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Mechanics of Drilling in Porous Brittle Solids
Get PDFThis thesis presents a detailed experimental programme on understanding the mechanics of drilling in porous brittle solids. Gypsum was used as a model material for this experimental study, in which the mechanics of drilling was decoupled into equivalent problems of indentation and cutting. A comprehensive understanding of the mechanics of indentation and cutting was gained by performing experiments in 2-D conditions. A camera and microscope assembly was used to capture images at high temporal and spatial resolution to measure the in situ deformation. Particle image velocimetry (PIV) algorithm was used to measure the deformation parameters such as velocity, strain-rate, strain and volume change. In the last part of this research, drilling experiments were performed in 3-D conditions and an attempt was made for understanding the mechanics of drilling by relating the drilling experiment results to that of indentation and cutting.
A series of wedge indentation experiments were performed under 2-D plane-strain conditions. Development of a parabolic zone of deformation, surrounding the indenter, was observed, wherein this size of the deformation zone and the strain accumulation in the deformation zone was a function of the geometry of the indenter. The maximum effective strain decreased and the overall strain field was more diffuse with increase in the wedge angle. Significant volume change was also observed in this deformation zone and the amount of volume change increased with increase in the porosity of the material. The zones of high volume change (compaction bands) were stacked in the form of layers oriented perpendicular to the direction of indentation. These compaction bands were more localized for the case of lower angles of wedge indenter. The extent of the compaction bands was also a function of porosity and spread over a larger area for the case of low porosity samples. A change in the material response was also observed with change in porosity and geometry of the indenter. The appearance of the crack was delayed with increase in porosity and reduction of wedge angle. The experimental results were also used to validate an analytical cavity expansion model. A better prediction of indentation pressure and the size of the deformation zone was possible after volume change corrections were incorporated into the cavity expansion formulation.
A series of orthogonal cutting experiments were performed in 2-D plane-strain conditions. The e ect of tool geometry and the depth of cut on the mechanics of cutting was studied with the help of image based measurements and cutting force signatures. Different types of cutting mechanisms were observed for the case of positive and negative rake angle tool. A cyclic increase and decrease in the cutting force was observed in case of positive rake angle cutting tool. The decrease in the cutting force corresponded to the initiation of crack from the tip of the tool. The crack traversed towards the surface of the material and resulted in the removal of a material chip. With progress of cutting, the tool scratched the material surface, giving rise to the gradual increase in the cutting force as it again reached local maxima when the tool completely re-engaged with the material. For the case of negative rake angle, apart from cyclic increase and decrease of the cutting force, there was a development of a triangular dead zone at the tip of the cutting tool. The size of the dead zone varied cyclically with the progress of cutting. The length of crack, which resulted in the removal of the chip from the material, was found to be a function of the rake angle and the depth of cut.
Drilling experiments were performed on gypsum samples in 3-D conditions. Two types of twist drills with different helix angles were used for this research work. Experiments were performed on the samples with two different porosities. Thrust force and torque signatures were recorded for five values of depth of cut per revolution. Since these experiments were performed in 3-D, image analysis was not performed. However, in order to ascertain a qualitative understanding of the drilling process, few experiments were performed on the edge of the material surface so that a cylindrical groove with semicircular cross section is made and the exposed surface of the material and the drill were imaged. The normalized thrust force and normalized torque were compared with indentation pressure and cutting force signatures and remarkable similarities between them was found. A transition from ductile to brittle type of response was observed with increase in the depth of cut per revolution, which was similar to what was observed in case of indentation. The magnitude of torque was found to be higher for high helix angle drills, which was counter to what was observed in cutting, which was due to the deposition of the material in helix for high helix angle drills, resulting in the reduction of the effective helix angle. An approximate estimate of the effective helix angle was made with the help of analytical solutions as well as from the qualitative analysis of the images
Weakly cemented granular materials: study at multiple length scales
No full textCemented granular systems are encountered at various scales in nature and artificially. We present an experimental study carried out on the structure and mechanical behaviour of weakly cemented granular materials. We study the cemented granular materials at two scales -- micro (particle-bond-particle) scale and macro (ensemble) scale.
At the micro-scale studies, a set of x-ray computed tomography experiments are performed. We characterize the structure of initial configuration of weakly cemented granular materials. We discuss, in detail, quantification of fabric and structure such as coordination number, fabric tensor, directional distribution of contact normal and particles, and grain size distribution. An alternative approach to arrive at the fabric tensor is also discussed. To obtain these characteristics, the scanned volume from XCT is segmented into particles and contacts (bonds - for a contact bound structure). For the segmentation, watershed along with h-minima or h-maxima transform are used. The algorithm is presented in detail for a two dimensional example image. From the segmentation results, it is observed that the particles of cemented granular materials orient themselves away from the direction of the gravity or body force whereas the contact normals have a tendency to orient along the direction of gravity. Further, we perform a set of uni-axial compression tests inside the X-ray computed tomograph. It is observed that the initial structure of cemented granular material does not changes significantly before the peak load is reached. The average coordination number increases at lower strains due to contraction of the specimen however at larger strains, continuous reduction in coordination number is observed. The evolution of average porosity field has similar trend to the volumetric strain. Further, the particle and contact align themselves along the direction of load at lower strains whereas at higher strains, they orient themselves away from the loading direction.
At macro-scale, we perform a set of triaxial and hollow cylinder shear tests to understand the effect of confining pressure, intermediate principal stress ratio, and density on weakly cemented sands. These results are analyzed in the framework of plasticity theory. We present the extraction of gross yield points of bonds, plastic work contours or yield curves, plastic strain increments, and failure. Further, we calibrate and validate the Lade's single hardening elastic-plastic model. The details of model parameter calibration and integration algorithms for prediction of behaviour are provided. The Lade's model uses stress transformation for accommodation of cementation in the model. Stress transformation implies the translation of elastic-plastic surface along the hydrostatic axis in the stress-space by bond strength of cemented sands. With this stress transformation, the stress-strain response is predicted satisfactorily however, the volumetric predictions only show contraction. In contrast, the experimental volumetric behaviour is initially contractive followed by a dilative response (in the range of confining pressure tested). To validate the applicability of stress transformation, we perform a set of experiments with cemented sands and sands (equivalent sand) subjected to elevated confining pressure (increased by the bond strength). The response suggest that the stress transformation is satisfactory at small strain however due to bond breakage, a deviation in the cemented sands and equivalent sand is observed. This behaviour suggest that the inclusion of bond degradation with stress transformation should work successfully. To verify this, we include the bond degradation in the stress-dilatancy relation for prediction of stress-dilatancy behaviour of cemented sands. With inclusion of bond degradation, the Rowe's and Zhang-Salgado's stress dilatancy relation successfully predict the stress-dilatancy behaviour of cemented sands. We provide microstructural insights from our tomography experiments to the macro level response observed under various stress conditios.
Further, a set of scaling studies are also performed on unconfined compressive strength with varying particle sizes (particle size effect), specimen sizes (specimen size effect), and controlled study (scaling specimen and particle sizes proportionally to keep number of particles fixed). With increase in the specimen size, the peak compressive strength of weakly cemented granular material increases. The peak compressive strength decreases with increase in the size of particle. In controlled study, the strength is insensitive to proportional scaling of specimen and particles. The scaling in these contact bound granular materials is significantly different from brittle and quasi-brittle solids such as rocks and concrete. To understand the emergence of the scaling, we use microstructural characteristics obtained from XCT. In particular, we obtain the geometric clusters which are akin to force chains i.e. geometric clusters are able to predict the force distribution in a granular material from its structure. Using the percolation probability of these geometric cluster and normalized cluster size, similar trends as strength are obtained. The primary source of these scaling is results of entanglement of force chains which is presented here by entanglement of geometric cluster.Department of Science and Technolog
Mechanics of cutting in granular media
No full textCutting is an important deformation process encountered during many instances in the engineering of infrastructure, such as trawling, trenching, and excavation. This thesis presents the results of an experimental program on the orthogonal cutting of granular materials, performed under plane strain conditions. The kinematics of granular materials, when subjected to large deformations, is understood through direct imaging and concomitant image analysis. Three suites of experiments were performed to understand the effects of boundary conditions, material systems, and other constraints on the mechanics of cutting.
In the first suite of experiments, we perform cutting on two sets of granular materials, spherical glass beads and angular Cauvery delta sand. We vary the three main parameters of the orthogonal cutting problem, the rake angle/ angle of attack of the tool, the initial depth of cut with respect to the free surface of the granular ensemble, and the cutting speed. The overall deformation of the material reaches a steady state after an initial transient phase, which can be observed from the angle of the repose of the pile that forms in-front of the tool. Instantaneous velocity fields show a bifurcation of flow occurring at the tool tip which results in a velocity jump across planes whose direction coincides with the Coulomb failure surfaces. These velocity jumps manifest as regions of intense shear emanating from the tool tip and has a ``band" like appearance when the effective strain rate contours are plotted. The velocity profiles across the shear band assume a sigmoidal shape which can be fit using an error function and allows us to extract the length scale associated with this velocity jump. We observe that the normalized width of the shear band decreases as particle size increases. The width of the shear band for both granular materials was observed to be insensitive to changes in the rake angle, cutting speed, and also the initial depth of cut and hence is a material constant for a given system size. In some cases, we also observe multiple shear bands forming in the system which run parallel to one another and have similar thickness. The measured volumetric strain rate within the shear bands is much lower than the shear rate. The dilation angle measured is always less than the friction angle suggesting that plastic flow occurring within the shear bands is in a non-associative state. We further observe a dead zone of granular material at the tool interface.
In the second suite of experiments, we use a suite of granular materials such as cylindrical steel beads, rice grains, clay aggregates, polystyrene beads, and smaller glass beads. At the ensemble level, we find that the volume of material removed during cutting increases linearly in time for all granular materials. At the meso-scale, we once again observe similar sigmoidal velocity profiles across the shear band and is further used to determine the shear band width of each granular material. We find the normalized width of the shear bands is a function of particle size. Upon re-scaling the velocity profiles for different granular materials by their corresponding shear band widths, they all collapse onto a single master curve given by the scaled error function. Thus, we witness a remarkable universality in the kinematics of granular materials at multiple length scales during orthogonal cutting. We further measure the temperature within the shear bands and observe a power-law relationship between the temperature and shear rate, with an exponent ~ 0.55, a significant deviation from the predictions of kinetic theory. In-order to further probe the origin of this inherent length scale at the meso-level, we measure the mean squared displacements of the particles in the shear bands and observe that the particles are not diffusive and hence conclude that the length scale corresponding to the shear bands does not have its origin in a diffusion like process.
In the third suite of experiments, a series of constrained cutting experiments were carried out wherein we force the cut granular material to flow through narrow channels by placing a rigid constraint across the cutting tool. We study the formation and evolution of shear bands under such circumstances. By the addition of a constraint, we pin the shear bands between the tool tip and the constraint corner. The separation distance between the tool and the constraint has a direct consequence on the characteristics of the shear bands generated within the ensemble. As the separation between the tool and plate increases, we recover the behavior of unconstrained cutting. Within the channel, the flow fields are very similar to what is observed in vertical hoppers, though the motion of particles is directed against gravity. Like in hopper flows, we observe dead zones along the length of tool and constraint boundaries. We further measure the thickness of this boundary layer using the same error function fit and find that the thickness of the boundary layer is constant across the channel height and nearly symmetric at the two boundaries for small channel widths. As the channel width increases, the boundary layer thickness also increases and becomes asymmetric at the two boundaries.
We also present the results of cutting carried out on a constrained granular ensemble, i.e. two granular chains with different numbers of beads. This addition of a constraint to the granular ensemble suppresses the formation of localized shear bands within the ensemble and the deformation is diffuse and homogeneous. The deformation also propagates to regions faraway from the tool.
Lastly, we provide a numerical underpinning to the cutting experiments using contact dynamics simulations. As in the case of the experiments, we vary the rake angle, initial depth of cut, and cutting speed in simulation. We compare the simulation results with analytical solutions provided by a 2-D model of cutting. We compare the deformation fields obtained from the simulations with our experimental results and deduce the forces on the tool and along the shear band.IISc, DB
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Rheological Studies on Slow Flow in Dense Granular Ensembles
No full textAn experimental study on the rheology and kinematics of dense granular materials is presented in this thesis in the slow flow regime. In this study, we employ an often used experimental set up of the cylindrical Couette Cell, for understanding the rheology of a host of model granular materials, especially under slow flow conditions.
Using a multi axis transducer capable of measuring one normal and two shear stress components, we study the wall stress profiles in granular ensembles. In a static granular bed, the stress saturates over certain depth as has been well documented. In the case of slow flow of dense granular media, the stress on the outer wall neither saturates nor follows linear profile of fluids but increases exponentially with the depth, and vertical shear stress acts in the opposite direction upon shearing, as has been first reported by Mehandia et al [1]. With this premise, we further investigate this interesting anomaly in the stress profile through a series of experiments on a modified Couette cell set up. We also observe that the stresses neither saturate like in static granular column nor increase linearly over depth like in fluids, but exponentially rise in slowly sheared granular ensembles.
We probe this anomaly of stress by examining influence of height of granular column, wall friction, and the effect of initial depositional fabric. No significant influence of column height or proximity of the base plate was found on the stress profile. The initial depositional fabric which was explored through four types of filling in the Couette Cell, while shows slight differences in the static stress profiles, reach a state of plastic flow or a critical state shear stress irrespective of the initial fabric. The wall friction effect was studied by shearing the sample under three different boundary roughness conditions. The results show that rheology is influenced by the boundary condition specially at high roughness and high shear rates. We examine the kinematics of granular flow through imaging and report the establishment of an secondary circulation of granular particles orthogonal to the velocity gradient which explains this anomaly in the direction of the wall stress.
We observe periodic stress fluctuations with a frequency nearly equaling to the native frequency of rotation of the inner cylinder. The cause of these fluctuations was investigated through a series of rheological experiments by enforcing a magnified compliance on the inner cylinder. Stress relaxation, stress transients studied through stop-reshear and stop-reverse experiments, and amplitude of stress fluctuations all show depth or mean stress dependence.
Stress measurements are extended to soft granule systems and particles with varied morphology. We use granules of three different shapes and three different rigidities, and compare their response. The rheology of different shaped granules and soft granules show rate independence similar to rigid particle flows. The results from the rheology of different shaped particles suggests a reduction in the shear
stress specimen when compared to spherical particles. The rheology of soft particles in slow flow regime exhibited qualitative similarity to rigid particles but had show higher exponential rise of normal stress with depth. Overall, we find the rheological behaviour to be a function of particle shape and rigidity in slow flow regime
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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