a wave-guide nearby the pile. In both the direct and indirect monitorings, AE sources are located, applying one-dimensional source location. Locations estimated are compared with the real damaged zone. Finally, conditions of AE generation of the damaged RC piles are discussed with respect to crack width and crack orientation.

Assessment of Construction Joint Effect in Full-Scale Concrete Beams by Acoustic Emission Activity

In the present paper the mechanical and acoustic emission (AE) behaviors of full-scale reinforced concrete beams are evaluated. One of the beams was constructed in two parts, which were assembled later in order to evaluate the effect of the joints in the structural behavior. The load was applied by means of a four-point-bending configuration. It is revealed that at initial stages of loading, the conventional measurements of strain and deflection, as well as pulse velocity, do not show any discrepancy, although the structural performance of the two beams is eventually proven to be quite different. On the contrary, AE parameters, even from early load steps, indicate that the damage accumulation is much faster in the assembled beam. This is confirmed by the calculated sources of AE events which are close to the construction joints. The results show that the AE technique is suitable to monitor the deterioration process of full-scale structures and yields valuable information that cannot be obtained at the early stages of damage by any other way.

Effect of Inhomogeneity Parameters on Wave Propagation in Cementitious Material

In this study, the relationship between inhomogeneity in cementitious material and stress wave parameters is investigated by measuring parameters of through transmission measurements of ultrasonic waves. Except from the inherent inhomogeneity of this type of material, the presence of damage in the form of cracks can lead to even more highlighted velocity dispersion and attenuation phenomena for specific bands of frequencies. Therefore, different contents of crack-like, film-shaped particles are included during casting of concrete to evaluate the contribution of distributed damage in the observed wave parameters. Experiments are carried out using low- and high-frequency sensors with the range of frequencies also covering those used for in-place application.

Large-scale evaluation of concrete repair by three-dimensional elastic-wave-based visualization technique

The development of elastic wave tomography technique using three-dimensional ray tracing and identification algorithm is discussed. The technique introduces hexahedral elements for isoparametric mapping of irregular structural shapes, for which the distribution of slowness of ray path is computed using simultaneous iterative reconstruction technique. In addition, the three-dimensional computation capability of the technique facilitates detailed assessment of any cross section within the measured perimeter, which is a main advantage compared to the more conventional two-dimensional elastic wave tomography. The newly developed technique was applied for evaluating the repair of concrete piers of a water intake facility by taking the transit time of primary waves (P-waves) as data for reconstruction of velocity distribution of the measured region. In this particular field implementation, multichannel elastic wave tomography measurements were carried out at selected areas on the piers before and after the repair to study the change of velocity distribution. Simple quantitative analysis for improvement of concrete Young’s modulus due to the repair was also examined. Findings obtained from field measurement and analysis confirmed the usefulness of the technique for large-scale in situ implementations.

Characterization of Deep Surface-Opening Cracks in Concrete: Feasibility of Impact-Generated Rayleigh-Waves

This paper studies the feasibility of impact-generated Rayleigh waves (R-waves) for measuring deep surface-opening cracks in concrete structures. The aim is to contribute to a methodology for simple and effective in-place crack depth estimation. Specimens induced with vertical slits of different depths were prepared for measurement. A 2-sensor array was implemented and elastic waves of different central frequencies were generated by mechanical impacts with steel- ball hammers of different ball diameters. R-wave amplitudes were extracted from the waveforms. Attenuation of R-waves due to diffraction and scattering by the slits and the trend of amplitude decaying with slit depth were examined. A reasonable correlation between the amplitude factor and slit depth-to-wavelength ratio was established, indicating a loss of sensitivity in the change of amplitude factor with regard to dominant wavelengths smaller than the slit depth. By comparing results of the P-wave time-offlight (TOF) method, the results by measuring again using the proposed method confirmed the feasibility of R-wave attenuation as an alternative parameter for characterizing surface-opening cracks. In addition, the potential problems associated with the reliability of P-wave TOF method in estimating a crack with limited length were also demonstrated.

Damage Evaluation for Railway Structures by Means of Acoustic Emission

Sudden ground motions such as earthquakes sometimes cause serious damage in railway structures. For evaluating the structural integrity after the shock, damage condition of superstructures (or the upper part of substructures) is primarily investigated with the unaided eye. The results are basically subjective, and the structural integrity is evaluated only from the region that could be observed. Accordingly, in order to quantify the structural integrity objectively, an AE testing is tried to apply for the railway structures. In damaged concrete materials, it is known that secondary AE activity from existing defects are readily generated, and based on the Kaiser effect, damage degree could be quantified. In the monitoring of railway structures, however, except for AE signals from the defect, directly induced AE signals by the acceleration of the train passage could be detected. In the present paper, frequency characteristics of such AE signals as secondary AE signals and noise signals are elucidated through laboratory and in-situ experiments. The damage is quantified in an actual concrete pier based on the AE activity of raw AE data including noise signals, and extracted AE data generated from existing cracks, respectively. The results suggest that the quantitative evaluation of damage degree might be possible with the extracted AE activity, however, more data should be collected to classify damage degree reasonably. Introduction Civil structures in Japan were started to construct since 1950s to support the rapid increase of the economic and social infrastructure. At present they reached more than 50 years and aging or deterioration due to weathering, corrosion, earthquake, and so forth, have been inevitable among those. Accordingly, the maintenance of those aging structures has been becoming the key in civil engineering. As it has been expected to be a promising health monitoring technique, an Acoustic Emission (AE) technique has been studied since 1960s. Currently strong movement to make the AE monitoring standard in concrete engineering can be seen e.g., in the recommended practice for in-situ monitoring of concrete structures (NDIS 2421[1]). Since the standard was based on the laboratory studies [2], in practical AE applications however, there are some problems to be resolved. In the present paper, AE activity due to evolution of damage is shown schematically, and damage indices using in-situ AE activity are proposed. In order to extract AE signals from only defects (damaged areas), AE signals directly excited by mobile load are studied for the frequency characteristics. Finally the comparison is made in estimated damage levels between raw AE data and the extracted AE data, and the possibility for damage quantification using the AE activity is discussed. Key Engineering Materials Online: 2004-08-15 ISSN: 1662-9795, Vols. 270-273, pp 1622-1630 doi:10.4028/www.scientific.net/KEM.270-273.1622