Showing papers on "Acoustic emission published in 2019"

Recent Advances in Piezoelectric Wafer Active Sensors for Structural Health Monitoring Applications

Abstract: In this paper, some recent piezoelectric wafer active sensors (PWAS) progress achieved in our laboratory for active materials and smart structures (LAMSS) at the University of South Carolina: http: //www.me.sc.edu/research/lamss/ group is presented. First, the characterization of the PWAS materials shows that no significant change in the microstructure after exposure to high temperature and nuclear radiation, and the PWAS transducer can be used in harsh environments for structural health monitoring (SHM) applications. Next, PWAS active sensing of various damage types in aluminum and composite structures are explored. PWAS transducers can successfully detect the simulated crack and corrosion damage in aluminum plates through the wavefield analysis, and the simulated delamination damage in composite plates through the damage imaging method. Finally, the novel use of PWAS transducers as acoustic emission (AE) sensors for in situ AE detection during fatigue crack growth is presented. The time of arrival of AE signals at multiple PWAS transducers confirms that the AE signals are originating from the crack, and that the amplitude decay due to geometric spreading is observed.

Experiment on Rockburst Process of Borehole and Its Acoustic Emission Characteristics

Abstract: In the present study, structural model test using rectangular prismatic granite specimen 200 mm × 200 mm × 200 mm with a horizontal central circular hole of 78 mm diameter was conducted to investigate a rockburst process of borehole. Strain measurement system and high-speed camera were used to capture the rock responses during rockburst. Acoustic emission (AE) system was adopted to monitor the associated AE signals during the rockburst process, and to locate the positions of micro-cracks and subsequently quantitatively investigate the cracking mechanisms. In addition, scanning electron microscope (SEM) was also used to identify the micro-cracks of fragments. The experimental results indicate that rockburst process is characterized by significant spatial distribution and structural responses. As the circumferential stress of surrounding rocks increase, there are some local rockbursts interlacing at different regions of compressive stress concentration along the opening axis direction before overall rockburst. These local rockbursts continued to develop and coalesce, eventually forming overall rockburst. A local rockburst in the present test can be composed of several bursts and persist for a longer period of time than that in true-triaxial tests using rectangular prismatic specimen. Hoop effect, stress gradient around the opening, and last V-shaped bands were accurately simulated. According to AE analysis results, quiet period characterized by few AE hits with high amplitude and a sharp increase in AE energy can be used as an early warning signal for overall rockburst. The time and position of rockburst are related to the spatiotemporal distribution of AE event density, which can be used as a potential indicator for rockburst prediction. During the rockburst process, tensile cracks occupied most of the total micro-cracks, and tensile splitting dominated the failure process. Shear cracks due to tensile cracks interaction initiated at 67% of the spalling strength (tangential stress for spalling failure at the opening boundary) of the borehole.

Shear Rate Effects on the Post-peak Shear Behaviour and Acoustic Emission Characteristics of Artificially Split Granite Joints

Abstract: Rock joints may be sheared at different rates under quasi-static or dynamic loading. Understanding the mechanical response of rock joints at different shear rates is of great importance for the mitigation of dynamic geo-hazards such as earthquakes, fault slip rockbursts and landslides. In this study, direct shear tests at various shear rates (0.001–0.1 mm/s) under different normal stresses (3–40 MPa) are conducted on split granite joints, and the influences of shear rates on the shear strength, post-peak shear behaviour and acoustic emission (AE) characteristics are analysed and discussed. The research findings suggest that both peak and residual shear strengths tend to decrease with increasing shear rate. Stick–slip occurs on all the joints, during which stress drop values increase with increasing shear displacement and normal stress. The stress drop magnitudes during stick–slip decrease with shear rate, while the time intervals between stress drops during stick–slip increase with shear rate. Further, the energy rate tends to increase while the AE events decrease with increasing shear rate, which is caused by the time-dependent deformation behaviour. The AE b value decreases linearly with the shear rate on a logarithmic scale, and the influence is more significant under high normal stress conditions. The variations in the b value can reflect the evolution process (first loading at lower and then higher shear rates) of dynamic geo-hazards and can be used as an effective indicator to predict the dynamic shear failure of granite joints in a temporal sequence. The results of this study will encourage better understanding of the rate-dependent shear behaviour of rough granite joints, particularly under high normal stress, and will provide some references for the monitoring and prediction of dynamic geo-hazards with respect to the AE (or micro-seismic) technique.

Effect of High Temperature on Deformation Failure Behavior of Granite Specimen Containing a Single Fissure Under Uniaxial Compression

Abstract: To investigate the role of fissure angle and heat treatment temperature on the mechanical properties and deformation failure behavior, uniaxial compression tests were carried out on granite specimens containing a single fissure. Using stress–strain curves, the peak strength, peak strain, and elastic modulus of the one-fissured granite specimens were analyzed in detail. The mechanical parameters are closely related to the fissure angle and the high temperature. As the fissure angle increases from 0° to 90°, the peak strength and elastic modulus first decrease and then increase, while the peak strain increases slowly. However, the peak strength and elastic modulus first increase and then decrease, while the peak strain first decreases and then increases with increasing treatment temperature. During the experiments, the crack evolution process and acoustic emission (AE) counts were obtained using real-time photography and the AE monitoring technique. In the granite specimens containing a pre-existing fissure, large AE counts are clearly observed before the peak strength, which indicates crack initiation and propagation. The accumulated AE count first increases slowly, but is followed by a sharp increase, with increasing deformation. The AE events in the one-fissured specimen also depend on the heat treatment temperature. As the temperature increases, the rate of increase of the accumulated AE count curve is reduced. Finally, using a digital image correlation method, the full fields of surface deformation were obtained for the entire testing process. In addition, the local strain around the pre-existing fissure was measured using strain gauges. The full strain field and local strain concentration are discussed to describe the fracture mechanism of brittle granite.

Water-Weakening Effects on the Mechanical Behavior of Different Rock Types: Phenomena and Mechanisms

Abstract: The presence of water strongly affects rock properties and would be related to a series of geological disasters. To understand water saturation effects on the mechanical behavior of different rock types and interpret the underlying mechanisms of differences in water sensitivity, three kinds of rocks, namely sandstone, granite and marble, were selected for tests. Uniaxial compression experiments were conducted on specimens under oven-dried and water-saturated conditions. Acoustic emission (AE) techniques were also applied to monitor and record AE signals during tests. Experimental results reveal that water weakens the mechanical parameters of the three tested rocks, such as uniaxial compressive strength (UCS), elastic modulus and critical strain. The sandstone undergoes the greatest weakening with the addition of pore water, the mechanical properties of the granite exhibit relatively minor reductions, while the marble is the least affected by water saturation. The water-weakening degree of rock properties depends on the porosity as well as the mineralogy, especially the proportion of quartz and swelling clays. Moreover, after water saturation, the failure pattern of the sandstone and the granite tends to transform into the shear-dominant mode from the tensile one in dry state, probably due to frictional reduction. However, the water presence does not change the failure mode of the marble.

Bonded-particle model-based simulation of artificial rock subjected to cyclic loading

Abstract: A nonlinear parallel-bonded stress corrosion (NPSC) model is proposed to simulate the fatigue characteristics of artificial rock (concrete) during cyclic loading. Numerical simulations of fatigue tests replicate the main mechanical features of concrete specimens subjected to cyclic loading observed in the laboratory. A nonlinear reduction speed of the bond diameter between two bonded particles represents the damage rate induced by the fatigue load. The damage rate is proportional to the maximum cyclic load level when the minimum cyclic load level is fixed. Compared with laboratory data, a logarithmic function of bond diameter in the NPSC model resulted in the best fit to simulate the fatigue behaviour of concrete. The simulation includes acoustic emission (AE) monitoring during fatigue tests. The axial strain of the assembly is governed by the evolution of bond breakages. The sum of released bond strain energy is documented as value proportional to cumulative AE energy. The simulation results show very similar evolution compared with laboratory data, which verifies the effectiveness of AE energy simulation.

Damage characterization in composite materials using acoustic emission signal-based and parameter-based data

Abstract: Damage propagation in DCB specimens is characterized by Acoustic Emission (AE) Technique. Signal-based AE data is used to relate the Mechanical Property of the Material with the Acoustic Activity; Sentry function is used to relate the Strain Energy and Acoustic Energy. The cumulative acoustic energy provides information about the critical points before failure in each specimen. Nonetheless, it was observed that the Sentry function reveals concealed information about the damage propagation and material deterioration which are concealed in the Load-Displacement curve. Sentry function can be used to characterize the damage propagation in the material even before its critical point of failure. The signal-based data, the wavelets, are decomposed into different levels using the Wavelet Packet Transform (WPT). These WPT results discriminate the different frequency band associated with the different types of damage propagation. The false signals from both the mechanical results and acoustic results can be identified using the WPT results.

Deformation failure characteristics of coal–rock combined body under uniaxial compression: experimental and numerical investigations

Abstract: The deformation and failure behaviour of coal–rock combined body under uniaxial compression were investigated experimentally and numerically. The mechanical parameters, including the uniaxial compressive strength (UCS), elastic modulus and full-scale stress–strain curves, were obtained. A detailed analysis of the evolution of the internal cracks based on X-ray computed tomography (CT) observations and acoustic emission (AE) locations is presented. The experimental results show that the mechanical properties and deformation failure characteristics of the coal–rock combined body were governed mainly by the coal. The UCS and elastic modulus of the coal–rock combined body were slightly larger than those of the coal and most of the cracks occurring in the coal were a result of the uniaxial compression. Furthermore, a numerical simulation was conducted to validate the experimental evidence. Finally, based on this understanding, a constitutive relationship was proposed using the natural strain described in Hooke’s law for accurate modelling of the deformation of the coal–rock body. A good agreement was obtained between the numerical results and experimental data during the pre-peak regime.

Strainburst process of marble in tunnel-excavation-induced stress path considering intermediate principal stress

Abstract: Strainburst is one type of rockburst that generally occurs in deep tunnel. In this study, the strainburst behaviors of marble specimens were investigated under tunnel-excavation-induced stress condition, and two stress paths were designed, a commonly used stress path in true triaxial unloading rockburst tests and a new test path in which the intermediate principal stress was varied. During the tests, a high-speed camera was used to record the strainburst process, and an acoustic emission (AE) monitoring system was used to monitor the AE characteristics of failure. In these two stress paths, all the marble specimens exhibited strainbursts; however, when the intermediate principal stress was varied, the rockburst became more violent. The obtained results indicate that the intermediate principal stress has a significant effect on rockburst behavior of marble. Under a higher intermediate principal stress before the unloading, more elastic strain energy was accumulated in the specimen, and the cumulative AE energy was higher in the rockburst-induced failure, i.e., more elastic strain energy was released during the failure. Therefore, more violent failure was observed: more rock fragments with a higher mass and larger size were ejected outward.

Rational-Dilation Wavelet Transform Based Torque Estimation from Acoustic Signals for Fault Diagnosis in a Three-Phase Induction Motor

Abstract: Condition monitoring of electric drives play a significant role for a safe working environment. Induction motors are widely used in industries and any fault in it leads to interruption in the process or complete shutdown of the equipment. In this paper, acoustic-based fault detection in a three-phase induction motor is done by estimating the torque from the acoustic signals released by the machine. The fault detection is possible as acoustic emission is different for faults such as single phasing, bearing cage damage, and broken rotor bars. The acoustic signals are processed using rational-dilation wavelet transform (RADWT) technique to extract the fault features and thereby diagnose the fault type. The torque estimation is done using multiple regression method by extracting the energy possessed in the processed acoustic signal and the faults are diagnosed precisely. An experimental setup comprising of a three-phase induction motor with brake drum loading is used to validate this approach. The RADWT has adjustable frequency resolution in comparison with other wavelet methods. When high Q-factor filters are employed in the RADWT, better representation of different faults are obtained in the decomposed sub-bands. In addition, characteristic frequencies of different faults are calculated analytically and validated by observing the frequencies in the FFT spectrum of acoustic fault signals.