Experimental identification of the transition from elasticity to inelasticity from ultrasonic attenuation analyses

Abstract

The transition from recoverable elastic to permanent inelastic deformation is marked by the onset of fracturing in the brittle field. Detection of this transition in materials is crucial to predict imminent failure/fracturing. We have used an ultrasonic pulse transmission method to record the change in waveform across this transition during fracturing experiments. The transition from elastic to inelastic deformation coincides with a minimum in ultrasonic attenuation (i.e., maximum wave amplitude). Prior to this attenuation minimum, the existing microfractures close. After this minimum, new microfractures form and attenuation increases until peak stress conditions, at which point, larger fractures form leading to complete sample failure. In our experiments, velocity changes are not sensitive enough to be indicative for the transition from elastic to inelastic deformation. Analysis of attenuation, not velocity, may thus detect imminent failure in materials. Our results may help detect fracturing in borehole casings or the near-wellbore area, or they may help predict imminent release of energy by seismic rupture.

Experimental Relationship Between Compressional Wave Attenuation and Surface Strains in Brittle Rock

“…This can be further confirmed from the near‐linear relationship between the ε NE AT and the ultrasonic wave amplitude above the CI (Figure b), coherent with the findings of Wulff et al (). This observation is consistent with the studies of Wulff et al (), Couvreur et al (), and Barnhoorn et al (), who found that the changes in ultrasonic signals can be an objective signature of onset (not a precursor) of microcracking in rocks where the damage is diffuse (randomly distributed) and small relative to the ultrasonic wavelength (identical to the scenario in the present study).…”

“…The results in present work show that the amplitudes drop by 40%–80% from their initial value over the course of loading (Figure ). This observation is consistent with the conclusions of several other studies that attenuation is sensitive to flaws in the medium through which a wave propagates (Barnhoorn et al, ; Gowd, ; Gupta, ; Lockner et al, ; Modiriasari et al, ; Rao & Raman, ; Walsh, ). This is also consistent with the fact that at high frequencies (in the range of MHz) the ultrasonic wave amplitude is more sensitive to damage than the ultrasonic wave velocity (Barnhoorn et al, ; Chen et al, ; Modiriasari et al, ; Pyrak‐Nolte et al, , Pyrak‐Nolte & Cook, ; Pyrak‐Nolte et al, , ).…”

“…Studies have concluded that changes in ultrasonic amplitude can be a precursor to the evolution of damage in rock specimens with macroscopic flaws (Hedayat, ; Hedayat et al, ; Modiriasari et al, , ). In contrast, there are studies that have shown the capability of ultrasonic signals to objectively define onset of cracking in rocks without any precursors to damage being indicated by the ultrasonic signals (Barnhoorn et al, ; Couvreur et al, ; Wulff et al, ). In the tests conducted in present study, the attenuation in ultrasonic amplitude begins around 25%–30% of UCS for the three specimens (Figure ), which is close to the CI limit of Lyons sandstone (25%–30% of UCS).…”

“…Several experimental studies have been conducted over the years utilizing ultrasonic attributes (velocity and attenuation) as a tool for implicit characterization of microcrack evolution in laboratory-scale rock specimens (Barnhoorn et al, 2018;Couvreur et al, 2001;Ghazvinian, 2015;Gupta, 1973;Harnett et al, 2018;Lockner et al, 1977;Modiriasari et al, 2017;Nur, 1971;Nur & Simmons, 1969;Pellet & Fabre, 2007;Rao & Raman, 1974;Sayers et al, 1990;Scott et al, 1993;Shirole et al, 2017;Walsh, 1966;Wulff et al, 1999). Based on these studies, it can be concluded that wave attenuation (amplitude) is a potential indicator of the extent of damage in rocks with attenuation increasing as damage proliferates in a rock volume.…”

Coda-wave monitoring of continuously evolving material properties and the precursory detection of yielding

“…Monitoring the evolution of a rock matrix towards dynamic failure with a wavelet at the mesoscopic scale (Li et al, 1998;Heap and Faulkner, 2008;Barnhoorn et al, 2010Barnhoorn et al, , 2018, is characterised by the continuous evolution of its wavefield propagation properties. For Unconfined Compressive Strength tests this can be illustrated by considering the different stages of material deformation as shown in figure 2 for the BNT1 core sample.…”