Dependence of elastic nonlinearity on aligned inhomogeneities

Newman, Jacob Edward (2021) Dependence of elastic nonlinearity on aligned inhomogeneities. Masters thesis, Memorial University of Newfoundland.

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Abstract

The nonlinear elasticity of geomaterials is a prominent indicator of the materials’ internal structure (micro-fractures, grain-to-grain boundaries, etc.), both microscopically and macroscopically. Dynamic acousto-elastic testing (DAET) has become a standard tool to assess the nonlinear elasticity of materials. Here, we use a transient wave variation of DAET (TW-DAET) to evaluate whether this type of testing can (1) distinguish the presence of varying types of inhomogeneities and (2) determine the direction in which said inhomogeneities align. We explore two main types of inhomogeneities with TW-DAET; micro-fracturing in sandstone samples (which we attempt to induce via freeze-thaw cycles) and embedded objects within an elastically linear background cement. The sandstone results remain largely inconclusive though evidence for a potential two-mechanism (one increasing and one decreasing nonlinearity) system is presented in a subset of the data. We observe the presence and alignment of inhomogeneities for a sub-set of the cement samples we examine with TW-DAET. We also find evidence for a potential dependence of the nonlinearity on the relative sizes of the probe’s wavelength and the spatial characteristics of the inhomogeneities.

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