1,167 research outputs found

    A micromechanical study of the Standard Penetration Test

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    This thesis explores the potential of models based on the discrete element method (DEM) to study dynamic probing of granular materials, considering realistic particle-scale properties. The virtual calibration chamber technique, based on the discrete element method, is applied to study the standard penetration test (SPT). A macro-element approach is used to represent a rod driven with an impact like those applied to perform SPT. The rod is driven into a chamber filled with a scaled discrete analogue of a quartz sand. The contact properties of the discrete analogue are calibrated simulating two low-pressure triaxial tests. The rod is driven changing input energy and controlling initial density and confinement stress. Energy-based blowcount normalization is shown to be effective. Results obtained are in good quantitative agreement with well-accepted experimentally-based relations between blowcount, density and overburden. A comprehensive energetic balance of the virtual calibration chamber is conducted. Energy balance is applied separately to the driven rod and the chamber system, giving a detailed account of all the different energy terms. The characterization of the evolution and distribution of each energy component is investigated. It appears that the SPT test input energy is mainly dissipated in friction. The energy-based interpretation of SPT dynamic response proposed by Schnaid et al. (2017) is then validated in comparisons between static and dynamic penetration results. Moreover, microscale investigation provides important information on energy dissipation mechanisms. A well-established DEM crushing contact model and a rough Hertzian contact model are combined to incorporate both effects in a single contact model. The efficient user defined contact model (UDCM) technique is used for the contact model implementation. Parametric studies explore the effect of particle roughness on single particle crushing event. The model is then used to recalibrate the contact properties of the quartz sand, being able to use realistic contact properties and then correctly capture both load-unload behaviour and particle size distribution evolution. The calibration chamber results are exploited to investigate the relation between static and dynamic penetration test. This is done first for unbreakable materials and later for crushable and rough-crushable ones. It is shown that the tip resistance measured under impact dynamic penetration conditions is very close to that under constant velocity conditions, hence supporting recent proposals to relate CPT and SPT results. It is also shown that penetration resistance reduces if particles are allowed to break, particularly when roughness is also considered.Esta tesis explora el potencial de los modelos basados en el m√©todo de elementos discretos (DEM) para estudiar el sondeo din√°mico de materiales granulares, considerando propiedades realistas a escala de part√≠culas. La t√©cnica de c√°mara de calibraci√≥n virtual, basada en el m√©todo de elemento discreto, se aplica para estudiar la prueba de penetraci√≥n est√°ndar (SPT). Se utiliza un enfoque de macroelemento para representar una barra impulsada con un impacto como los aplicados para realizar SPT. La varilla se introduce en una c√°mara llena de un an√°logo discreto escalado de arena de cuarzo. Las propiedades de contacto del an√°logo discreto se calibran simulando dos pruebas triaxiales de baja presi√≥n. La varilla se acciona cambiando la energ√≠a de entrada y controlando la densidad inicial y el estr√©s de confinamiento. La normalizaci√≥n del recuento de golpes basado en energ√≠a se muestra efectiva. Los resultados obtenidos est√°n en buen acuerdo cuantitativo con relaciones basadas en experimentos bien aceptadas entre recuento de golpes, densidad y sobrecarga. Se realiza un balance energ√©tico integral de la c√°mara de calibraci√≥n virtual. El balance de energ√≠a se aplica por separado a la varilla impulsada y al sistema de c√°mara, dando una descripci√≥n detallada de todos los diferentes t√©rminos de energ√≠a. Se investiga la caracterizaci√≥n de la evoluci√≥n y distribuci√≥n de cada componente energ√©tico. Parece que la energ√≠a de entrada de prueba SPT se disipa principalmente en fricci√≥n. La interpretaci√≥n basada en la energ√≠a de la respuesta din√°mica SPT propuesta por Schnaid et al. (2017) luego se valida en comparaciones entre los resultados de penetraci√≥n est√°tica y din√°mica. Adem√°s, la investigaci√≥n en microescala proporciona informaci√≥n importante sobre los mecanismos de disipaci√≥n de energ√≠a. Un modelo de contacto de trituraci√≥n DEM bien establecido y un modelo de contacto hertziano aproximado se combinan para incorporar ambos efectos en un modelo de contacto √ļnico. La t√©cnica eficiente de modelo de contacto definido por el usuario (UDCM) se utiliza para la implementaci√≥n del modelo de contacto. Los estudios param√©tricos exploran el efecto de la rugosidad de las part√≠culas en el evento de trituraci√≥n de part√≠culas individuales. El modelo se usa para recalibrar las propiedades de contacto de la arena de cuarzo, pudiendo usar propiedades de contacto realistas y luego capturar correctamente el comportamiento de carga y descarga y la evoluci√≥n de la distribuci√≥n del tama√Īo de part√≠cula. Los resultados de la c√°mara de calibraci√≥n se explotan para investigar la relaci√≥n entre la prueba de penetraci√≥n est√°tica y din√°mica. Esto se hace primero para materiales irrompibles y luego para materiales triturables y desmenuzables. Se muestra que la resistencia de la punta medida en condiciones de penetraci√≥n din√°mica de impacto es muy cercana a la de condiciones de velocidad constante, por lo tanto, respalda propuestas recientes para relacionar los resultados de CPT y SPT. Tambi√©n se muestra que la resistencia a la penetraci√≥n se reduce si se permite que las part√≠culas se rompan, particularmente cuando tambi√©n se considera la aspereza.Postprint (published version

    A micromechanical study of the Standard Penetration Test

    Get PDF
    This thesis explores the potential of models based on the discrete element method (DEM) to study dynamic probing of granular materials, considering realistic particle-scale properties. The virtual calibration chamber technique, based on the discrete element method, is applied to study the standard penetration test (SPT). A macro-element approach is used to represent a rod driven with an impact like those applied to perform SPT. The rod is driven into a chamber filled with a scaled discrete analogue of a quartz sand. The contact properties of the discrete analogue are calibrated simulating two low-pressure triaxial tests. The rod is driven changing input energy and controlling initial density and confinement stress. Energy-based blowcount normalization is shown to be effective. Results obtained are in good quantitative agreement with well-accepted experimentally-based relations between blowcount, density and overburden. A comprehensive energetic balance of the virtual calibration chamber is conducted. Energy balance is applied separately to the driven rod and the chamber system, giving a detailed account of all the different energy terms. The characterization of the evolution and distribution of each energy component is investigated. It appears that the SPT test input energy is mainly dissipated in friction. The energy-based interpretation of SPT dynamic response proposed by Schnaid et al. (2017) is then validated in comparisons between static and dynamic penetration results. Moreover, microscale investigation provides important information on energy dissipation mechanisms. A well-established DEM crushing contact model and a rough Hertzian contact model are combined to incorporate both effects in a single contact model. The efficient user defined contact model (UDCM) technique is used for the contact model implementation. Parametric studies explore the effect of particle roughness on single particle crushing event. The model is then used to recalibrate the contact properties of the quartz sand, being able to use realistic contact properties and then correctly capture both load-unload behaviour and particle size distribution evolution. The calibration chamber results are exploited to investigate the relation between static and dynamic penetration test. This is done first for unbreakable materials and later for crushable and rough-crushable ones. It is shown that the tip resistance measured under impact dynamic penetration conditions is very close to that under constant velocity conditions, hence supporting recent proposals to relate CPT and SPT results. It is also shown that penetration resistance reduces if particles are allowed to break, particularly when roughness is also considered.Esta tesis explora el potencial de los modelos basados en el m√©todo de elementos discretos (DEM) para estudiar el sondeo din√°mico de materiales granulares, considerando propiedades realistas a escala de part√≠culas. La t√©cnica de c√°mara de calibraci√≥n virtual, basada en el m√©todo de elemento discreto, se aplica para estudiar la prueba de penetraci√≥n est√°ndar (SPT). Se utiliza un enfoque de macroelemento para representar una barra impulsada con un impacto como los aplicados para realizar SPT. La varilla se introduce en una c√°mara llena de un an√°logo discreto escalado de arena de cuarzo. Las propiedades de contacto del an√°logo discreto se calibran simulando dos pruebas triaxiales de baja presi√≥n. La varilla se acciona cambiando la energ√≠a de entrada y controlando la densidad inicial y el estr√©s de confinamiento. La normalizaci√≥n del recuento de golpes basado en energ√≠a se muestra efectiva. Los resultados obtenidos est√°n en buen acuerdo cuantitativo con relaciones basadas en experimentos bien aceptadas entre recuento de golpes, densidad y sobrecarga. Se realiza un balance energ√©tico integral de la c√°mara de calibraci√≥n virtual. El balance de energ√≠a se aplica por separado a la varilla impulsada y al sistema de c√°mara, dando una descripci√≥n detallada de todos los diferentes t√©rminos de energ√≠a. Se investiga la caracterizaci√≥n de la evoluci√≥n y distribuci√≥n de cada componente energ√©tico. Parece que la energ√≠a de entrada de prueba SPT se disipa principalmente en fricci√≥n. La interpretaci√≥n basada en la energ√≠a de la respuesta din√°mica SPT propuesta por Schnaid et al. (2017) luego se valida en comparaciones entre los resultados de penetraci√≥n est√°tica y din√°mica. Adem√°s, la investigaci√≥n en microescala proporciona informaci√≥n importante sobre los mecanismos de disipaci√≥n de energ√≠a. Un modelo de contacto de trituraci√≥n DEM bien establecido y un modelo de contacto hertziano aproximado se combinan para incorporar ambos efectos en un modelo de contacto √ļnico. La t√©cnica eficiente de modelo de contacto definido por el usuario (UDCM) se utiliza para la implementaci√≥n del modelo de contacto. Los estudios param√©tricos exploran el efecto de la rugosidad de las part√≠culas en el evento de trituraci√≥n de part√≠culas individuales. El modelo se usa para recalibrar las propiedades de contacto de la arena de cuarzo, pudiendo usar propiedades de contacto realistas y luego capturar correctamente el comportamiento de carga y descarga y la evoluci√≥n de la distribuci√≥n del tama√Īo de part√≠cula. Los resultados de la c√°mara de calibraci√≥n se explotan para investigar la relaci√≥n entre la prueba de penetraci√≥n est√°tica y din√°mica. Esto se hace primero para materiales irrompibles y luego para materiales triturables y desmenuzables. Se muestra que la resistencia de la punta medida en condiciones de penetraci√≥n din√°mica de impacto es muy cercana a la de condiciones de velocidad constante, por lo tanto, respalda propuestas recientes para relacionar los resultados de CPT y SPT. Tambi√©n se muestra que la resistencia a la penetraci√≥n se reduce si se permite que las part√≠culas se rompan, particularmente cuando tambi√©n se considera la aspereza

    Topological winding properties of spin edge states in Kane-Mele graphene model

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    We study the spin edge states in the quantum spin-Hall (QSH) effect on a single-atomic layer graphene ribbon system with both intrinsic and Rashba spin-orbit couplings. The Harper equation for solving the energies of the spin edge states is derived. The results show that in the QSH phase, there are always two pairs of gapless spin-filtered edge states in the bulk energy gap, corresponding to two pairs of zero points of the Bloch function on the complex-energy Riemann surface (RS). The topological aspect of the QSH phase can be distinguished by the difference of the winding numbers of the spin edge states with different polarized directions cross the holes of the RS, which is equivalent to the Z2 topological invariance proposed by Kane and Mele [Phys. Rev. Lett. 95, 146802 (2005)].Comment: 9 pages, 10 figure

    Topological crystalline antiferromagnetic state in tetragonal FeS

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    Integration between magnetism and topology is an exotic phenomenon in condensed-matter physics. Here, we propose an exotic phase named topological crystalline antiferromagnetic state, in which antiferromagnetism intrinsically integrates with nontrivial topology, and we suggest such a state can be realized in tetragonal FeS. A combination of first-principles calculations and symmetry analyses shows that the topological crystalline antiferromagnetic state arises from band reconstruction induced by pair checker-board antiferromagnetic order together with band-gap opening induced by intrinsic spin-orbit coupling in tetragonal FeS. The topological crystalline antiferromagnetic state is protected by the product of fractional translation symmetry, mirror symmetry, and time-reversal symmetry, and present some unique features. In contrast to strong topological insulators, the topological robustness is surface-dependent. These findings indicate that non-trivial topological states could emerge in pure antiferromagnetic materials, which sheds new light on potential applications of topological properties in fast-developing antiferromagnetic spintronics.Comment: 8 pages, 6 figure

    Chiral topological excitonic insulator in semiconductor quantum wells

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    We present a scheme to realize the chiral topological excitonic insulator in semiconductor heterostructures which can be experimentally fabricated with a coupled quantum well adjacent to twoferromagnetic insulating films. The different mean-field chiral topological orders, which are due to the change in the directions of the magnetization of the ferromagnetic films, can be characterized by the TKNN numbers in the bulk system as well as by the winding numbers of the gapless states in the edged system. Furthermore, we propose an experimental scheme to detect the emergence of the chiral gapless edge state and distinguish different chiral topological orders by measuring the thermal conductance.Comment: 14 pages, 4 figure

    Topological edge states and quantum Hall effect in the Haldane model

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    We study the topological edge states of the Haldane's graphene model with zigzag/armchair lattice edges. The Harper equation for solving the energies of the edge states is derived. The results show that there are two edge states in the bulk energy gap, corresponding to the two zero points of the Bloch function on the complex-energy Riemann surface. The edge-state energy loops move around the hole of the Riemann surface in appropriate system parameter regimes. The quantized Hall conductance can be expressed by the winding numbers of the edge states, which reflects the topological feature of the Haldane model.Comment: 5 pages, 6 figure

    Quantiled conditional variance, skewness, and kurtosis by Cornish-Fisher expansion

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    The conditional variance, skewness, and kurtosis play a central role in time series analysis. These three conditional moments (CMs) are often studied by some parametric models but with two big issues: the risk of model mis-specification and the instability of model estimation. To avoid the above two issues, this paper proposes a novel method to estimate these three CMs by the so-called quantiled CMs (QCMs). The QCM method first adopts the idea of Cornish-Fisher expansion to construct a linear regression model, based on nn different estimated conditional quantiles. Next, it computes the QCMs simply and simultaneously by using the ordinary least squares estimator of this regression model, without any prior estimation of the conditional mean. Under certain conditions that allow estimated conditional quantiles to be biased, the QCMs are shown to be consistent with the convergence rate n‚ąí1/2n^{-1/2}. Simulation studies indicate that the QCMs perform well under different scenarios of estimated conditional quantiles. In the application, the study of QCMs for eight major stock indexes demonstrates the effectiveness of financial rescue plans during the COVID-19 pandemic outbreak, and unveils a new ``non-zero kink'' phenomenon in the ``news impact curve'' function for the conditional kurtosis
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