On calibration of piezoelectric sensors with laser doppler vibrometer

We present a method for calibrating piezoelectric sensors using a laser Doppler vibrometer. Our method uses an average of Fourier transform terms of the recorded signal from the piezoelectric sensor, which is compared with the laser probe measurement in the overlapping frequency range. We use our method to calibrate the response of miniature needle sensors employed in acoustic emission testing to several different excitation sources of stress waves in the frequency range of 20-300 kHz. We demonstrate that the output of the piezoelectric sensors can be accurately scaled with particle velocity.

An ultrasound probe array for a high-pressure, high-temperature solid medium deformation apparatus

Inelastic deformation of minerals and rocks is associated with activation of various defects such as fractures, twins, and dislocations. Active and passive ultrasound probes are potential tools to examine the nature of these defects under a broad range of pressure and temperature conditions. Here, we report on the development of an ultrasound probe array that allows us to monitor deforming samples in a high-pressure, high-temperature solid medium apparatus (a modified Griggs rig). We utilize several broadband miniature piezoelectric sensors that are placed above and below the sample to record acoustic emissions accompanying deformation and determine their locations in 1D. The emissions are recorded at 50 MS/s with a 12 bit resolution. Proper grounding and electric insulation of the sensors, together with optimized power delivery from the heating system, tremendously reduces electromagnetic interference and allows for a background noise level of ≈90 mV at a full range of ±2 V and 60 dB amplification. The system is capable of recording acoustic waves from 80 kHz to 2.5 MHz at sample temperatures up to 1100 °C and confining pressure up to 2.5 GPa.

Microscopic Evolution of Laboratory Volcanic Hybrid Earthquakes

Characterizing the interaction between fluids and microscopic defects is one of the long-standing challenges in understanding a broad range of cracking processes, in part because they are so difficult to study experimentally. We address this issue by reexamining records of emitted acoustic phonon events during rock mechanics experiments under wet and dry conditions. The frequency spectrum of these events provides direct information regarding the state of the system. Such events are typically subdivided into high frequency (HF) and low frequency (LF) events, whereas intermediate "Hybrid" events, have HF onsets followed by LF ringing. At a larger scale in volcanic terranes, hybrid events are used empirically to predict eruptions, but their ambiguous physical origin limits their diagnostic use. By studying acoustic phonon emissions from individual microcracking events we show that the onset of a secondary instability-related to the transition from HF to LF-occurs during the fast equilibration phase of the system, leading to sudden increase of fluid pressure in the process zone. As a result of this squeezing process, a secondary instability akin to the LF event occurs. This mechanism is consistent with observations of hybrid earthquakes.

Observation of the Kibble-Zurek Mechanism in Microscopic Acoustic Crackling Noises

Characterizing the fast evolution of microstructural defects is key to understanding "crackling" phenomena during the deformation of solid materials. For example, it has been proposed using atomistic simulations of crack propagation in elastic materials that the formation of a nonlinear hyperelastic or plastic zone around moving crack tips controls crack velocity. To date, progress in understanding the physics of this critical zone has been limited due to the lack of data describing the complex physical processes that operate near microscopic crack tips. We show, by analyzing many acoustic emission events during rock deformation experiments, that the signature of this nonlinear zone maps directly to crackling noises. In particular, we characterize a weakening zone that forms near the moving crack tips using functional networks, and we determine the scaling law between the formation of damages (defects) and the traversal rate across the critical point of transition. Moreover, we show that the correlation length near the transition remains effectively frozen. This is the main underlying hypothesis behind the Kibble-Zurek mechanism (KZM) and the obtained power-law scaling verifies the main prediction of KZM.