The Experts below are selected from a list of 270 Experts worldwide ranked by ideXlab platform
Budiman Minasny - One of the best experts on this subject based on the ideXlab platform.
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digital mapping of soil carbon
Advances in Agronomy, 2013Co-Authors: Budiman Minasny, Alex B Mcbratney, Brendan P Malone, Ichsani WheelerAbstract:There is a global demand for soil data and information for food security and global environmental management. There is also great interest in recognizing the soil system as a significant terrestrial sink of carbon. The reliable assessment of soil carbon (C) stocks is of key importance for soil conservation and in mitigation strategies for increased atmospheric carbon. In this article, we review and discuss the recent advances in digital mapping of soil C. The challenge to map carbon is demonstrated with the large variation of soil C concentration at a field, continental, and global scale. This article reviews recent studies in mapping soil C using digital soil mapping approaches. The general activities in digital soil mapping involve collection of a database of soil carbon observations over the area of interest; compilation of relevant covariates (scorpan factors) for the area; calibration or training of a spatial prediction function based on the observed dataset; interpolation and/or extrapolation of the prediction function over the whole area; and finally validation using existing or independent datasets. We discuss several relevant aspects in digital mapping: carbon concentration and carbon density, source of data, sampling density and resolution, depth of investigation, map validation, map uncertainty, and environmental covariates. We demonstrate harmonization of soil depths using the equal-area spline and the use of a Material Coordinate system to take into consideration the varying bulk density due to management practices. Soil C mapping has evolved from 2-D mapping of soil C stock at particular depth ranges to a semi-3-D soil map allowing the estimation of continuous soil C concentration or density with depth. This review then discusses the dynamics of soil C and the consequences for prediction and mapping of soil C change. Finally, we illustrate the prediction of soil carbon change using a semidynamic scorpan approach.
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Comment on “Determining soil carbon stock changes: Simple bulk density corrections fail” [Agric. Ecosyst. Environ. 134 (2009) 251–256]
Agriculture Ecosystems & Environment, 2010Co-Authors: Alex B Mcbratney, Budiman MinasnyAbstract:Lee et al. (2009) showed the equivalent soil mass (ESM) approach for correcting bulk density changes when estimating soil carbon stock or density. The ESM approach from Ellert and Bethany (1995) attempts to correct for differences in bulk density from different sampling dates by calculating the mass of soil C in an equivalent soil mass per unit area. This is done by first designating the mass of the heaviest soil layer as the equivalent mass. The C density from subsequent sampling is then calculated by estimating the thickness of the deepest soil layer required to attain the equivalent mass. Lee et al. (2009) showed various calculations of the ESM approach. In this comment, we should like to point out that the Material Coordinate system, which is simpler andmore general, is better for handling this issue. In fact, Gifford and Roderick (2003) have proposed the use of the mass (or Material) Coordinate system for soil C accounting. The Material Coordinate or Lagrange system was proposed by Smiles and Rosenthal (1968) for calculating water flux in swelling soils. It has been applied in calculatingwater flows in swelling soils (McGarry and Malafant, 1987; Ringrose-Voase et al., 2000).For C accounting, we based the C density on the mass of the soil mineral Material. First, we calculate the mineral mass of each sampling
W. Brocks - One of the best experts on this subject based on the ideXlab platform.
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On a finite-strain viscoplastic law coupled with anisotropic damage: theoretical formulations and numerical applications
Archive of Applied Mechanics, 2006Co-Authors: W. BrocksAbstract:Based on a dissipation inequality at finite strains and the effective stress concept, a Chaboche-type infinitesimal viscoplastic theory is extended to finite-strain cases coupled with anisotropic damage. The anisotropic damage is described by a rank-two symmetric tensor. The constitutive law is formulated in the corotational Material Coordinate system. Thus, the evolution equations of all internal variables can be expressed in terms of their Material time derivatives. The numerical algorithm for implementing the Material model in a finite element programme is also formulated, and several numerical examples are shown. Comparing the numerical simulations with experimental observations indicates that the present Material model can describe well the primary, secondary and tertiary creep. It can also predict the anisotropic damage modes observed in experiments correctly.
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On a finite strain viscoplastic theory based on a new internal dissipation inequality
International Journal of Plasticity, 2004Co-Authors: R.c. Lin, W. BrocksAbstract:Abstract This work is focused on the theoretical development and numerical implementation of a viscoplastic law. According to the second law of thermodynamics a dissipation inequality described in the rotated Material Coordinate system is developed. Based on this dissipation inequality and the principle of maximum dissipation a finite strain viscoplastic model described also in the rotated Material Coordinate system is formulated. The evolution equations are expressed in terms of the Material time derivatives of the rotated elastic logarithmic strain, the accumulated plastic strain and the strain-like tensor conjugate to the rotated back stress. The mathematical structure of this theory is concise and similar to that of the infinitesimal viscoplastic theory. These characteristics make the numerical implementation of this theory easy. The stress integration algorithm and the algorithmic tangent moduli for the infinitesimal theory can be applied to the numerical implementation of the present finite strain theory with a little reformulation. The complicated algorithmic formulations for most of other finite plastic laws can be therefore circumvented. In order to check the effectivity of the present finite strain theory a set of numerical examples under strict deformation conditions are presented. These numerical examples prove the excellent performance of the present viscoplastic Material law at describing the finite strain elastoplastic and viscoplastic problems.
Glen L Niebur - One of the best experts on this subject based on the ideXlab platform.
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preparation of on axis cylindrical trabecular bone specimens using micro ct imaging
Journal of Biomechanical Engineering-transactions of The Asme, 2004Co-Authors: Xiang Wang, Xiangyi Liu, Glen L NieburAbstract:The Orientation of trabecular bone specimens for mechanical testing must be carefully controlled. A method for accurately preparing on-axis cylindrical specimens using high-resolution micro-CT imaging was developed. Sixteen cylindrical specimens were prepared from eight bovine tibiae. High-resolution finite element models were generated from micro-CT images of parallelepipeds and used to determine the principal Material Coordinate system of each parallelepiped. A cylindrical specimen was then machined with a diamond coring bit. The resulting specimens were scanned again to evaluate the orientation. The average deviation between the principal fabric orientation and the longitudinal axis of the cylindrical specimen was only 4.70 +/- 3.11 degrees.
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Effects of damage on the orthotropic Material symmetry of bovine tibial trabecular bone.
Journal of biomechanics, 2003Co-Authors: Xiangyi Liu, Xiang Wang, Glen L NieburAbstract:The macroscopic mechanical properties of trabecular bone can be predicted by its architecture using theoretical relationships between the elastic and architectural properties. Microdamage caused by overloading or fatigue decreases the apparent elastic moduli of trabecular bone requiring these relationships to be modified to predict the damaged elastic properties. In the case of isotropic damage, the apparent level elastic properties could be determined by multiplying all of the elastic constants by a single scalar factor. If the damage is anisotropic, the elastic constants may change by differing factors and the Material Coordinate system could become misaligned with the fabric Coordinate system. High-resolution finite element models were used to simulate damage overloading on seven trabecular bone specimens subjected to pure shear strain in two planes. Comparison of the apparent elastic moduli of the specimens before and after damage showed that the reduction of the elastic moduli was anisotropic. This suggests that the microdamage within the specimens was inhomogeneous. However, after damage the specimens exhibited nearly orthotropic Material symmetry as they did before damage. Changes in the orientation of the orthotropic Material Coordinate system were also small and occurred primarily in the transverse plane. Thus, while damage in trabecular bone is anisotropic, the Material Coordinate system remains aligned with the fabric tensor.
Alex B Mcbratney - One of the best experts on this subject based on the ideXlab platform.
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digital mapping of soil carbon
Advances in Agronomy, 2013Co-Authors: Budiman Minasny, Alex B Mcbratney, Brendan P Malone, Ichsani WheelerAbstract:There is a global demand for soil data and information for food security and global environmental management. There is also great interest in recognizing the soil system as a significant terrestrial sink of carbon. The reliable assessment of soil carbon (C) stocks is of key importance for soil conservation and in mitigation strategies for increased atmospheric carbon. In this article, we review and discuss the recent advances in digital mapping of soil C. The challenge to map carbon is demonstrated with the large variation of soil C concentration at a field, continental, and global scale. This article reviews recent studies in mapping soil C using digital soil mapping approaches. The general activities in digital soil mapping involve collection of a database of soil carbon observations over the area of interest; compilation of relevant covariates (scorpan factors) for the area; calibration or training of a spatial prediction function based on the observed dataset; interpolation and/or extrapolation of the prediction function over the whole area; and finally validation using existing or independent datasets. We discuss several relevant aspects in digital mapping: carbon concentration and carbon density, source of data, sampling density and resolution, depth of investigation, map validation, map uncertainty, and environmental covariates. We demonstrate harmonization of soil depths using the equal-area spline and the use of a Material Coordinate system to take into consideration the varying bulk density due to management practices. Soil C mapping has evolved from 2-D mapping of soil C stock at particular depth ranges to a semi-3-D soil map allowing the estimation of continuous soil C concentration or density with depth. This review then discusses the dynamics of soil C and the consequences for prediction and mapping of soil C change. Finally, we illustrate the prediction of soil carbon change using a semidynamic scorpan approach.
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Comment on “Determining soil carbon stock changes: Simple bulk density corrections fail” [Agric. Ecosyst. Environ. 134 (2009) 251–256]
Agriculture Ecosystems & Environment, 2010Co-Authors: Alex B Mcbratney, Budiman MinasnyAbstract:Lee et al. (2009) showed the equivalent soil mass (ESM) approach for correcting bulk density changes when estimating soil carbon stock or density. The ESM approach from Ellert and Bethany (1995) attempts to correct for differences in bulk density from different sampling dates by calculating the mass of soil C in an equivalent soil mass per unit area. This is done by first designating the mass of the heaviest soil layer as the equivalent mass. The C density from subsequent sampling is then calculated by estimating the thickness of the deepest soil layer required to attain the equivalent mass. Lee et al. (2009) showed various calculations of the ESM approach. In this comment, we should like to point out that the Material Coordinate system, which is simpler andmore general, is better for handling this issue. In fact, Gifford and Roderick (2003) have proposed the use of the mass (or Material) Coordinate system for soil C accounting. The Material Coordinate or Lagrange system was proposed by Smiles and Rosenthal (1968) for calculating water flux in swelling soils. It has been applied in calculatingwater flows in swelling soils (McGarry and Malafant, 1987; Ringrose-Voase et al., 2000).For C accounting, we based the C density on the mass of the soil mineral Material. First, we calculate the mineral mass of each sampling
Ryszard Staroszczyk - One of the best experts on this subject based on the ideXlab platform.
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A Material Coordinate treatment of the sea–ice dynamics equations
Proceedings of the Royal Society of London. Series A: Mathematical Physical and Engineering Sciences, 1998Co-Authors: Leslie Morland, Ryszard StaroszczykAbstract:A finite–element algorithm is constructed for a Material Coordinate formulation of the equations of sea–ice dynamics, using quadratic elements and fully implicit time steps. The Material Coordinate description allows the nodes of a fixed finite–element mesh to define the same Material elements as time proceeds, which avoids interpolation of nodal values on a changing spatial mesh as the pack evolves, quadratic elements preserve continuity of second derivatives, and this time stepping is stable and accurate in standard problems. An earlier finite–element study of a wind–driven pack with two free boundary sections, using spatial Coordinates and implicit time steps without iteration, gave rise to numerical instability when the constitutive law for the ice stress induced by floe interactions imposes zero stress in diverging flow. The present more accurate study of the same problem, using a Material description and fully implicit time steps, with a smoothed transition to zero stress in diverging flow, significantly extends the time over which a stable solution is obtained. Stability and accuracy of the present algorithm is first demonstrated by comparison with a class of exact solutions to specific problems using linearly viscous relations in converging flow and abrupt transition to zero stress in diverging flow, for which an expanding region of diverging flow is initiated after a finite time at an interior point, either following convergence everywhere, or following an expanded region of neutral flow. The previous problem with two free boundary sections is then solved with the same rheology to demonstrate a stable solution over an extended time period. Next, a general nonlinearly viscous relation is constructed which ensures that the stress lies close to a yield envelope during strongly converging flow, to reflect the commonly used viscous–plastic model without the disjoint stress relations in different regimes. This is applied to a pack flow with a dramatically deforming free boundary driven by a vortex wind, which demonstrates how well the present Material formulation can capture large deformations.
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a Material Coordinate treatment of the sea ice dynamics equations
Proceedings of The Royal Society A: Mathematical Physical and Engineering Sciences, 1998Co-Authors: Leslie Morland, Ryszard StaroszczykAbstract:A finite–element algorithm is constructed for a Material Coordinate formulation of the equations of sea–ice dynamics, using quadratic elements and fully implicit time steps. The Material Coordinate description allows the nodes of a fixed finite–element mesh to define the same Material elements as time proceeds, which avoids interpolation of nodal values on a changing spatial mesh as the pack evolves, quadratic elements preserve continuity of second derivatives, and this time stepping is stable and accurate in standard problems. An earlier finite–element study of a wind–driven pack with two free boundary sections, using spatial Coordinates and implicit time steps without iteration, gave rise to numerical instability when the constitutive law for the ice stress induced by floe interactions imposes zero stress in diverging flow. The present more accurate study of the same problem, using a Material description and fully implicit time steps, with a smoothed transition to zero stress in diverging flow, significantly extends the time over which a stable solution is obtained. Stability and accuracy of the present algorithm is first demonstrated by comparison with a class of exact solutions to specific problems using linearly viscous relations in converging flow and abrupt transition to zero stress in diverging flow, for which an expanding region of diverging flow is initiated after a finite time at an interior point, either following convergence everywhere, or following an expanded region of neutral flow. The previous problem with two free boundary sections is then solved with the same rheology to demonstrate a stable solution over an extended time period. Next, a general nonlinearly viscous relation is constructed which ensures that the stress lies close to a yield envelope during strongly converging flow, to reflect the commonly used viscous–plastic model without the disjoint stress relations in different regimes. This is applied to a pack flow with a dramatically deforming free boundary driven by a vortex wind, which demonstrates how well the present Material formulation can capture large deformations.