Different Values Of Stiffness Research Articles

Spatial-Time Inhomogeneity Due to the Portevin-Le Chatelier Effect Depending on Stiffness

This work is devoted to the study of the influence of the rigidity of the loading system on the kinetics of the initiation and propagation of the Portevin-Le Chatelier (PLC) strain bands due to the jerky flow in Al-Mg alloy. To estimate the influence of the loading system, the original loading attachment, which allows reducing the stiffness in a given range, was used. Registration of displacement and strain fields on the specimen surface was carried out with the Vic-3D non-contacting deformation measurement system based on the Digital Image Correlation (DIC) technique. The mechanical uniaxial tension tests were carried out using samples of Al-Mg alloy at the biaxial servo-hydraulic testing system Instron 8850. As a result of tensile tests, deformation diagrams were obtained for Al-Mg alloy samples tested at different values of stiffness of the loading system: 120 MN/m (nominal value), 50 MN/m, 18 MN/m, and 5 MN/m. All diagrams show discontinuous plastic deformations (the Portevin-Le Chatelier effect). It is noted that a decrease in the rigidity of the loading system leads to a change in the type of jerky flow. At constant parameters of the loading rate, temperature, and chemical composition of the material, the PLC effects of types A, B, and C are recorded in tests.

Acoustic Response of a Vibrating Elongated Cylinder in a Hydrodynamic Turbulent Flow

The present paper contains the results of the numerical analysis of the interaction between a Newtonian incompressible turbulent flow and a linear elastic slender body, together with the influence of the fluid–structure interaction (FSI) on the noise generation and propagation. The purpose is to evaluate the differences in term of acoustic pressure between the case where the solid body is rigid (infinite stiffness) and the case where it is elastic (finite stiffness). A partitioned and implicit algorithm with the arbitrary Lagrangian–Eulerian method (ALE) is used for the interaction between the fluid and solid. For the evaluation of the turbulent fluid motion, we use a large eddy simulation (LES) with the Smagorinsky subgrid scale model. The equation for the solid is solved through the Lagrangian description of the momentum equation and the second Piola–Kirchoff stress tensor. In addition, the acoustic analogy of Lighthill is used to characterize the acoustic source (the slender body) by directly using the fluid dynamic fields. In particular, we use the Ffowcs Williams and Hawkings (FW-H) equation for the evaluation of the acoustic pressure in the fluid medium. As a first numerical experiment, we analyze a square cylinder immersed in a turbulent flow characterized by two different values of stiffness: one infinite (rigid case) and one finite (elastic case). In the latter case, the body stiffness and mean flow velocity are such that they induce the lock-in phenomenon. Finally, we evaluate the differences in terms of acoustic pressure between the two different cases.

Improved algorithm for a system with different stiffness for positive and negative displacements

In the process of investigation of dynamics of elements of robots, systems with different values of stiffness for positive and negative displacements play an important role. This change of the value of stiffness has an effect to the results of numerical calculation of dynamics of the system. For more precise investigation of dynamics a special numerical procedure is proposed in the paper. Numerical results for two values of time steps: large and medium ones, are presented for two cases: without application of the proposed procedure and with application of it. Comparison of the presented results indicates advantages of the proposed numerical procedure.

Simulation of a Passive Knee Exoskeleton for Vertical Jump Using Optimal Control.

Research on exoskeletons designed to augment human activities and the attendant exoskeleton industry are both rapidly growing areas of endeavor. However, progress in the field is currently being hindered by a lack of understanding of human-exoskeleton interactions. At present, the main method applied to reach such an understanding is to build and test prototypes or end-effectors (that simulate the devices), but this is a very time-consuming and costly process. In this study, we aimed to address this problem by simulating passive exoskeleton-human interactions during a vertical jump. The simulation is based on theoretical and empirical models. Using the simulation, we performed a numerical optimization procedure to determine the muscle excitations and starting postures that would give the maximum jump height. The simulation used a planar 4-DOF dynamic model. The muscles at the joints were modeled as torque actuators, with a flexor and an extensor for each joint and passive torque representing the tendon and muscle properties. We then simulated jumps with a passive knee exoskeleton with five different values of stiffness with the aim to study their effect on the jump height. The optimal excitation for the maximum jump height was found by using a genetic algorithm (GA). To improve our optimization performance and to test the convergence of the GA, the GA optimization was performed several times. For each exoskeleton condition, the GA found the optimal jump more than 400 times, and out of these solutions the one that achieved the highest jump was chosen. The result revealed an increase in jump height as the spring became stiffer. In addition, it was found that the energy that was stored in the spring of the exoskeleton was not fully converted to jump height.

Study of the braking regime of rolling stock mining suspended monorail taking into account clearances in the coupling

Introduction. Currently, mine suspended monorails are becoming more common in coal mines for auxiliary cargo and people transportation. Compared to ground transport, monorail has significant differences, which does not allow to fully use the previously obtained results of braking modes studies. Research aims to establish regularities in the processes that occur during the mine suspended monorail road rolling stock braking mode, taking into account clearances in the couplings. Methodology. To substantiate the parameters and designs of brake and coupling devices of a mine suspended monorail, a mathematical model of rolling stock movement along the monorail has been developed. Results. Dependencies are established that describe the process of rolling stock braking on a monorail. Motion parameters for different values of stiffness, damping and clearances in the couplers are analyzed. It is established that the gaps in rolling stock coupling influence the value of undercarriage and suspension units longitudinal vibrations amplitudes. Conclusions. A mathematical model of rolling stock movement is developed with allowance for clearances in the coupling making it possible to simulate mine suspended monorail braking mode.

Optimal linear control design applied to the synchronism of vocal folds with asymmetric stiffness

The emission of sounds is an important means of communication between animals, which helps them to find a partner, establish their territory, and even alert other members of a population about a possible threat. In human beings, the emission of sounds allows an even more specialized communication, because it possibly render a wide exchange of ideas and knowledge. The most important process related to voice emission is the movement of vocal folds. Therefore, their proper functioning is essential for the emission of sounds. Vocal folds possess a muscular tissue, and are located inside the larynx. When air passes through them, they vibrate, thus emitting the sound by which we communicate. The vocal folds are elastic fibers that distend or relax under the action of the larynx muscles, thus modulating and modifying the sound as we speak or sing, for example. This complex process is modeled by a system of differential equations. In this paper, we present a study about asymmetric vocal folds, where a difference in their stiffness is considered. The objective of this study is to synchronize the movement of vocal folds caused by that asymmetry through the optimal linear control method, showing their functioning for different values of stiffness for each vocal fold. In this way, the control design has shown to be efficient, producing a good functioning of the vocal folds and rendering the emission of sound possible even in diseased vocal folds.

Concurrent topological design of composite structures and materials containing multiple phases of distinct Poisson’s ratios

ABSTRACTMost studies on composites assume that the constituent phases have different values of stiffness. Little attention has been paid to the effect of constituent phases having distinct Poisson’s ratios. This research focuses on a concurrent optimization method for simultaneously designing composite structures and materials with distinct Poisson’s ratios. The proposed method aims to minimize the mean compliance of the macrostructure with a given mass of base materials. In contrast to the traditional interpolation of the stiffness matrix through numerical results, an interpolation scheme of the Young’s modulus and Poisson’s ratio using different parameters is adopted. The numerical results demonstrate that the Poisson effect plays a key role in reducing the mean compliance of the final design. An important contribution of the present study is that the proposed concurrent optimization method can automatically distribute base materials with distinct Poisson’s ratios between the macrostructural and microstructural levels under a single constraint of the total mass.

Methodology for clamping load measurement of locking systems based on white light digitalization

This paper presents a new methodology to measure clamping force of locking clips of a weatherproof luminary from the previous determination of its stiffness and the elongation they suffer under assembly conditions. Application of the methodology includes determination of the clip stiffness both experimentally and by FEM analysis, and measurement of the elongation by structured white light digitalization techniques.Elongation values measured by white light are similar to those analyzed by conventional methods. The relationship between clips elongation and load applied is linear, so stiffness can be deduced from the slope of the curve. Results show that functionality of the clips can be achieved with different values of stiffness. There is not any direct relationship between clip stiffness and load performed by them because of the influence of initial length on the final load performed. Therefore, manufacturing tolerances must be taken into account to assure the proper performance of the clips.

Dynamics of beams on non-uniform nonlinear foundations subjected to moving loads

This paper presents a study on the dynamic response of beams on nonlinear elastic foundations, subjected to moving loads. The effects of the load’s intensity and velocity and of the foundation’s stiffness are investigated. Critical velocities are determined for beams on uniform foundations or foundations composed of two sub-domains with different values of stiffness, with or without viscous damping. In the case of non-uniform linear foundations, a very good agreement is obtained between the results of the model and available analytical solutions. The goal of this study is to generalize, for more realistic foundation behaviors, the analyses obtained by other authors.

Performance of soft dielectric laminated composites

This paper contains a thorough investigation of the performance of electrically activated layered soft dielectric composite actuators under plane deformation. Noting that the activation can be induced by controlling either the voltage or the surface charge, the overall behaviour of the system is obtained via homogenization at large strains, taking either the macroscopic electric field or the macroscopic electric displacement field as independent electrical variables. The performance of a two-phase composite actuator compared to that of the homogeneous case is highlighted for few boundary-value problems and for different values of stiffness and permittivity ratios between constituents being significant for applications, where the soft matrix is reinforced by a relatively small volume fraction of a stiff and high-permittivity phase. For charge-controlled devices, it is shown that some composite layouts admit, on one hand, the occurrence of pull-in/snap-through instabilities that can be exploited to design release-actuated systems, and on the other hand, the possibility of thickening at increasing surface charge density.

Reliability analysis of TSG19-type pantograph based on time-dependent parameters

The aim of the present paper is to study the damage mechanism of the TSG19-type pantographs. Firstly, a rigid-flexible coupled dynamics model is established to predict the pantograph–catenary contact forces with different values of stiffness and damping. A three-dimensional finite element model of a pantograph is then built up to investigate the time history of the stress in a critical section of the pantograph. Dynamics simulations provide the contact forces for the finite element analysis, which the authors needed to extract the stress amplitude and the stress cycles for different values of stiffness and damping. Finally, a reliability model based on the Miner cumulative damage theory is proposed for the TSG19-type pantograph in which the stiffness and damping characteristics degenerate with time. The proposal also takes into account the pantograph life under a given reliability. The results show that the stress amplitude and stress cycles increase when so does the stiffness characteristic and damping decreases. In such way, the stiffness and the damping of the pantograph sets the operating life and the reliability of the component. Therefore, the reliability model appears as a useful tool in the design process of the TSG19-type pantograph since its parameters change with time.

Handheld Microtactile Sensor for Elasticity Measurement

This paper presents a novel MEMS tactile sensor that can provide elasticity measurement of a variety of target objects. This sensor can be integrated on a handheld probe or on minimally invasive surgical tools for in-vivo tissue elasticity measurement. The tactile sensor consists of a pair of contact bumps that have different values of stiffness. Two capacitive force gauges are integrated underneath the bumps. The sensor readout is composed of two channels of capacitance values and the ratio of these capacitance values can be used to calculate the elasticity of target objects. To obtain reliable measurement of capacitance ratios, an estimation algorithm is developed by using a recursive least squares method with adaptive forgetting factors. Experimental characterization of the sensor shows that this tactile sensor provides reliable elasticity measurement of polymer specimens and quickly detects changes in elasticity.

Theoretical Analysis of Vibrations of a Mass Connected with a Support through a Chain of Elastic Elements

Vibrations of a mass are analyzed when the mass is connected with the support in succession of two elastic elements characterized by stiffness and damping coefficients. It is demonstrated that upon harmonic excitation of the mass and considering different values of stiffness and damping coefficients the system can respond differently in comparison to the case when the mass is connected to the support by means of single elastic element. Reduction of stiffness and damping coefficients of a pair of elements to a single one (as it is proposed in calculations of considered systems) can lead to incorrect results. This is confirmed by presented calculations and dependences.

Effet de l'instruction sur la régulation de la raideur des membres inférieurs lors de sauts de contre-haut

Goal. – We tested in this study the possibility of influencing leg stiffness through instructions on the knee flexion in drop jump (30 and 60 cm). Method. – Twelve basket players were instructed to jump with three different instructions: 1) “jump as high as you can”; 2) jump high with a larger knee flexion at touch-down and 3) jump high with a smaller knee flexion at touch-down. The ground reaction force were measured with an AMTI force plate (500 Hz). The kinematics of the jump was recorded using two digital cameras (50 Hz). Results. – The results show that the ground reaction force pattern depended more on the instruction than on the height of the box. The active peak decreased from 6 times the body weight (BW) to 2.9 times BW. Bending the knee appears to be an efficient strategy to increase the leg stiffness [ R =0.86; P <0.05]. Conclusion. – The instructions high knee flexion decreases the leg stiffness. The same vertical jump performance could be achieved with different values of stiffness. The maximum knee flexion does not modify the vertical performance.

Constant Pressure and Constant Volume Direct Shear Tests on Reinforced Sand

In order to investigate the deformation and strength characteristics of reinforced sand, a series of relatively large-inscale direct shear tests was performed. Phosphor bronze strips and sheets with different configurations were used as reinforcement to study the effects of the shape and dimensions of reinforcement. Constant pressure tests and constant volume tests were conducted with different forms of reinforcement and different values of stiffness, surface area, spatial dispersion and surface friction. Test results show that for a given amount of reinforcement material, the reinforcing efficiency is higher with reinforcement having a larger surface area, a rougher surface texture and a larger degree of dispersion. The dilatancy at the residual state depends on the reinforcement property and geometry, and it is larger in more effectively reinforced sand with a larger shear zone. More dilative characteristics of reinforced soil results in larger shear strength under constant volume conditions at large strains.