Sharif Digital Repository

We present fabrication, characterization, and experimental results describing electrical actuation and readout of the mechanical vibratory response of electrospun ZnO nanofibers. For a fiber with an approximate radius of 200 nm and a length of 70 ìm, a resonance frequency around 3.62 MHz with a quality factor (Q) of about 235 in air at ambient conditions is observed. It is shown that the measured frequency of the resonance is consistent with results from finite element simulations. Also, the measurements were performed in an enclosed chamber with controlled levels of ethanol vapor. The adsorption of ethanol causes a shift in the resonance frequency of the fibers, which can be related to the... 

In this paper we present experimental results describing electrical readout of the mechanical vibratory response of graphene-doped fibers by employing electrical actuation. For a fiber resonator with an approximate radius of 850 nm and length of 100 m, we observed a resonance frequency around 580 kHz with a quality factor (Q) of about 2511 in air at ambient conditions. Through the use of finite element simulations, we show that the reported frequency of resonance is relevant. We also show that the resonance frequency of the fiber resonators decreases as the bias potential is increased due to the electrostatic spring-softening effect  

Asymmetric sheet rolling is simulated by using an elastic-plastic arbitrary Lagrangian-Eulerian (ALE) finite element method. The ALE finite element approach easily predicts the curvature development due to inequality in work roll/plate surface finish (interface friction) and speed mismatch. An isotropic work hardening material is assumed. Reasonable agreements were found between the numerical method and the experimental results. © 2006 Elsevier B.V. All rights reserved  

This article presents the cyclic behavior of the A356.0 aluminum alloy under low-cycle fatigue (or isothermal) and thermo-mechanical fatigue loadings. Since the thermo-mechanical fatigue (TMF) test is time consuming and has high costs in comparison to low-cycle fatigue (LCF) tests, the purpose of this research is to use LCF test results to predict the TMF behavior of the material. A time-independent model, considering the combined nonlinear isotropic/kinematic hardening law, was used to predict the TMF behavior of the material. Material constants of this model were calibrated based on room-temperature and high-temperature low-cycle fatigue tests. The nonlinear isotropic/kinematic hardening... 

In this paper the effect of the re-autofrettage process on the residual stress distribution at the wall of a thick-walled tube is considered. For accurate material behavior modeling, it is assumed that the yield surface is a function of all the stress invariants. Also for estimating the behavior of the material under loading-unloading process, a modified Chaboche's hardening model is applied. For evaluation of this unloading behavior model a series of loading-unloading tests are conducted on specimens that are made of the high strength steel, DIN1.6959. In addition the finite element simulations are implemented to simulate the re-autofrettage process and to estimate the residual stresses.... 

Fretting fatigue occurs in engineering applications where two interacting surfaces are subjected to fluctuating forces. The fluctuating forces occasionally act as a combined loading condition. The present paper investigates fretting fatigue behaviour of 316L stainless steel under such combined (i.e. tensile and bending) loading conditions. A new fixture was developed in order to apply bending and tensile loads simultaneously. Using this fixture, the effect of different bending-to-tension ratios was investigated. The results showed that increasing the contribution of bending load improves fretting fatigue life. On the other hand, increasing grain size decreases fretting fatigue life.... 

Durability of the vehicle components needs special attention in the design step due to this fact that the loads on a vehicle are dynamic by their nature. Also, fatigue resistance of the vehicle body is quite important as it is the main load-bearing component among others. The main purpose of the present research is to simulate the spot weld failures of the vehicle body structure due to fatigue damage induced on the body during standardized maneuvers. This was accomplished by using a combination of multi-body dynamics and finite element analyses. To enhance the precision of the analysis, a thickness-dependent nugget diameter was utilized to model the spot welds. To validate the finite element... 

In-stent restenosis (ISR) is one of the main drawbacks of stent implementation which limits the long-term success of the procedure. Morphological changes occurring within the arterial wall due to stent-induced mechanical injury are a major cause for activation of vascular smooth muscle cells (VSMCs), and the subsequent development of ISR. Considering the theory of volumetric mass growth and adopting a multiplicative decomposition of the deformation gradient into an elastic part and a growth part, we present a mechanobiological model for ISR. An evolution equation is developed for mass growth of the neointima, in which the activation of VSMCs due to stent-induced damage (injury) and the... 

Coverage is one of the most important parameters which is always used in practice to characterize a shot peening process. At the same time however, it is the most missing parameter in the finite element simulations of this process. This study aims to relate shot peening simulation to the actual coverage that is developed during the process. Accordingly, two important models from literature are re-simulated and their capability to predict an actual coverage is assessed. Results of this study illustrate that full coverage situation is not captured by these models. Thereafter, a random finite element simulation along with a step by step examination of the treated surface is adopted in order to... 

In this paper, the effect of clamping time on welding distortion is studied. The welding process has been done in austenitic stainless steel 304L butt-welded joint by TIG welding method. The numerical thermo-mechanical analysis is performed using commercial finite element software ABAQUS. The welding simulation is performed using the birth and death of elements technique. The heat source model that used in this study is the Goldak double-ellipsoidal model. At last, for verifying numerical results, a set of experimental tests have been carried out and welding distortion has been measured. The experimental measured data are compared with numerical (FEM) results. The results showed that the... 

In the present study, a special heat treatment cycle (step quenching) was used to produce a dual-phase (DP) microstructure in low carbon steel. By producing this DP microstructure, the mechanical properties of the investigated steel such as yield stress, tensile strength, and Vickers hardness were increased 14, 55, and 38%, respectively. In order to investigate the effect of heat treatment on formability of the steel, Nakazima forming test was applied and subsequently finite element base modeling was used to predict the outcome on forming limit diagrams. The results show that the DP microstructure also has a positive effect on formability. The results of finite element simulations are in a... 

Forming limit diagram is often used as a criterion to predict necking initiation in sheet metal forming processes. In this study, the forming limit diagram was obtained through the inclusion of the Marciniak–Kaczynski model in the Nakazima out-of-plane test finite element model and also a flat model. The effect of bending on the forming limit diagram was investigated numerically and experimentally. Data required for this simulation were determined through a simple tension test in three directions. After comparing the results of the flat and Nakazima finite element models with the experimental results, the forming limit diagram computed by the Nakazima finite element model was more convenient... 

Experimental and numerical study regarding the uniaxial tensile test and the forming limit diagram are addressed in this paper for AL2024 with the face-centered cube structure. First, representation of a grain structure can be obtained directly by mapping metallographic observations via scanning electron microscopy approach. Artificial grain microstructures produced by Voronoi Tessellation method are employed in the model using VGRAIN software. By resorting to the finite element software (ABAQUS) capabilities, the constitutive equations of the crystal plasticity were utilized and implemented as a user subroutine material UMAT code. The hardening parameters were calibrated by a trial and... 

In this study, a periodic beam-like structure consisting of the horizontal and inclined beam elements is proposed. Three models with three different angles of inclination are considered. The generalized differential quadrature (GDQ) and generalized differential quadrature rule (GDQR) methods are used to solve the differential equations of longitudinal and transverse vibrations, respectively. The effects of two geometrical parameters on the first three band gaps of each model are investigated, comprehensively. Results show that this periodic structure has wide band gaps at low frequency ranges. Furthermore, the band gaps can get close to each other by changing the geometrical parameters.... 

In this study, a periodic beam-like structure consisting of the horizontal and inclined beam elements is proposed. Three models with three different angles of inclination are considered. The generalized differential quadrature (GDQ) and generalized differential quadrature rule (GDQR) methods are used to solve the differential equations of longitudinal and transverse vibrations, respectively. The effects of two geometrical parameters on the first three band gaps of each model are investigated, comprehensively. Results show that this periodic structure has wide band gaps at low frequency ranges. Furthermore, the band gaps can get close to each other by changing the geometrical parameters.... 

In this study, a periodic beam-like structure consisting of the horizontal and inclined beam elements is proposed. Three models with three different angles of inclination are considered. The generalized differential quadrature (GDQ) and generalized differential quadrature rule (GDQR) methods are used to solve the differential equations of longitudinal and transverse vibrations, respectively. The effects of two geometrical parameters on the first three band gaps of each model are investigated, comprehensively. Results show that this periodic structure has wide band gaps at low frequency ranges. Furthermore, the band gaps can get close to each other by changing the geometrical parameters.... 

Chatter vibration is a major cause of limitations in increasing material removal rate in machining. Machining chatter is caused by the interaction between tool and work-piece. Various nonlinear phenomena in the machining process affect the occurrence of chatter. Finite element simulation of chip formation allows for incorporation of various factors in the chip formation process. It can therefore be used to simulate the occurrence of chatter in practical conditions and to predict the conditions that lead to stable cutting. In this paper, a one degree of freedom dynamic model of the cutting tool is used to simulate the interaction of machining dynamics with the thermo-mechanical chip formation... 

In this study, the effect of a compressive normal stress has been considered in the determination of forming limit diagrams and forming limit stress diagrams to predict neck initiation failure in tube hydroforming of T-shaped parts. Computation of the forming limit diagrams and FLSDs is based on the generalized Marciniak and Kuczynski method to consider the existence of through-thickness compressive normal stress. The proposed forming limit diagrams and FLSDs were used in conjunction with ABAQUS/EXPLICIT finite element simulations to predict the onset of necking in tube hydroforming of T-shaped part. The amount of calibration pressure and axial feeding required to produce an acceptable part... 

This paper presents a dynamic model for brushless doubly fed machine including core loss. Using the space vector notation, the conventional equivalent circuit model of the machine is modified to take the core loss effect into account. The proposed method introduces resistances corresponding to stator/rotor core loss components designated to the power/control windings. This model can be used for enhanced time-domain simulations, design of high performance control strategies with core loss detuning effect compensation and also as a tool for stability analysis of the machine drive under different control methods. The dynamic model of a prototype machine is simulated with and without... 

The present study aims to challenge the existing finite element models in terms of one of the most important practical parameters, i.e. coverage. Important models from the literature are re-simulated and their resulted treated surfaces are carefully examined. Result of this study shows that existing finite element models could not reflect the realistic coverage. A variable dimension symmetry cell is developed in order to acquire full coverage and at the same time not increasing the computational cost. This model can successfully simulate the surface nanocrystallization by severe shot peening in which the amount of coverage is much higher than conventional shot peening