The collapse of a mine pillar is a catastrophic event with great consequences for a mining operation. These events are not uncommon, and have been reported to produce air blasts able to knock down, seriously injury or kill miners; cause cascade pillar failures which involve the collapse of neighboring pillars; produce surface subsidence; and sterilize valuable reserves. In spite of the low probability of occurrence for a pillar collapse in comparison to other ground control instability issues, these consequences make these events high risk. Therefore, the design of these structures should be considered from a risk perspective rather than from a factor-of-safety deterministic approach, as it has been traditionally done. Discontinuities are one of the main failure drivers in underground stone pillars. Regardless of this, traditional pillar strength equations do not consider the effect of these. Recently, the NIOSH pillar strength equation introduced a Large Discontinuity Factor that acknowledges the effect of discontinuities in pillar strength. However, this parameter only considers "averaged" parameters in a deterministic way, failing to account for the spatial variability of fracture networks. This work presents a risk-based pillar design framework that enables to characterize the effect of discontinuities in pillar strength, as well as account for the possible range of stresses that will be acting on pillars. The proposed method was evaluated in an underground dipping stone mine. Discontinuities were characterized by integrating Laser Scanning and virtual discontinuity mapping. Information obtained from the discontinuity mapping process was used to generate discrete fracture networks (DFNs) for each discontinuity set. The Discrete Element Modeling Software 3DEC was used along with the DFNs to simulate fractured rock pillars. Different fractured pillar strength modeling approaches were evaluated, and the most adequate in terms of pillar strength values, failure mechanisms representation, and processing times, was selected. The selected model was tested stochastically, and these results were used to characterize pillar strength variability due to the presence of discontinuities. Pillar stress distributions were estimated using an stochastic finite volume continuous numerical model that accounted for the dipping nature of the deposit and the case study mine design. A pillar probability of failure baseline was defined by contrasting resulting pillar strength and stress distributions using the reliability method. Results from this design framework provide additional decision-making tools to prevent pillar failure from the design stages by reducing the uncertainty. The proposed method enables the integration of pillar design into the risk analysis framework of the mining operation, ultimately improving safety by preventing future pillar collapses. / Doctor of Philosophy / Underground mining operations involve the removal of rock material from the ground. Engineers are required to design structural elements to ensure the stability of the openings as the material is extracted. These structural elements are known as pillars, and are usually carefully-designed regular chunks of rock left unmined. The pressures that the mined rock was carrying are shed to these pillars, which sizes and dimensions must provide enough strength to ensure the overall stability of the mine and avoid a collapse. Failure of mine pillars are events that have occurred, causing serious consequences such as injuring and killing mine workers, producing ground surface sinking affecting neighboring communities, and halting the regular mine operation. Due to the severity of the consequences of pillar collapses, these events are classified as high risk. Therefore, pillar design should be addressed from a perspective that estimates the likelihood of pillar failure given all possible hazards during their design process. The rock material that composes mine pillars present fractures and weakness planes that have an influence on pillar strength. Even though it has been widely demonstrated that these features have a direct impact on pillar strength, most of the commonly used pillar design methods fail to consider such effect, producing uncertainty about the possible range of values for the actual strength of the pillars. This work introduces a pillar design framework that enables to characterize the effect of discontinuities in pillar strength, as well as account for the possible range of stresses that will be acting on pillars. The proposed method was evaluated in an underground inclined stone mine. Laser scanning was used to map and characterize rock fractures. Fracturing information was used to generate virtual three-dimensional fracture models referred to as discrete fracture networks (DFNs). A computational mechanical model of the mine pillar was done using the software 3DEC to evaluate the compressive strength of the fractured pillar. Multiple fracturing scenarios were tested and distributions of possible pillar strengths were estimated from these tests. An additional computational model to estimate the distribution of the stresses in the pillar was performed considering the mine designs and geological conditions. Results from both analyses allowed to estimate a pillar probability of failure baseline. This design framework provides additional decision-making tools to prevent pillar failure from the design stages by reducing uncertainty. The proposed method enables the integration of pillar design into the risk analysis framework of the mining operation, ultimately improving safety by preventing future pillar collapses.

The study of the shear behavior of particulate (soil) – continuum (man-made material) interfaces has received significant attention during the last three decades. The historical belief that the particulate – continuum interface represents the weak link in most geotechnical systems has been shown to be incorrect for many situations. Namely, prescribing properties of the continuum material, such as its surface roughness and hardness, can result in interface strengths that are equal to the contacting soil mass internal shear strength. This research expands the engineering implications of these findings by studying the response of interface systems in different loading conditions. Specifically, the axial and torsional shear modes are studied in detail. Throughout this thesis it is shown that taking an engineering approach to design the loading conditions induced to the interface system can result in interface strengths that exceed the previously considered limiting shear strength of the contacting soil. Fundamental experimental and numerical studies on specimens of different types of sand subjected to torsional and axial interface shear highlighted the inherent differences of these processes. Specifically, micro-scale soil deformation measurements showed that torsional shear induces larger soil deformations as compared to axial shear, as well as complex volume-change tendencies consisting of dilation and contraction in the primary and secondary shear zones. Studies on the global response of torsional and axial shear tests showed that they are affected differently by soil properties such as particle angularity and roughness. This difference in global behavior highlights the benefits of making systems that transfer load to the contacting soil in different manners available for use in geotechnical engineering. Discrete Element Modeling (DEM) simulations allowed for internal information of the specimens to be studied, such as their fabric and shear-induced loading conditions. These findings allowed for the development of links between the measured micro-scale behavior and the observed global-scale response. The understanding of the behavior of torsional and axial interfaces has allowed provides a framework for the development of enhanced geotechnical systems and applications. The global response of torsional shear found to induce larger cyclic contractive tendencies within the contacting soil mass. Therefore, this shear mode is more desirable than the conventional axial shear for the study of phenomena that depend on soil contractive behavior, such as liquefaction. A study on the influence of surface roughness form revealed that surfaces with periodic profiles of protruding elements that prevent clogging are capable of mobilizing interface friction angles that are 20 to 60% larger than the soil friction angle. These findings have direct implications in engineering design since their implementation can result in more resilient and sustainable geotechnical systems.

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2011. / ENGLISH ABSTRACT: The removal of grape berries from the stems is an important step in the wine making process. Various problems are experienced using the destemming machines currently available, where the berries are mechanically removed and separated from the stems by a rotating beater shaft and drum. Not all berries are removed from the stems and broken stems can end up with the removed berries which can result in unwanted characters and flavours in the wine. The development of these machines is currently limited to experimental tests. In this study, the destemming process was investigated experimentally. The ability of the Discrete Element Method (DEM) to simulate this process was also investigated. A range of experiments was designed to obtain the material properties of the grapes. These experiments included the measurement of the stem stiffness and break strength, the berry stiffness, and the force needed to remove a berry from the stem. Experiments were conducted to gain further insight into the destemming process. Firstly, a simplified destemming machine with only a beater shaft and a single grape bunch was built. The influence of the bunch size and the speed of the beater shaft on the number of berries removed from the stems were investigated. Secondly, field tests on a commercial destemming machine were conducted and the performance of the machine was measured. A DEM model of both the simplified and the commercial destemming machine were built. Commercial DEM software was used with linear contact and bond models. The stems were built from spherical particles bonded together and a single spherical particle was used to represent each berry. The measured stiffnesses and break strengths were used to set the particle and bond properties. Modelling the simplified destemming machine, it was found that the DEM model could accurately predict the effect of the bunch size and the speed of the beater shaft on the number of berries removed from the stems. The model of the commercial destemming machine could accurately predict the machine’s performance in terms of the number of berries removed as well as the number of broken stems. / AFRIKAANSE OPSOMMING: Die verwydering van druiwekorrels vanaf die stingels is ʼn belangrike stap tydens die wynmaak proses. Verskeie probleme word ondervind met huidige beskikbare ontstingelaars, waar die korrels meganies verwyder en skei word vanaf die stingels deur middel van ʼn roterende klop-as en drom. Nie alle korrels word vanaf die stingels verwyder nie en gebreekte stingels kan saam met die verwyderde korrels beland, wat ongewensde karakters en geure in die wyn kan veroorsaak. Die ontwikkeling van ontstingelaars is tans beperk tot eksperimentele toetse. In hierdie studie is die ontstingel proses eksperimenteel ondersoek Die vermoë van die Diskrete Element Metode (DEM) om hierdie proses te simuleer is ook ondersoek. ʼn Reeks eksperimente is ontwikkel om die materiaal eienskappe van die druiwe te bepaal. Hierdie eksperimente sluit in die meet van die styfheid en breeksterkte van die stingel, die korrel styfheid, en die krag benodig om ʼn korrel vanaf die stingel te verwyder. Eksperimente is gedoen om verdere insig oor die ontstingel proses te bekom. Eerstens is ʼn vereenvoudigde ontstingelaar gebou, met slegs ʼn klop-as en een tros. Die invloed van die trosgrootte en die klop-as spoed op die aantal korrels wat verwyder is, is ondersoek. Tweedens is ʼn toets in die veld gedoen met ʼn kommersiële ontstingelaar om die werkverrigting van die masjien te bepaal. ʼn DEM model van beide die vereenvoudigde en kommersiële ontstingelaar is gebou. Kommersiële DEM sagteware is gebruik met lineêre kontak- en bindingsmodelle. Die stingels is gebou deur sferiese partikels aan mekaar te bind en ʼn enkele sferiese partikel is gebruik om ʼn druiwe korrel voor te stel. Die gemete styfhede en breeksterktes is gebruik om die partikel- en bindingseienskappe te spesifiseer. Die modellering van die vereenvoudigde ontstingelaar het getoon dat die DEM model akkuraat kan voorspel wat die invloed is van die trosgrootte en die klop-as spoed op die aantal korrels wat verwyder is. Die model van die kommersiële ontstingelaar kon die werkverrigting van die masjien akkuraat voorspel in terme van die aantal korrels wat verwyder is asook die aantal gebreekte stingels.

A discrete element model (DEM) capable of reproducing the mechanistic behavior of a triaxial compressive test performed on a Vosges sandstone specimen is presented considering similar experimental testing conditions and densely packed spherical elements with low lock-in stress. The main aim of this paper is to illustrate the calibration process of the model‟s micro-parameters when obtained from the experimental meso-parameters measured in the lab. For this purpose, a probabilistic inverse method is introduced to fully define the micro-parameters of the particle models through a joint probability density function, which is conditioned on the experimental observations obtained during a series of tests performed at the L3S-R France. The DEM captures successfully some of the rock mechanical behavior features, including the global stress-strain and failure mechanisms. Results include a detailed parametric analysis consisting of varying each DEM parameter at the time and measuring the model response on the strain-stress domain. First order statistics on probabilistic results show the adequacy of the model to capture the experimental data, including a measure of the DEM performance for different parameter combinations. Also, joint probability density functions and cross-correlations among the micro-parameters are presented.

In 2006 resin accounted for approximately 17% of the direct manufacturing costs for oriented strand board (OSB). Because of their increased dependency on pMDI-resins, this percentage is likely greater for oriented strand lumber (OSL) and laminated strand lumber (LSL). The cost of PF- and pMDI-resins is expected to face upward pressure as the cost of their primary constituents, natural gas and crude oil, continue to reach new highs. Therefore, there is strong economic incentive to optimize the use of resin in the production of these three products. This can be accomplished by addressing two key issues: reducing resin wastage and optimizing resin distribution on the strands. Both issues will be overcome by focusing on the blending process, where resin is applied to the strands. This work focused on development and use of a discrete element model (DEM) for simulating strand flow in a rotary drum blender using the EDEM software package. EDEM required the input of three material and three interaction properties. Development of the model involved creating the simulated environment (i.e. physical dimensions) and assigning appropriate material and interaction properties given this environment and the assumptions that were made. This was accomplished in two steps, completing baseline bench-top experiments and a literature review to determine appropriate parameters and initial value ranges for these properties, and then fine-tuning these values based on a validation process. Using the validated model, an exploratory study was conducted to determine the effect of four blender design and operating parameters (flight height, number of flights, blender rotational speed, and blender fill level) on bulk strand flow. The results were analyzed with regards to overall trends and by focusing on two perspectives, end users and blender manufacturers. It was found that there was a strong relationship between these key parameters and bulk strand flow. These results suggest that operating parameters of a blender, namely rotational speed and tilt angle, should be linked directly to the blender feed rate to ensure an optimal blending environment is maintained. In addition, manufacturers of blenders must take into consideration the range in final operating conditions when designing and positioning flights.

Previous research has shown that the sample preparation method used to reconstitute specimens for granular materials can have a significant impact on its mechanistic behavior. As the Discrete Element Method becomes a more popular choice for modeling multiphysics problems involving granular materials, the sample heterogeneity should be correctly characterized in order to obtain accurate results. In order to capture the effect of sample preparation on the homogeneity of the sample, standard procedures were used to reconstitute samples composed of a homogeneous granular material. X-ray computed tomography and image analysis techniques were then used to characterize the spatial heterogeneity of a typical sample. The sample preparation method was modeled numerically using the Discrete Element program PFC3D. The resulting microstructure of the numerical sample was compared to the results of the image analysis to determine if the heterogeneity of the sample could be reproduced correctly for use in Discrete Element Modeling.

In 2006 resin accounted for approximately 17% of the direct manufacturing costs for oriented strand board (OSB). Because of their increased dependency on pMDI-resins, this percentage is likely greater for oriented strand lumber (OSL) and laminated strand lumber (LSL). The cost of PF- and pMDI-resins is expected to face upward pressure as the cost of their primary constituents, natural gas and crude oil, continue to reach new highs. Therefore, there is strong economic incentive to optimize the use of resin in the production of these three products. This can be accomplished by addressing two key issues: reducing resin wastage and optimizing resin distribution on the strands. Both issues will be overcome by focusing on the blending process, where resin is applied to the strands. This work focused on development and use of a discrete element model (DEM) for simulating strand flow in a rotary drum blender using the EDEM software package. EDEM required the input of three material and three interaction properties. Development of the model involved creating the simulated environment (i.e. physical dimensions) and assigning appropriate material and interaction properties given this environment and the assumptions that were made. This was accomplished in two steps, completing baseline bench-top experiments and a literature review to determine appropriate parameters and initial value ranges for these properties, and then fine-tuning these values based on a validation process. Using the validated model, an exploratory study was conducted to determine the effect of four blender design and operating parameters (flight height, number of flights, blender rotational speed, and blender fill level) on bulk strand flow. The results were analyzed with regards to overall trends and by focusing on two perspectives, end users and blender manufacturers. It was found that there was a strong relationship between these key parameters and bulk strand flow. These results suggest that operating parameters of a blender, namely rotational speed and tilt angle, should be linked directly to the blender feed rate to ensure an optimal blending environment is maintained. In addition, manufacturers of blenders must take into consideration the range in final operating conditions when designing and positioning flights.

In 2006 resin accounted for approximately 17% of the direct manufacturing costs for oriented strand board (OSB). Because of their increased dependency on pMDI-resins, this percentage is likely greater for oriented strand lumber (OSL) and laminated strand lumber (LSL). The cost of PF- and pMDI-resins is expected to face upward pressure as the cost of their primary constituents, natural gas and crude oil, continue to reach new highs. Therefore, there is strong economic incentive to optimize the use of resin in the production of these three products. This can be accomplished by addressing two key issues: reducing resin wastage and optimizing resin distribution on the strands. Both issues will be overcome by focusing on the blending process, where resin is applied to the strands. This work focused on development and use of a discrete element model (DEM) for simulating strand flow in a rotary drum blender using the EDEM software package. EDEM required the input of three material and three interaction properties. Development of the model involved creating the simulated environment (i.e. physical dimensions) and assigning appropriate material and interaction properties given this environment and the assumptions that were made. This was accomplished in two steps, completing baseline bench-top experiments and a literature review to determine appropriate parameters and initial value ranges for these properties, and then fine-tuning these values based on a validation process. Using the validated model, an exploratory study was conducted to determine the effect of four blender design and operating parameters (flight height, number of flights, blender rotational speed, and blender fill level) on bulk strand flow. The results were analyzed with regards to overall trends and by focusing on two perspectives, end users and blender manufacturers. It was found that there was a strong relationship between these key parameters and bulk strand flow. These results suggest that operating parameters of a blender, namely rotational speed and tilt angle, should be linked directly to the blender feed rate to ensure an optimal blending environment is maintained. In addition, manufacturers of blenders must take into consideration the range in final operating conditions when designing and positioning flights. / Forestry, Faculty of / Graduate

<div>Thermal barrier coatings (TBCs) made of low thermal conductivity ceramic topcoats have been extensively used in hot sections of gas turbine engines, in aircraft propulsion and power generation applications. TBC damage may occur during gas turbine operations, due to either time- and cycle-dependent degradation phenomena, external foreign object damage, and/or erosion. The damaged TBCs, therefore, need to be removed and repaired during engine maintenance cycles. Although several coating removal practices have been established which are based on the trial-and-error approach, a fundamental understanding of coating fracture mechanisms during the removal process is still limited, which hinders further development of the process.</div><div>The objective of the thesis is to develop a particle-based coating removal modeling framework, using both the smoothed particle hydrodynamics (SPH) and discrete element modeling (DEM) methods. The thesis systematically investigates the processing-property relationships in the TBC removal processes using a modeling approach, thus providing a scientific tool for process design and optimization.</div><div>To achieve the above-mentioned objective, the following research tasks are identified. First a comprehensive literature review of major coating removal techniques is presented in Chapter 2. Chapter 3 discusses an improved SPH model to simulate the high-velocity particle impact behaviors on TBCs. In Chapter 4, the abrasive water jet (AWJ) removal process is modeled using the SPH method. In Chapter 5, an SPH model of the cutting process with regular electron beam physical vapor deposition (EB-PVD) columnar grains is presented. In Chapter 6, a 3D DEM cutting model with regular EB-PVD column grains is discussed. In Chapter 7, a 2D DEM cutting model based on the realistic coating microstructure is developed. Finally, in Chapter 8, based on the particle-based coating removal modeling framework results and analytical solutions, a new fracture mechanism map is proposed, which correlates the processing parameters and coating fracture modes.</div><div>The particle-based modeling results show that: (1) for the SPH impact model, the impact hole penetration depth is mainly controlled by the vertical velocity component. (2) The SPH AWJ simulation results demonstrate that the ceramic removal rate increases with incident angle, which is consistent with the fracture mechanics-based analytic solution. (3) The SPH model with regular EB-PVD columnar grains shows that it is capable to examine the stress evolutions in the coating with columnar grain structures, which is not available if a uniform bulk coating model was used. Additional analysis reveals that the fracture of the columnar grains during the cutting process is achieved through deflection and fracture of the grains, followed by pushing against neighboring grains. (4) The 3D DEM model with regular coating columnar grains shows that, during the coating removal process, a ductile-to-brittle transition is identified which depends on the cutting depth. The transition occurs at the critical cutting depth, which is based on the Griffith fracture criterion. At small cutting depths, the ductile failure mode dominates the cutting process, leading to fine cut particles. As the cutting depth exceeds the critical cutting depth, a brittle failure mode is observed with the formation of chunk-like chips. (5) The 2D DEM model with the realistic coating microstructure shows that there are densification and fracture during the foreign object compaction process, which qualitatively agrees with the experimental observations. (6) The newly proposed coating fracture mechanism map provides guidance to predict three fracture modes, i.e., ductile brittle, and mixed ductile-brittle, as a function of processing parameters, including the cutting depth and cutting speed. The map can be used to determine the processing conditions based on required TBC removal operations: rough cut (brittle mode), semi-finish (mixed ductile-brittle mode), and finish (ductile mode).</div><div><br></div>

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