Microphysical Mechanisms Behind the Nonlinear Elastic Response of Granular Media
Open Access
- Author:
- Gao, Linying
- Graduate Program:
- Engineering Science and Mechanics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 08, 2023
- Committee Members:
- Albert Segall, Program Head/Chair
Chris Marone, Outside Unit & Field Member
Parisa Shokouhi, Major Field Member
Clifford Lissenden, Major Field Member
Jacques Riviere, Chair & Dissertation Advisor - Keywords:
- Nonlinear acoustics
granular media
ultrasonics
DAET
geophysics
relative humidity
grain shape - Abstract:
- This study focuses on unraveling the microphysical origins of the nonlinear elastic effects in granular and poorly consolidated materials. Understanding the origins of such effects is relevant to numerous applications and domains, from the nondestructive evaluation of industrial and civil materials and structures to the physics of earthquakes, itself linked to a variety of geophysical applications (geothermal energy, oil and gas, CO2 sequestration). Previous work has shown that the nonlinear elastic response of consolidated granular media like rocks likely arises from two distinct mechanisms, however, we do not have a clear understanding of their physical origins at the microscopic scale. Connections between the nonlinear elastic properties and the microstructural features are difficult to establish with certainty because of the complex composition of rock samples. For instance, the presence of minerals like clay – even in small amounts – might greatly alter the elastic properties of the samples. Therefore, here we use granular media of simple composition and study the influence of relative humidity (RH) and grain shape, and on the nonlinear elastic response of granular media. Here, we conduct Dynamic acousto-elastic testing (DAET) on samples of glass beads and quartz sand under different RH conditions at room temperature and a range of static stresses. Compared to other nonlinear ultrasonic methods, DAET allows us to retrieve the full nonlinear elastic response, including transient softening and hysteretic effects. In samples of (spherical) glass beads, we find that the elastic nonlinearity of humid samples is an order of magnitude larger than dry samples. Moreover, we find that all extracted nonlinear parameters increase with RH. This overall increase in nonlinearity is consistent with findings from previous studies and the hypothesis that water adsorption on the grains makes the contact junctions weaker and prone to greater disturbances when subjected to dynamic stressing. In the investigation examining the influence of grain shape on the elastic nonlinearity of granular assemblies, we find that the elastic nonlinearity of angular sand particles is of the same order of magnitude as that measured in spherical glass beads. However, while the elastic nonlinearity of glass beads increases by an order of magnitude with RH, that of sand particles is rather RH independent. We attribute this difference to the angularity of sand particles: adsorbed water on the spherical grains weakens the junctions making them more nonlinear, while no such effect occurs in sand due to grain interlocking. Additionally, for one of the nonlinear parameters that likely arises from shearing/partial slip of the grain junctions, we observe a sharp amplitude threshold in sand which is not observed in glass beads. In the remainder of the study, we explore the use of an alternative experimental setup to try to reduce measurements’ uncertainties. And to better understand the origins of this amplitude threshold, we also study the effect of static stress on the nonlinear elastic properties. We find that the overall nonlinearity as the static stress level increases, as expected. However, we are unable to consistently reproduce the amplitude thresholds with this new setup. We list some possible explanations and also offer guidance reducing uncertainties in future work.