Showing papers on "Direct stiffness method published in 2017"

Flexible Medical Devices: Review of Controllable Stiffness Solutions

Abstract: In the medical field and in soft robotics, flexible devices are required for safe human interaction, while rigid structures are required to withstand the force application and accuracy in motion. This paper aims at presenting controllable stiffness mechanisms described in the literature for applications with or without shape-locking performances. A classification of the solutions based on their working principle is proposed. The intrinsic properties of these adaptive structures can be modified to change their mechanical characteristics from a geometrical point of view or equivalent elastic properties (with internal mechanisms or with a change in material properties). These solutions are compared quantitatively, based on selected criteria linked to the medical field as the stiffness range, the activation time and the working conditions. Depending on the application and its requirements, the most suitable solution can be selected following the quantitative comparisons. Several applications of these tunable stiffness structures are proposed and illustrated by examples of the literature.

Higher-order multi-resolution topology optimization using the finite cell method

Abstract: This article presents a detailed study on the potential and limitations of performing higher-order multi-resolution topology optimization with the finite cell method. To circumvent stiffness overestimation in high-contrast topologies, a length-scale is applied on the solution using filter methods. The relations between stiffness overestimation, the analysis system, and the applied length-scale are examined, while a high-resolution topology is maintained. The computational cost associated with nested topology optimization is reduced significantly compared with the use of first-order finite elements. This reduction is caused by exploiting the decoupling of density and analysis mesh, and by condensing the higher-order modes out of the stiffness matrix.

Stiffness Modeling of Parallel Mechanisms at Limb and Joint/Link Levels

Abstract: Drawing on screw theory and the virtual joint method, this paper presents a general and hierarchical approach for semianalytical stiffness modeling of parallel mechanisms. The stiffness model is built by two essential steps: 1) formulating the map between the stiffness matrices of platform and limbs using the duality of wrench and twist of the platform; and 2) formulating the map between stiffness matrices of a limb and a number of elastic elements in that limb using the duality of the wrench attributed to the limb and the twist of the endlink of that limb. By merging these two threads, the Cartesian stiffness matrix can be explicitly expressed in terms of the compliance matrices of joints and links. The proposed approach bridges the gap between two currently available approaches and is thereby very useful for evaluating stiffness over the entire workspace and investigating the influences of joint/link compliances on those of the platform in a quick and precise manner. A stiffness analysis for a 3- P RS parallel mechanism is presented as an example to illustrate the effectiveness of the proposed approach.

Prediction of sound transmission through, and radiation from, panels using a wave and finite element method

Abstract: This paper describes the extension of a wave and finite element (WFE) method to the prediction of noise transmission through, and radiation from, infinite panels. The WFE method starts with a conventional finite element model of a small segment of the panel. For a given frequency, the mass and stiffness matrices of the segment are used to form the structural dynamic stiffness matrix. The acoustic responses of the fluids surrounding the structure are modelled analytically. The dynamic stiffness matrix of the segment is post-processed using periodic structure theory, and coupled with those of the fluids. The total dynamic stiffness matrix is used to obtain the response of the medium to an incident acoustic pressure. Excitation of the structure by oblique plane waves and a diffuse sound field are considered. The response to structural excitation and the consequent radiation are determined. Since the size of the WFE model is small, computational times are small. Various example applications are presented to illustrate the approach, including a thin isotropic panel, an antisymmetric, cross-ply sandwich panel and a symmetric panel with an orthotropic core.

Stiffness modeling, analysis and evaluation of a 5 degree of freedom hybrid manipulator for friction stir welding:

Abstract: In order to meet the requirements of large downward force and high stiffness performance for the friction stir welding process, this paper proposes a 5 degree-of-freedom hybrid manipulator as friction stir welding robot. It is composed of a 3 degree-of-freedom redundant parallel module and a 2 degree-of-freedom rotating head. Semi-analytical stiffness model of the hybrid manipulator is firstly established by compliance models of the two substructures. Virtual work principle, deformation superposition principle and twist/wrench mapping model are applied to this compliance modeling process. A novel instantaneous stiffness performance index is then proposed on the basis of instantaneous energy defined by reciprocal product of external payload screw and corresponding deformation screw. It solves the problems of inconsistent physical unit of linear/angular stiffness and is able to evaluate overall and worst-case stiffness performance. Next, stiffness/compliance experiments are carried out to verify the stiffne...

Criterion for the Design of Low-Power Variable Stiffness Mechanisms

Abstract: Designing robotic systems capable of low-power operation, inherent to their compliant actuation, has been elusive in practical application. In this paper, we propose a physical measure to mathematically define mechanical designs that are suitable to realize stiffness modulation with low power cost. Using this measure, we present a mathematical formulation of an ideal variable stiffness mechanism unaffected by the external load during its operation. We then analyze several existing mechanisms from the literature to relate design features with analytical conditions inherent to low power stiffness modulation in practical designs. Through this analysis, we identify an approximate practical realization of an ideal actuator capable of stiffness modulation with inherently low power cost. Similar to a number of existing efficient variable stiffness mechanisms, this mechanism is able to hold a given stiffness setting with zero input force under no external load. However, unlike many other previously designed mechanisms, it enables infinite range stiffness modulation using finite control forces. A practical variable stiffness mechanism that is capable of infinite range stiffness modulation using finite control forces leads to lower power cost and reduced energy consumption.

Calculation model of the lateral stiffness of high‐rise diagrid tube structures based on the modular method

Abstract: Summary High-rise diagrid tube structures are widely used in high-rise buildings because of their strong lateral stiffness and flexible arrangement of plane layout. The lateral stiffness of rectangular diagrid tube structures is studied by many researchers, but it seldom attracts attention for arbitrary polygonal ones. Therefore, it is necessary to propose a calculation model for lateral stiffness of arbitrary polygonal diagrid tube structures. First, the basic concept of modular method is defined. Assuming that diagonals are only subjected to axial force, a calculation model of lateral stiffness of arbitrary polygonal diagrid tube structures is presented. The lateral displacements of structures are calculated by modular method. Then the accuracy of modular method and calculation model of lateral stiffness are verified by finite element method. Intersection law of structural lateral displacement curves is achieved. Taking the top displacement of structures as the reference index and based on the principle of equivalent material, diagonal angle optimization method is proposed, whose rationality is validated by finite element method. Based on the design method of top displacement control, preliminary design method of cross section of diagonals is suggested. Results in this paper are expected to provide a theoretical reference for preliminary engineering design.

Transverse shear stiffness of corrugated core steel sandwich panels with dual weld lines

Abstract: Advances in the field of laser welding have recently enabled the commercial production of all-steel sandwich panels with a continuous and robust connection between the core and the face plates, even for plate thicknesses over 10 mm. This allows for the application of the high-performance steel sandwich panel in several fields, such as civil structures. In this paper, an analytical model is presented to determine the transverse shear stiffness of corrugated core steel sandwich panels with dual weld lines. The derivation is based on the direct stiffness method (DSM). At the welded connection between the core and the faces, a rotational spring is included in the structural model, as the idealised rigid connection is unable to properly capture the mechanical interaction between the constituent plates. Both bending and shear deformation in the cross-sectional constituent members is taken into account. The model is shown to have high precision in terms of predicting the transverse shear stiffness when compared with numerical analyses. Furthermore, high precision is also shown when it comes to predicting normal stresses in the constituent members of the panel with respect to shear action. In a case study included in this paper, the impact of having two weld lines compared with a single weld was studied, together with the effect of the distance between the welds. The results show a large impact with respect to shear stiffness and stresses in the constituent plates. This paper focuses on laser-welded corrugated core steel sandwich panels, but the presented model can also be used for analyses of general continuous core shapes and other isotropic materials.

Study on Finite Element Model Updating in Highway Bridge Static Loading Test Using Spatially-Distributed Optical Fiber Sensors.

Abstract: A finite model updating method that combines dynamic-static long-gauge strain responses is proposed for highway bridge static loading tests. For this method, the objective function consisting of static long-gauge stains and the first order modal macro-strain parameter (frequency) is established, wherein the local bending stiffness, density and boundary conditions of the structures are selected as the design variables. The relationship between the macro-strain and local element stiffness was studied first. It is revealed that the macro-strain is inversely proportional to the local stiffness covered by the long-gauge strain sensor. This corresponding relation is important for the modification of the local stiffness based on the macro-strain. The local and global parameters can be simultaneously updated. Then, a series of numerical simulation and experiments were conducted to verify the effectiveness of the proposed method. The results show that the static deformation, macro-strain and macro-strain modal can be predicted well by using the proposed updating model.

Identification of contact stiffness of shrink-fit tool-holder joint based on fractal theory

Abstract: Shrink-fit holders are used widely in high-speed machining because of their high concentric and rigid clamp performance. In this paper, a method to determine the contact stiffness and contact force of shrink-fit tool-holder joint is proposed. The relationship between the contact stiffness and the normal contact load is deduced based on Hertz contact theory and fractal geometry theory. The contact force at the interface is obtained through finite element contact analysis. The identified contact stiffness is incorporated into the finite element model of the tool-holder assembly, which is used to predict the tool point frequency response function of the assembly. The predicted frequency response function agrees with that of the experiment, which verifies the proposed identification method. The effect of radial interference, tool insertion length, and rotational speed on contact behaviors between the shrink-fit tool-holder connection is determined, which is informative for the application and design of the tool-holder assembly.

A novel variable stiffness mechanism for dielectric elastomer actuators

Abstract: In this paper, a novel variable stiffness mechanism is proposed for the design of a variable stiffness dielectric elastomer actuator (VSDEA) which combines a flexible strip with a DEA in a dielectric elastomer minimum energy structure The DEA induces an analog tuning of the transverse curvature of the strip, thus conveniently providing a voltage-controllable flexural rigidity The VSDEA tends to be a fully flexible and compact structure with the advantages of simplicity and fast response Both experimental and theoretical investigations are carried out to reveal the variable stiffness performances of the VSDEA The effect of the clamped location on the bending stiffness of the VSDEA is analyzed, and then effects of the lengths, the loading points and the applied voltages on the bending stiffness are experimentally investigated An analytical model is developed to verify the availability of this variable stiffness mechanism, and the theoretical results demonstrate that the bending stiffness of the VSDEA decreases as the applied voltage increases, which agree well with the experimental data Moreover, the experimental results show that the maximum change of the relative stiffness can reach about 8880% It can be useful for the design and optimization of active variable stiffness structures and DEAs for soft robots, vibration control, and morphing applications

Diffraction of SH-waves by topographic features in a layered transversely isotropic half-space

Abstract: The scattering of plane SH-waves by topographic features in a layered transversely isotropic (TI) half-space is investigated by using an indirect boundary element method (IBEM). Firstly, the anti-plane dynamic stiffness matrix of the layered TI half-space is established and the free fields are solved by using the direct stiffness method. Then, Green’s functions are derived for uniformly distributed loads acting on an inclined line in a layered TI half-space and the scattered fields are constructed with the deduced Green’s functions. Finally, the free fields are added to the scattered ones to obtain the global dynamic responses. The method is verified by comparing results with the published isotropic ones. Both the steady-state and transient dynamic responses are evaluated and discussed. Numerical results in the frequency domain show that surface motions for the TI media can be significantly different from those for the isotropic case, which are strongly dependent on the anisotropy property, incident angle and incident frequency. Results in the time domain show that the material anisotropy has important effects on the maximum duration and maximum amplitudes of the time histories.

Vector form Intrinsic Finite Element Based Approach to Simulate Crack Propagation

Abstract: This paper presents a new approach to simulate the propagation of elastic and cohesive cracks under mode-I loading based on the vector form intrinsic finite element method. The proposed approach can handle crack propagation without requiring global stiffness matrices and extra weak stiffness elements. The structure is simulated by mass particles whose motions are governed by the Newton's second law. Elastic and cohesive crack propagation are simulated by proposed VFIFE-J-integral and VFIFE-FCM methods, respectively. The VFIFE-J-integral method is based on vector form intrinsic finite element (VFIFE) and J-integral methods to calculate the stress intensity factors at the crack tips, and the VFIFE-FCM method combines VFIFE and fictitious crack models (FCM). When the stress state at the crack tip meets the fracture criterion, the mass particle at the crack tip is separated into two particles. The crack then extends in the plate until the plate splits into two parts. The proposed VFIFE-J-integral method was validated by elastic crack simulation of a notched plate, and the VFIFE-FCM method by cohesive crack propagation of a three point bending beam. As assembly of the global stiffness matrix is avoided and each mass particle motion is calculated independently, the proposed method is easy and efficient. Numerical comparisons demonstrate that the present results predicted by the VFIFE method are in agreement with previous analytical, numerical and experimental works.