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Takato Takemura - One of the best experts on this subject based on the ideXlab platform.

  • changes in crack density and wave velocity in association with crack growth in triaxial tests of inada granite
    2006
    Co-Authors: Takato Takemura
    Abstract:

    A non-dimensional Second Rank Tensor Fij, called the crack Tensor, has successfully been introduced to deal with geometrical aspects of microcracks (fabric) such as anisotropy and crack density. Unfortunately, however, its usage for practical purposes is rather limited because its determination involves tedious and time-consuming laboratory work. We seek the possibility of using the directional change of longitudinal wave velocities to conquer the difficulty associated with the determination of crack Tensors. A new Second-Rank Tensor Vij is introduced, such that the directional change in the longitudinal wave velocities is represented in terms of the Tensor, and the crack Tensor Fij is then given as a function of Vij. Based on the analyses of the crack Tensors for one intact and several damaged samples of Inada granite, we then discuss how microcracks grow through the whole inelastic process, terminating at brittle failure. The conclusions are summarized as follows: The Second-Rank symmetrical Tensor Vij (or its inversion Tensor Vij -1 ) can be determined experimentally, with sufficient accuracy from the directional change in the squared longitudinal wave velocity. It is found that the Tensor changes markedly so as to reflect the fabric of the damaged Inada granite formed by open microcracks. The principal axes of Vij -1 are coaxial with the principal axes of Fij so that both Tensors are correlated in terms of their principal values Fi and Vi -1 .

  • changes in crack density and wave velocity in association with crack growth in triaxial tests of inada granite
    2005
    Co-Authors: Takato Takemura
    Abstract:

    [1] A nondimensional Second-Rank Tensor Fij, called the crack Tensor, has successfully been introduced to deal with geometrical aspects of microcracks (fabric) such as anisotropy and crack density. Unfortunately, however, its usage for practical purposes is rather limited because its determination involves tedious and time-consuming laboratory work. We seek the possibility of using the directional change of longitudinal wave velocities to conquer the difficulty associated with the determination of crack Tensors. A new Second-Rank Tensor Vij is introduced, such that the directional change in the longitudinal wave velocities is represented in terms of the Tensor, and the crack Tensor Fij is then given as a function of Vij. On the basis of the analyses of the crack Tensors for one intact and several damaged samples of Inada granite, we then discuss how microcracks grow through the whole inelastic process, terminating at brittle failure. The conclusions are summarized as follows: The Second-Rank symmetrical Tensor Vij (or its inversion Tensor Vij−1) can be determined experimentally, with sufficient accuracy from the directional change in the squared longitudinal wave velocity. It is found that the Tensor changes markedly so as to reflect the fabric of the damaged Inada granite formed by open microcracks. The principal axes of Vij−1 are coaxial with the principal axes of Fij so that both Tensors are correlated in terms of their principal values Fi and Vi−1. Four successive stages can be distinguished in regard to the crack growth as follows: In stage 1, the rock behaves like an elastic solid. In stage 2, microcracks start to grow so that inelastic volumetric strain is slowly accumulated, along with microcracking. However, crack growth does not occur globally but rather is limited within some local zones (probably in each grain). In stage 3, microcracking is considerably accelerated, suggesting that the micromechanism leading to crack growth changes substantially at the boundary stress between stages 2 and 3. In stage 4, the crack density, as well as the dilatancy, increases explosively in association with a drop of a few percent in the differential stress after the peak stress is reached. Interestingly, this explosive increase is always associated with the development of a few fault zones. Experimental evidence seems to support the postulate that Inada granite starts to collapse once the crack density, F0(f), the first invariant of Fij, attains a threshold value of 7–8, regardless of the applied confining pressure. This can be a failure criterion in terms of the crack density, and be an extended expression for the so-called “critical dilatancy” for creep failure suggested by Kranz and Scholz (1977).

  • preferred orientations of open microcracks in granite and their relation with anisotropic elasticity
    2003
    Co-Authors: Takato Takemura, Aliakbar Golshani, Masanobu Oda, Kenichiro Suzuki
    Abstract:

    Abstract In order to study how to deal with open microcracks in rock, anisotropic behaviors of Oshima granite were investigated by carrying out wave velocity tests and uniaxial compression tests, together with observations of microcracks under an optical microscope equipped with a universal stage. Anisotropy in the longitudinal wave velocity VL and secant deformation modulus E10 at 10% strength is caused by pre-existing open microcracks, not by pre-existing healed microcracks. The structural anisotropy formed by open microcracks, which is quantitatively represented by a Second-Rank Tensor (called crack Tensor), is in good agreement with the directional changes of E10 and VL. The mechanical, as well as structural, anisotropy shows rhombic symmetry with orthogonal symmetry axes in the directions roughly normal to the rift, grain and hardway planes, which are parallel to the major joint sets in the field. Since longitudinal wave velocity changes drastically depending on the density and orientation of open microcracks in granitic rocks, it is suggested that the crack Tensor can be determined from non-destructive wave velocity tests. The elastic modulus Tensor theoretically formulated in terms of the Second-Rank crack Tensor can be used, as a first-order approximation at least, to describe the anisotropic elasticity of Oshima granite induced by pre-existing open microcracks. It is of particular importance to point out that the micro-scale structure by open microcracks is geometrically similar to the macro-scale structure by joints and faults (scale independent). This finding strongly suggests that some of the conclusions related to open microcracks are applicable to deal with macro-scale cracks in rock masses.

Peter C M Van Zijl - One of the best experts on this subject based on the ideXlab platform.

  • mapping magnetic susceptibility anisotropies of white matter in vivo in the human brain at 7 t
    2012
    Co-Authors: Deepti S Vikram, Issel Anne L Lim, Craig K Jones, Jonathan A D Farrell, Peter C M Van Zijl
    Abstract:

    High-resolution magnetic resonance phase- or frequency-shift images acquired at high field show contrast related to magnetic susceptibility differences between tissues. Such contrast varies with the orientation of the organ in the field, but the development of quantitative susceptibility mapping (QSM) has made it possible to reproducibly image the intrinsic tissue susceptibility contrast. However, recent studies indicate that magnetic susceptibility is anisotropic in brain white matter and, as such, needs to be described by a symmetric Second-Rank Tensor( χ). To fully determine the elements of this Tensor, it would be necessary to acquire frequency data at six or more orientations. Assuming cylindrical symmetry of the susceptibility Tensor in myelinated white matter fibers, we propose a simplified method to reconstruct the susceptibility Tensor in terms of a mean magnetic susceptibility, MMS=(χ(//)+2 χ(⊥))/3 and a magnetic susceptibility anisotropy, MSA=χ(//)-χ(⊥), where χ(//) and χ(⊥) are susceptibility parallel and perpendicular to the white matter fiber direction, respectively. Computer simulations show that with a practical head rotation angle of around 20°-30°, four head orientations suffice to reproducibly reconstruct the Tensor with good accuracy. We tested this approach on whole brain 1 × 1 × 1 mm(3) frequency data acquired from five healthy subjects at 7 T. The frequency information from phase images collected at four head orientations was combined with the fiber direction information extracted from diffusion Tensor imaging (DTI) to map the white matter susceptibility Tensor. The MMS and MSA were quantified for regions in several large white matter fiber structures, including the corona radiata, posterior thalamic radiation and corpus callosum. MMS ranged from -0.037 to -0.053 ppm (referenced to CSF being about zero). MSA values could be quantified without the need for a reference and ranged between 0.004 and 0.029 ppm, in line with the expectation that the susceptibility perpendicular to the fiber is more diamagnetic than the one parallel to it.

Jiang Bin - One of the best experts on this subject based on the ideXlab platform.

  • Symmetry-adapted high dimensional neural network representation of electronic friction Tensor of adsorbates on metals
    2020
    Co-Authors: Zhang Yaolong, Maurer, Reinhard J., Jiang Bin
    Abstract:

    Nonadiabatic effects in chemical reaction at metal surfaces, due to excitation of electron–hole pairs, stand at the frontier of the studies of gas-surface reaction dynamics. However, the first-principles description of electronic excitation remains challenging. In an efficient molecular dynamics with electronic friction (MDEF) method, the nonadiabatic couplings are effectively included in a so-called electronic friction Tensor (EFT), which can be computed from first-order time-dependent perturbation theory (TDPT) in terms of density functional theory (DFT) orbitals. This Second-Rank Tensor depends on adsorbate position and features a complicated transformation with regard to the intrinsic symmetry operations of the system. In this work, we develop a new symmetry-adapted neural network representation of EFT, based on our recently proposed embedded atom neural network (EANN) framework. Inspired by the derivation of the nonadiabatic coupling matrix, we represent the Tensorial friction by the first and Second derivatives of multiple outputs of NNs with respect to atomic Cartesian coordinates. This rigorously preserves the positive semidefiniteness, directional property, and correct symmetry-equivariance of EFT. Unlike previous methods, our new approach can readily include both molecular and surface degrees of freedom, regardless of the type of surface. Tests on the H2 + Ag(111) system show that this approach yields an accurate, efficient, and continuous representation of EFT, making it possible to perform large scale TDPT-based MDEF simulations to study both adiabatic and nonadiabatic energy dissipation in a unified framework

  • Symmetry-Adapted High Dimensional Neural Network Representation of Electronic Friction Tensor of Adsorbates on Metals
    2019
    Co-Authors: Zhang Yaolong, Maurer, Reinhard J., Jiang Bin
    Abstract:

    Nonadiabatic effects in chemical reaction at metal surfaces, due to excitation of electron-hole pairs, stand at the frontier of the studies of gas-surface reaction dynamics. However, the first principles description of electronic excitation remains challenging. In an efficient molecular dynamics with electronic friction (MDEF) method, the nonadiabatic couplings are effectively included in a so-called electronic friction Tensor (EFT), which can be computed from first-order time-dependent perturbation theory (TDPT) in terms of density functional theory (DFT) orbitals. This Second-Rank Tensor depends on adsorbate position and features a complicated transformation with regard to the intrinsic symmetry operations of the system. In this work, we develop a new symmetry-adapted neural network representation of EFT, based on our recently proposed embedded atom neural network (EANN) framework. Inspired by the derivation of the nonadiabatic coupling matrix, we represent the Tensorial friction by the first and Second derivatives of multiple outputs of NNs with respect to atomic Cartesian coordinates. This rigorously preserves the positive semidefiniteness, directional property, and correct symmetry-equivariance of EFT. Unlike previous methods, our new approach can readily include both molecular and surface degrees of freedom, regardless of the type of surface. Tests on the H2+Ag(111) system show that this approach yields an accurate, efficient, and continuous representation of EFT, making it possible to perform large scale TDPT-based MDEF simulations to study both adiabatic and nonadiabatic energy dissipation in a unified framework.Comment: The paper have been published on the journal of physical chemistry

Deepti S Vikram - One of the best experts on this subject based on the ideXlab platform.

  • mapping magnetic susceptibility anisotropies of white matter in vivo in the human brain at 7 t
    2012
    Co-Authors: Deepti S Vikram, Issel Anne L Lim, Craig K Jones, Jonathan A D Farrell, Peter C M Van Zijl
    Abstract:

    High-resolution magnetic resonance phase- or frequency-shift images acquired at high field show contrast related to magnetic susceptibility differences between tissues. Such contrast varies with the orientation of the organ in the field, but the development of quantitative susceptibility mapping (QSM) has made it possible to reproducibly image the intrinsic tissue susceptibility contrast. However, recent studies indicate that magnetic susceptibility is anisotropic in brain white matter and, as such, needs to be described by a symmetric Second-Rank Tensor( χ). To fully determine the elements of this Tensor, it would be necessary to acquire frequency data at six or more orientations. Assuming cylindrical symmetry of the susceptibility Tensor in myelinated white matter fibers, we propose a simplified method to reconstruct the susceptibility Tensor in terms of a mean magnetic susceptibility, MMS=(χ(//)+2 χ(⊥))/3 and a magnetic susceptibility anisotropy, MSA=χ(//)-χ(⊥), where χ(//) and χ(⊥) are susceptibility parallel and perpendicular to the white matter fiber direction, respectively. Computer simulations show that with a practical head rotation angle of around 20°-30°, four head orientations suffice to reproducibly reconstruct the Tensor with good accuracy. We tested this approach on whole brain 1 × 1 × 1 mm(3) frequency data acquired from five healthy subjects at 7 T. The frequency information from phase images collected at four head orientations was combined with the fiber direction information extracted from diffusion Tensor imaging (DTI) to map the white matter susceptibility Tensor. The MMS and MSA were quantified for regions in several large white matter fiber structures, including the corona radiata, posterior thalamic radiation and corpus callosum. MMS ranged from -0.037 to -0.053 ppm (referenced to CSF being about zero). MSA values could be quantified without the need for a reference and ranged between 0.004 and 0.029 ppm, in line with the expectation that the susceptibility perpendicular to the fiber is more diamagnetic than the one parallel to it.

W H Owens - One of the best experts on this subject based on the ideXlab platform.