Showing papers on "Acoustic emission published in 2014"

Influence of Unloading Rate on the Strainburst Characteristics of Beishan Granite Under True-Triaxial Unloading Conditions

Abstract: Rockburst is a sudden and violent failure of rocks and it often occurs in hard rocks in highly stressed ground. Strainburst is classified as one type of rockburst and it often occurs in rocks near or at the excavation boundary. Deep insight into the strainburst phenomenon is essential for safe underground construction at depth. In this paper, an experimental laboratory study on the strainburst behavior of Beishan granite is presented. Based on in-situ stress measurement data from the Beishan area in China, a series of tests under different unloading rates were performed to investigate the strainburst process using a true-triaxial strainburst test system which was equipped with an acoustic emission (AE) monitoring system. In addition, a high-speed video camera was used to record and visualize the initiation and ejection of rock fragments as well as the sudden dynamic failure (strainburst) of the test samples. AE characteristics associated with the cumulative energy and frequency–amplitude distributions were analyzed. Characteristics of the microscopic structure of a fragment generated from one test were observed using a scanning electron microscope. The experimental results indicate that the degree of violence during failure and the associated AE energy release in the strainburst process are dependent on the unloading rate. When the unloading rate is high, the rock is prone to strainburst. On the other hand, as the unloading rate decreases, the failure mode changes from strainburst to spalling. In addition, the cumulative AE energy is not sensitive to unloading rates greater than 0.05 MPa/s. When the unloading rate is less than 0.05 MPa/s, the cumulative AE energy shows a marked decreasing trend during rock failure.

Gearbox tooth cut fault diagnostics using acoustic emission and vibration sensors--a comparative study.

Abstract: In recent years, acoustic emission (AE) sensors and AE-based techniques have been developed and tested for gearbox fault diagnosis. In general, AE-based techniques require much higher sampling rates than vibration analysis-based techniques for gearbox fault diagnosis. Therefore, it is questionable whether an AE-based technique would give a better or at least the same performance as the vibration analysis-based techniques using the same sampling rate. To answer the question, this paper presents a comparative study for gearbox tooth damage level diagnostics using AE and vibration measurements, the first known attempt to compare the gearbox fault diagnostic performance of AE- and vibration analysis-based approaches using the same sampling rate. Partial tooth cut faults are seeded in a gearbox test rig and experimentally tested in a laboratory. Results have shown that the AE-based approach has the potential to differentiate gear tooth damage levels in comparison with the vibration-based approach. While vibration signals are easily affected by mechanical resonance, the AE signals show more stable performance.

Onset of Hydraulic Fracture Initiation Monitored by Acoustic Emission and Volumetric Deformation Measurements

Abstract: In this paper, the results of laboratory studies of fracture initiation, early propagation and breakdown are reported. Three experiments were conducted on a low permeability sandstone block, loaded in a polyaxial test frame, to representative effective in situ stress conditions. The blocks were instrumented with acoustic emission (AE) and volumetric deformation sensors. In two experiments, fluids of different viscosity were injected into the wellbore, fluid injection was interrupted soon after the breakdown pressure had been reached. This allowed us to investigate hydraulic fracture initiation. In the third test, fracture initiation criteria were applied to stop hydraulic fracture propagation significantly earlier, prior to breakdown, and as it propagated a short distance from the wellbore. The analysis of AE results shows an increase in AE activity and a change in the AE spatial correlation, during the fracture initiation. This early stage of fracturing correlates strongly with the onset of rock volumetric deformation, and is confirmed by the analysis of ultrasonic transmission monitoring. The rock microstructure, after the test, was investigated by analysis of scanning electron microscope images. These indicated the development of leak-off zone near the wellbore and a dry hydraulic fracture at the farther distance from the wellbore.

Acoustic emission detection in concrete specimens: Experimental analysis and lattice model simulations:

Abstract: In civil engineering, a quantitative evaluation of damage in materials subjected to stress or strain states is of great importance due to the critical character of these phenomena, which may sudden...

Brittleness Effect on Rock Fatigue Damage Evolution

Abstract: The damage evolution mechanism of rocks is one of the most important aspects in studying of rock fatigue behavior. Fatigue damage evolution of three rock types (onyx marble, sandstone and soft limestone) with different brittleness were considered in the present study. Intensive experimental tests were conducted on the chosen rock samples and acoustic emission (AE) sensors were used in some of them to monitor the fracturing process. Experimental tests indicated that brittleness strongly influences damage evolution of rocks in the course of static and dynamic loading. AE monitoring revealed that micro-crack density induced by the applied loads during different stages of the failure processes increases as rock brittleness increases. Also, results of fatigue tests on the three rock types indicated that the rock with the most induced micro-cracks during loading cycles has the least fatigue life. Furthermore, the condition of failure surfaces of the studied rocks samples, subjected to dynamic and static loading, were evaluated and it was concluded that the roughness of failure surfaces is influenced by loading types and rock brittleness. Dynamic failure surfaces were rougher than static ones and low brittle rock demonstrate a smoother failure surface compared to high brittle rock.

Tool Condition Monitoring of Single-Point Dresser Using Acoustic Emission and Neural Networks Models

Abstract: Identification and online monitoring of the dresser wear are necessary to guarantee a desired wheel surface and improve the effectiveness of grinding process to a satisfactory level. However, tool wear is a complex phenomenon occurring in several and different ways in cutting processes, and there is a lack of analytical models that can represent the tool condition. On the other hand, neural networks are considered as a good approach to resolve the absence of an analytical or empirical model. This paper describes a method to characterize the dresser wear condition from acoustic emission (AE) signal. To achieve this, some neural network models are proposed. Initially, a study on the frequency content of the raw AE signal was carried out to determine features that correlate the signal and dresser wear. The features of the signal were obtained from the root mean square and ratio of power statistics at nine frequency bands selected from AE spectra. Combinations of two frequency bands were evaluated as inputs to eight neural networks models, which have been compared with their classification ability. It could be verified that the combination of the frequency bands of 28-33 and 42-50 kHz best characterized the dresser wear condition. Some of the models produced very good results and can therefore ensure the ground part will be within project specifications.

Effect of graphite addition on mechanical behavior of Al6061/TiB2 hybrid composite using acoustic emission

Abstract: The present work includes the effect of addition of graphite for improving the properties of Al6061–TiB2 composite using the high-energy stir casting method. The characterization was performed with X-ray diffraction, energy dispersive spectrum and scanning electron microscope. The thermal and mechanical behaviors such as thermo gravimetric analysis/differential thermal analysis, hardness, tensile strength using acoustic emission and fatigue behaviors were investigated. The composite of composition Al6061–20% TiB2 with 2% graphite shows the greatest improvement in mechanical behavior. An Acoustic Emission system was employed in all the tests to monitor the acoustic energy release during the whole deformation process and some useful conclusions were drawn.

Experimental investigation of aerofoil tonal noise generation

Abstract: The present study investigates the mechanisms associated with tonal noise emission from a NACA 0012 aerofoil at moderate incidence (0; 1; 2 and 4 angle of attack) and with Reynolds numbers ranging from 100 000 to 270 000. Simultaneous time-resolved particle image velocimetry (PIV) of the aeroacoustic source region near the trailing edge and acoustic measurements in the far field are performed in order to establish the correspondence between the flow structure and acoustic emissions. Results of these experiments are presented and analysed in view of past research for a number of selected cases. Characteristics of the acoustic emission and principal features of the average flow field agree with data presented in previous studies on the topic. Time-resolved analysis shows that downstream convecting vortical structures, resulting from growing shear layer instabilities, coherently pass the trailing edge at a frequency equal to that of the dominant tone. Therefore, the scattering of the vortical structures and their associated wall pressure fluctuations are identified as tone generating mechanisms for the cases investigated here. Moreover, wavelet analysis of the acoustic pressure and velocity signals near the trailing edge show a similar periodic amplitude modulation which is associated with multiple tonal peaks in the acoustic spectrum. Periodic amplitude modulation of the acoustic pressure and velocity fluctuations on the pressure side are also observed when transition is forced on the suction side, showing that pressure-side events alone can be the cause.

Study of the loading mode dependence of the twinning in random textured cast magnesium by acoustic emission and neutron diffraction methods

Abstract: The twinning activity in random textured cast magnesium during monotonic, room temperature tension and compression tests was monitored by acoustic emission (AE) and neutron diffraction (ND). The AE detected higher twin nucleation activity in tension than in compression. The correlation of AE with the ND data indicates that in compression the nucleation of twins is followed by rapid growth, unlike in tension in which twin variants with limited growth were observed.

Acoustic emission analysis for characterisation of damage mechanisms in fibre reinforced thermosetting polyurethane and epoxy

Abstract: Acoustic emission analysis is used to investigate microscopic damage mechanisms and damage progress in unidirectional glass and carbon fibre reinforced composites. Under static loading the influence of fibre orientation on damage initiation and propagation is determined. A novel polyurethane matrix system significantly enhances material performance in terms of crack initiation load levels, crack growth, damage tolerance and off-axis tensile strength. Hysteresis measurements during stepwise increasing dynamic load tests highlight the effect of fibre–matrix-adhesion and resin fracture toughness in unidirectional 0° fibre reinforced composites. Acoustic detection of beginning fibre breakage correlates with a significant increase of loss work per cycle.

A laboratory acoustic emission experiment under in situ conditions

Abstract: In this paper, we revisit acoustic emission (AE) data from an in situ rock fracture experiment conducted at the Underground Research Laboratory (URL) in Manitoba, Canada. The Mine-By experiment, a large-scale excavation response test, was undertaken at a depth of 420 m and involved the mechanical excavation of a cylindrical tunnel. During the experiment a small array of 16 Panametrics V103 AE sensors enclosed a 0.7 m × 0.7 m × 1.1 m rectangular prism of Lac du Bonnet granite located in the tunnel wall. The V103 sensors were later calibrated in the laboratory, and a source parameter analysis was undertaken using a spectral fitting method. Corner frequency and moment magnitude were found to be inside the ranges 250 kHz

Seismic moment tensor and b value variations over successive seismic cycles in laboratory stick‐slip experiments

Abstract: The formation of fault damage due to slip under high normal stresses can rarely be monitored under in situ conditions. To advance our understanding of microfracture processes, we investigated stick-slip events on Westerly granite samples containing the following: (1) a planar saw cut fault and (2) a fault developed from a fresh fracture surface. We examined temporal changes of seismic moment tensors and b values of acoustic emission (AE) events. During experiment on the saw cut surface, small AEs exhibiting non-double-couple components were observed continuously and strong AEs displaying double-couple components were visible only when approaching the slip onsets. Sliding on naturally fractured surfaces showed, in addition to double-couple components, significant volumetric contributions, especially during the interslip periods and immediately after stick-slip events indicating substantial shear-enhanced compaction within a relatively broad damage zone. The obtained results shed light on how differences in fault structure control the kinematics of microseismicity during different periods of the seismic cycle.

Avalanches in compressed porous SiO(2)-based materials.

Abstract: The failure dynamics in SiO(2)-based porous materials under compression, namely the synthetic glass Gelsil and three natural sandstones, has been studied for slowly increasing compressive uniaxial stress with rates between 0.2 and 2.8 kPa/s. The measured collapsed dynamics is similar to Vycor, which is another synthetic porous SiO(2) glass similar to Gelsil but with a different porous mesostructure. Compression occurs by jerks of strain release and a major collapse at the failure point. The acoustic emission and shrinking of the samples during jerks are measured and analyzed. The energy of acoustic emission events, its duration, and waiting times between events show that the failure process follows avalanche criticality with power law statistics over ca. 4 decades with a power law exponent ɛ≃ 1.4 for the energy distribution. This exponent is consistent with the mean-field value for the collapse of granular media. Besides the absence of length, energy, and time scales, we demonstrate the existence of aftershock correlations during the failure process.

Damage monitoring and identification in SiC/SiC minicomposites using combined acousto-ultrasonics and acoustic emission

Abstract: The acoustic emission (AE)-based technique is considered to be a promising way to real-time monitoring of microstructural changes and damage evolution in Ceramic Matrix Composites (CMCs). The present paper proposes a testing protocol that combines acousto-ultrasonics (AU) and acoustic emission (AE) monitoring, with a view to obtain both global and local definite characteristics on damage modes and kinetics. It is developed and assessed on SiC/SiC minicomposites, which are appropriate test specimens to establish sound relations between mechanical behavior and damage modes. AU wave velocity measurements provide a global measure of matrix cracking damage and the relations between crack growth and damage characteristics. AE monitoring allows accurate localization of AE sources taking into account wave velocity dependence to damage as well as differentiation of the damage modes, which control the mechanical behavior. Finally, multivariate analysis of AE data allowed classification of signals into clusters, which were successfully associated to the various damage modes.

Early Corrosion Detection in Prestressed Concrete Girders Using Acoustic Emission

Abstract: Long-term corrosion tests were performed on prestressed T-girders measuring 498 m (16 ft, 4 in) The test program was composed of four specimens, including one control The specimens were subjected to wet/dry cycles using 3% NaCl solution to accelerate corrosion Two of the specimens were precracked prior to conditioning to examine the effect of crack presence and/or width The specimens were continuously monitored using acoustic emission (AE) Half-cell potential (HCP) measurements and linear polarization resistance (LPR) were performed daily to serve as a benchmark for corrosion detection Acoustic emission can detect the onset of corrosion comparable in time to conventional electrochemical methods Furthermore, AE intensity analysis has the ability to distinguish between different levels of corrosion The specimens were load-tested to measure the residual capacity, which can give an indication of AE’s ability to detect corrosion damage before it affects the strength of prestressed structures

Identification of Cracking Mechanisms in Scaled FRP Reinforced Concrete Beams using Acoustic Emission

Abstract: Acoustic emission was used to monitor the cracking mechanisms leading to the failure of scaled concrete beams having Glass Fiber Reinforced Polymer (GFRP) longitudinal reinforcement and no shear reinforcement. Dimensional scaling included that of the effective depth of the cross section, which is a key parameter associated with the scaling of shear strength; and maximum aggregate size, which affects the shear-resisting mechanism of aggregate interlock along shear (inclined) cracks. Five GFRP reinforced concrete (RC) beams with effective depth up to 290 mm and constant shear span-to-effective depth ratio of 3.1 were load tested under four-point bending. Two types of failures were observed: flexural, due to rupture of the GFRP reinforcement in the constant moment region; and shear, due to inclined cracking in either constant shear region through the entire section depth. Acoustic emission (AE) analyses were performed to classify crack types occurring at different points in the load history. The results of this study indicate that appropriate AE parameters can be used to discriminate between developing flexural and shear cracks irrespective of scale, and provide warning of impending failure irrespective of the failure mode (flexural and shear). In addition, AE source location enabled to accurately map crack growth and identify areas of significant damage activity. These outcomes attest to the potential of AE as a viable technique for structural health monitoring and prognosis systems and techniques.