13,687 research outputs found
Geometric Morphology of Granular Materials
We present a new method to transform the spectral pixel information of a
micrograph into an affine geometric description, which allows us to analyze the
morphology of granular materials. We use spectral and pulse-coupled neural
network based segmentation techniques to generate blobs, and a newly developed
algorithm to extract dilated contours. A constrained Delaunay tesselation of
the contour points results in a triangular mesh. This mesh is the basic
ingredient of the Chodal Axis Transform, which provides a morphological
decomposition of shapes. Such decomposition allows for grain separation and the
efficient computation of the statistical features of granular materials.Comment: 6 pages, 9 figures. For more information visit
http://www.nis.lanl.gov/~bschlei/labvis/index.htm
Decoding Geometric Origin of Geomechanical Properties
Granular materials such as soil and aggregate, are ubiquitous in nature and the understanding of their mechanical behavior is of great importance to better predict and design the civil infrastructure. The particle geometry is a key information to robustly establish the link between the underlying grain-scale mechanisms and the macroscopic behavior of granular materials. However, the characteristics of the particle geometry remain to be better understood. For example, we do not know how the volume is related to the surface area for irregularly shaped particles in general. Their relation clearly depends on the morphology, dictating that volume, surface area, and morphology are interrelated. Then, the remaining question is how the size of a particle would be related to those three geometric properties. The interrelation of these four geometry parameters is the key information to fundamentally understand their concerted influence on the complex behavior of granular materials, but we do not have the answer in the body of knowledge yet.
The research in this dissertation advances the understanding of grain-scale origin of the complex macroscale behavior of granular materials and creates a set of new knowledge as follows: (i) This study systematically addresses the influence of coarse aggregate angularity on cemented granular materials. It shows that cemented granular materials with round aggregates have superior small-strain performance, while the materials with angular aggregates have superior large-strain performance; (ii) This study develops a new theory for comprehensive 3D particle geometry characterization by proposing a formulation M = A/VĂ—L/6, which translates the 3D particle morphology M as a function of surface area A, volume V, and size L; (iii) This dissertation is benefited by the early adoption of 3D-printing for geomechanical testing. Laboratory direct shear tests have been conducted on 3D-printed synthetic particles with different geometry, to robustly correlate the geometric properties of particles to geomechanical properties of the granular materials. (iv) This study unravels, for the first time, the power law relationship between A/V ratio and V for coarse aggregate in nature. This relationship is the key to predict morphology using volume measurement only, thus significantly reducing the effort of particle geometry characterization
Minkowski Tensors of Anisotropic Spatial Structure
This article describes the theoretical foundation of and explicit algorithms
for a novel approach to morphology and anisotropy analysis of complex spatial
structure using tensor-valued Minkowski functionals, the so-called Minkowski
tensors. Minkowski tensors are generalisations of the well-known scalar
Minkowski functionals and are explicitly sensitive to anisotropic aspects of
morphology, relevant for example for elastic moduli or permeability of
microstructured materials. Here we derive explicit linear-time algorithms to
compute these tensorial measures for three-dimensional shapes. These apply to
representations of any object that can be represented by a triangulation of its
bounding surface; their application is illustrated for the polyhedral Voronoi
cellular complexes of jammed sphere configurations, and for triangulations of a
biopolymer fibre network obtained by confocal microscopy. The article further
bridges the substantial notational and conceptual gap between the different but
equivalent approaches to scalar or tensorial Minkowski functionals in
mathematics and in physics, hence making the mathematical measure theoretic
method more readily accessible for future application in the physical sciences
Scaling forces to asteroid surfaces: The role of cohesion
The scaling of physical forces to the extremely low ambient gravitational
acceleration regimes found on the surfaces of small asteroids is performed.
Resulting from this, it is found that van der Waals cohesive forces between
regolith grains on asteroid surfaces should be a dominant force and compete
with particle weights and be greater, in general, than electrostatic and solar
radiation pressure forces. Based on this scaling, we interpret previous
experiments performed on cohesive powders in the terrestrial environment as
being relevant for the understanding of processes on asteroid surfaces. The
implications of these terrestrial experiments for interpreting observations of
asteroid surfaces and macro-porosity are considered, and yield interpretations
that differ from previously assumed processes for these environments. Based on
this understanding, we propose a new model for the end state of small, rapidly
rotating asteroids which allows them to be comprised of relatively fine
regolith grains held together by van der Waals cohesive forces.Comment: 54 pages, 7 figure
Spatiotemporal evolution, mineralogical composition, and transport mechanisms of long-runout landslides in Valles Marineris, Mars
Long-runout landslides with transport distances of >50 km are ubiquitous in Valles Marineris (VM), yet the transport mechanisms remain poorly understood. Four decades of studies reveal significant variation in landslide morphology and emplacement age, but how these variations are related to landslide transport mechanisms is not clear. In this study, we address this question by conducting systematic geological mapping and compositional analysis of VM long-runout landslides using high-resolution Mars Reconnaissance Orbiter imagery and spectral data. Our work shows that: (1) a two-zone morphological division (i.e., an inner zone characterized by rotated blocks and an outer zone expressed by a thin sheet with a nearly flat surface) characterizes all major VM landslides; (2) landslide mobility is broadly dependent on landslide mass; and (3) the maximum width of the outer zone and its transport distance are inversely related to the basal friction that was estimated from the surface slope angle of the outer zone. Our comprehensive Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) compositional analysis indicates that hydrated silicates are common in landslide outer zones and nearby trough-floor deposits. Furthermore, outer zones containing hydrated minerals are sometimes associated with longer runout and increased lateral spreading compared to those without detectable hydrated minerals. Finally, with one exception we find that hydrated minerals are absent in the inner zones of the investigated VM landslides. These results as whole suggest that hydrated minerals may have contributed to the magnitude of lateral spreading and long-distance forward transport of major VM landslides
A Terradynamics of Legged Locomotion on Granular Media
The theories of aero- and hydrodynamics predict animal movement and device
design in air and water through the computation of lift, drag, and thrust
forces. Although models of terrestrial legged locomotion have focused on
interactions with solid ground, many animals move on substrates that flow in
response to intrusion. However, locomotor-ground interaction models on such
flowable ground are often unavailable. We developed a force model for
arbitrarily-shaped legs and bodies moving freely in granular media, and used
this "terradynamics" to predict a small legged robot's locomotion on granular
media using various leg shapes and stride frequencies. Our study reveals a
complex but generic dependence of stresses in granular media on intruder depth,
orientation, and movement direction and gives insight into the effects of leg
morphology and kinematics on movement
Modelling of material cutting with a material microstructure-level (MML) model
In this research work a material microstructure-level cutting model (MML cutting model) is presented. The crystal plasticity theory is adopted for modeling the cutting of the titanium alloy Ti–6Al–4V in orthogonal case. In this model, the grains of the studied material are explicitly presented, and their orientation angles and slip system strength anisotropy are considered as the main source of the microstructure heterogeneity in the cutting material. To obtain the material degradation process, the continuum self-consistent intragranular damage model and discrete cohesive zone inter-granular damage model, were developed, wherein the zero thickness cohesive element is implemented to simulate the bond between grain interfaces. This model was validated by a comparison with compression tests from literature. Results demonstrate the possibility to capture the influence of the microstructure on the material removal in terms of chip formation. Particularly, it is demonstrated that the grain orientation angle plays an important role for the chip segmentation and its periodicity during the cutting process
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