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.

Keywords

acoustic emission / waveform / Hilbert-Huang transform / coal rock

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