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External Load (external + load)
Selected AbstractsOptimization-based dynamic human walking prediction: One step formulationINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 6 2009Yujiang Xiang Abstract A new methodology is introduced in this work to simulate normal walking using a spatial digital human model. The proposed methodology is based on an optimization formulation that minimizes the dynamic effort of people during walking while considering associated physical and kinematical constraints. Normal walking is formulated as a symmetric and cyclic motion. Recursive Lagrangian dynamics with analytical gradients for all the constraints and objective function are incorporated in the optimization process. Dynamic balance of the model is enforced by direct use of the equations of motion. In addition, the ground reaction forces are calculated using a new algorithm that enforces overall equilibrium of the human skeletal model. External loads on the human body, such as backpacks, are also included in the formulation. Simulation results with the present methodology show good correlation with the experimental data obtained from human subjects and the existing literature. Copyright © 2009 John Wiley & Sons, Ltd. [source] Failure Mechanism of Deformed Concrete Tunnels Subject to Diagonally Concentrated LoadsCOMPUTER-AIDED CIVIL AND INFRASTRUCTURE ENGINEERING, Issue 6 2009Wei He Based on the experimental findings, an extended discussion is carried out to select a rational compressive model for concrete that represents the dominant failure modes of deformed concrete tunnels. Three main dominant final failure modes are described: structural failure due to the plastic rotation of softening hinges, tensile failure caused by localized cracks, and material failure due to concrete deterioration. A parametric analysis of the material properties of concrete shows that the compressive strength of concrete has a dominant effect on the load-carrying capacity, although the compressive fracture energy of concrete remarkably influences the post-peak deformation behavior of the tunnel. Moreover, the soil pressure, which is regarded as a distributed external load, plays an important role in controlling the final failure modes and the deformation behavior of concrete tunnels. The size effect on the load-carrying capacities of different-sized concrete tunnels is also discussed based on the numerical simulations. [source] The effect of strength training on the force of twitches evoked by corticospinal stimulation in humansACTA PHYSIOLOGICA, Issue 2 2009T. J. Carroll Abstract Aim:, Although there is considerable evidence that strength training causes adaptations in the central nervous system, many details remain unclear. Here we studied neuromuscular responses to strength training of the wrist by recording electromyographic and twitch responses to transcranial magnetic stimulation (TMS) and cervicomedullary stimulation of the corticospinal tract. Methods:, Seventeen participants performed 4 weeks (12 sessions) of strength training for the radial deviator (RD) muscles of the wrist (n = 8) or control training without external load (n = 9). TMS recruitment curves were constructed from stimuli at five to eight intensities ranging between 15% below resting motor threshold and maximal stimulator output, both at rest and during isometric wrist extension (EXT) and RD at 10% and 50% of maximal voluntary contraction (MVC). Responses to weak TMS and cervicomedullary stimulation (set to produce a response of 10% maximal M wave amplitude during 10% MVC EXT contraction) were also compared at contraction strengths ranging from 10% to 75% MVC. Results:, Isometric strength increased following strength training (10.7% for the RD MVC, 8.8% for the EXT MVC), but not control training. Strength training also significantly increased the amplitude of TMS- and cervicomedullary-evoked twitches during low-force contractions. Increases in the force-generating capacity of the wrist extensor muscles are unlikely to account for this finding because training did not affect the amplitude of twitches elicited by supra-maximal nerve stimulation. Conclusion:, The data suggest that strength training induces adaptations that increase the net gain of corticospinal-motor neuronal projections to the trained muscles. [source] Derivation of Recovery Kinetics From Stress Relaxation Tests,ADVANCED ENGINEERING MATERIALS, Issue 3 2010Sheila Bhaumik The recovery behavior of a commercial aluminum alloy 3103 was investigated by the means of two alternative experimental methods: stress relaxation (SR) and double tension tests (DT). In case of SR, the stress,time evolution after deformation was recorded, and for DT the yield stress after several recovery times were measured. The DT tests were further sub-divided into tests with and without external load during recovery. The results revealed that the recovery kinetics is clearly accelerated by the external stress during the SR. However, the difference between the DT and SR stresses is much larger. It is caused by continued dislocation glide after the deformation, which causes continued plastic elongation of the specimens. This is demonstrated quantitatively by appropriate evaluation models for both experiments. In contrast to DT, the SR evaluation accounts for the elastic SR due to plastic elongation, but the recovery parameters are the same ones as for DT. This makes it possible to replace DT by SR experiments, which are materially less laborious. [source] Stress-Dependent Elastic Properties of Porous Microcracked Ceramics,ADVANCED ENGINEERING MATERIALS, Issue 12 2009Irina Pozdnyakova Abstract Although ceramics are considered linear elastic materials, we have observed a non-linear pseudo-elastic behavior in porous cellular microcracked ceramics such as , -eucryptite. This is attributed to the evolution of microstructure in these materials. This behavior is particularly different from that of non-microcracked ceramics such as silicon carbide. It is shown that in microcracked materials two processes, namely stiffening and softening, always compete when a compressive external load is applied. The first regime is attributed to microcrack closure, and the second to microcracks opening, i.e. to a damage introduced by the applied stress. On the other hand rather a continuous damage is observed in the non-microcracked case. A comparison has been done between the microscopic (as measured by neutron diffraction) and the macroscopic stress-strain response. Also, it has been found that at constant load a significant strain relaxation occurs, which has two timescales, possibly driven by the two phenomena quoted above. Indeed, no such relaxation is observed for non-microcracked SiC. Implications of these findings are discussed. [source] Mechanically Powered Transparent Flexible Charge-Generating Nanodevices with Piezoelectric ZnO NanorodsADVANCED MATERIALS, Issue 21 2009Min-Yeol Choi Transparent flexible charge-generating piezoelectric nanodevices are developed. The resulting integrated nanodevice generates a noticeable current when it is pushed by application of an external load. Piezoelectric ZnO nanorod-based nanodevices with embossed PdAu top electrodes produce the highest output current density of approximately 10 ,A cm,2 at a load of 0.9 kgf. [source] A mixed integer programming for robust truss topology optimization with stress constraintsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 13 2010Yoshihiro Kanno Abstract This paper presents a mixed integer programming (MIP) formulation for robust topology optimization of trusses subjected to the stress constraints under the uncertain load. A design-dependent uncertainty model of the external load is proposed for dealing with the variation of truss topology in the course of optimization. For a truss with the discrete member cross-sectional areas, it is shown that the robust topology optimization problem can be reduced to an MIP problem, which is solved globally. Numerical examples illustrate that the robust optimal topology of a truss depends on the magnitude of uncertainty. Copyright © 2010 John Wiley & Sons, Ltd. [source] If bone is the answer, then what is the question?JOURNAL OF ANATOMY, Issue 2 2000R. HUISKES In the 19th century, several scientists attempted to relate bone trabecular morphology to its mechanical, load-bearing function. It was suggested that bone architecture was an answer to requirements of optimal stress transfer, pairing maximal strength to minimal weight, according to particular mathematical design rules. Using contemporary methods of analysis, stress transfer in bones was studied and compared with anatomical specimens, from which it was hypothesised that trabecular architecture is associated with stress trajectories. Others focused on the biological processes by which trabecular architectures are formed and on the question of how bone could maintain the relationship between external load and architecture in a variable functional environment. Wilhelm Roux introduced the principle of functional adaptation as a self-organising process based in the tissues. Julius Wolff, anatomist and orthopaedic surgeon, entwined these 3 issues in his book The Law of Bone Remodeling (translation), which set the stage for biomechanical research goals in our day. ,Wolff's Law' is a question rather than a law, asking for the requirements of structural optimisation. In this article, based on finite element analysis (FEA) results of stress transfer in bones, it is argued that it was the wrong question, putting us on the wrong foot. The maximal strength/minimal weight principle does not provide a rationale for architectural formation or adaptation; the similarity between trabecular orientation and stress trajectories is circumstantial, not causal. Based on computer simulations of bone remodelling as a regulatory process, governed by mechanical usage and orchestrated by osteocyte mechanosensitivity, it is shown that Roux's paradigm, conversely, is a realistic proposition. Put in a quantitative regulatory context, it can predict both trabecular formation and adaptation. Hence, trabecular architecture is not an answer to Wolff's question, in the sense of this article's title. There are no mathematical optimisation rules for bone architecture; there is just a biological regulatory process, producing a structure adapted to mechanical demands by the nature of its characteristics, adequate for evolutionary endurance. It is predicted that computer simulation of this process can help us to unravel its secrets. [source] On singular behaviors of impedance-based repeatable control for redundant robotsJOURNAL OF FIELD ROBOTICS (FORMERLY JOURNAL OF ROBOTIC SYSTEMS), Issue 4 2001Chau-Chang Wang This article addresses the association between the unstiffening phenomena in structural mechanics and the algorithmic singularities encountered in the impedance-based repeatable control algorithms used to command redundant manipulators. It is well known that velocity control schemes such as the pseudoinverse control schemes do not guarantee repeatability for redundant manipulators. In other words, for a closed end-effector trajectory, the joints do not, in general, exhibit a closed trajectory. One way to overcome this problem is to model each joint with compliance and incorporate a second-order correction term for the pseudoinverse. With this model, the joint configuration adopted by the manipulator at a given point in task space is one which minimizes the artificial potential energy of the system and is locally unique. In terms of statics, this is equivalent to saying that the elastic structure reaches its static equilibrium under external load. Keep this analogy in mind. We know that the impedance control commands the manipulator to mimic the behavior of an elastic articulated chain. For any phenomena observable on a real elastic structure, we should be able to find its counterpart embedded in the impedance control. In this article, we analyze the performance of such repeatable control algorithms from the point of view of structure mechanics. Singularities in the algorithm are examined and their significance in mechanics are also discussed. © 2001 John Wiley & Sons, Inc. [source] In vivo assessment of regenerate axial stiffness in distraction osteogenesisJOURNAL OF ORTHOPAEDIC RESEARCH, Issue 2 2005Gudrun Trøite Aarnes Abstract This paper presents an in vivo test for assessment of regenerate axial stiffness after the distraction phase of lengthening therapy. The test result supplements radiography in evaluating bone healing and assists in determining when the regenerate stiffness is sufficient for removal of the external fixator. The test is non-invasive and does not require fixator removal. The theoretical basis for the method is that an externally applied load is shared between the fixator and the regenerating bone. The amount of load carried by the regenerate depends on its axial stiffness, which increases with advanced mineralization. By measuring the force in the fixator while applying a known external load to the limb, the load-share ratio between fixator and limb can be assessed. A load-share ratio of 100% indicates that the entire load is carried by the fixator. The ratio decreases as the regenerate structure gradually stiffens. In a clinical trial of 22 individuals with tibial lengthening, the fixator was removed when the load-share ratio dropped below 10%. None of the patients experienced fracture after removal of the fixator. © 2004 Orthopaedic Research Society. Published by Elsevier Ltd. All rights reserved. [source] Measurement of Viscosity of Densifying Glass-Based Systems by Isothermal Cyclic Loading DilatometryJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 2 2004Aravind Mohanram This study describes the isothermal cyclic loading dilatometry (ICLD) technique to measure the viscosity of glass-based materials. We demonstrate its merit relative to constant-load techniques in minimizing the stress history effects (changes in shrinkage anisotropy and sample microstructure) that arise due to the application of an external load. A constant-load test overestimates the viscosity by an order of magnitude compared with a cyclic load test. To obtain accurate viscosity data, maximum loading rates and longer unloading periods are desirable as they reduce effects of shrinkage anisotropy on viscosity values. Representative data for a low-temperature cofired ceramic (LTCC) material are reported. Nonparametric statistical tests revealed insignificant differences between the viscosity data sets at 5% significance level and thus indicate good reproducibility of the testing methodology. [source] Compatibilized poly(ether imide)/LCP blends: drawing ability and mechanical properties of the ribbonsPOLYMERS FOR ADVANCED TECHNOLOGIES, Issue 7 2002F. J. Vallejo Abstract The effects of the addition of polyarylate (PAr) on the drawing ability of poly(ether imide) (PEI)/Rodrun liquid-crystalline 5000 (Ro LC5000) (Ro) blends and on the mechanical properties of their ribbons were studied. The compatibilizing effect of the PAr led to an increase in the drawing ability of the blends, as seen by the fact that the maximum Ro content compatible with the drawing process increased from 15 to 30%. This may lead to new applications in the field of gas barrier materials. The presence of PAr also led to an increase in the adhesion between the two phases of the blends and consequent improved ductility. However, the very high modulus of elasticity and tensile strength in the direction of orientation (up to three-fold those of the matrix) due to the less developed fibrillation were smaller than the corresponding values before compatibilization. These very large modulus of elasticity and tensile strength values and those perpendicular to the direction of orientation, which were similar to those of the matrix, led to a high anisotropy in the extruded ribbons that increased with the draw ratio (DR) and the Ro contents. The high mechanical properties of the ribbons may be used and the anisotropy diminished by layering the ribbons in, and perpendicular to, the direction of orientation, according to the expected external load. Copyright © 2002 John Wiley & Sons, Ltd. [source] Improved design of sliding mode control for civil structures with saturation problemEARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 11 2004Sang-Hyun Lee Abstract A systematic and improved design procedure for sliding mode control (SMC) of seismically excited civil structures with saturation problem is provided in this paper. In order to restrict the control force to a certain level, a procedure for determining the upper limits of the control forces for single or multiple control units is proposed based on the design response spectrum of external loads. Further, an efficient procedure using the LQR method for determining sliding surfaces appropriate for different controller types is provided through the parametric evaluation of the dynamic characteristics of sliding surfaces in terms of SMC controller performance. Finally, a systematic design procedure for SMC required to achieve a given performance level is provided and its effectiveness is verified by applying it to multi-degree-of-freedom (MDOF) systems. Copyright © 2004 John Wiley & Sons, Ltd. [source] The perturbation method and the extended finite element method.FATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 8 2006An application to fracture mechanics problems ABSTRACT The extended finite element method has been successful in the numerical simulation of fracture mechanics problems. With this methodology, different to the conventional finite element method, discretization of the domain with a mesh adapted to the geometry of the discontinuity is not required. On the other hand, in traditional fracture mechanics all variables have been considered to be deterministic (uniquely defined by a given numerical value). However, the uncertainty associated with these variables (external loads, geometry and material properties, among others) it is well known. This paper presents a novel application of the perturbation method along with the extended finite element method to treat these uncertainties. The methodology has been implemented in a commercial software and results are compared with those obtained by means of a Monte Carlo simulation. [source] A rational elasto-plastic spatially curved thin-walled beam elementINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 3 2007Yong-Lin Pi Abstract Torsion is one of the primary actions in members curved in space, and so an accurate spatially curved-beam element needs to be able to predict the elasto-plastic torsional behaviour of such members correctly. However, there are two major difficulties in most existing finite thin-walled beam elements, such as in ABAQUS and ANSYS, which may lead to incorrect predictions of the elasto-plastic behaviour of members curved in space. Firstly, the integration sample point scheme cannot capture the shear strain and stress information resulting from uniform torsion. Secondly, the higher-order twists are ignored which leads to loss of the significant effects of Wagner moments on the large twist torsional behaviour. In addition, the initial geometric imperfections and residual stresses are significant for the elasto-plastic behaviour of members curved in space. Many existing finite thin-walled beam element models do not provide facilities to deal with initial geometric imperfections. Although ABAQUS and ANSYS have facilities for the input of residual stresses as initial stresses, they cannot describe the complicated distribution patterns of residual stresses in thin-walled members. Furthermore, external loads and elastic restraints may be applied remote from shear centres or centroids. The effects of the load (and restraint) positions are important, but are not considered in many beam elements. This paper presents an elasto-plastic spatially curved element with arbitrary thin-walled cross-sections that can correctly capture the uniform shear strain and stress information for integration, and includes initial geometric imperfections, residual stresses and the effects of the load and restraint positions. The element also includes elastic restraints and supports, which have to be modelled separately as spring elements in some other finite thin-walled beam elements. Comparisons with existing experimental and analytical results show that the elasto-plastic spatially curved-beam element is accurate and efficient. Copyright © 2006 John Wiley & Sons, Ltd. [source] The spatial conservative congruence transformation for manipulator stiffness modeling with coordinate and noncoordinate basesJOURNAL OF FIELD ROBOTICS (FORMERLY JOURNAL OF ROBOTIC SYSTEMS), Issue 1 2005Shih-Feng Chen This paper presents systematic methods to approach the spatial conservative congruence transformation (SCCT) and applies it to the stiffness transformation and matrices of robotic systems via the strategy of changing basis. Through geometrical analysis, the realization of the Cartesian stiffness of manipulators is shown to be basis dependent. We illustrate that the SCCT is conservative as well as directly represents the spatial stiffness mapping relationship for robotic stiffness control. Examples of the serial manipulators are given to help one to better understand the source of the asymmetric 6×6 Cartesian stiffness matrix in the presence of external loads using the SCCT algorithm. The SCCT suggests an explicit and generalized stiffness transformation relationship for robotic systems. © 2005 Wiley Periodicals, Inc. [source] Influence of muscular activity on local mineralization patterns in metatarsals of the embryonic mouseJOURNAL OF ORTHOPAEDIC RESEARCH, Issue 4 2000E. Tanck This study addressed the theory that local mechanical loading may influence the development of embryonic long bones. Embryonic mouse metatarsal rudiments were cultured as whole organs, and the geometry of the primary ossification center was compared with that of rudiments that had developed in utero. The mineralization front in vivo was found to be nearly straight, whereas in vitro it acquired a more convex shape due to a slower mineralization rate at the periphery of the mineralized cylinder. A poroelastic finite element analysis was performed to calculate the local distributions of distortional strain and fluid pressure at the mineralization front in the metatarsal during loading in vivo as a result of muscle contractions in the embryonic hindlimbs. The distribution of fluid pressure from the finite element analysis could not explain the difference in mineralization shape. The most likely candidate for the difference was the distortional strain, resulting from muscle contraction, which is absent in vitro, because its value at the periphery was significantly higher than in the center of the tissue. Without external loads, the mineralization process may be considered as pre-programmed, starting at the center of the tissue and resulting in a spherical mineralization front. Strain modulates the rate of the mineralization process in vivo, resulting in the straight mineralization front. These results confirm that disturbances in muscle development are likely to produce disturbed mineralization patterns, resulting in a disordered osteogenic process. [source] Oesophageal morphometry and residual strain in a mouse model of osteogenesis imperfectaNEUROGASTROENTEROLOGY & MOTILITY, Issue 5 2001H. Gregersen Recently, it was demonstrated in the oesophagus that the zero-stress state is not a closed cylinder but an open circular cylindrical sector. The closed cylinder with no external loads applied is called the no-load state and residual strain is the difference in strain between the no-load state and the zero-stress state. To understand the physiology and pathology of the oesophagus, it is necessary to know the zero-stress state and the stress,strain relationships of the tissues in the oesophagus, and the changes of these states and relationships due to biological remodelling of the tissues under stress. The aim of this study was to investigate the morphological and biomechanical remodelling at the no-load and zero-stress states in mutant osteogenesis imperfecta murine (oim) mice with collagen deficiency. The oesophagi of seven oim and seven normal wild-type mice were excised, cleaned, and sectioned into rings in an organ bath containing calcium-free Krebs solution with dextran and EGTA. The rings were photographed in the no-load state and cut radially to obtain the zero-stress state. Equilibrium was awaited for 30 min and the specimens were photographed again. Circumferences, submucosa and muscle layer thicknesses and areas, and the opening angle were measured from the digitized images. The oesophagi in oim mice had smaller layer thicknesses and areas compared to the wild types. The largest reduction in layer thickness in oim mice was found in the submucosa (approximately 36%). Oim mice had significantly larger opening angles (120.2 ± 4.5°) than wild-type mice (93.0 ± 11.2°). The residual strain was compressive at the mucosal surface and tensile at the serosal surface in both oim and wild types. In the oim mice, the residual strains at the serosal and mucosal surfaces and the mucosa-submucosal,muscle layer interface were higher than in the wild types (P < 0.05). The gradient of residual strain per unit thickness was higher in oim mice than in wild-type mice, and was highest in submucosa (P < 0.05). The only morphometric measure that was similar in oim and wild-type mice was the inner circumference in the no-load state. In conclusion, our data show significant differences in the residual strain distribution and morphometry between oim mice and wild-type mice. The data suggest that the residual stress in oesophagus is caused by the tension in the muscle layer rather than the stiffness of the submucosa in compression and that the remodelling process in the oim oesophagus is due mainly to morphometric and biomechanical alterations in the submucosa. [source] A Holistic Simulation Approach from a Measured Load to Element Stress Using Combined Multi-body Simulation and Finite Element ModellingPROCEEDINGS IN APPLIED MATHEMATICS & MECHANICS, Issue 1 2009Matthias Harter The design of vehicle bodies requires the knowledge of the vehicle's structural response to external loads and disturbances. In rigid multi-body simulation the dynamic behaviour of complex systems is calculated with rigid bodies and neglect of body elasticity. On the other hand, in finite element models large degree of freedom numbers are used to represent the elastic properties of a single body. Both simulation methods can be combined, if the finite element model size is reduced to a degree of freedom number feasible to multi-body simulation. The application to practical purposes requires the use and interconnection of several different software tools. In this contribution a holistic method is presented, which starts with the measurement or synthesis of loads and excitations, continues with the integration of a reduced finite element model into a multi-body system, the dynamic response calculation of this combined model, and concludes with the result expansion to the full finite element model for calculating strain and stress values at any point of the finite element mesh. The applied software tools are Simpack, Nastran, and Matlab. An example is given with a railway vehicle simulated on measured track geometry. (© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||||||||