DEM modelling of sequential fragmentation of zeolite granules under oedometric compression based on XCT observations

Copyright © 2019 Elsevier B.V.The objective of our research is to define a new Discrete Element Method (DEM) that can describe the processes involved in particle breakage and the resulting macroscopic behaviour of the particulate assembly, by directly observing and characterizing breakage mechanisms. To this aim, an oedometer compression test is performed on a dry granular assembly of zeolite, while acquiring 3D images of the specimen at several strain levels with an x-ray computed tomography device. We construct a DEM model that reproduces experimental observations, mainly: axial splitting is the main breakage mode; fragments are subjected to further breakage; very few fragments pass through the breakage plane. A fragment size limit is defined to reduce the computational cost associated with large numbers of breakage generations. We simulate the oedometer test for the same initial microstructure as in the lab test and with realistic particle mechanical properties, and compare the re-ults to the 3D images. The numerical results show that our proposed model can capture the size evolution, shape change and mechanical response of the tested specimens

Evolution of particle breakage studied using x-ray tomography and the discrete element method

Particle breakage can significantly change the fabric (size and shape of particles and contact network) of a granular material, affecting highly the material's macroscopic response. In this paper, oedometric compression tests are performed on zeolite specimens and x-ray computed micro-tomography is employed, to acquire high resolution 3D images of the specimens throughout the test. The images are processed, to describe breakage spatially and quantify it throughout the test and gain information about the mechanisms leading to particle breakage. In addition to the image processing, the discrete element method (DEM) is used to study the initiation and likelihood of particle breakage, by simulating the experimental test during the early stages of loading and using quantitative results from the images to inform and validate the DEM model. A discrete digital image correlation is used, in order to incrementally identify intact grains and simultaneously get results about the strain field within the specimen, as well as the kinematics of individual grains and fragments. In the initial stages of breakage, there is a clear boundary effect on the spatial distribution of breakage, as it is concentrated at the moving boundary (more than 90% of total breakage) and circumferentially (more than 70% of total breakage) close to the apparatus cell. The DEM model can reproduce the bulk response of the material until the point where substantial breakage governs the macroscopic response and it starts to soften. Additionally, there is an initial indication that the spatial distribution of the force network matches the localisation of breakage radially, but it does not seem to localise close to the loading platen. This analysis will enrich our understanding of the mechanisms and evolution of particle breakage

Study of temporal and spatial evolution of deformation and breakage of dry granular materials using x-ray computed tomography and the discrete element method

Particles exist in great abundance in nature, such as in sands and clays, and they also constitute 75% of the materials used in industry (e.g., mineral ores, formulated pharmaceuticals, dyes, detergent powders). When a load is applied to a bulk assembly of soil particles, the response of a geomaterial at the bulk (macro) scale, originates from the changes that take place at the particle scale. If particle breakage occurs, the shape and size of the particles comprising the bulk are changed; this induces changes in the contact network through which applied loads are transmitted. As a result, changes at the micro-scale can significantly affect the mechanical behaviour of a geomaterial at a macro-scale. It is therefore unsurprising that the mechanisms leading to particle breakage are a subject of intense research interest in several fields, including geomechanics. In this thesis, particle breakage of two dry granular materials is studied, both experimentally and numerically. The response of the materials is investigated under different stress paths and in all the tests grain breakage occurs. High resolution x-ray computed micro-tomography (XCT) is used to obtain 3D images of entire specimens during high confinement triaxial compression tests and strain controlled oedometric compression tests. The acquired images are processed and measurements are made of the temporal and spatial evolution of breakage, local variations of porosity, volumetric and shear strain and grading. The evolution and spatial distribution of quantified breakage including the resulting particle size distribution for the whole specimen and for specific areas, are presented and further related to the localised shear and volumetric strains that developed in the specimens. In addition, the discrete element method (DEM) was used to provide further micro-mechanical insight of the underlying mechanisms leading to particle breakage. Classical DEM simulations, using a Hertz-Mindlin contact model and non-breakable spheres, was first deployed to study the initiation and likelihood of particle breakage under oedometric compression. Moreover, a bonded DEM model was used to create clumps that represent each particle and simulate breakage of particles under single particle compression. The DEM model parameters were obtained from results of single particle compression test and the models were validated against the quantitative 3D information of the micro-scale, acquired from the XCT analysis

Simulation of laboratory strength tests on intact rock with the distinct element method

142 σ.Στην παρούσα διπλωματική εργασία, έγινε μια προσπάθεια προσομοίωσης εργαστηριακών δοκιμών αντοχής σε άρρηκτο πέτρωμα, με τη μέθοδο διακριτών στοιχείων. Η προσομοίωση πραγματοποιήθηκε με τον αριθμητικό κώδικα PFC2D. Στόχο της εργασίας αποτέλεσε η όσο το δυνατόν καλύτερη προσομοίωση της μικροδομής του πετρώματος, ώστε η συμπεριφορά του μοντέλου σε θλίψη, να είναι συγκρίσιμη με τη συμπεριφορά πρισματικού δοκιμίου μαρμάρου Διονύσου, διαστάσεων 10cm ύψος και 5cm πλάτος, κατά τη διεξαγωγή εργαστηριακών δοκιμών αντοχής σε αυτό. Πιο συγκεκριμένα, κατασκευάστηκαν αριθμητικά μοντέλα που υποβλήθηκαν σε δοκιμές μονοαξονικής θλίψης, τριαξονικής θλίψης, αντιδιαμετρικής θλίψης (brazilian) και δοκιμές άμεσου εφελκυσμού. Τα αποτελέσματα για τη μακροσκοπική απόκριση του συνθετικού πετρώματος σε μονοαξονική θλίψη (μέτρο ελαστικότητας, αντοχή σε μονοαξονική θλίψη, λόγος του Poisson) συγκρίνονται με αυτά του πραγματικού πετρώματος. Με τον όρο «συνθετικό πέτρωμα», εννοούμε το προσομοίωμα ενός πετρώματος που αποτελείται από ένα σύνολο δομικών μονάδων (π.χ. άκαμπτων κυκλικών σφαιρών, δίσκων πεπερασμένου πάχους ή και συσσωματωμάτων τους). Εναλλακτικά, καλείται και ως «μοντέλο συνδεδεμένων σωματιδίων» (Bonded Particles Model, BPM). Καθοριστικής σημασίας για την επίτευξη του στόχου της εργασίας ήταν η βέλτιστη επιλογή των παραμέτρων του αριθμητικού μοντέλου (μικρομηχανικές παράμετροι). Για την επιλογή αυτή ακολουθήθηκαν βασικά τρεις μεθοδολογίες. Αρχικά, κατασκευάστηκε ένα μοντέλο BPM βάσει της μεθοδολογίας του Yoon (2007), που βασίζεται στην εύρεση των μικρομηχανικών παραμέτρων με σχεδιασμό πειράματος και βελτιστοποίηση. Η διερεύνηση συνεχίστηκε κατασκευάζοντας 102 μοντέλα BPM, θεωρώντας τις μικρο-παραμέτρους τους ως τυχαίες μεταβλητές (προσομοίωση Latin Hypercube). Στο τελευταίο στάδιο κατασκευάστηκαν 112 μοντέλα συνδεδεμένων κόκκων (Grain Based Model, GBM), εφαπτόμενων σε κάθε πλευρά τους, ακολουθώντας την ίδια μεθοδολογία καθορισμού των μικρο-παραμέτρων, όπως στα 102 μοντέλα BPM. Στα εισαγωγικά κεφάλαια της εργασίας γίνεται μια σύντομη περιγραφή του προγράμματος PFC2D, για την πληρέστερη και αρτιότερη ενημέρωση των αναγνωστών. Τέλος, στο τελευταίο κεφάλαιο συμπτύσσονται τα συμπεράσματα που εξήχθησαν από τη διπλωματική εργασία.In the present thesis, there has been an attempt to simulate laboratory tests on intact rock specimens, using the Discrete Element Method. The simulation was completed using the two dimensional discontinuum program PFC2D. Objective of this effort was the best possible simulation of the microstructure of a rock, so that its behaviour could be compared to the behaviour of a prismatic specimen of Dionysus marble, with dimensions 10cm height and 5cm width, after undergoing laboratory tests. Specifically, a set of numerical models were developed in order to be submitted to unconfined and confined compression tests, but also direct – tension and Brazilian tests. The results of the macroscopic response (unconfined compression strength, Young’s modulus, Poisson's ratio) of the synthetic rock will be compared with the macroscopic response of a real rock. Therefore the optimum selection of its micro-parameters is of great importance. The term "synthetic rock" refers to a simulation of a rock that consists of a set of rigid circular or spherical particles or even clumps. Alternatively, it is called Bonded Particles Model, BPM. Three different methodologies were presented during the completion of this thesis. Initially, we constructed a BPM based on Yoon’s (2007) methodology, using experimental design and optimization to investigate the micromechanisms that produce the macroscopic response of a rock. Furthermore, 102 BPM’s were developed, considering the micro-parameters to be random variables (Latin Hypercube simulation). In the last stage of our simulations we constructed 112 Grain Based Models, GBM’s consisting of deformable, breakable polygonal grains cemented on their adjoining sides. For the determination of the models’ micro-parameters the same methodology was followed, such as the 102 BPM models’. At the initial chapter a brief description of the program PFC2D is implemented, for the understanding of the readers. Finally, in the last chapter the conclusions reached during the whole thesis are outlined.Ζεϋνέπ Τ. Καράτζ

Effect of wetting and drying on meniscus structures in hydrophobic sands

Hydrophobic soils can occur either naturally when particles are coated with plant-derived hydropho-bic organic compounds or if exposed to very high temperatures, or artificially if treated with contaminated water or chemicals in the laboratory. Hydrophobic soils can resist water infiltration, are associated with preferential flow and may lead to increased surface runoff and soil erosion. Traditional understanding of unsaturated hy-drophobic soils suggests that convex water menisci, and so positive water pressures, should form between soil particles, due to contact angles > 90◦. However, experimental results do not support this theory. The objective of this work was to study the changes in meniscus structures in hydrophobic sand specimens, as well as the overall response of the sand to wetting and drying cycles. A very uniform, fine silica sand was mixed with Dimethyldichlorosilane to induce water repellence. Successive images captured in an environmental scanning electron microscope are presented, to examine the response of the sand in two distinct drying and wetting cycles. Preliminary results show that the non-spherical nature of the sand particles prevent or hinder the formation of convex liquid bridges, despite the high contact angles. Rather, water droplets appear to expand only through droplet coalescence, which prevents structures from contracting on drying

Comparison of bonded-particle numerical model results with indirect tension experimental results for Dionysos marble

Summarization: In this study, results from two series of indirect tensile tests on Dionysos marble specimens, the Brazilian test and the ring test, are compared to those evaluated numerically by using the distinct element code PFC2D. Circular disc specimens with a diameter of 54 mm were prepared and tested in the laboratory. Specimens with different hole diameters were tested with respect to the ring test. The numerical simulations of this study were performed using the Bonded Particle Model (BPM) in PFC2D. The initial selection of the BPM micro-parameters was based on diagrams relating the PFC2D parameters and the synthetic rock properties. The models were calibrated by numerically simulating uniaxial compression tests and Brazilian tests. The ring-test PFC2D models were developed with the same specimen geometries as those of the laboratory tests and by applying the calibrated BPM micro-parameters. The numerically obtained fracture loads are compared to those measured experimentally. Furthermore, the evolution of bonds breakage during the simulation is compared to the rock fracture patterns observed during failure of the specimens. The simulation results demonstrate that both the macro-mechanical response and the failure process can be modeled using BPMs. Differences between the numerical results and the macroscopic marble behavior are discussed.Παρουσιάστηκε στο: 53rd U.S. Rock Mechanics/Geomechanics Symposiu