The Experts below are selected from a list of 81444 Experts worldwide ranked by ideXlab platform
Somnath Ghosh - One of the best experts on this subject based on the ideXlab platform.
-
microstructural parameters affecting creep Induced Load shedding in ti 6242 by a size dependent crystal plasticity fe model
International Journal of Plasticity, 2008Co-Authors: Gayathri Venkataramani, Kedar Kirane, Somnath GhoshAbstract:This paper is aimed at identifying critical microstructural parameters that cause local stress concentration due to Load shedding between microstructural regions of varying strengths. This stress is viewed as one of the fundamental reasons for crack initiation in Ti-6242. A rate dependent, anisotropic, elasto-crystal plasticity based finite element model (CPFEM) for poly-phase Ti-6242 is used in this study to identify the critical variables responsible for localized stress concentration due to Load shedding. The model can account for various microstructural features like grain size, orientation and misorientation distributions. Various microstructural variables, such as crystal orientation, misorientation, grain size, Schmid factor and composition of phases, are considered in a detailed parametric study. Critical combinations of these parameters that result in high stress due to Load shedding are identified. Finally, Load shedding in a microstructure model of polycrystalline Ti-6242 is discussed from the results of CPFEM simulations. The model is statistically equivalent with respect to features observed in OIM scans.
-
a size dependent crystal plasticity finite element model for creep and Load shedding in polycrystalline titanium alloys
Acta Materialia, 2007Co-Authors: G Venkatramani, Somnath Ghosh, Michael J. MillsAbstract:A rate-dependent anisotropic elastic-crystal plasticity based finite-element (FE) model with size-dependent yield strength is developed for polycrystalline Ti-6242. The initial slip system deformation resistances in the crystal plasticity relations are expressed as Hall–Petch type relations, where the grain size, lath size and colony size are chosen as characteristic lengths depending on the nature of slip. The FE model incorporates accurate phase volume fractions and orientation distributions that are statistically equivalent to those observed in orientation imaging microscopy (OIM) maps of the microstructure. The model is validated with experimental results on constant strain rate and creep tests. A relationship between the macroscopic flow stress and grain size and lath size for two-phase Ti-6242 is proposed. The effect of grain morphology on creep-Induced Load shedding and localization is discussed.
-
crystal plasticity based fe model for understanding microstructural effects on creep and dwell fatigue in ti 6242
Journal of Engineering Materials and Technology-transactions of The Asme, 2006Co-Authors: Gayathri Venkataramani, Dhyanjyoti Deka, Somnath GhoshAbstract:This paper is aimed at identifying key microstructural parameters that play important roles in the failure initiation of polycrystalline Ti-6242 subjected to creep and dwell Loading. A finite element model, incorporating rate dependent elastocrystal plasticity, is developed for analyzing evolving variables in material microstructure. The crystal plasticity parameters are characterized by a combination of microtesting, orientation imaging microscopy, computational simulations, and minimization process involving Genetic algorithms (Ga). Accurate phase volume fractions and orientation distributions that are statistically equivalent to those observed in orientation imaging microscope scans are incorporated in the computational model of polycrystalline Ti-6242 for constant strain rate, creep, and dwell tests. The computational model is used for the identification of possible microstructural variables that may result in local crack initiation. Basal normal stress, equivalent plastic strain, and stress in Loading direction are considered as candidate parameters, of which the former is chosen as most probable from results of creep and dwell experiments and simulations. Creep Induced Load shedding phenomena is observed to lead to high value stresses that cause failure. The role of grain orientation with respect to the Loading axis and misorientation with its neighbors, in causing Load shedding and stress localizations is explored.
-
Crystal plasticity modeling of deformation and creep in polycrystalline Ti-6242
Metallurgical and Materials Transactions A, 2006Co-Authors: Dhyanjyoti Deka, Deepu S. Joseph, Somnath Ghosh, Michael J. MillsAbstract:This paper develops an experimentally validated computational model based on crystal plasticity for the analysis of two-phase α/β Ti-6242 polycrystalline alloys. A rate-dependent elastic-crystal plasticity model is incorporated in this model to accommodate anisotropy in material behavior and tension-compression asymmetry inherent to this alloy. A combination of microtesting, orientation imaging microscopy, computational simulations, and minimization process, involving genetic algorithms, is implemented in this study for careful characterization and calibration of the material parameters. Size effects are considered in this analysis through a simple scaling process. A homogenized equivalent model of the primary α with transformed β colonies is developed for incorporation in the Ti-6242 FE model. The polycrystalline Ti-6242 computational model incorporates accurate phase volume fractions, as well as statistically equivalent orientation distributions to those observed in the orientation imaging microscopy scans. The effects of orientation, misorientations, and microtexture distributions are investigated through simulations by this computational model. The model is used to simulate constant strain rate and creep tests in compression and tension, and the results are compared with experiments. The effects of microstructure and creep-Induced Load-shedding on the localization of microstructural stresses and strains are studied for potential crack initiation criteria.
Saurabh Amin - One of the best experts on this subject based on the ideXlab platform.
-
security assessment of electricity distribution networks under der node compromises
IEEE Transactions on Control of Network Systems, 2017Co-Authors: Devendra Shelar, Saurabh AminAbstract:This paper focuses on the security assessment of electricity distribution networks (DNs) with vulnerable distributed energy resource (DER) nodes. The adversary model is a simultaneous compromise of DER nodes by strategic manipulation of generation setpoints. The loss to the defender (DN operator) includes loss of voltage regulation and cost of Induced Load control under supply-demand mismatch caused by the attack. A three-stage defender-attacker-defender (DAD) game is formulated: in Stage 1, the defender chooses a security strategy to secure a subset of DER nodes; in Stage 2, the attacker compromises a set of vulnerable DERs and injects false generation setpoints; in Stage 3, the defender responds by controlling Loads and non-compromised DERs. Solving this trilevel optimization problem is hard due to nonlinear power flows and mixed-integer decision variables. To address this challenge, the problem is approximated by a tractable formulation based on an $\epsilon$ -linear power-flow model. The set of critical DER nodes and the setpoint manipulations characterizing the optimal attack strategy are computed. An iterative greedy approach to computing attacker-defender strategies for the original nonlinear problem is proposed. These results provide guidelines for optimal security investment and defender response in preattack and postattack conditions, respectively.
-
security assessment of electricity distribution networks under der node compromises
arXiv: Optimization and Control, 2016Co-Authors: Devendra Shelar, Saurabh AminAbstract:This article focuses on the security assessment of electricity Distribution Networks (DNs) with vulnerable Distributed Energy Resource (DER) nodes. The adversary model is simultaneous compromise of DER nodes by strategic manipulation of generation set-points. The loss to the defender (DN operator) includes loss of voltage regulation and cost of Induced Load control under supply-demand mismatch caused by the attack. A 3-stage Defender-Attacker-Defender (DAD) game is formulated: in Stage 1, the defender chooses a security strategy to secure a subset of DER nodes; in Stage 2, the attacker compromises a set of vulnerable DERs and injects false generation set-points; in Stage 3, the defender responds by controlling Loads and uncompromised DERs. Solving this trilevel optimization problem is hard due to nonlinear power flows and mixed-integer decision variables. To address this challenge, the problem is approximated by a tractable formulation based on linear power flows. The set of critical DER nodes and the set-point manipulations characterizing the optimal attack strategy are computed. An iterative greedy approach to compute attacker-defender strategies for the original nonlinear problem is proposed. These results provide guidelines for optimal security investment and defender response in pre- and post-attack conditions, respectively.
Joachim Hermsdorfer - One of the best experts on this subject based on the ideXlab platform.
-
grip force behavior during object manipulation in neurological disorders toward an objective evaluation of manual performance deficits
Movement Disorders, 2005Co-Authors: Dennis A. Nowak, Joachim HermsdorferAbstract:The control of prehensile finger forces is an essential feature of skilled manual performance. The basic aspects of healthy grip force behavior have been well documented. In healthy subjects, grip force is precisely adjusted to the mechanical object properties. Grip force is always slightly higher than the minimum necessary to prevent the object from slipping. When we move a hand-held object, grip force is modulated in parallel with movements-Induced Load fluctuations without an obvious delay. The absence of a temporal delay between grip and Load force profiles suggests that the central nervous system is able to predict the Load variations before the intended manipulation and consequently regulates grip force in anticipation. Feedback from the grasping fingertips is used to adjust the level of applied fingertip force efficiently to the actual Loading requirements. Pathologic grip force control affects the efficiency of produced force and the precision of the temporal coupling between grip and Load force profiles. Here, we review the characteristics of pathologic grip force behavior in various neurological disorders. Detailed examination of grip force control is simple and well suited for the objective evaluation of impaired motor function of the hand and its rehabilitation.
-
how predictive is grip force control in the complete absence of somatosensory feedback
Brain, 2004Co-Authors: Dennis A. Nowak, Stefan Glasauer, Joachim HermsdorferAbstract:Grip force control relies on accurate internal models of the dynamics of our motor system and the external objects we manipulate. Internal models are not fixed entities, but rather are trained and updated by sensory experience. Sensory feedback signals relevant object properties and mechanical events, e.g. at the skin-object interface, to modify motor commands and update internal representations automatically. Here we prove that intact sensory feedback is essential for predictive grip force regulation. The efficiency and precision of grip force adjustments to Load fluctuations arising from vertical and horizontal point-to-point arm movements with a hand-held object were analysed in a chronically deafferented subject (G.L.) and three healthy control subjects. Point-to-point movements started and ended with the object being held stationary. G.L. and healthy controls produced similar accelerations of the grasped object and consequently similar Load magnitudes during vertical and horizontal movements. Compared with healthy controls, G.L. employed inefficiently high grip forces when holding and moving the object, indicating inaccurate force scaling to object weight and inertial Loads. For healthy controls, the grip force profile was precisely timed to the movement-Induced Load fluctuations during vertical and horizontal movements. However, G.L.'s grip force profile was not processed to match differential Loading requirements of movement direction. We conclude that predictive grip force control requires at least intermittent sensory feedback to signal the effectiveness of descending motor commands and to update internal models.
-
the effects of digital anaesthesia on predictive grip force adjustments during vertical movements of a grasped object
European Journal of Neuroscience, 2001Co-Authors: Dennis A. Nowak, Joachim Hermsdorfer, Stefan Glasauer, Jens Philipp, Ludger Meyer, Norbert MaiAbstract:Grip force adjustments to fluctuations of inertial Loads Induced by vertical arm movements with a grasped object were analysed during normal and impaired finger sensibility. Normally grip force is modulated in a highly economical way in parallel with fluctuations of Load force. Two subjects performed vertical up and down movements of a grasped object, both with normal finger sensibility and then cutaneously anaesthetized finger sensibility. Short breaks were taken in between single movements, during which the object was held stationary. After digital anaesthesia was applied to the grasping fingers, both subjects substantially increased the grip force. The grip force amplitude and timing still anticipated changes in Load force, although the established grip force had already overcome movement-Induced Load force peaks. This implies that the increase of grip force and consequently the elevated force ratio between maximum grip and maximum Load force are not processed to alter the feedforward system of grip force control. Cutaneous afferent information from the grasping digits appears to be necessary for economic scaling of the grip force level, but it plays a subordinate role in the precise anticipatory temporal coupling of grip and Load forces during voluntary object manipulation.
Michael J. Mills - One of the best experts on this subject based on the ideXlab platform.
-
a size dependent crystal plasticity finite element model for creep and Load shedding in polycrystalline titanium alloys
Acta Materialia, 2007Co-Authors: G Venkatramani, Somnath Ghosh, Michael J. MillsAbstract:A rate-dependent anisotropic elastic-crystal plasticity based finite-element (FE) model with size-dependent yield strength is developed for polycrystalline Ti-6242. The initial slip system deformation resistances in the crystal plasticity relations are expressed as Hall–Petch type relations, where the grain size, lath size and colony size are chosen as characteristic lengths depending on the nature of slip. The FE model incorporates accurate phase volume fractions and orientation distributions that are statistically equivalent to those observed in orientation imaging microscopy (OIM) maps of the microstructure. The model is validated with experimental results on constant strain rate and creep tests. A relationship between the macroscopic flow stress and grain size and lath size for two-phase Ti-6242 is proposed. The effect of grain morphology on creep-Induced Load shedding and localization is discussed.
-
Crystal plasticity modeling of deformation and creep in polycrystalline Ti-6242
Metallurgical and Materials Transactions A, 2006Co-Authors: Dhyanjyoti Deka, Deepu S. Joseph, Somnath Ghosh, Michael J. MillsAbstract:This paper develops an experimentally validated computational model based on crystal plasticity for the analysis of two-phase α/β Ti-6242 polycrystalline alloys. A rate-dependent elastic-crystal plasticity model is incorporated in this model to accommodate anisotropy in material behavior and tension-compression asymmetry inherent to this alloy. A combination of microtesting, orientation imaging microscopy, computational simulations, and minimization process, involving genetic algorithms, is implemented in this study for careful characterization and calibration of the material parameters. Size effects are considered in this analysis through a simple scaling process. A homogenized equivalent model of the primary α with transformed β colonies is developed for incorporation in the Ti-6242 FE model. The polycrystalline Ti-6242 computational model incorporates accurate phase volume fractions, as well as statistically equivalent orientation distributions to those observed in the orientation imaging microscopy scans. The effects of orientation, misorientations, and microtexture distributions are investigated through simulations by this computational model. The model is used to simulate constant strain rate and creep tests in compression and tension, and the results are compared with experiments. The effects of microstructure and creep-Induced Load-shedding on the localization of microstructural stresses and strains are studied for potential crack initiation criteria.
Dennis A. Nowak - One of the best experts on this subject based on the ideXlab platform.
-
grip force behavior during object manipulation in neurological disorders toward an objective evaluation of manual performance deficits
Movement Disorders, 2005Co-Authors: Dennis A. Nowak, Joachim HermsdorferAbstract:The control of prehensile finger forces is an essential feature of skilled manual performance. The basic aspects of healthy grip force behavior have been well documented. In healthy subjects, grip force is precisely adjusted to the mechanical object properties. Grip force is always slightly higher than the minimum necessary to prevent the object from slipping. When we move a hand-held object, grip force is modulated in parallel with movements-Induced Load fluctuations without an obvious delay. The absence of a temporal delay between grip and Load force profiles suggests that the central nervous system is able to predict the Load variations before the intended manipulation and consequently regulates grip force in anticipation. Feedback from the grasping fingertips is used to adjust the level of applied fingertip force efficiently to the actual Loading requirements. Pathologic grip force control affects the efficiency of produced force and the precision of the temporal coupling between grip and Load force profiles. Here, we review the characteristics of pathologic grip force behavior in various neurological disorders. Detailed examination of grip force control is simple and well suited for the objective evaluation of impaired motor function of the hand and its rehabilitation.
-
how predictive is grip force control in the complete absence of somatosensory feedback
Brain, 2004Co-Authors: Dennis A. Nowak, Stefan Glasauer, Joachim HermsdorferAbstract:Grip force control relies on accurate internal models of the dynamics of our motor system and the external objects we manipulate. Internal models are not fixed entities, but rather are trained and updated by sensory experience. Sensory feedback signals relevant object properties and mechanical events, e.g. at the skin-object interface, to modify motor commands and update internal representations automatically. Here we prove that intact sensory feedback is essential for predictive grip force regulation. The efficiency and precision of grip force adjustments to Load fluctuations arising from vertical and horizontal point-to-point arm movements with a hand-held object were analysed in a chronically deafferented subject (G.L.) and three healthy control subjects. Point-to-point movements started and ended with the object being held stationary. G.L. and healthy controls produced similar accelerations of the grasped object and consequently similar Load magnitudes during vertical and horizontal movements. Compared with healthy controls, G.L. employed inefficiently high grip forces when holding and moving the object, indicating inaccurate force scaling to object weight and inertial Loads. For healthy controls, the grip force profile was precisely timed to the movement-Induced Load fluctuations during vertical and horizontal movements. However, G.L.'s grip force profile was not processed to match differential Loading requirements of movement direction. We conclude that predictive grip force control requires at least intermittent sensory feedback to signal the effectiveness of descending motor commands and to update internal models.
-
the effects of digital anaesthesia on predictive grip force adjustments during vertical movements of a grasped object
European Journal of Neuroscience, 2001Co-Authors: Dennis A. Nowak, Joachim Hermsdorfer, Stefan Glasauer, Jens Philipp, Ludger Meyer, Norbert MaiAbstract:Grip force adjustments to fluctuations of inertial Loads Induced by vertical arm movements with a grasped object were analysed during normal and impaired finger sensibility. Normally grip force is modulated in a highly economical way in parallel with fluctuations of Load force. Two subjects performed vertical up and down movements of a grasped object, both with normal finger sensibility and then cutaneously anaesthetized finger sensibility. Short breaks were taken in between single movements, during which the object was held stationary. After digital anaesthesia was applied to the grasping fingers, both subjects substantially increased the grip force. The grip force amplitude and timing still anticipated changes in Load force, although the established grip force had already overcome movement-Induced Load force peaks. This implies that the increase of grip force and consequently the elevated force ratio between maximum grip and maximum Load force are not processed to alter the feedforward system of grip force control. Cutaneous afferent information from the grasping digits appears to be necessary for economic scaling of the grip force level, but it plays a subordinate role in the precise anticipatory temporal coupling of grip and Load forces during voluntary object manipulation.