Showing papers on "Acoustic emission published in 2021"

AE waveform characteristics of rock mass under uniaxial loading based on Hilbert-Huang transform

Abstract: Acoustic Emission (AE) waveforms contain information on microscopic structural features that can be related with damage of coal rock masses. In this paper, the Hilbert-Huang transform (HHT) method is used to obtain detailed structural characteristics of coal rock masses associated with damage, at different loading stages, from the analyses of the characteristics of AE waveforms. The results show that the HHT method can be used to decompose the target waveform into multiple intrinsic mode function (IMF) components, with the energy mainly concentrated in the C1–C4 IMF components, where the C1 component has the highest frequency and the largest amount of energy. As the loading continues, the proportion of energy occupied by the low-frequency IMF component shows an increasing trend. In the initial compaction stage, the Hilbert marginal spectrum is mainly concentrated in the low frequency range of 0–40 kHz. The plastic deformation stage is associated to energy accumulation in the frequency range of 0–25 kHz and 200–350 kHz, while the instability damage stage is mainly concentrated in the frequency range of 0–25 kHz. At 20 kHz, the instability damage reaches its maximum value. There is a relatively clear instantaneous energy peak at each stage, albeit being more distinct at the beginning and at the end of the compaction phase. Since the effective duration of the waveform is short, its resulting energy is small, and so there is a relatively high value from the instantaneous energy peak. The waveform lasts a relatively long time after the peak that coincides with failure, which is the period where the waveform reaches its maximum energy level. The Hilbert three-dimensional energy spectrum is generally zero in the region where the real energy is zero. In addition, its energy spectrum is intermittent rather than continuous. It is therefore consistent with the characteristics of the several dynamic ranges mentioned above, and it indicates more clearly the low-frequency energy concentration in the critical stage of instability failure. This study well reflects the response law of geophysical signals in the process of coal rock instability and failure, providing a basis for monitoring coal rock dynamic disasters.

Implications for rock instability precursors and principal stress direction from rock acoustic experiments

Abstract: The characteristics of rock instability precursors and the principal stress direction are very crucial for the prevention of geological disasters. This study investigated the qualitative relationship between rock instability precursors and principal stress direction through wave velocity in rock acoustic emission (AE) experiments. Results show that the wave velocity variation exhibits obvious anisotropic characteristics in 0%–20% and 60%–90% of peak strength due to the differences of stress-induced microcrack types. The amplitude of wave velocity variation is related to the azimuth and position of wave propagation path, which indicates that the principal stress direction can be identified by the anisotropic characteristics of wave velocity variations. Furthermore, the experiments also demonstrate that the AE event rate and wave velocity show quiet and stable variations in the elastic stage of rock samples, while they present a trend of active and unstable variations in the plastic stage. It implies that both the AE event rate and wave velocity are effective monitoring parameters for rock instability. The anisotropic characteristics of the wave velocity variation and AE event rate are beneficial complements for identifying the rock instability precursors and determining the principal stress direction, which provides a new analysis method for stability monitoring in practical rock engineering.

Experimental Study on Damage and Fracture Characteristics of Beishan Granite Subjected to High-temperature Treatment with DIC and AE Techniques

Abstract: The effect of high temperatures up to 800 °C on the physical and mechanical characteristics and fracturing behaviour of Beishan granite is experimentally studied with a combination of acoustic emission (AE), digital image correlation (DIC) and optical microscope observations. The experimental results show that the responses of the P-wave velocity, the effective porosity, Young’s modulus, and the uniaxial compressive strength (UCS) of the granite to temperature are different. The critical temperature for the brittle–ductile transition of Beishan granite is between 500 and 600 °C. The counts, cumulative energy, b value, and waveform from AE and the full-field deformation evolution from DIC are combined to investigate the damage evolution and fracture mechanism in heated granite. It is found that a rise in temperature increases the number of AE events but reduces the cumulative energy release. Tensile microcracking mainly occurs in granite exposed to low temperatures, while shear fracturing gradually dominates in granite exposed to higher temperatures. With increasing temperature from 25 to 800 °C, the failure mode of granite specimens changes from being controlled by longitudinal splitting cracks to a single shear fracture and finally to multiple conjugate shear fractures. Microscopic observation of granite thin sections is conducted to further reveal the essential mechanism driving the physical–mechanical response and fracturing behaviour of heated Beishan granite.

Fracture behavior and acoustic emission characteristics of sandstone samples with inclined precracks

Abstract: Sandstone samples with precracks of different dip angles were collected from a coal mine roof and subjected to uniaxial compression tests, and acoustic emission (AE) and scanning electron microscopy (SEM) were used to study how the crack dip angle affected the fracture mechanism. In the precracked sandstone samples, as the dip angle between the crack line and loading direction decreased, so did the peak stress and its completion time. The SEM observations revealed a fracture transition from tensile cleavage to shear slip, which was manifested by a microstructure change from aggregate to staggered. According to energy conversion, a decreased crack dip angle results in gradually decreasing total and dissipative peak energies, whose variation amplitudes at different stages are consistent with those of the peak stress of the samples. The decreased crack dip angle lowered the stress required to trigger the first appearance of AE energy peaks and ring-down counts, as well as shortening the period before the occurrence of the first AE peak signal. However, the AE energy and ring-down count during the failure stage after the stress peak increased gradually. A stepped increase was observed in the AE ring-down count curves, with each step corresponding to a jump in the stress-strain curve. From the characteristics of the AE signal of the fracture of a precracked rock sample, the occurrence of joints or faults in the rock mass can be reasonably inferred. This is expected to provide a new method and approach for predicting coal and rock dynamic disasters.

Applications of the Acoustic Method in Partial Discharge Measurement: A Review

Abstract: This paper reviews the main features of acoustic emission technique. First, the characteristics of acoustic signals and their physics are addressed. Then, the structure of acoustic sensors employed for capturing signals is studied. In the next step, the acoustic method of PD measurement is compared with the standard electrical method. Afterward, the applications of AET in PD measurement in different equipment are summarized. All acoustic method and combined acoustic-electrical method for PD localization in transformers are discussed. Moreover, the applications of AET in the monitoring of GIS systems are explained, along with the acoustic behaviors of moving particles in these systems. Finally, the general considerations in the signal processing of the acoustic signals are reviewed.

Acoustic emission wave classification for rail crack monitoring based on synchrosqueezed wavelet transform and multi-branch convolutional neural network:

Abstract: This study focuses on the acoustic emission wave classification for the sake of more accurate and comprehensive rail crack monitoring in the field typically with complex cracking conditions, high-o...

Characteristic strength and acoustic emission properties of weakly cemented sandstone at different depths under uniaxial compression

Abstract: As coal mining is extended from shallow to deep areas along the western coalfield, it is of great significance to study weakly cemented sandstone at different depths for underground mining engineering. Sandstones from depths of 101.5, 203.2, 317.3, 406.9, 509.9 and 589.8 m at the Buertai Coal Mine were collected. The characteristic strength, acoustic emission (AE), and energy evolution of sandstone during uniaxial compression tests were analyzed. The results show that the intermediate frequency (125–275 kHz) of shallow rock mainly occurs in the postpeak stage, while deep rock occurs in the prepeak stage. The initiation strength and damage strength of the sandstone at different depths range from 0.23 to 0.50 and 0.63 to 0.84 of peak strength (σc), respectively, decrease exponentially and are a power function with depth. The precursor strength ranges from 0.88σc to 0.99σc, increases with depth before reaching a depth of 300 m, and tends to stabilize after 300 m. The ratio of the initiation strength to the damage strength (k) ranges from 0.25 to 0.62 and decreases exponentially with depth. The failure modes of sandstone at different depths are tension-dominated mixed tensile-shear failure. Shear failure mainly occurs at the unstable crack propagation stage. The count of the shear failure bands before the peak strength increases gradually, and increases first and then decreases after the peak strength with burial depth. The cumulative input energy, released elastic energy and dissipated energy increase with depth. The elastic release rate ranges from 0.46 × 10–3 to 198.57 × 10–3 J/(cm3 s) and increases exponentially with depth.

Mechanical properties and damage constitutive model for uniaxial compression of salt rock at different loading rates

Abstract: To study the effect of loading rate on the mechanical properties of salt rock, uniaxial compression tests and acoustic emission tests at different loading rates were carried out on salt rock specim...

Fracture response and mechanisms of brittle rock with different numbers of openings under uniaxial loading

Abstract: Hazardous failure phenomena such as rock bursts and slabbing failure frequently occur in deep hardrock tunnels, thus understanding the failure phenomena and mechanisms of the stress regime on tunnels is extremely critical. In this study, the tunnel system in a rock mass was physically modelled as a number of scaled openings in rock specimens, and the mechanical behavior of specimens having one to four horseshoe-shaped openings under uniaxial compression were investigated systematically. During the tests, the digital image correlation (DIC) and acoustic emission (AE) techniques were jointly employed to monitor the fracture response of specimens. After which, the stress distributions in the specimens were numerically analyzed and the stress concentration factor on the periphery of the opening was calculated. The results show that the number of openings have a significant impact on the weakening effect of rock mechanical properties. The progressive cracking process of the specimens with openings evolves from first-tensile cracks through second-tensile cracks and spalling cracks to shear cracks, and the crack threshold stresses are measured. Two failure modes are formed: shear failure and shear-tensile failure. According to the stress distribution law around the opening, the crack initiation mechanism can be fully explained. This research provides an insight to failure mechanism of hardrock tunnel.

Early Fault Detection of Planetary Gearbox Based on Acoustic Emission and Improved Variational Mode Decomposition

Abstract: Planetary gearbox is applied more and more extensively in the modern industry. Its condition directly determines the availability and the operation safety of the machinery system that utilizes it for power transmission. To enhance its reliability, an early fault detection method is proposed in this study, which is based on the acoustic emission (AE) technique and the improved variational mode decomposition (VMD). AE data are expected to contain more valuable information about the early fault than the vibration data. Mutual information is used to determine its layer that can avoid over-decomposition or under-decomposition. Then, energy entropy is used to analyze intrinsic mode functions extracted by the improved VMD. By comparing values of energy entropy, it is easy to distinguish different working conditions. The main contributions of this study include: 1) An improved VMD method for fault detection of planetary gearbox is proposed. 2) This method provides a novel strategy for detecting early fault in different positions of planetary gearbox by utilizing the advantages of AE technique.