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71.
Mir Masoud Seyyed Fakhrabadi Abbas Rastgoo Mohammad Taghi Ahmadian Mahmoud Mosavi Mashhadi 《Acta Mechanica》2014,225(6):1523-1535
Modified couple stress theory is a size-dependent theorem capturing the micro/nanoscale effects influencing the mechanical behaviors of the micro- and nanostructures. In this paper, it is applied to investigate the nonlinear vibration of carbon nanotubes under step DC voltage. The vibration, natural frequencies and dynamic pull-in characteristics of the carbon nanotubes are studied in detail. Moreover, the effects of various boundary conditions and geometries are scrutinized on the dynamic characteristics. The results reveal that application of this theory leads to the higher values of the natural frequencies and dynamic pull-in voltages. 相似文献
72.
We present a three-dimensional model for flow pumping in a channel induced by two moving contractions from the upper wall. This pumping model is inspired by insect respiration processes, specifically, the rhythmic collapses that take place within their tracheal tube networks. The present work is a natural extension of our previous theoretical and numerical investigations of a two-dimensional insect-inspired micropumping model, which accounts for three-dimensional effects and further validates our insect-inspired pumping paradigm (Aboelkassem and Staples in Acta Mech 223(3):463–480, 2012a; Theor Comput Fluid Dyn, 2012b. doi:10.1007/s00162-012-0269-7). The formal goal of this article is to compare three-dimensional Stokeslets-meshfree numerical results with results from our previous two-dimensional analytical pumping model. We use regularized Stokeslets-meshfree computations in three dimensions to reconstruct the flow motions induced by wall contractions and to calculate the time-averaged net flow pumping rate. The results show that, although the net flow rate distribution as a function of the wall motion time (phase) lag parameter for the three-dimensional Stokeslets-meshfree computations and the two-dimensional analytical model displays some differences, the same basic features appear in both cases, leading to the same general conclusions about the proposed pumping paradigm. 相似文献
73.
SangUk Han Farzaneh Saba SangHyun Lee Yasser Mohamed Feniosky Peña-Mora 《Accident; analysis and prevention》2014
It is not unusual to observe that actual schedule and quality performances are different from planned performances (e.g., schedule delay and rework) during a construction project. Such differences often result in production pressure (e.g., being pressed to work faster). Previous studies demonstrated that such production pressure negatively affects safety performance. However, the process by which production pressure influences safety performance, and to what extent, has not been fully investigated. As a result, the impact of production pressure has not been incorporated much into safety management in practice. In an effort to address this issue, this paper examines how production pressure relates to safety performance over time by identifying their feedback processes. A conceptual causal loop diagram is created to identify the relationship between schedule and quality performances (e.g., schedule delays and rework) and the components related to a safety program (e.g., workers’ perceptions of safety, safety training, safety supervision, and crew size). A case study is then experimentally undertaken to investigate this relationship with accident occurrence with the use of data collected from a construction site; the case study is used to build a System Dynamics (SD) model. The SD model, then, is validated through inequality statistics analysis. Sensitivity analysis and statistical screening techniques further permit an evaluation of the impact of the managerial components on accident occurrence. The results of the case study indicate that schedule delays and rework are the critical factors affecting accident occurrence for the monitored project. 相似文献
74.
Ali Azadeh Seyyed Farid Ghaderi Mohammad Sheikhalishahi Behnaz Pourvalikhan Nokhandan 《Optimization and Engineering》2014,15(2):485-508
Accurate short-term load forecasting (STLF) is one of the essential requirements for power systems. In this paper, two different seasonal artificial neural networks (ANNs) are designed and compared in terms of model complexity, robustness, and forecasting accuracy. Furthermore, the performance of ANN partitioning is evaluated. The first model is a daily forecasting model which is used for forecasting hourly load of the next day. The second model is composed of 24 sub-networks which are used for forecasting hourly load of the next day. In fact, the second model is partitioning of the first model. Time, temperature, and historical loads are taken as inputs for ANN models. The neural network models are based on feed-forward back propagation which are trained and tested using data from electricity market of Iran during 2003 to 2005. Results show a good correlation between actual data and ANN outcomes. Moreover, it is shown that the first designed model consisting of single ANN is more appropriate than the second model consisting of 24 distinct ANNs. Finally ANN results are compared to conventional regression models. It is observed that in most cases ANN models are superior to regression models in terms of mean absolute percentage error (MAPE). 相似文献
75.
76.
Strontium hexaferrite SrFe12O19 thin films have been synthesized on the Si (100) substrate using spin coating sol-gel process. The influence of different citric acid to metal ions (CA/MN) molar ratios of 0.5, 1 and 1.5 on the formation of SrFe12O19 was investigated. Fourier transform infrared analysis and theoretical calculations were conducted to determine and help control the concentration of metal citrates in solution precursors. X-ray diffraction, scanning electron microscopy and vibrating sample magnetometer techniques were also applied to evaluate the composition, microstructure, crystallite size and magnetic properties of the SrFe12O19 thin films. The results showed that single phase strontium hexaferrite thin films with optimum physical properties can be obtained from the solution with the CA/MN molar ratio of 1 after calcination at 800 °C. The SrFe12O19 thin films synthesized with the CA/MN molar ratio of 1 exhibited crystallite size of 42 nm and isotropically magnetic properties of Ms = 267.5 kA/m (at 795.8 kA/m), Mr = 134 kA/m, and Hc = 341.4 kA/m. 相似文献
77.
Structural reliability theory is a useful tool for estimating the risks associated with deteriorating structures. The aim of this study is to develop and demonstrate a procedure for the assessment of box girder bridge ultimate strength reliability with the degradation of plate members due to general corrosion taken into account. A probabilistic model for ultimate steel-box girder strength is established on the basis of an analytic formula that considers corrosion-related, time-dependent strength degradation. The study involves the selection of representative structures, formulation of limit state functions, development of resistance models for corroded steel-box girders, development of load models, development of a reliability analysis method, reliability analysis of the selected bridges and development of the time-dependant reliability profiles, including deterioration due to corrosion. The results of this study can be used for the better prediction of the service life of deteriorating steel-box girder bridges and the development of optimal reliability-based maintenance strategies. 相似文献
78.
79.
This paper presents a case of introducing new technology to a single stage in a maintenance operation composed of sequence of stages. The process - Thermal tile replacement - is a low volume, high value operation. A method for data collection at each stage, to estimate the variability in process quality, cost and duration is presented. The method involves: Identifying key product features, accuracy measure for each, rate of product rejection by feature and the associated probability density functions at each stage. The method relates accuracy variability by feature, “effect” to the contributing stage in the process “cause”. Simulation is used to justify the introduction of a new technology and to predict the percentage of product conformity in a “before” and “after” scenarios for the implementation of the new technology. The simulation model enables the quantification of technology impact on the product quality, overall productivity and the associated cost savings. 相似文献
80.
Kadah YM El-Sharkawy Ael-M Youssef AB 《IEEE transactions on bio-medical engineering》2005,52(1):127-131
We develop a simple yet effective technique for motion artifact suppression in ultrasound images reconstructed from multiple acquisitions. Assuming a rigid-body motion model, a navigator echo is computed for each acquisition and then registered to estimate the motion in between acquisitions. By detecting this motion, it is possible to compensate for it in the reconstruction step to obtain images that are free of lateral motion artifacts. The theory and practical implementation details are described and the performance is analyzed using computer simulations as well as real data. The results indicate the potential of the new method for real-time implementation in lower cost ultrasound imaging systems. 相似文献