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11.
Itzhak Levit 《Computers & Structures》1983,16(6):749-753
The standard truss design problem with stress and displacements constraints is formulated as an unconstrained minimax problem which is then solved using the Norm Optimization Method. Truncated Gauss-Seidel iterations for the solution of the compatibility equations are used as an approximate reanalysis tool. The method is demonstrated on known examples from the literature. Results indicate that the technique is simple to implement and gives excellent results. 相似文献
12.
A selection of WC-Co and Cr3C2-25%NiCr coatings deposited by plasma spraying and high velocity oxygen fuel (HVOF) were tested. The microstructures of the
coatings were characterized, and their mechanical properties were assessed using Knoop microindentation procedures. The coatings
were also subjected to various wear tests. All of the coatings were at least 200 μm thick and were deposited onto stainless
steel substrates. The wear tests simulated abrasion, cavitation wear, sliding wear, and particle erosion wear.
In this first part of a two-part contribution, the microstructures of the coatings are characterized and a discussion on the
evaluation of mechanical properties from the microindentation response is presented. The nature of microhardness testing as
applied to thermal spray coatings is evaluated as a means of assessing resistance to plastic flow, elasticity, and brittleness.
In Part 2, the results of the various wear simulations are reported, and the utility of microhardness as an indicator of wear
resistance is examined. 相似文献
13.
This work models statistically elasto-plastic contact between two rough surfaces using the results of a previous finite element analysis of an elasto-plastic sphere in contact with a rigid flat. The individual asperity contact model used accounts for a varying geometrical hardness effect that has recently been documented in previous works (where geometrical hardness is defined as the uniform pressure found during fully plastic contact). The contact between real surfaces with known material and surface properties, such as the elastic modulus, yield strength, and roughness are modeled. The asperity is modeled as an elastic-perfectly plastic material. The model produces predictions for contact area, contact force, and surface separation. The results of this model are compared to other existing models of asperity contact. Agreement exists in some cases and in other cases it corrects flaws, especially at large deformations. The model developed by Chang, Etsion and Bogy is also shown to have serious flaws when compared to the others. This work also identifies significant limitations of the statistical models (including that of Greenwood and Williamson). 相似文献
14.
Goren Y Davrath LR Pinhas I Toledo E Akselrod S 《IEEE transactions on bio-medical engineering》2006,53(1):35-42
Heart rate variability (HRV) is a major noninvasive technique for evaluating the autonomic nervous system (ANS). Use of time-frequency approach to analyze HRV allows investigating the ANS behavior from the power integrals, as a function of time, in both steady-state and non steady-state. Power integrals are examined mainly in the low-frequency and the high-frequency bands. Traditionally, constant boundaries are chosen to determine the frequency bands of interest. However, these ranges are individual, and can be strongly affected by physiologic conditions (body position, breathing frequency). In order to determine the dynamic boundaries of the frequency bands more accurately, especially during autonomic challenges, we developed an algorithm for the detection of individual time-dependent spectral boundaries (ITSB). The ITSB was tested on recordings from a series of standard autonomic maneuvers with rest periods between them, and the response to stand was compared to the known physiological response. A major advantage of the ITSB is the ability to reliably define the mid-frequency range, which provides the potential to investigate the physiologic importance of this range. 相似文献
15.
Kevin van de Ruit Racheli Itzhak Cohen Dirk Bollen Ton van Mol Rachel Yerushalmi‐Rozen René A. J. Janssen Martijn Kemerink 《Advanced functional materials》2013,23(46):5778-5786
The mechanism and magnitude of the in‐plane conductivity of poly(3,4‐ethy‐lenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) thin films is determined using temperature dependent conductivity measurements for various PEDOT:PSS weight ratios with and without a high boiling solvent (HBS). Without the HBS the in‐plane conductivity of PEDOT:PSS is lower and for all studied weight ratios well described by the relation $ \sigma = \sigma _0 {\rm exp}[- \left({{{{T_0}}\over{T}}} \right)^{0.5}] $ with T0 a characteristic temperature. The exponent 0.5 indicates quasi‐one dimensional (quasi‐1D) variable range hopping (VRH). The conductivity prefactor σ0 varies over three orders of magnitudes and follows a power law σ0∝c3.5PEDOT with cPEDOT the weight fraction of PEDOT in PEDOT:PSS. The field dependent conductivity is consistent with quasi‐1D VRH. Combined, these observations suggest that conductance takes place via a percolating network of quasi‐1D filaments. Using transmission electron microscopy (TEM) filamentary structures are observed in vitrified dispersions and dried films. For PEDOT:PSS films with HBS, the conductivity also exhibits quasi‐1D VRH behavior when the temperature is less than 200 K. The low characteristic temperature T0 indicates that HBS‐treated films are close to the critical regime between a metal and an insulator. In this case, the conductivity prefactor scales linearly with cPEDOT, indicating the conduction is no longer limited by a percolation of filaments. The lack of observable changes in TEM upon processing with the HBS suggests that the changes in conductivity are due to a smaller spread in the conductivities of individual filaments, or a higher probability for neighboring filaments to be connected rather than being caused by major morphological modification of the material. 相似文献
16.
In this study, an experimental investigation on the effects of grooves on thrust washer bearings is investigated. Eight equally sized grooves are machined about 100 μ m deep into one side of a flat-faced steel washer. This thrust washer bearing is located between a helical gear and its carrier and is tested on a test rig capable of measuring frictional torque and the temperature of the bearing at different speeds. It is found that the grooved washers had lower bearing temperatures and failed at significantly higher loads than the control washer with no grooves. For a test procedure with varying operating conditions, the coefficient of friction is also significantly lower for the grooved washers. However, the grooved washers had about the same coefficient of friction as the control washers at each step when the speeds are very high. The results from various tests suggest that the increased amount of lubricant passing through the grooved surface of the washer removes heat from the washer bearing by convection. This decrease in stored heat conducted from friction deters thermoelastic instabilities and the reduction of hydrodynamic stiffness due to the decrease in viscosity. Enhanced hydrodynamic load-carrying capacity is also evident in the grooved washers test results. 相似文献
17.
In this paper an extensive CFD simulation of the exhaust stroke of a single-cylinder fourstroke ICE, including the entire exhaust manifold is described. Guidelines for the implementation of the full threedimensional model of the discussed process are included. The simulation involves the time-dependent flow of exhaust gases through the exhaust valve and the flow dynamics within the 2.2-m-long, straight exhaust pipe during the period when the valve is closed. Also the intake port with the intake valve is being coupled during the valves’ overlap period. The model geometry corresponds exactly to the actual engine geometry. The movement of the mesh follows the measured kinematics of the piston and the valves. The data obtained from the experimental environment was used for both the initialization and the validation of the computations. It was found that the phenomena affecting the dynamics of the exhaust flow are extremely three-dimensional and should be treated as such. In particular, the flow through the exhaust valve and the heat transfer along the exhaust pipe were influenced greatly by the effects of cold, fresh air breaking into the exhaust pipe in the period after the EVC. The presented study is the basis for future three-dimensional investigations of the entropy-generation rate along the exhaust system, including the exhaust valve. 相似文献
18.
A sequence of three variables mini-max problems is introduced and implemented for numerically solving standard symmetric eigenvalue problems. The procedure is shown to possess considerable advantages over similar procedures introduced by the authors of this paper in the past. Finally, a general strategy for handling large scale symmetric eigenvalue problems is presented and implemented together with the new numerical solution procedure. 相似文献
19.
Melamed Samuel; Shirom Arie; Toker Sharon; Berliner Shlomo; Shapira Itzhak 《Canadian Metallurgical Quarterly》2006,132(3):327
Burnout is characterized by emotional exhaustion, physical fatigue, and cognitive weariness, resulting from prolonged exposure to work-related stress. The authors review the accumulated evidence suggesting that burnout and the related concept of vital exhaustion are associated with increased risk of cardiovascular disease and cardiovascular-related events. The authors present evidence supporting several potential mechanisms linking burnout with ill health, including the metabolic syndrome, dysregulation of the hypothalamic-pituitary-adrenal axis along with sympathetic nervous system activation, sleep disturbances, systemic inflammation, impaired immunity functions, blood coagulation and fibrinolysis, and poor health behaviors. The association of burnout and vital exhaustion with these disease mediators suggests that their impact on health may be more extensive than currently indicated. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
20.