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George Z. Voyiadjis - One of the best experts on this subject based on the ideXlab platform.
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Studying the effect of a hydrostatic Stress/strain Reduction Factor on damage mechanics of concrete materials
Journal of the Mechanical Behavior of Materials, 2013Co-Authors: Ziad N. Taqieddin, George Z. VoyiadjisAbstract:In the Nonlinear Finite Element Analysis (NFEA) of concrete materials, Continuum Damage Mechanics (CDM) provide a powerful framework for the derivation of constitutive models capable of describing the mechanical behavior of such materials. The internal state variables of CDM can be introduced to the elastic analysis of concrete to form elastic-damage models (no inelastic strains), or to the elastic-plastic analysis in order to form coupled/uncoupled elastic-plastic-damage models. Experimental evidence that is well documented in literature shows that concrete’s susceptibility to damage and failure is distinguished under deviatoric loading from that corresponding to hydrostatic loading. A Reduction Factor is usually introduced into a CDM model to reduce the susceptibility of concrete to hydrostatic Stresses/strains. In this work, the effect of a hydrostatic Stress Reduction Factor on the performances of two NFEA concrete models will be studied. These (independently published) models did not provide any results showing such effect. One of these two models is an elastic-damage model while the other is an uncoupled elastic-plastic-damage model. Comparisons are carried out between the performances of the two models under tensile and compressive loadings, clearly showing the effect of the Reduction Factor on the numerically depicted behaviors of concrete materials. In order to have rational comparisons, the hydrostatic Stress Reduction Factor applied to each model is chosen to be a function of the internal state variables common to both models. Therefore, once the two models are calibrated to simulate the experimental behaviors, their corresponding Reduction Factors are readily available at every increment of the iterative NFEA procedures.
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studying the effect of a hydrostatic Stress strain Reduction Factor on damage mechanics of concrete materials
Journal of the mechanical behavior of materials, 2013Co-Authors: Ziad N. Taqieddin, George Z. VoyiadjisAbstract:In the Nonlinear Finite Element Analysis (NFEA) of concrete materials, Continuum Damage Mechanics (CDM) provide a powerful framework for the derivation of constitutive models capable of describing the mechanical behavior of such materials. The internal state variables of CDM can be introduced to the elastic analysis of concrete to form elastic-damage models (no inelastic strains), or to the elastic-plastic analysis in order to form coupled/uncoupled elastic-plastic-damage models. Experimental evidence that is well documented in literature shows that concrete’s susceptibility to damage and failure is distinguished under deviatoric loading from that corresponding to hydrostatic loading. A Reduction Factor is usually introduced into a CDM model to reduce the susceptibility of concrete to hydrostatic Stresses/strains. In this work, the effect of a hydrostatic Stress Reduction Factor on the performances of two NFEA concrete models will be studied. These (independently published) models did not provide any results showing such effect. One of these two models is an elastic-damage model while the other is an uncoupled elastic-plastic-damage model. Comparisons are carried out between the performances of the two models under tensile and compressive loadings, clearly showing the effect of the Reduction Factor on the numerically depicted behaviors of concrete materials. In order to have rational comparisons, the hydrostatic Stress Reduction Factor applied to each model is chosen to be a function of the internal state variables common to both models. Therefore, once the two models are calibrated to simulate the experimental behaviors, their corresponding Reduction Factors are readily available at every increment of the iterative NFEA procedures.
Piltan Tabatabaie Shourijeh - One of the best experts on this subject based on the ideXlab platform.
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a comparison of micromechanical assessments with internal stability instability criteria for soils
Powder Technology, 2015Co-Authors: Mojtaba Farahnak Langroudi, Abbas Soroush, Piltan Tabatabaie ShourijehAbstract:Suffusion is the erosion of small particles through the skeleton of coarse grains. Soils susceptible to suffusion are described as internally unstable. In this study three dimensional discrete element modeling is employed to investigate internal instability in soils. The simulation is achieved by assessing contacts distributions, forces analysis and transmitted Stresses between particles. Three types of gradations have been selected for the analyses: linear, concave upward and gap-graded. Observations of mechanical coordination number and contact distributions during isotropic compression show that the number of fine particles with low connectivity is comparatively higher for gap-graded and concave upward gradations. The evolution of contact force networks confirms that internally stable soils have a relatively homogeneous network of contact forces compared to internally unstable soil. Force distribution analyses reflect higher percent of weak contacts and low connectivity for fine particles in internal instability. In addition four commonly used internal instability assessment criteria were contrasted with micromechanical parameters, and findings revealed reasonable compliance between stability indices and micromechanical measures. Finally the Stress Reduction Factor of the soils is calculated, confirming previous experimental and numerical studies that α is higher for internally stable soils.
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A comparison of micromechanical assessments with internal stability/instability criteria for soils
Powder Technology, 2015Co-Authors: Mojtaba Farahnak Langroudi, Abbas Soroush, Piltan Tabatabaie ShourijehAbstract:Suffusion is the erosion of small particles through the skeleton of coarse grains. Soils susceptible to suffusion are described as internally unstable. In this study three dimensional discrete element modeling is employed to investigate internal instability in soils. The simulation is achieved by assessing contacts distributions, forces analysis and transmitted Stresses between particles. Three types of gradations have been selected for the analyses: linear, concave upward and gap-graded. Observations of mechanical coordination number and contact distributions during isotropic compression show that the number of fine particles with low connectivity is comparatively higher for gap-graded and concave upward gradations. The evolution of contact force networks confirms that internally stable soils have a relatively homogeneous network of contact forces compared to internally unstable soil. Force distribution analyses reflect higher percent of weak contacts and low connectivity for fine particles in internal instability. In addition four commonly used internal instability assessment criteria were contrasted with micromechanical parameters, and findings revealed reasonable compliance between stability indices and micromechanical measures. Finally the Stress Reduction Factor of the soils is calculated, confirming previous experimental and numerical studies that α is higher for internally stable soils.
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Stress transmission in internally unstable gap-graded soils using discrete element modeling
Powder Technology, 2013Co-Authors: Mojtaba Farahnak Langroudi, Abbas Soroush, Piltan Tabatabaie Shourijeh, Roozbeh ShafipourAbstract:Abstract Three dimensional discrete element modeling is employed to investigate micromechanical behavior of internally unstable/stable gap-graded soils under isotropic compression. Two gap-graded soils, one internally stable and the other unstable, are modeled by assemblies of spherical particles and the micromechanical parameters in Stress transmission are inspected. The variation of coordination number and contacts per particle during isotropic compression suggests higher coordination number for internally stable soil. Mechanical coordination number reveals that floating particles with low number of contacts are more frequent in the internally unstable soil. The evolution of contact force networks during compression shows that internal instability corresponds to a more heterogeneous contact force network. The results confirm the hypothesis that loose particles nest within pores of the primary fabric (coarser fraction) which is preponderant in transferring Stresses. Finally, the variation of Stress Reduction Factor (α) with confining pressure and void ratio, confirms previous experimental and numerical studies that α is higher for internally stable soil.
Daining Fang - One of the best experts on this subject based on the ideXlab platform.
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Theoretical Research on Thermal Shock Resistance of Ultra-High Temperature Ceramics Focusing on the Adjustment of Stress Reduction Factor
Materials, 2013Co-Authors: Yushan Shi, Daining FangAbstract:The thermal shock resistance of ceramics depends on not only the mechanical and thermal properties of materials, but also the external constraint and thermal condition. So, in order to study the actual situation in its service process, a temperature-dependent thermal shock resistance model for ultra-high temperature ceramics considering the effects of the thermal environment and external constraint was established based on the existing theory. The present work mainly focused on the adjustment of the Stress Reduction Factor according to different thermal shock situations. The influences of external constraint on both critical rupture temperature difference and the second thermal shock resistance parameter in either case of rapid heating or cooling conditions had been studied based on this model. The results show the necessity of adjustment of the Stress Reduction Factor in different thermal shock situations and the limitations of the applicable range of the second thermal shock resistance parameter. Furthermore, the model was validated by the finite element method.
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Properties and appropriate conditions of Stress Reduction Factor and thermal shock resistance parameters for ceramics
Applied Mathematics and Mechanics-english Edition, 2012Co-Authors: Weiguo Li, Tianbao Cheng, Rubing Zhang, Daining FangAbstract:Through introducing the analytical solution of the transient heat conduction problem of the plate with convection into the thermal Stress field model of the elastic plate, the Stress Reduction Factor is presented explicitly in its dimensionless form. A new Stress Reduction Factor is introduced for the purpose of comparison. The properties and appropriate conditions of the Stress Reduction Factor, the first and second thermal shock resistance (TSR) parameters for the high and low Biot numbers, respectively, and the approximation formulas for the intermediate Biot number-interval are discussed. To investigate the TSR of ceramics more accurately, it is recommended to combine the heat transfer theory with the theory of thermoelasticity or fracture mechanics or use a numerical method. The critical rupture temperature difference and the critical rupture dimensionless time can be used to characterize the TSR of ceramics intuitively and legibly.
Han Kyu Yoo - One of the best experts on this subject based on the ideXlab platform.
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Weightage Effect during Back-Calculation of Rock-Mass Quality from the Installed Tunnel Support in Rock-Mass Rating and Tunneling Quality Index System
MDPI AG, 2019Co-Authors: Jonguk Kim, Hafeezur Rehman, Wahid Ali, Abdul Muntaqim Naji, Han Kyu YooAbstract:In extensively used empirical rock-mass classification systems, the rock-mass rating (RMR) and tunneling quality index (Q) system, rock-mass quality, and tunnel span are used for the selection of rock bolt length and spacing and shotcrete thickness. In both systems, the rock bolt spacing and shotcrete thickness selection are based on the same principle, which is used for the back-calculation of the rock-mass quality. For back-calculation, there is no criterion for the selection of rock-bolt-spacing-based rock-mass quality weightage and shotcrete thickness along with tunnel-span-based rock-mass quality weightage. To determine this weightage effect during the back-calculation, five weightage cases are selected, explained through example, and applied using published data. In the RMR system, the weightage effect is expressed in terms of the difference between the calculated and back-calculated rock-mass quality in the two versions of RMR. In the Q system, the weightage effect is presented in plots of Stress Reduction Factor versus relative block size. The results show that the weightage effect during back-calculation not only depends on the difference in rock-bolt-spacing-based rock-mass quality and shotcrete along with tunnel-span-based rock-mass quality, but also on their corresponding values
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Extension of tunneling quality index and rock mass rating systems for tunnel support design through back calculations in highly Stressed jointed rock mass: An empirical approach based on tunneling data from Himalaya
Tunnelling and Underground Space Technology, 2019Co-Authors: Hafeezur Rehman, Jung Joo Kim, Abdul Muntaqim Naji, Han Kyu YooAbstract:Abstract A preliminary support design is the basic output of an empirical rock mass classification system. Tunneling quality index (Q) and rock mass rating (RMR) systems are used specifically for tunnel support in moderately jointed and massive rocks; however, they do not provide any information/guidance for jointed rock masses in a highly Stressed environment. Hence, this study focuses on the already supported drill and blast tunnel sections mapping data of four tunneling projects in Pakistan. The study extends the application of these systems to the tunnel support design for highly Stressed jointed rock mass through an empirical approach. In each empirical approach, the parameters for the Stress condition are suggested. Rock mass quality (Q or Qc) is determined from back calculations using an installed support and tunnel span. Empirical equations and charts are proposed for the Stress Reduction Factor (SRF) characterization. In the proposed equations and charts, the SRF is a function of intact rock strength, relative block size, and strength–Stress ratio. In the case of the RMR system, approximately 90% of the sections show that the actual supports are heavier than the suggested supports by RMR89. The RMR was determined from the installed support for each tunnel section through the back calculations. To select the adjustment rating Factor for Stress, three hypotheses are considered based on the intact rock strength to the major principal Stress ratio. The RMR14 suggested support revealed that all sections are heavily supported. A strong correlation exists between RMR89 and RMR14. Three hypotheses are considered for the Stress adjustment Factor rating selection for RMR14 based on the correlation equation between RMR89 and RMR14. For the evaluation, the application of modified Q and RMR systems are used in the tunnel support in a case study. The exploration reports show that the tunnel will pass through a jointed rock mass under high in-situ Stress environments. The comparison shows that heavy support is recommended from the modified systems for tunnel stability.
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An Empirical Approach for Tunnel Support Design through Q and RMi Systems in Fractured Rock Mass
Applied Sciences, 2018Co-Authors: Jae-kook Lee, Hafeezur Rehman, Jung Joo Kim, Abdul Muntaqim Naji, Han Kyu YooAbstract:Empirical systems for the classification of rock mass are used primarily for preliminary support design in tunneling. When applying the existing acceptable international systems for tunnel preliminary supports in high-Stress environments, the tunneling quality index (Q) and the rock mass index (RMi) systems that are preferred over geomechanical classification due to the Stress characterization parameters that are incorporated into the two systems. However, these two systems are not appropriate when applied in a location where the rock is jointed and experiencing high Stresses. This paper empirically extends the application of the two systems to tunnel support design in excavations in such locations. Here, the rock mass characterizations and installed support data of six tunnel projects are used. The back-calculation approach is used to determine the Q value using the Q-system support chart, and these values are then used to develop the equations and charts to characterize the Stress Reduction Factor (SRF), which is also numerically evaluated. These equations and charts reveal that the SRF is a function of relative block size, strength–Stress ratio, and intact rock compressive strength. Furthermore, the RMi-suggested supports were heavier than the actual installed ones. If the approximate inverse relation between Stress level (SL) and SRF is used, the difference between the actual and the recommended supports increases when using the RMi-recommended rock support chart for blocky ground. An alternate system is made for support recommendation using a Q-system support chart. In this system, the ground condition Factor is modified from the available parameters, and a correlation is developed with a modified Q system.
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Stress Reduction Factor characterization for highly Stressed jointed rock based on tunneling data from pakistan
2017Co-Authors: Hafeezur Rehman, Jung Joo Kim, Han Kyu YooAbstract:This research was supported by Development of Design and Construction Technology for Double Deck Tunnel in Great Depth Underground Space(14SCIP-B088624-01) from Construction Technology Research Program funded by Ministry of Land, Infrastructure and Transport of Korean government.
Ziad N. Taqieddin - One of the best experts on this subject based on the ideXlab platform.
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Studying the effect of a hydrostatic Stress/strain Reduction Factor on damage mechanics of concrete materials
Journal of the Mechanical Behavior of Materials, 2013Co-Authors: Ziad N. Taqieddin, George Z. VoyiadjisAbstract:In the Nonlinear Finite Element Analysis (NFEA) of concrete materials, Continuum Damage Mechanics (CDM) provide a powerful framework for the derivation of constitutive models capable of describing the mechanical behavior of such materials. The internal state variables of CDM can be introduced to the elastic analysis of concrete to form elastic-damage models (no inelastic strains), or to the elastic-plastic analysis in order to form coupled/uncoupled elastic-plastic-damage models. Experimental evidence that is well documented in literature shows that concrete’s susceptibility to damage and failure is distinguished under deviatoric loading from that corresponding to hydrostatic loading. A Reduction Factor is usually introduced into a CDM model to reduce the susceptibility of concrete to hydrostatic Stresses/strains. In this work, the effect of a hydrostatic Stress Reduction Factor on the performances of two NFEA concrete models will be studied. These (independently published) models did not provide any results showing such effect. One of these two models is an elastic-damage model while the other is an uncoupled elastic-plastic-damage model. Comparisons are carried out between the performances of the two models under tensile and compressive loadings, clearly showing the effect of the Reduction Factor on the numerically depicted behaviors of concrete materials. In order to have rational comparisons, the hydrostatic Stress Reduction Factor applied to each model is chosen to be a function of the internal state variables common to both models. Therefore, once the two models are calibrated to simulate the experimental behaviors, their corresponding Reduction Factors are readily available at every increment of the iterative NFEA procedures.
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studying the effect of a hydrostatic Stress strain Reduction Factor on damage mechanics of concrete materials
Journal of the mechanical behavior of materials, 2013Co-Authors: Ziad N. Taqieddin, George Z. VoyiadjisAbstract:In the Nonlinear Finite Element Analysis (NFEA) of concrete materials, Continuum Damage Mechanics (CDM) provide a powerful framework for the derivation of constitutive models capable of describing the mechanical behavior of such materials. The internal state variables of CDM can be introduced to the elastic analysis of concrete to form elastic-damage models (no inelastic strains), or to the elastic-plastic analysis in order to form coupled/uncoupled elastic-plastic-damage models. Experimental evidence that is well documented in literature shows that concrete’s susceptibility to damage and failure is distinguished under deviatoric loading from that corresponding to hydrostatic loading. A Reduction Factor is usually introduced into a CDM model to reduce the susceptibility of concrete to hydrostatic Stresses/strains. In this work, the effect of a hydrostatic Stress Reduction Factor on the performances of two NFEA concrete models will be studied. These (independently published) models did not provide any results showing such effect. One of these two models is an elastic-damage model while the other is an uncoupled elastic-plastic-damage model. Comparisons are carried out between the performances of the two models under tensile and compressive loadings, clearly showing the effect of the Reduction Factor on the numerically depicted behaviors of concrete materials. In order to have rational comparisons, the hydrostatic Stress Reduction Factor applied to each model is chosen to be a function of the internal state variables common to both models. Therefore, once the two models are calibrated to simulate the experimental behaviors, their corresponding Reduction Factors are readily available at every increment of the iterative NFEA procedures.