Quasi-static State Research Articles

Mechanical behavior of polyimide filament tows under high strain rate tension

The tensile properties of polyimide (PI) filament tows were measured under quasi-static state and at high strain rates with a universal tensile testing machine and a split Hopkinson tension bar, respectively. Experimental results showed that mechanical behaviors of the tows were rather sensitive to strain rate, with failure stress and modulus increasing distinctly but the elongation at break declining as the strain rate increased. Besides, the PI filament tows exhibited a higher growth rate of fracture stress than para-aramid fiber and aramid III fiber did, and scanning electronic microscopy observation on the fracture surface indicated a ductile fracture mode. With the increase of strain rate, the axial splitting of fiber intensified. Further, strength distributions of the PI filament tows were evaluated by a single Weibull distribution function, and the curve predicted was in good accordance with the experimental data obtained.

Experimental Study on Dynamic Compression Mechanical Properties of Aluminum Honeycomb Structures

In this paper, dynamic compression tests are developed to investigate the dynamic compression mechanical properties of the aluminum honeycomb structures at different strain rates, especially at the high strain rates. The difficulties at the high strain rates exist due to the large deformation, the low wave resistance and the size effect of the honeycomb structures. The Split Hopkinson Pressure Bar (SPHB) test method is carried out and special measures such as the adoption of waveform shaper, the size optimization of the impact bar and the specimen, and employment of the semiconductor strain gauge, etc. are taken to overcome the difficulties. It is discovered that the dynamic compression mechanical properties possess a stress hardening effect at a high strain rate from 1.3 × 103 s−1 to 2.0 × 103 s−1, but then a stress softening effect at a high strain rate of 4.6 × 103 s−1. It is also discovered that the yield strength and the average plateau stress at the strain rate of 2.0 × 103 s−1 is higher than that at the strain rate of 1.3 × 103 s−1. However, the yield strength and the average plateau stress at the strain rate of 4.6 × 103 s−1 is lower than that at the strain rate of 2.0 × 103 s−1 and 1.3 × 103 s−1, but higher than that at a quasi-static state. This indicates that the aluminum honeycomb structure is sensitive to the strain rate. Additionally, the damage mode of the aluminum honeycomb structure is plastic buckling, collapse and folding of the cell wall, which is carried out using dynamic compression tests. The folding length of the cell wall at a higher strain rate is found to be longer than that at a lower strain rate. The test results can also be used as the stress–strain curves of the honeycomb constitutive model at the high strain rates to carry out the numerical simulation of high-speed impact.

On the Linearization of Flat Two-Input Systems by Prolongations and Applications to Control Design

In this paper we consider (x,u)-flat nonlinear control systems with two inputs, and show that every such system can be rendered static feedback linearizable by prolongations of a suitably chosen (new) control. This result is not only of theoretical interest, but has also important implications on the design of flatness based tracking controls. We show that a tracking control based on quasi-static state feedback can always be designed in such a way that only measurements of a (classical) state of the system, and not measurements of a generalized Brunovsky state, as reported in the literature, are required.

СТЕХИОМЕТРИЧЕСКИЕ МОДЕЛИ МЕТАБОЛИЗМА ЖИВОТНОЙ КЛЕТКИ: ПОНЯТИЕ И ПРИМЕНЕНИЕ В БИОМЕДИЦИНСКИХ ИССЛЕДОВАНИЯХ

The ability to obtain large amounts of heterogeneous data on the components of an animal cell using high-perfor- mance molecular biological technologies necessitates the development of computer data processing methods. One of these methods is the representation of an animal cell as a single biological system through a stoichiometric model of the metabolic network of the cell. The process of cell metabolism over time is considered in such a model as a series of sequential quasistatic states. The model validation procedure includes a number of successive stages: analysis of the metabolic network; confirmation of the conclusions of model analysis by experimental data on a living cell; tuning of model parameters aimed at more accurate imitation of cell metabolism. The model is represented by a directed graph and a mathematical matrix, which display the simultaneous state of stoichiometric equations of enzymatic reactions forming the metabolic network of the cell. When determining the functional state of the metabolic system of a cell, matrix calculations are used; the tasks of optimizing the metabolic functions of a cell are solved by linear program- ming methods and graph theory. An example of the practical application of human cell metabolism models are Recon models. They found practical application in research when determining biomarkers of the action of biologically active substances, studying birth defects in metabolism, identifying side effects of a drug action, determining targets for expo- sure to biologically active substances, and studying cancer cell metabolism. Today, metabolic models of various cells have been created for use in various fields of biomedical research: hepatocytes, cardiomyocytes, astrocytes, kidney cells, adipocytes, red blood cells, blood mononuclear cells, mesenchymal stem cells, platelets, myocytes, sperm cells, enterocytes, endothelial cells, cancer cells, brain neurons.

Evaluating the Stress and Vibration Conditions of the Rotor of Steam Turbine with the Defect

Consideration was given to the vibrations of the two-bearing weighty rotor of the steam turbine with the acquired defect in the form of cross crack caused by the working fluid temperature field. Rotor vibration equations were derived during the three-D simulation using the method of finite elements taking thus into consideration the temperature field in the form of additional volume forces. The equations are supplemented by a special mass matrix whose introduction allows us to take into consideration a change in centrifugal forces in a linear statement caused by the offsets and retain their radial directionality. The alternating two-dimensional temperature field is defined by the solution of the nonstationary problem of heat conductivity. The stressed state, the coefficients of stress intensity along the crack front and the crack edge contact conditions were defined for different rotor rotation phases. The stress intensity coefficient values calculated using asymptotic formulas for the rotor with the 50 % cross-section crack depth are much lower than critical and present no danger for the quasistatic state. However, these values can increase significantly in the case of the resonance state. The rotor vibration state was studied for different rotation frequencies in the presence and absence of temperature field. A maximum effect of the influence of nonstationary temperature field in the rotor with the cross crack on its vibrations is manifested at a maximum radial temperature difference. The temperature field reduces considerably maximum inflection values and peak-to-peak values. The superresonance 2/1 that occurred in the rotor in the absence of temperature field disappears and the vibrations approach a monoharmonic type and it is explained by the constancy of the contact area of crack edges during the rotor rotation.

Semi-analytical solution for deformation of elastic/viscoelastic two-layered films pressed on partially-opened substrate

This paper proposes an efficient solution procedure for the deformation of a viscoelastic film pressed by another elastic film in cases wherein the films are on partially-opened substrates. The problem is formulated as a quasi-static stress state, where the stress equilibrium is satisfied at each time instance. The instantaneous stress equilibrium is expressed by biharmonic and harmonic functions for the stress and displacement of those general solutions that can be obtained in the Fourier domain. By applying the boundary conditions mathematically everywhere but the surface, which is in direct contact with the partially-opened substrate, the total problem could be discretized and solved using the Fourier collocation method. In this manner, the total number of unknowns can be greatly reduced. The validity and effectiveness of the proposed method was confirmed by comparing the results with those obtained by a finite element method (FEM) solver. The viscoelastic deformation obtained by the proposed method was almost in complete agreement with the result obtained by the FEM. The elapsed computational time required by the proposed method was approximately 1/100 of that required by the FEM.

A constitutive model of the compressive mechanical properties of ultra high molecular weight polyethylene (UHMWPE) at different temperatures and different strain rates

The impact resistance of UHMWPE is related to temperature and strain rate. In order to determine the compressive mechanical properties of UHMWPE, quasi-static and dynamic compression tests were conducted on UHMWPE using an MTS universal testing machine and split Hopkinson pressure bar equipment, respectively. Stress–strain curves for UHMWPE over the temperature range of −40 °C∼100 °C and strain rate range of 0.001∼3300 s−1 was generated. By comparing the stress–strain curves of UHMWPE with molecular weights (MWs) of 3, 6 and 9 million in a quasi-static state, it was found that UHMWPE with a MW of 6 million had better compressive mechanical properties than other UHMWPE samples. Using the stress–strain curves for UHMWPE with a MW of 6 million at different strain rates and temperatures, the yield strength of UHMWPE increased with an increasing strain rate and decreased with increasing temperature were obtained. Based on the quasi-static and dynamic experimental results for UHMWPE with a MW of 6 million, a constitutive model of plasticity at different temperatures and strain rates was established. This model agreed well with the experimental results. This study provides both theoretical and empirical reference for the study of the mechanical properties of UHMWPE materials.

Assessment and structural improvement on the performance of soil chamber system of EPB shield assisted with DEM modeling

This paper assesses the adaptability of the soil chamber system of a earth pressure balanced (EPB) shield machine to be used in sandy pebble stratum by the discrete element method (DEM) modelling. Furthermore, measures for the structural improvement for better fluidity and higher wear resistance are proposed based on the analysis results from DEM accordingly. Firstly, the flow characteristics of particles inside the chamber was obtained through investigation of velocity field, motion trajectory and mass flow rate. Secondly, the abrasion loss on cutterhead and the maximum tunneling distance without changing tools was predicted. Thirdly, through particle-structure interaction (PSI) analysis, the stress and deformation information about cutterhead in a quasi-static state was output. Fourthly, on-site measured data indicate that the power supply device can fulfill the requirement of thrust force and cutterhead torque. Finally, the strength, abrasion loss and internal pressure of screw conveyor were checked. Overall, the selected chamber system shows a satisfactory performance. Furthermore, the proposed improvement measures were verified through DEM simulation, which can provide references for further optimization design of shield chamber.

Thermodynamic and Electrochemical Mitochondrial Cycles (in the Direct and Reverse Directions)

A meta-analysis of the functional states of mitochondria was carried out using a mathematical model based on changes in the direction and speed of thermodynamic (TD) and electrochemical (EC) parameters (pressure, volume, temperature, entropy, Gibbs potential, exergy, etc.), which are interrelated. A mathematical model of the TC and EC cycle of mitochondria was constructed using the basic principle of supramolecular functional interactions. The model describes the TD and EC reactions that occur in the mitochondrial matrix–inner membrane–intermembrane space system and are accompanied by an accumulation of electrochemical potential and charge separation (ionization). Four functional states of mitochondria were studied in the situation where the speed and direction of electron movement along the respiratory chain changed in the thickness of the inner membrane in association with fluctuations in heat flow. The TD and EC cycle of mitochondria is a continuous succession of transitions between quasi-static states, in which quasi-equilibrium states change recurrently, displaying typical coherent stability of non-equilibrium processes according to I. Prigogine. The mathematical model of the TD behavior of mitochondria and the limitations imposed by laws of physical and chemical thermodynamics indicate that the highest degree of TD efficiency in the process of mitochondrial respiration corresponds to the state where Gibbs energy and exergy are at a minimum and entropy is at a maximum; the functional state was consequently identified as a basic one.

Constitutive relation and mechanical mechanism analysis in granular lubrication

PurposeGranular lubrication can solve some lubrication problems under many extreme operating conditions. Meanwhile, the flow constitutive relation is one of its unsolved problems in fully understanding its rheological mechanism.Design/methodology/approachIn this paper, a plane shear cell under granular lubrication is established by the discrete element method to study the flow constitutive relation and its mechanical mechanism of the hard granular lubricants.FindingsResearch results show that the flow regimes in granular flow lubrication strongly rely on the dimensionless parameter I, in which it is called the inertial coefficient. When the inertial coefficient I increase, the flow regimes of the granular lubricants also evolve from a quasi-static state to a collisional state accordingly. Comparing to the influence of the restitution coefficient, the friction coefficient of the hard granular lubricants has a strong influence on its constitutive relation of the granular flow lubrication. Finally, it is shows that the dimensionless parameter I has strong influence on the contacts and flow states of this granular lubrication system than the influence of the dimensionless parameter R.Originality/valueThese findings reveal the constitutive relation and mechanical mechanism of granular lubrication and can also offer the helpful reference for the design of the new granular lubrication bearing.

Indentation size and loading rate sensitivities on mechanical properties and creep behavior of solid bitumen

Creep behavior of rocks could impair fracture conductivity and wellbore stability during gas production from highly matured organic-rich shales in South China, of which the organic matter is mainly in the form as solid bitumen and is thought to be a major contributor for the creep deformation. To get a better insight into this phenomenon, this paper for the first time characterizes the mechanical properties and creep behavior of a millimeter-sized solid bitumen sample by using quasi-static state creep tests and Dynamic Mechanical Analysis in nanoindentation, and reports their dependences on indentation size and loading rate, respectively. Mechanical properties (including hardness and Young's modulus) are found to be negatively related with both indentation size and loading rate. The extremely small creep strain rate sensitivity (m) of solid bitumen indicates a localized shear flow inside. And m exhibits slightly positive dependences on indentation size and loading rate. The potential mechanisms controlling the deformation of solid bitumen under indentation are also discussed.

Experimental research on dynamic response of PZT-5H under impact load

In order to improve the stability of PZT-based sensors, the mechanical, dielectric, ferroelectric and piezoelectric properties of PZT-5H under impact load were studied experimentally by using the separated Hopkinson pressure bar (SHPB) with an electrical output measurement device. At the same time, the experimental study on the material properties of PZT-5H before and after the impact was carried out. The effect of impact cracks on the output voltage of PZT-5H was also analyzed. The results show that the dynamic piezoelectric constants of PZT-5H under low stress impact (10–50 MPa) are different from those under quasi-static state, and the empirical relationship between them and the peak stress is obtained through experiments. The dielectric properties of PZT-5H did not change under low stress impact, but micro-cracks occurred in the material and dielectric loss increased at high frequencies. Under short circuit, the residual polarization intensity of PZT-5H decreases sharply due to impact load. While the impact load causes the secondary polarization and the increase of the residual polarization intensity of PZT under open circuit. When the stress is over 45 MPa, the PZT-5H breaks. The formation of cracks causes abnormal discharge voltage and gap discharge.

Numerical Calculation of Hip Squeak Over the Normal Gait Cycle

This paper present the numerical and analytical hip squeak model using the normal gait data acquired from the experiment. The axis of the ground reaction force is defined on the hip joint at the successive position of the right hip during the gait cycle. The contact pressure and its distribution on the ball are calculated by the Hertz theory at the corresponding cycle. The eigenvalue sensitivity analysis is conducted at the quasi-static state of the successive position of the right hip for both the full lubrication and the mixed lubrication. For the full lubrication case, the hip joint system becomes dynamically unstable only when the hip extends. In the mixed lubrication, the 2nd bending mode has the very high squeak propensity and its frequency drastically varies with the gait cycle.

A Note on the Fracture Behavior of Concrete Beams within Water Pressure under Different Loading Rates

This study examined the experiment of the water fracture interactions in the concrete. In order to create water pressure in the crack, a sealing device consisting of a mechanical clamp device and a waterproof membrane was proposed. Firstly, eighteen three-point bending tests were carried out to verify the proposed sealing device under three different loading rates. Subsequently, twelve hydraulic tests were performed under different loading rates. The results showed that the sealing device had minimal influence on the fracture behavior of concrete. Besides, with the increase of internal water pressure, the carrying capacity, fracture parameters and crack opening rate of concrete beam were evidently decreased. Meanwhile, the decrease of the carrying capacity, fracture parameters, and crack opening rate induced by internal water pressure decreased with the increase of loading rate. In addition, a significant hydraulic fracturing behavior caused by water pressure was observed in the quasi-static state.

Asymmetric Hardening Behavior of AZ31B Magnesium Alloy Sheet with Large Strain at Various Strain Rates

This paper investigates asymmetric hardening behavior of the magnesium alloy sheet of AZ31B at different strain rates when deformation is large. Tensile tests are carried out at the quasi-static state of 0.001 s−1 and the intermediate strain rate of 100 s−1 utilizing the Instron 5583 and the high speed material testing machine (HSMTM), respectively. Compression tests are conducted at the strain rate of 0.001 s−1 and 100 s−1 using a jig fixture mounted on the testing machines to change the loading direction into the opposite direction. A compression testing method is developed to obtain compressive properties with large strain, which includes the attachment of a fork-type clamping device and the design of specimen dimensions to achieve large strain without buckling during the test. Experimental results reveal the asymmetric behavior showing that the tensile yield stress is much larger than the compressive one at both strain rates. The difference between the flow stress level in tension and compression becomes larger when the strain rate is higher. The compressive hardening behavior is very much different from the tensile hardening behavior showing that the compressive flow stress increases remarkably as the deformation proceeds while the tensile flow stress asymptotes to a certain value.

A dynamic model for underwater robotic fish with a servo actuated pectoral fin

The main objective of this work is to mimic and model pectoral fins based locomotion mechanisms observed in various labriform fishes like box-fish, angel fish, etc. accounting the previous problems using rigid joint connection for fins. The hydrodynamic forces acting on the pectoral fin are evaluated for a quasi-static state using blade element approach while taking into account the added mass. Implementing Newton–Euler equations, the kinematics and the dynamics of a free-swimming robot-fish are obtained. Thus, an analytical model is derived to calculate net forces and torques generated throughout the gait. Apart from a parametric study, simulations are performed to predict various output parameters like thrust, acceleration and swimming speed for some specific locomotion. For experimental validation, a rigid fish-like-body is fabricated with a pair of fins attached symmetrically on lateral sides of the body. Each fin is actuated using three servo-motors in [Ry Rz Ry] configurations to exhibit both the rowing and flapping gaits. A micro-controller is deployed to control the actuators by generating pulse-width modulation and read inertial sensors (a gyroscope and an accelerometer mounted at the centroid of the body) using I2C protocol. In addition, a Bluetooth module is also attached to it for communicating the sensor’s readouts to a computer (running MATLAB script). To achieve some specific motions of rowing gait, the fins are actuated appropriately while the robot is freely submerged underwater and the sensor’s data are recorded in real-time for parallel observations. Based on video motion analysis and the sensed data, the values of acceleration are calculated and compared with the theoretical values predicted by the model.

Characteristics of the fluid–structure interaction within Darrieus water turbines with highly flexible blades

Vertical-axis water turbines (VAWT), the target of the present study, provide a higher area-based power density when used in arrays, and are a promising alternative to horizontal-axis hydro-kinetic turbines (HAWT). Nevertheless, they operate under highly dynamic conditions near or even beyond dynamic stall at their best-efficiency-point. The abrupt loss of lift and strong increase of drag associated with hydrofoil stall can produce cyclic loads and possible damage of turbomachines due to material fatigue.The effect of flexible structures in a highly dynamic flow regime including separation and stall is here studied systematically in an experimental setup which permits observations of all regimes ranging from quasi-static state up to the occurrence of deep dynamic stall and beyond. The process is studied using a surrogate model consisting of an oscillating NACA0018 hydrofoil in a closed water channel, following a motion law comparable to the real angle of incidence of a Darrieus turbine blade along its rotation. The investigated parameters are the oscillation frequency and tip speed ratio, for one rigid as well as for three flexible hydrofoils of different stiffnesses. The coupling process is therefore investigated for multiple machine designs and working points. Lift and drag measurements have been carried out in a systematic manner.Results show that at tip speed ratios for which highly dynamic flow regimes occur, flexible blades provide not only higher thrust, but also reduced normal forces and reduced peak-to-peak cyclic normal force variations. This reduction of stress loads would translate into significantly increased turbine lifetime. This supports the need for further investigations in order to identify optimal blade flexibility and check further turbine designs.

Nonlinear vibration of crowned gear pairs considering the effect of Hertzian contact stiffness

This study aims to analyze the influence of lead crowning modification of teeth on the vibration behavior of a spur gear pair. Two dynamic rotational models including an uncrowned and crowned gear are examined. Hertzian mesh stiffness is computed using tooth contact analysis in quasi-static state along a complete mesh cycle of teeth mesh. The dynamic orbits of the system are observed using some useful attractors which expand our understanding about the influence of crown modification on the vibration behavior of the gear pair. Nonlinear impact damper consists of non-integer compliance exponents identify energy dissipation of the system beneath the surface layer. By augmenting tooth crown modification, the surface penetration increases and consequently normal pressure of the contact area becomes noticeable. Finally, the results show modification prevents gear pair to experience period doubling bifurcation as the numerical results proved. Using this new method in dynamic analysis of contact, broaden the new horizon in analyzing of the surface of bodies in contact.

A Soft Ring-Shaped Actuator for Radial Contracting Deformation: Design and Modeling.

Bioinspired movements such as elongation/shortening and bending are extensively studied in the field of soft robotics. However, radial contracting deformation has been overlooked, another typical deformation in nature, such as the constrictive deformation in the human esophagus and bladder. We present a novel soft ring-shaped actuator that generates radial contraction and examine its quasi-static performance under pressurization. This actuator consists of a soft ring-shaped body and a rigid casing. A single circular air chamber embedded concentrically inside the actuator is created through the lost-wax method. A theoretical model of the quasi-static state of the actuator's contracting deformation was formulated in light of the minimum total potential energy principle. Meanwhile, a finite-element method (FEM) model was constructed to stimulate the radial contraction of the actuator. In addition, experiments were conducted to identify the maximum input pressure and measure the deformations at the specific input pressure. When the actuators were pressurized under 20.0 kPa, axisymmetric contractions were achieved. The theoretical model and FEM models produced the predictions in good agreement with experimental results, with slight mean relative differences regarding peak points, 3.4% and 5.4%, respectively. In addition, pilot tests suggest that the ring-shaped actuator can achieve the key tasks of robotic manipulators, such as gripping and holding.

Equilibration of quasi-integrable systems.

We study the slow relaxation of isolated quasi-integrable systems, focusing on the classical problem of Fermi-Pasta-Ulam-Tsingou (FPU) chain. It is well known that the initial energy sharing between different linear modes can be inferred by the integrable Toda chain. Using numerical simulations, we show explicitly how the relaxation of the FPU chain toward equilibration is determined by a slow drift within the space of Toda's integrals of motion. We analyze the whole spectrum of Toda modes and show how they dictate, via a generalized Gibbs ensemble, the quasistatic states along the FPU evolution. This picture is employed to devise a fast numerical integration, which can be generalized to other quasi-integrable models. In addition, we discuss how a fluctuation theorem, recently derived in Goldfriend and Kurchan [Europhys. Lett. 124, 10002 (2018)10.1209/0295-5075/124/10002], describes the large deviations as the system flows in the entropy landscape.