Changes In Elastic Anisotropy Research Articles

Deformation Behavior and Seismic Characteristics of Sandy Facies Opalinus Clay During Triaxial Deformation Under Dry and Wet Conditions

AbstractUnconsolidated, undrained triaxial deformation tests were performed on sandy facies Opalinus Clay at 50 MPa confining pressure to characterize the effect of water and microfabric orientation on the deformation behavior, mechanical properties, and P-wave velocity evolution. Dry and wet (≈ 8 and > 95% initial water saturation, respectively) samples with 12.6 ± 0.4 vol% porosity were deformed parallel and perpendicular to the bedding direction at a constant strain rate of 5 × 10–6 s−1. Dry samples revealed semi-brittle behavior and exhibited strain localization at failure, while deformation was more ductile at saturated conditions, promoting stable, slow faulting. Peak strength, Young’s modulus, and number of cumulative acoustic emissions decreased significantly for wet samples compared to dry samples; the opposite was observed for Poisson’s ratio. P-wave velocity anisotropy was significantly altered by differential stress, primarily due to the interplay between pore and fracture closure and stress-induced microcrack formation. For samples that were deformed perpendicular to bedding, we observed a reduction and reversal of P-wave velocity anisotropy with increasing differential stress, whereas anisotropy of parallel samples increased. The results suggest that water saturation reduces the pressure at the brittle-ductile transition and that the elastic properties and anisotropy of sandy facies Opalinus Clay can be significantly altered in an anisotropic stress field, e.g., adjacent to fault zones or tunnel excavations. Changes in elastic anisotropy are primarily controlled by the orientation between the pre-existing microfabric and the maximum principal stress direction, stress magnitude, and the degree of water saturation.

Valence electron concentration- and N vacancy-induced elasticity in cubic early transition metal nitrides

Motivated by frequently reported deviations from stoichiometry in cubic transition metal nitride (TMNx) thin films, the effect of N-vacancy concentration on the elastic properties of cubic TiNx, ZrNx, VNx, NbNx, and MoNx (0.72≤x≤1.00) is systematically studied by density functional theory (DFT) calculations. The predictions are validated experimentally for VNx (0.77≤x≤0.97). The DFT results indicate that the elastic behavior of the TMNx depends on both the N-vacancy concentration and the valence electron concentration (VEC) of the transition metal: While TiNx and ZrNx exhibit vacancy-induced reductions in elastic modulus, VNx and NbNx show an increase. These trends can be rationalized by considering vacancy-induced changes in elastic anisotropy and bonding. While introduction of N-vacancies in TiNx results in a significant reduction of elastic modulus along all directions and a lower average bond strength of Ti–N, the vacancy-induced reduction in [001] direction of VNx is overcompensated by the higher stiffness along [011] and [111] directions, resulting in a higher average bond strength of V–N. To validate the predicted vacancy-induced changes in elasticity experimentally, close-to-single-crystal VNx (0.77≤x≤0.97) are grown on MgO(001) substrates. As the N-content is reduced, the relaxed lattice parameter a0, as probed by X-ray diffraction, decreases from 4.128 Å to 4.096 Å. This reduction in lattice parameter is accompanied by an anomalous 11% increase in elastic modulus, as determined by nanoindentation. As the experimental data agree with the predictions, the elasticity enhancement in VNx upon N-vacancy formation can be understood based on the concomitant changes in elastic anisotropy and bonding.

Stress-induced anisotropy in brine saturated shale

SUMMARY This paper reports the intrinsic and crack-induced anisotropic properties of a set of preserved, brine-saturated shale samples and their response to external stresses. We used undrained multistage triaxial tests to evaluate how the ultrasonic wave velocities and their anisotropy changed with increasing isotropic and differential stress conditions. In addition, the impact of stress orientation with respect to fabric orientation was evaluated. An array of ultrasonic transducers allowed to measure five independent wave velocities which were used to calculate the elastic properties of the shale. Results indicate that in this shale P- and S-wave velocities vary with stress in a different manner dependent on the maximum principal stress orientation with respect to the fabric. Where the maximum stress is normal to bedding, Vpv and Vs1 increase monotonically with increasing effective stress. However Vph and Vsh decrease during individual loading stages but increase from stage to stage as confining pressure increases. The reverse occurs when the microfabric is parallel to the maximum principal stress. Where the maximum stress is bedding normal, velocity anisotropy decreases as differential stress increases; when maximum stress is fabric parallel, anisotropy increases. Elastic anisotropy is related to the initial composition and the spatial distribution of the different minerals (fabric) in the sediment and the presence of microfractures, while changes in elastic anisotropy result from the applied stresses, their orientation with respect to the rock fabric and the degree of stress anisotropy.

Temporal changes in seismic wave propagation towards the end of the 2002 Mt Etna eruption

SUMMARY Stress can undergo rapid temporal changes in volcanic environments, and this is particularly true during eruptions. We use two independent methods, coda wave interferometry (CWI) and shear wave splitting (SWS) analysis to track stress related wave propagation effects during the waning phase of the 2002 NE fissure eruption at Mt Etna. CWI is used to estimate temporal changes in seismic wave velocity, while SWS is employed to monitor changes in elastic anisotropy. We analyse seismic doublets, detecting temporal changes both in wave velocities and anisotropy, consistent with observed eruptive activity. In particular, syn-eruptive wave propagation changes indicate a depressurization of the system, heralding the termination of the eruption, which occurs three days later.

Elastic Properties of Shales with Respect to Silt Fraction

Elastic properties of shales are important for quantitative interpretation of seismic response, detecting high pore pressure before drilling and understanding overburden response during production. Modelling shale properties is difficult however as elastic properties of individual clay minerals are hard to measure directly. We use a transversely isotropic differential effective medium approach to calculate elastic constants of shales with different silt fractions. Increasing silt fraction from 0 to 25% results in an increase of elastic constants C11, C33, C13, C44 and C66 by 15%, 25%, -0.5%, 33% and 75%, respectively, whereas a 50% increase of silt fraction increases the elastic constants by 36%, 70%, -7%, 83% and 220%, respectively, in comparison with the constants of pure clay. These non-uniform alterations of elastic constants might lead to changes in elastic anisotropy. Thomsen?s anisotropy parameters e, ?, and d which describe the variation of P- and SH-wave velocities as a function of polar angle with respect to symmetry axis are also calculated. e decreases by 18% and 43% with increasing silt fraction from 0 to 25% and 0 to 50%, respectively. ? decreases by 15% and 36% by adding 25% and 50% of silt, respectively. d decreases by 60% with increasing silt fraction from 0 to 25% and changes sign when silt fraction reaches 42%. The results show that the presence of silt in shales cannot be neglected as it substantially increases compressional and shear velocities and reduces anisotropy. Elastic moduli of shales are predicted using known silt fractions and elastic moduli of clays and compared with the results of laboratory measurements.

Porosity and elastic anisotropy of rocks under tectonic stress and pore-pressure changes

Elastic properties of rocks depend on tectonic stress. Using the theory of poroelasticity as a constraint, we analyze features of these dependencies related to changes in rock pore-space geometry. We develop a formalism describing elastic moduli and anisotropy of rocks as nonlinear functions of confining stress and pore pressure. This formalism appears to agree with laboratory observations. To a first approximation, elastic moduli and seismic velocities as well as porosity depend only on the difference between the confining tectonic stress and pore pressure. However, in general, both the confining stress tensor and the pore pressure must be taken into account as independent variables. The stress-dependent geometry of the pore space fully controls the stress-induced changes in elastic moduli and seismic velocities. Specifically, the compliant porosity plays the most important role, despite the fact that in many rocks the compliant porosity is a very small part of total porosity. Changes in compliant porosity with pressure and stress explain the often observed behavior of elastic moduli: in the low compressive stress regime — say, below 50 to 100 MPa — moduli increase rapidly and then taper exponentially into a flat and linear increase with increasing load. Taking into account the strain of the compliant pore space, we introduce a tensor quantity defining the sensitivity of elastic moduli of rocks to the difference between confining stress and pore pressure. We call it the stress sensitivity tensor. This tensor is an important physical characteristic directly related to elastic nonlinearity of rocks. The stress sensitivity tensor governs the changes in elastic anisotropy of a drained poroelastic system as it depends upon the applied load.

Total induced anisotropy due to tilted cracks in anisotropic medium

This paper is concerned with the relationship between the effective stiffness tensor and the intensity of damage in individual modes for an anisotropic material containing tilted cracks. The predictions are compared favourably with the experimentally measured load-induced changes of the 13 stiffnesses of a 2D C/C-SiC ceramic-matrix composite subjected to off-axis loading. By taking into account the thickness of the cracks, it is possible to understand the change in elastic anisotropy in material and of its inelastic strain.

Some remarks on the origin of seismic anisotropy in the D” layer

Physical mechanisms of seismic anisotropy in the D” layer are examined based on seismological and mineral physics observations. The results of body-wave seismology on the fine structure of the D” layer and of mineral physics studies on the elastic constants and the lattice preferred orientation in lower mantle minerals as well as the shape preferred orientation of melt pockets are taken into account. Evidence of large but depth (pressure)-dependent elastic anisotropy of lower mantle minerals, particularly (Mg,Fe)O, and of tilted shape preferred orientation of sheared partial melts is summarized. It is shown that both shape preferred orientation of partial melts (or iron-rich secondary phases) and lattice preferred orientation of minerals with well-documented slip systems are difficult to reconcile with seismological observations. However, lattice preferred orientation of highly anisotropic mineral, (Mg,Fe)O, is consistent with most of the seismic observations if the dominant glide plane under the D” layer conditions is 100 rather than 110 as observed at lower pressures. Such a change in glide plane in MgO (or (Mg,Fe)O) is likely to occur as a result of pressure-induced change in elastic anisotropy and/or in the nature of chemical bonding (and possibly due to high temperatures). Both solid-state and partial melt mechanisms of anisotropy imply that the VSH > VSV (VSV > VSH) polarization anisotropy means horizontal (vertical) flow. In the solid-state mechanism, significant VSH > VSV in the D” layer beneath the circum-Pacific (Alaska and the Caribbean) implies horizontal shear at high stress caused presumably by the collision of subducting materials with the core-mantle boundary. Highly variable anisotropy beneath the central-Pacific can be attributed to solid-state fabrics caused by a complicated three-dimensional flow presumably related to the upwelling of plumes, but anisotropy in this region could also be attributed to the shape preferred orientation of melt pockets the presence of which is suggested by very low average velocities.

Thermal Depoling Effects on Anisotropy of Lead Zirconate Titanate Materials

Changes in elastic anisotropy and fracture toughness anisotropy of a lead zirconate titanate (PZT) piezoelectric material as a function of depoling temperature were investigated using ultrasound and Vickers indentation. A gradual transition from anisotropic to isotropic properties occurs with increased depoling temperatures at and above the Curie temperature. Depoling is also evident through changes in domain character revealed by X‐ray diffraction. The relationship between the mechanical properties and domain structure variation with respect to different depoling conditions is discussed.

Assessment of Damage in Ceramics and Ceramic Matrix Composites Using Ultrasonic Techniques

This paper addresses the application of ultrasonic methods to damage assessment in ceramics and ceramic matrix composites. It focuses on damage caused by thermal shock and oxidation at elevated temperatures. The damage-induced changes in elastic constants and elastic anisotropy are determined by measuring the velocities of ultrasonic waves in different propagation directions within the sample. Thermal shock damage measurement is performed in ceramic samples of reaction bonded silicon nitride (RBSN) and aluminum oxide. Thermal shock treatment from different temperatures up to 1000°C is applied to produce the microcracks. Both surface and bulk ultrasonic wave methods are used to correlate the change of elastic constants to microstructural degradation and to determine the change in elastic anisotropy induced by microcrack damage. Oxidation damage is studied in silicon carbide fiber/reaction bonded silicon nitride matrix (SCS-6/RBSN) composites. The oxidation is done by exposing the samples in a flowing oxygen environment at elevated temperatures, up to 1400°C, for 100 hours. Significant changes of ultrasonic velocities were observed for composites before and after oxidation. The elastic constants of the composites were determined from the measured velocity data. The Young’s modulus in the fiber direction as obtained from ultrasonic measurements decrases significantly at 600°C but retains its original value at temperatures above 1200°C. This agrees well with the results of destructive tests by other authors. The transverse longitudinal and shear moduli obtained from ultrasonic measurements decrease continually until 1200°C. The results of this work show that the damage-induced anisotropy in both ceramics and ceramic matrix composites can be determined successfully by ultrasonic methods. This suggests the possibility of assessing damage severity using ultrasonic techniques.

Ultrasonic study on the change in elastic anisotropy with plastic compressive deformation

The macroscopic elasticity of a polycrystalline material is more or less anisotropic due to the presence of texture. In this Paper, the change in such elastic anisotropy with plastic deformation was studied ultrasonically. Test specimens prepared from a rolled plate of mild steel were deformed plastically by uni-axial compression, and the relative retardation and polarization directions of two independent transverse waves were measured by analysing frequency spectra of ultrasonic waves. It was confirmed from the results that the plastic deformation changes not only the degree of the elastic anisotropy but also its principal axes towards those of the plastic strain. A preliminary consideration was presented concerning the relation between the elastic stiffness and small plastic strain.