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Manuel Malheiro - One of the best experts on this subject based on the ideXlab platform.
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magnetic dipole Moment of soft gamma ray repeaters and anomalous x ray pulsars described as massive and magnetic white dwarfs
Publications of the Astronomical Society of Japan, 2014Co-Authors: Jaziel G Coelho, Manuel MalheiroAbstract:The Anomalous X-ray Pulsars (AXPs) and Soft Gamma-ray Repeaters (SGRs) are some of the most interesting groups of pulsars that have been intensively studied in the recent years. They are understood as neutron stars (NSs) with super strong magnetic fields, namely $B\gtrsim10^{14}$ G. However, in the last two years two SGRs with low magnetic fields $B\sim(10^{12}-10^{13})$ G have been detected. Moreover, three fast and very {\it magnetic} white dwarfs (WDs) have also been observed in the last years. Based on these new pulsar discoveries, we compare and contrast the magnetic fields, magnetic dipole Moment, Characteristic ages, and X-ray steady luminosities of these two SGRs (in the WD model) with three fast white dwarfs, to conclude that they show strong similarities corroborating an alternative description of several SGRs/AXPs as very massive and magnetic white dwarfs. The pulsar magnetic dipole Moment $m$ depending only on the Momentum of inertia $I$, and observational properties, such as the period $P$ and its first time derivative $\dot{P}$, can help to identify the scale of $I$ for SGRs/AXPs. We analyze the pulsar magnetic dipole Moment $m$ of SGRs and AXPs when a model based on a massive fast rotating highly magnetized white dwarf is considered. We show that the values for $m$ obtained for several SGRs and AXPs are in agreement with the observed range $10^{34}{\rm emu}\leq m \leq10^{36}{\rm emu}$ of isolated and polar magnetic white dwarfs. This result together with the fact that for {\it magnetic} white dwarfs $B\sim(10^6-10^8)$ G their magnetic dipole Moments are almost independent of the star rotation period ($10^{4}\lesssim P \lesssim10^{6} {\rm s}$) - a phenomenology not shared by neutron stars pulsars - suggests a possible {\it magnetic} white dwarf nature for some of SGRs/AXPs that have much smaller periods ($P\sim 10$ s).
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magnetic dipole Moment of sgrs and axps described as massive and magnetic white dwarfs
arXiv: Solar and Stellar Astrophysics, 2012Co-Authors: Jaziel G Coelho, Manuel MalheiroAbstract:The Anomalous X-ray Pulsars (AXPs) and Soft Gamma-ray Repeaters (SGRs) are some of the most interesting groups of pulsars that have been intensively studied in the recent years. They are understood as neutron stars (NSs) with super strong magnetic fields, namely $B\gtrsim10^{14}$ G. However, in the last two years two SGRs with low magnetic fields $B\sim(10^{12}-10^{13})$ G have been detected. Moreover, three fast and very {\it magnetic} white dwarfs (WDs) have also been observed in the last years. Based on these new pulsar discoveries, we compare and contrast the magnetic fields, magnetic dipole Moment, Characteristic ages, and X-ray steady luminosities of these two SGRs (in the WD model) with three fast white dwarfs, to conclude that they show strong similarities corroborating an alternative description of several SGRs/AXPs as very massive and magnetic white dwarfs. The pulsar magnetic dipole Moment $m$ depending only on the Momentum of inertia $I$, and observational properties, such as the period $P$ and its first time derivative $\dot{P}$, can help to identify the scale of $I$ for SGRs/AXPs. We analyze the pulsar magnetic dipole Moment $m$ of SGRs and AXPs when a model based on a massive fast rotating highly magnetized white dwarf is considered. We show that the values for $m$ obtained for several SGRs and AXPs are in agreement with the observed range $10^{34}{\rm emu}\leq m \leq10^{36}{\rm emu}$ of isolated and polar magnetic white dwarfs. This result together with the fact that for {\it magnetic} white dwarfs $B\sim(10^6-10^8)$ G their magnetic dipole Moments are almost independent of the star rotation period ($10^{4}\lesssim P \lesssim10^{6} {\rm s}$) - a phenomenology not shared by neutron stars pulsars - suggests a possible {\it magnetic} white dwarf nature for some of SGRs/AXPs that have much smaller periods ($P\sim 10$ s).
Jaziel G Coelho - One of the best experts on this subject based on the ideXlab platform.
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magnetic dipole Moment of soft gamma ray repeaters and anomalous x ray pulsars described as massive and magnetic white dwarfs
Publications of the Astronomical Society of Japan, 2014Co-Authors: Jaziel G Coelho, Manuel MalheiroAbstract:The Anomalous X-ray Pulsars (AXPs) and Soft Gamma-ray Repeaters (SGRs) are some of the most interesting groups of pulsars that have been intensively studied in the recent years. They are understood as neutron stars (NSs) with super strong magnetic fields, namely $B\gtrsim10^{14}$ G. However, in the last two years two SGRs with low magnetic fields $B\sim(10^{12}-10^{13})$ G have been detected. Moreover, three fast and very {\it magnetic} white dwarfs (WDs) have also been observed in the last years. Based on these new pulsar discoveries, we compare and contrast the magnetic fields, magnetic dipole Moment, Characteristic ages, and X-ray steady luminosities of these two SGRs (in the WD model) with three fast white dwarfs, to conclude that they show strong similarities corroborating an alternative description of several SGRs/AXPs as very massive and magnetic white dwarfs. The pulsar magnetic dipole Moment $m$ depending only on the Momentum of inertia $I$, and observational properties, such as the period $P$ and its first time derivative $\dot{P}$, can help to identify the scale of $I$ for SGRs/AXPs. We analyze the pulsar magnetic dipole Moment $m$ of SGRs and AXPs when a model based on a massive fast rotating highly magnetized white dwarf is considered. We show that the values for $m$ obtained for several SGRs and AXPs are in agreement with the observed range $10^{34}{\rm emu}\leq m \leq10^{36}{\rm emu}$ of isolated and polar magnetic white dwarfs. This result together with the fact that for {\it magnetic} white dwarfs $B\sim(10^6-10^8)$ G their magnetic dipole Moments are almost independent of the star rotation period ($10^{4}\lesssim P \lesssim10^{6} {\rm s}$) - a phenomenology not shared by neutron stars pulsars - suggests a possible {\it magnetic} white dwarf nature for some of SGRs/AXPs that have much smaller periods ($P\sim 10$ s).
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magnetic dipole Moment of sgrs and axps described as massive and magnetic white dwarfs
arXiv: Solar and Stellar Astrophysics, 2012Co-Authors: Jaziel G Coelho, Manuel MalheiroAbstract:The Anomalous X-ray Pulsars (AXPs) and Soft Gamma-ray Repeaters (SGRs) are some of the most interesting groups of pulsars that have been intensively studied in the recent years. They are understood as neutron stars (NSs) with super strong magnetic fields, namely $B\gtrsim10^{14}$ G. However, in the last two years two SGRs with low magnetic fields $B\sim(10^{12}-10^{13})$ G have been detected. Moreover, three fast and very {\it magnetic} white dwarfs (WDs) have also been observed in the last years. Based on these new pulsar discoveries, we compare and contrast the magnetic fields, magnetic dipole Moment, Characteristic ages, and X-ray steady luminosities of these two SGRs (in the WD model) with three fast white dwarfs, to conclude that they show strong similarities corroborating an alternative description of several SGRs/AXPs as very massive and magnetic white dwarfs. The pulsar magnetic dipole Moment $m$ depending only on the Momentum of inertia $I$, and observational properties, such as the period $P$ and its first time derivative $\dot{P}$, can help to identify the scale of $I$ for SGRs/AXPs. We analyze the pulsar magnetic dipole Moment $m$ of SGRs and AXPs when a model based on a massive fast rotating highly magnetized white dwarf is considered. We show that the values for $m$ obtained for several SGRs and AXPs are in agreement with the observed range $10^{34}{\rm emu}\leq m \leq10^{36}{\rm emu}$ of isolated and polar magnetic white dwarfs. This result together with the fact that for {\it magnetic} white dwarfs $B\sim(10^6-10^8)$ G their magnetic dipole Moments are almost independent of the star rotation period ($10^{4}\lesssim P \lesssim10^{6} {\rm s}$) - a phenomenology not shared by neutron stars pulsars - suggests a possible {\it magnetic} white dwarf nature for some of SGRs/AXPs that have much smaller periods ($P\sim 10$ s).
Radaelli G. - One of the best experts on this subject based on the ideXlab platform.
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Synthesis of mechanisms with prescribed elastic load-displacement Characteristics
2017Co-Authors: Radaelli G.Abstract:In this dissertation a collection of concepts to synthesise nonlinear springs is presented. Such springs can be useful in various application domains where, e.g., multi-stability or static balancing is desired. These behaviors are often sought to alleviate the effort required for actuation. The explored concepts are presented by showing the design methods, numerical or analytical models, and assessing their viability with experimental evaluations.In part~I two concepts show how the linear Moment Characteristic of torsion bars can be reshaped into a nonlinear one. Torsion bars are often suitable energy storage elements because they can be conveniently integrated within the hinge of a mechanism. In both examples the synthesised nonlinear Characteristic is determined such that it counteracts the Moment of a turning pendulum. The way how the Characteristic is reshaped is, however, very different. In the first concept multiple springs are employed, but activated or deactivated by mechanical stops in order to create a piecewise linear Characteristic. In the second concept the Characteristic is reshaped by a set of non-circular gears. These gears are arranged in a planetary way to obtain a compact transmission. In part~II the focus is on planar compliant mechanisms that by virtue of their optimized shape exhibit the desired behavior. A few examples demonstrate that, even with relatively simple topologies, complex Characteristics can be synthesised accurately. For example, a single beam clamped at one end and pivoted at the other end, is able to match a sinusoidal Moment Characteristic for a half period. In a second example we were able to produce a constant force by a doubly clamped optimally shaped beam. The constant force of this minimalistic design can be applied to balance a weight over a range of motion approximately equal to the largest dimension of the design. In another example it is shown that an optimized beam shape can emulate the behavior of zero free-length springs. These springs have ideal properties but are in practice difficult to make. We also show that a meta-material constituted by a lattice of zero free-length springs, exhibits very peculiar properties as zero Poisson's ratio, isotropy, and constant Young's modulus, up to large strains. Obtaining the required spring bahaviour at such small scale would become possible by the use of optimally shaped beam springs. In the last example of part~II a design consisting of four symmetric beams that move over a straight line of continuous static equilibrium is shown. As an aid to the design process, a representation of the elastokinematic behavior is introduced, based on the potential energy field (PEF). The PEFs characterise the behavior of compliant systems not only instantaneously, but over an area of possible displacement locations of the endpoint of the system. Part~III of this dissertation is dedicated to compliant shell mechanisms. The design of compliant mechanisms as spatial, thin walled, and possibly double curved structures has some interesting and promising aspects. Because of their inherent nonlinear behavior, for example, they lend themselves good for synthesising the nonlinear equilibrium path. With compliant shell mechanisms it is also possible to conveniently create anisotropic stiffness, such that some motion directions are travelled much easier with respect to others. This type of effects can be tailored to create a desired kinematic function. In applications as wearable devices and interactive structures, compliant shell mechanisms can yield to slender, lightweight, aesthetically pleasing, and highly functional solutions. In this dissertation some progresses are made in this infant field of research. As a showcase, in the first chapter of this part, a self-balanced shell is designed. The optimized doubly curved shape of this shell is in continuous equilibrium with its own weight over a fairly large range of motion. In the subsequent two chapters, a tailored Moment-angle Characteristic is realized by optimizing the parameters of a basic origami mechanism. In the last chapter of this part a spiral spring with various cross-sections is analyzed to understand the anisotropic stiffness behaviors that can be achieved. In particular, the out-of-plane spatial behavior is studied. This is done by using the PEFs, for the first time in three dimensions. In part~IV two application examples are shown. First a shell mechanism, designed to provide a constant force, is applied to the tip of a heart ablation catheter. The constant force at the tip of the catheter helps maintaining contact with the heart wall while preventing dangerously high forces. The second example shows the concept of a large scale collapsible wall, consisting of a doubly curved shell that balances its own weight. Such wall, employed as e.g. a sound barrier, could be hidden flat when not in use, and be lifted upright when it is needed. The concepts presented in this dissertation are applied to selected examples. However, they can be applied to synthesise a broader scope of desired Characteristics. Also, the ideas can be generalised by moving from springs to mechanisms, i.e. where input and output have distinct locations. A step even further is to apply distributed actuation, sensing, and control on the deforming bodies such to obtain real automata, where advantage is taken of the synthesised elastic behavior. It is also advisable to direct future research into the use of composites as spring material. It can be expected that their high strength, their tailorable anisotropy, and the possibility to deliberately introduce prestress will lead to springs with increased performance and improved control of the behavior. Future research should also be directed towards improving the available design aids, including PEFs, for compliant mechanism designers. Furthermore, it is expected that the developments of this dissertation can be beneficially applied in an increasing number of application areas
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Synthesis of mechanisms with prescribed elastic load-displacement Characteristics
2017Co-Authors: Radaelli G.Abstract:In this dissertation a collection of concepts to synthesise nonlinear springs is presented. Such springs can be useful in various application domains where, e.g., multi-stability or static balancing is desired. These behaviors are often sought to alleviate the effort required for actuation. The explored concepts are presented by showing the design methods, numerical or analytical models, and assessing their viability with experimental evaluations.In part~I two concepts show how the linear Moment Characteristic of torsion bars can be reshaped into a nonlinear one. Torsion bars are often suitable energy storage elements because they can be conveniently integrated within the hinge of a mechanism. In both examples the synthesised nonlinear Characteristic is determined such that it counteracts the Moment of a turning pendulum. The way how the Characteristic is reshaped is, however, very different. In the first concept multiple springs are employed, but activated or deactivated by mechanical stops in order to create a piecewise linear Characteristic. In the second concept the Characteristic is reshaped by a set of non-circular gears. These gears are arranged in a planetary way to obtain a compact transmission. In part~II the focus is on planar compliant mechanisms that by virtue of their optimized shape exhibit the desired behavior. A few examples demonstrate that, even with relatively simple topologies, complex Characteristics can be synthesised accurately. For example, a single beam clamped at one end and pivoted at the other end, is able to match a sinusoidal Moment Characteristic for a half period. In a second example we were able to produce a constant force by a doubly clamped optimally shaped beam. The constant force of this minimalistic design can be applied to balance a weight over a range of motion approximately equal to the largest dimension of the design. In another example it is shown that an optimized beam shape can emulate the behavior of zero free-length springs. These springs have ideal properties but are in practice difficult to make. We also show that a meta-material constituted by a lattice of zero free-length springs, exhibits very peculiar properties as zero Poisson's ratio, isotropy, and constant Young's modulus, up to large strains. Obtaining the required spring bahaviour at such small scale would become possible by the use of optimally shaped beam springs. In the last example of part~II a design consisting of four symmetric beams that move over a straight line of continuous static equilibrium is shown. As an aid to the design process, a representation of the elastokinematic behavior is introduced, based on the potential energy field (PEF). The PEFs characterise the behavior of compliant systems not only instantaneously, but over an area of possible displacement locations of the endpoint of the system. Part~III of this dissertation is dedicated to compliant shell mechanisms. The design of compliant mechanisms as spatial, thin walled, and possibly double curved structures has some interesting and promising aspects. Because of their inherent nonlinear behavior, for example, they lend themselves good for synthesising the nonlinear equilibrium path. With compliant shell mechanisms it is also possible to conveniently create anisotropic stiffness, such that some motion directions are travelled much easier with respect to others. This type of effects can be tailored to create a desired kinematic function. In applications as wearable devices and interactive structures, compliant shell mechanisms can yield to slender, lightweight, aesthetically pleasing, and highly functional solutions. In this dissertation some progresses are made in this infant field of research. As a showcase, in the first chapter of this part, a self-balanced shell is designed. The optimized doubly curved shape of this shell is in continuous equilibrium with its own weight over a fairly large range of motion. In the subsequent two chapters, a tailored Moment-angle Characteristic is realized by optimizing the parameters of a basic origami mechanism. In the last chapter of this part a spiral spring with various cross-sections is analyzed to understand the anisotropic stiffness behaviors that can be achieved. In particular, the out-of-plane spatial behavior is studied. This is done by using the PEFs, for the first time in three dimensions. In part~IV two application examples are shown. First a shell mechanism, designed to provide a constant force, is applied to the tip of a heart ablation catheter. The constant force at the tip of the catheter helps maintaining contact with the heart wall while preventing dangerously high forces. The second example shows the concept of a large scale collapsible wall, consisting of a doubly curved shell that balances its own weight. Such wall, employed as e.g. a sound barrier, could be hidden flat when not in use, and be lifted upright when it is needed. The concepts presented in this dissertation are applied to selected examples. However, they can be applied to synthesise a broader scope of desired Characteristics. Also, the ideas can be generalised by moving from springs to mechanisms, i.e. where input and output have distinct locations. A step even further is to apply distributed actuation, sensing, and control on the deforming bodies such to obtain real automata, where advantage is taken of the synthesised elastic behavior. It is also advisable to direct future research into the use of composites as spring material. It can be expected that their high strength, their tailorable anisotropy, and the possibility to deliberately introduce prestress will lead to springs with increased performance and improved control of the behavior. Future research should also be directed towards improving the available design aids, including PEFs, for compliant mechanism designers. Furthermore, it is expected that the developments of this dissertation can be beneficially applied in an increasing number of application areas.Mechatronic Systems Desig
Ali Abolmaali - One of the best experts on this subject based on the ideXlab platform.
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Moment rotation hysteresis behavior of top and seat angle steel frame connections
Journal of Structural Engineering-asce, 1999Co-Authors: Anant R Kukreti, Ali AbolmaaliAbstract:This paper presents an approach toward formulating analytical models to predict the Moment-rotation hysteresis behavior of top and seat angle connections. Experimental results obtained from 12 top and seat angle connection specimens are used to obtain the prediction equations for the parameters defining the Moment rotation hysteresis loops of a typical top and seat angle connection. These parameters include the initial stiffness, ultimate Moment capacity, ultimate rotation, the transition Moment, Characteristic Moment, and rigidity parameter. Regression analysis results and comparisons with test results are presented to demonstrate the acceptability of these prediction equations. The prediction equations obtained for these parameters are used to develop four different Moment rotation hysteresis models for the connection: the bilinear, elastoplastic, Ramberg-Osgood, and modified bilinear models. The results of the study show that the top and seat angle connection behaves as a semirigid connection. A wide range of initial stiffnesses and ultimate Moment capacities are possible to achieve by altering the connection geometry related variables within a practical range. For certain geometric configurations of the connection, significant transfer of Moment from the beam to the column can occur before the connection fails. Also, it is possible to design a connection with low stiffness and small Moment transfer capability, so that it behaves in a manner such that it is close to being classified as a pin connection. The prediction equations developed for the parameters characterizing the four hysteresis models give acceptable results when compared to experimental results. The degree to which the models idealized the actual behavior varies with the elastoplastic model being the least conservative and the modified bilinear modeling being the best. The Ramberg-Osgood model is the most accurate in just modeling the nonpinching Moment-rotation loops.
Anant R Kukreti - One of the best experts on this subject based on the ideXlab platform.
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Moment rotation hysteresis behavior of top and seat angle steel frame connections
Journal of Structural Engineering-asce, 1999Co-Authors: Anant R Kukreti, Ali AbolmaaliAbstract:This paper presents an approach toward formulating analytical models to predict the Moment-rotation hysteresis behavior of top and seat angle connections. Experimental results obtained from 12 top and seat angle connection specimens are used to obtain the prediction equations for the parameters defining the Moment rotation hysteresis loops of a typical top and seat angle connection. These parameters include the initial stiffness, ultimate Moment capacity, ultimate rotation, the transition Moment, Characteristic Moment, and rigidity parameter. Regression analysis results and comparisons with test results are presented to demonstrate the acceptability of these prediction equations. The prediction equations obtained for these parameters are used to develop four different Moment rotation hysteresis models for the connection: the bilinear, elastoplastic, Ramberg-Osgood, and modified bilinear models. The results of the study show that the top and seat angle connection behaves as a semirigid connection. A wide range of initial stiffnesses and ultimate Moment capacities are possible to achieve by altering the connection geometry related variables within a practical range. For certain geometric configurations of the connection, significant transfer of Moment from the beam to the column can occur before the connection fails. Also, it is possible to design a connection with low stiffness and small Moment transfer capability, so that it behaves in a manner such that it is close to being classified as a pin connection. The prediction equations developed for the parameters characterizing the four hysteresis models give acceptable results when compared to experimental results. The degree to which the models idealized the actual behavior varies with the elastoplastic model being the least conservative and the modified bilinear modeling being the best. The Ramberg-Osgood model is the most accurate in just modeling the nonpinching Moment-rotation loops.