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C S Manohar - One of the best experts on this subject based on the ideXlab platform.
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reliability based critical earthquake load models part 1 linear structures
Journal of Sound and Vibration, 2005Co-Authors: A M Abbas, C S ManoharAbstract:The problem of determining critical stochastic earthquake excitation models for simple nonlinear systems under single-point or multi-point nonstationary seismic inputs is considered. The earthquake acceleration components are obtained by multiplying known deterministic enveloping functions with zero mean Gaussian stationary random processes with unknown auto-power spectral density functions (for single-point excitations) and power spectral density matrix (for multi-point excitations). The definition of critical earthquake input is based on the notion of a performance function. The system is considered to have failed if the maximum response over a given time interval exceeds specified Limits. The critical excitations are defined as those that minimize the Hasofer-Lind reliability index associated with this performance function. The computation of this index, in turn, is based on the use of response Surface to model the Limit Surface near the check point. Here the quantity to be optimally determined is taken to be the unknown input power spectral density function (for single-point excitations) or the input power spectral density matrix (for multi-point excitations). The excitations are taken to satisfy constraints on total average energy, zero crossing rate, lower bounds on entropy rate and other positivity and bounding requirements that are of mathematical nature. The resulting constrained nonlinear optimization problems are solved using the sequential quadratic programming method. Illustrative examples for computing random critical excitations for singly supported and multiply supported oscillators that have cubic force-displacement relations are provided.
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Reliability-based critical earthquake load models. Part 2: nonlinear structures
Academic Press Inc Elsevier Science Ltd, 2005Co-Authors: A M Abbas, C S ManoharAbstract:The problem of determining critical stochastic earthquake excitation models for simple nonlinear systems under single-point or multi-point nonstationary seismic inputs is considered. The earthquake acceleration components are obtained by multiplying known deterministic enveloping functions with zero mean Gaussian stationary random processes with unknown auto-power spectral density functions (for single-point excitations) and power spectral density matrix (for multi-point excitations). The definition of critical earthquake input is based on the notion of a performance function. The system is considered to have failed if the maximum response over a given time interval exceeds specified Limits. The critical excitations are defined as those that minimize the Hasofer-Lind reliability index associated with this performance function. The computation of this index, in turn, is based on the use of response Surface to model the Limit Surface near the check point. Here the quantity to be optimally determined is taken to be the unknown input power spectral density function (for single-point excitations) or the input power spectral density matrix (for multi-point excitations). The excitations are taken to satisfy constraints on total average energy, zero crossing rate, lower bounds on entropy rate and other positivity and bounding requirements that are of mathematical nature. The resulting constrained nonlinear optimization problems are solved using the sequential quadratic programming method. Illustrative examples for computing random critical excitations for singly supported and multiply supported oscillators that have cubic force-displacement relations are provided
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an improved response Surface method for the determination of failure probability and importance measures
Structural Safety, 2004Co-Authors: Sayan Gupta, C S ManoharAbstract:The problem of response Surface modeling of Limit Surface lying within two hyper spheres of prescribed radii is considered. The relevance of this problem in structural reliability analysis involving performance functions with multiple design points and/or multiple regions that make significant contributions to failure probability is discussed. The paper also proposes global measures of sensitivity of failure probability with respect to the basic random variables. The performance of the proposed improvements is examined by comparing simulation based results with results from the proposed procedure with reference to two specific structural reliability analysis problems.
Martin Bertram - One of the best experts on this subject based on the ideXlab platform.
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generalized b spline subdivision Surface wavelets for geometry compression
IEEE Transactions on Visualization and Computer Graphics, 2004Co-Authors: Martin BertramAbstract:We present a new construction of lifted biorthogonal wavelets on Surfaces of arbitrary two-manifold topology for compression and multiresolution representation. Our method combines three approaches: subdivision Surfaces of arbitrary topology, B-spline wavelets, and the lifting scheme for biorthogonal wavelet construction. The simple building blocks of our wavelet transform are local lifting operations performed on polygonal meshes with subdivision hierarchy. Starting with a coarse, irregular polyhedral base mesh, our transform creates a subdivision hierarchy of meshes converging to a smooth Limit Surface. At every subdivision level, geometric detail is expanded from wavelet coefficients and added to the Surface. We present wavelet constructions for bilinear, bicubic, and biquintic B-spline subdivision. While the bilinear and bicubic constructions perform well in numerical experiments, the biquintic construction turns out to be unstable. For lossless compression, our transform is computed in integer arithmetic, mapping integer coordinates of control points to integer wavelet coefficients. Our approach provides a highly efficient and progressive representation for complex geometries of arbitrary topology.
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Resin Supply at the Crossroads
1999Co-Authors: Martin BertramAbstract:Surfaces. Abstract: We present a new construction of lifted biorthogonal wavelets on Surfaces of arbitrary two-manifold topology for compression and multiresolution representation. Our method combines three approaches: subdivision Surfaces of arbitrary topology, B-spline wavelets, and the lifting scheme for biorthogonal wavelet construction. The simple building blocks of our wavelet transform are local lifting operations performed on polygonal meshes with subdivision hierarchy. Starting with a coarse, irregular polyhedral base mesh, our transform creates a subdivision hierarchy of meshes converging to a smooth Limit Surface. At every subdivision level, geometric detail can be expanded from wavelet coefficients and added to the Surface. We present wavelet constructions for bilinear, bicubic, and biquintic B-Spline subdivision. While the bilinear and bicubic constructions perform well in numerical experiments, the biquintic construction turns out to be unstable. For lossless compression, our transform can be computed in integer arithmetic, mapping integer coordinates of control points to integer wavelet coefficients. Our approach provides a highly efficient and progressive representation for complex geometries of arbitrary topology.
David M Parks - One of the best experts on this subject based on the ideXlab platform.
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a comprehensive lattice stability Limit Surface for graphene
Journal of The Mechanics and Physics of Solids, 2016Co-Authors: Sandeep Kumar, David M ParksAbstract:Abstract The Limits of reversible deformation in graphene under various loadings are examined using lattice-dynamical stability analysis. This information is then used to construct a comprehensive lattice-stability Limit Surface for graphene, which provides an analytical description of incipient lattice instabilities of all kinds, for arbitrary deformations, parametrized in terms of symmetry-invariants of strain/stress. Symmetry-invariants allow obtaining an accurate parametrization with a minimal number of coefficients. Based on this Limit Surface, we deduce a general continuum criterion for the onset of all kinds of lattice-stabilities in graphene: an instability appears when the magnitude of the deviatoric strain γ reaches a critical value γc which depends upon the mean normal strain E ¯ and the directionality θ of the principal deviatoric stretch with respect to reference lattice orientation. We also distinguish between the distinct regions of the Limit Surface that correspond to fundamentally different mechanisms of lattice instabilities in graphene, such as structural versus material instabilities, and long-wave (elastic) versus short-wave instabilities. Utility of this Limit Surface is demonstrated in assessment of incipient failures in defect-free graphene via its implementation in a continuum finite elements analysis (FEA). The resulting scheme enables on-the-fly assessments of not only the macroscopic conditions (e.g., load and deflection) but also the microscopic conditions (e.g., local stress/strain, spatial location, temporal proximity, and nature of incipient lattice instability) at which an instability occurs in a defect-free graphene sheet subjected to an arbitrary loading condition.
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a comprehensive lattice stability Limit Surface for graphene
arXiv: Materials Science, 2015Co-Authors: Sandeep Kumar, David M ParksAbstract:The Limits of reversible deformation in graphene under various loadings are examined using lattice-dynamical stability analysis. This information is then used to construct a comprehensive lattice-stability Limit Surface for graphene, which provides an analytical description of incipient lattice instabilities of \textit{all kinds}, for arbitrary deformations, parametrized in terms of symmetry-invariants of strain/stress. Symmetry-invariants allow obtaining an accurate parametrization with a minimal number of coefficients. Based on this Limit Surface, we deduce a general continuum criterion for the onset of all kinds of lattice-stabilities in graphene: an instability appears when the magnitude of the deviatoric strain $\gamma$ reaches a critical value $\gamma^c$ which depends upon the mean hydrostatic strain $\bar {\mathcal E}$ and the directionality $\theta$ of the deviatoric stretch. We also distinguish between the distinct regions of the Limit Surface that correspond to fundamentally-different mechanisms of lattice instabilities in graphene, such as structural vs material instabilities, and long-wave (elastic) vs short-wave instabilities. Utility of this Limit Surface is demonstrated in assessment of incipient failures in defect-free graphene via its implementation in a continuum Finite Elements Analysis (FEA). The resulting scheme enables on-the-fly assessments of not only the macroscopic conditions (e.g., load; deflection) but also the microscopic conditions (e.g., local stress/strain; spatial location, temporal proximity, and nature of incipient lattice instability) at which an instability occurs in a defect-free graphene sheet subjected to an arbitrary loading condition.
Imin Kao - One of the best experts on this subject based on the ideXlab platform.
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Development of Realistic Pressure Distribution and Friction Limit Surface for Soft-Finger Contact Interface of Robotic Hands
Journal of Intelligent & Robotic Systems, 2016Co-Authors: Amin Fakhari, Mehdi Keshmiri, Imin KaoAbstract:Various models have been presented for pressures distribution in the contact interface of a soft finger and object in the literature. These models have been proposed without considering the effect of the tangential forces which are usually exerted in the contact interface of a soft finger and object during grasping and manipulation. Having an accurate pressures distribution model across the contact interface is important for designing tactile sensors and improving the modeling of the friction Limit Surface (LS). In this paper, a new and more accurate model is proposed to describe the asymmetry of the pressure distribution in the contact interface of a hemispherical soft finger under both normal and tangential forces. This model is derived based upon observations in the previous literature stating that the contact interface would move and skew toward the direction of the tangential force. According to the proposed pressure distribution model in this study, an improved and more accurate LS is presented. The LS profile obtained by this model is compared with the corresponding results based on the previous models. The new results show that the consideration of the skewness or asymmetry in the pressure distribution (due to the tangential force) causes the LS profile to shrink compared with that constructed with symmetric pressure distribution assumption. This shrinkage, as a result of the skewness and asymmetry of the pressure distribution, makes the contact interface more vulnerable. Furthermore, this new model can also provide a more accurate tool for the analysis of grasping and manipulation involving soft contact interface.
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Modeling of Contact Mechanics and Friction Limit Surfaces for Soft Fingers in Robotics, with Experimental Results
The International Journal of Robotics Research, 1999Co-Authors: Nicholas Xydas, Imin KaoAbstract:A new theory in contact mechanics for modeling of soft fingers is proposed to define the relationship between the normal force and the radius of contact for soft fingers by considering general soft-finger materials, including linearly and nonlinearly elastic materials. The results show that the radius of contact is proportional to the normal force raised to the power of, which ranges from 0 to 1/3. This new theory subsumes the Hertzian contact model for linear elastic materials, where D 1/3. Experiments are conducted to validate the theory using artificial soft fingers made of various materials such as rubber and silicone. Results for human fingers are also compared. This theory provides a basis for numerically constructing friction Limit Surfaces. The numerical friction Limit Surface can be approximated by an ellipse, with the major and minor axes as the maximum friction force and the maximum moment with respect to the normal axis of contact, respectively. Combining the results of the contactmechanics mo...
K. Kase - One of the best experts on this subject based on the ideXlab platform.
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three axis nc cutter path generation for subdivision Surface with z map
Jsme International Journal Series C-mechanical Systems Machine Elements and Manufacturing, 2005Co-Authors: W U Peng, H. Suzuki, K. KaseAbstract:In this paper we propose methodologies and algorithms of NC cutter path generation for subdivision Surfaces. We select Loop Surface as the subdivision Surfaces. A path plan including rough cut and finish-cut is developed based on the LoD (Level of Detail) property of subdivision Surface. We generate a coarse mesh that covers the Limit Surface to implement the rough cut. For finish-cut we use ball-end mills and offset cutter contact data to generate cutter location data. In these two steps we use a Z-map model and a collision detection/correction method is presented for the interference-free of these two steps. We implement our methods and present machining results. All of these two kinds of cutter paths are computed rapidly and automatically.
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Three-axis NC cutter path generation for subdivision Surface
Geometric Modeling and Processing 2004. Proceedings, 2004Co-Authors: Peng Wu, H. Suzuki, J. Kuragano, K. KaseAbstract:In this paper we propose methodologies and algorithms of NC cutter path generation for subdivision Surfaces. We select Loop Surface as the subdivision Surface. A path plan including rough cut and finish-cut is developed based on LoD (level of detail) property of the subdivision Surface. We generate a coarse mesh that covers the Limit Surface to implement rough cut. For finish-cut we use ball-end mills and offset cutter contact positions to generate cutter location. In these two steps we use a Z-map model and a collision detection and correction method is presented for the interference-free of these two steps. We implement our methods and present machining results. All of these two kinds of cutter paths are computed rapidly and automatically.