Sharif Digital Repository

An explicit dynamics extended finite element method (X-FEM) is applied to the problems with material interfaces. To this end, the available X-FEM element mass matrix lumping techniques have been identified and assessed. It is shown that all the methods have their own limitations and are not general enough to handle the enrichment functions specifically developed for the elements with material interfaces. Hence, a general element mass matrix lumping technique has been introduced which exactly preserves the kinetic energies of the basic rigid body and enrichment modes. The idea is borrowed from the previously published method for a specific range of enrichment functions and extended to... 

The present paper studies the failure of concrete from the mesoscopic point of view. Biphasic cubic concrete samples containing spherical aggregates embedded in a homogenized mortar have been simulated using standard finite element method. Linear elasticity and damage-plasticity hypotheses are considered for the aggregates and mortar, respectively. Various triaxial loading conditions are assumed for each sample to generate adequate discrete failure points within the stress space. In the next step, the approximated failure surfaces of specimens are constructed using the Delaunay triangulation technique. The effects of mesostructural features such as aggregate grading curve, aggregate... 

The crushing behaviour of dynamically loaded metal foam-filled square columns has been investigated using an extended version of the existing self-similar pressure dependent constitutive model for metal foams. The model has been implemented in ABAQUS/Explicit and analyses have been conducted using different approaches to model the uniaxial and hydrostatic hardening behaviour of metal foams. A practical and reliable procedure to approximate the observed anisotropic behaviour within the computational framework of isotropic plasticity is introduced. The comparison between the available experimental and newly generated numerical results is presented in order to illustrate the accuracy and... 

In this paper, the effects of foam density distribution and initial impactor inclination angle on crushing behaviour of metal foam-filled thin-walled square columns have been investigated. The simplified version of an existing pressure dependent self-similar constitutive model with tensile failure for metal foam has been implemented in ABAQUS/Explicit code. Random distribution of cubic regions with normal distribution of density has been created. The analyses have been accomplished for three distinct foam densities, two extreme random distributions (fine and coarse), and five different impactor inclination angles. A local effect on folding patterns of columns filled with low-density foam has... 

The effect of spot welding failure, flange location, sheet metal thickness, glue presence in foam-wall contact area, and foam filling on the bending crash performance and the collapse mechanism of empty and foam-filled box-beams is investigated. It is observed that the flange location in box-beams controls spot welds force, the energy absorption capacity of box-beams, and collapse behavior of the sections. As a result, the definition of spot weld failure criterion is inevitable in most cases. A survey on the effect of slight aluminium foam filling (12% mass increase) shows that the specific energy absorption value (SEA) and the energy dissipation performance of a filled section can even... 

In this paper, we propose an analytical approach to compute the probability of connectivity for one-dimensional ad hoc networks. The proposed analysis gives the exact probability of connectivity for an arbitrary distribution of nodes, provided that nodes are independently and identically distributed. We conduct separate analyses for two cases; in the first case, the number of nodes varies by time under a stationary distribution and in the second case, there is a fixed (known) number of nodes in the network. Using the approaches presented in this work, we are able to derive closed-form formulas for the probability of connectivity for some spatial distributions, while for more complicated... 

Wall walking robots are designed for different purposes, rescue operations, wall inspections and jobs such as painting and cleaning and fire fighting for tall buildings. These are some cases that these types of robots are extensively used. This paper describes a design of a new serial mechanism for wall climbing job. In deed we are seeking to define a minimum degree of freedom mechanism to be applied in a robot moving vertically on a surface. This mechanism has 5 links, but at any point it works with its 4 links, actually in each cycle of motion the linkage will be interchanged. Copyright © 2006 by ASME  

Orthogonal frequency division multiplexing (OFDM) has a large peak to mean envelope power ratio (PMEPR), which can result in significant signal distortion in presence of nonlinear amplifiers. In this paper, we will show that peak of the continuous envelope of OFDM signal can be bounded by maximum of corresponding oversampled sequence. This fact is then used as the corner stone of our study in two parts. In the first part, statistical analysis is used to find probability upper bound for complementary cumulative distribution function of PMEPR. It is also proven that for large number of subcarriers, only one redundant bit is necessary to limit PMEPR within %10 of its maximum value. In the... 

Recently, the tradeoff between low energy consumption and high fault-tolerance has attracted a lot of attention as a key issue in the design of real-time embedded systems. Dynamic Voltage Scaling (DVS) is known as one of the most effective low energy techniques for real-time systems. It has been observed that the use of control-theoretic methods can improve the effectiveness of DVS-enabled systems. In this paper, we have investigated reducing the energy consumption of fault-tolerant hard real-time systems using feedback control theory. Our proposed feedback-based DVS method makes the system capable of selecting the proper frequency and voltage settings in order to reduce the energy... 

It is widely accepted that the presence of a lossy network, such as a SAW filter or balun, at the input of LNA increases NF of the overall circuit by an amount equal to the insertion loss of the lossy network. In this work it is shown that this idea is not exactly true and it considerably underestimates the overall NF. To verify this claim, Z-model of the lossy network is added before the LNA and the overall noise figure is recalculated for some commonly used LNA topologies. It is shown that in common gate LNA and common source LNA with resistive feedback the accepted equation underestimates the overall NF by 1.3 dB for an insertion loss of 2 dB. In common source with resistive matching and... 

This paper investigates the performance of a MOS track-and-hold circuit as the front end of a wide input range and high-speed single-sided ADC. In order to reduce charge injection and clock feedthrough errors, a new technique for employing a dummy transistor in the bootstrapped switch of the track and hold circuit is introduced. Performance of the proposed circuit for the two cases of constant and non-linear hold capacitor is studied. It is also shown that how and when dummy switch and bulk switching method can improve the accuracy of theT/H. © IEICE 2007  

A divide-by-3 frequency divider for Inphase and Quadrature (I/Q) LO signal generation in a multi-band frequency synthesizer is presented. Using divisor numbers other than powers of 2 (2n) for quadrature signal generation, reduces the required frequency range of the VCO in multi-band frequency synthesizers. The divide-by-3 circuit is designed in a 0.18um CMOS technology. © 2008 IEEE  

In this paper a theoretical comparison between the performance of switched-capacitor (SC) and switched-current (SI) pipeline analog-to-digital converters (ADCs) is presented. Power dissipation and die area of SC and SI implementations are compared based on linearity and noise constraints. It is shown that if nonlinearity errors of the class AB SI ADCs are removed by calibration, their performance prevails that of the SC ADCs. Also it is shown that class AB SI ADCs occupy less die area than SC ADCs for a given resolution. © 2007 IEEE  

A high speed, high resolution switched-current sample and hold (SI S/H) based on a new class AB transconductance stage is presented. Simulations performed on the SI S/H in standard 0.18um CMOS technology with 1.5v supply voltage indicate low power dissipation, high sampling speed and high SNR. © 2006 IEEE  

A new architecture for analog sampled-data filters is presented. This architecture is based on implementing the difference equation of the filter using a neural network. It results in lower sensitivity of filter coefficients to hardware nonidealities. Different tradeoffs in the hardware implementation of the filter are discussed. A design example is also presented. © 2005 IEEE  

This study provides a stochastic dynamic programming model with a Markov chain that explicitly focuses on the customer as well as a new model for valuing the customers in the banking industry. The proposed framework calculates individual customer’s lifetime value dynamically. The study follows a stochastic dynamic programming model that is based on the Markov chain. The deduced findings are illustrated with supplementary context from an outstanding case study. The findings underline the importance of the stochastic model for calculating customer lifetime value based on customer behavior. The presented framework provides a beneficial way for future research and valuable insight for allocating...