Sharif Digital Repository

Analytical frequency analysis of a nonlinear viscoelastic microcantilever is performed based on strain gradient theory. The Kelvin–Voigt scheme is utilized to model the viscoelasticity effect. Due to the microcantilever shortening effect via Euler–Bernoulli inextensibility condition, geometric, inertia, stiffness, and inherent damping nonlinearities are considered. The equation of motion is derived from Hamilton’s principle, and discretized using Galerkin method. The multiple timescale perturbation method is performed to solve the time response equation. Implying steady-state condition, the nonlinear relation between detuning parameter and amplitude of the vibration of a nonlinear... 

Based on the first-order shear deformation (FSD) model and nonlocal elasticity theory, the simultaneous effects of shear and small scale on the nonlinear vibration behavior of functionally graded carbon nanotube-reinforced composite (FG-CNTRC) beams are investigated for the first time. To this end, the governing equations of bending and stretching with von Kármán geometric nonlinearity are decoupled into one fourth-order partial differential equation in terms of transverse deflection. A closed-form solution of the nonlinear natural frequency, which can be used in conceptual design and optimization algorithms of FG- CNTRC beams with different boundary conditions, is developed using a hybrid... 

In this paper, a size-dependent formulation is developed for Timoshenko beams made of functionally graded materials (FGMs). The developed formulation is based on the strain gradient theory; a non-classical continuum theory able to capture the size-effect in micro-scaled structures. Five new equivalent length scale parameters are introduced as functions of the constituents' length scale parameters. It is shown that the size-dependent static and dynamic behavior of FG micro-beams can be described using these equivalent length scales. The governing differential equations of motion and both classical and non-classical sets of boundary conditions are derived for the proposed strain gradient FG... 

In MEMS gyroscopes, it is essential to use matched resonance frequencies of the drive and sense vibrational modes for improving the sensitivity. For this end, the natural frequencies can be tuned by voltages. In this study, a new model is utilized to determine the natural frequencies of the doubly clamped beam microgyroscope. In the model, nonlinear electrostatic forces, fringing fields and mid-plane stretching of thebeam are considered. The system is actuated and sensed by electrostatic force and its natural frequencies and stiffness are detuned by DC voltages. The oscillatory problem of the gyroscope is analytically solved versus DC voltages for different design parameters. Copyright  

In this paper, a size-dependent formulation is developed for Timoshenko beams made of functionally graded materials (FGM). The developed formulation is based on the strain gradient theory;a non-classical continuum theory able to capture the size-effect in micro-scaled structures. Considering the material length scale parameters of the FG beams vary through the thickness, the new equivalent length scale parameters are proposed as functions of the constituents' length scale parameters to describe the size-dependent static and dynamic behavior of FG microbeams. The governing differential equations of equilibrium and both classical and nonclassical sets of boundary conditions are derived for the... 

This paper deals with the dynamic analysis of a delaminated Bernoulli-Euler beam with constrained model under the action of an external moving force. The beam is analyzed as four interconnected sub-beams using the delamination as their boundaries. The continuity and compatibility conditions are satisfied between adjoining beams. An analytical solution is presented in a basis of the series expansion of the unknown deflection. The delaminated beam response differences with the healthy beam have been investigated. Effect of the parameters like the moving speed of the force and the size, depth and spanwise location of the delamination on the dynamic response of the beam has been studied.... 

The governing differential equation and both classical and non-classical boundary conditions of strain gradient bars are derived using variational approach. A closed-form analytical solution is obtained for static torsion and the characteristic equation, which gives the natural frequencies, is derived and analytically solved for the free torsional vibrations of the strain gradient microbars. A fixed-fixed microbar is considered as a specific case to investigate the torsional size-dependent static and free-vibration behavior of strain gradient microbars. The results of the current model are compared to those of the modified couple stress and classical theories  

In this paper, two lowest resonant frequencies and sensitivities of an AFM V-Shaped microcantilever made of functionally graded materials are studied. The beam is modeled by Euler-Bernoulli beam theory in which rotary inertia and shear deformation is neglected. It is assumed that the beam is made of a mixture of metal and ceramic with properties varying through the thickness of the beam. This variation is function of volume fraction of beam material constituents. The interaction between AFM tip and surface is modeled by two linear springs which expresses the normal and lateral contact stiffness. A relationship is developed to evaluate the sensitivity of FGM micro cantilever beam. Effect of... 

Self-excited and forced vibrations are important topics in machining processes because their occurrence results in poor surface finish, increase in tool wear and hampers productivity. In this paper, turning process is modeled as a SDOF dynamic system including quadratic and cubic structural nonlinearities. The effect of tool flank wear, as a contact force between the work-piece and tool, is addressed vigorously. Multiple scale method is used to find the solution of the nonlinear dynamic equation including regenerative chatter, forced excitation and tool wear. It is shown that, width of cut can be considered as the bifurcation parameter of the system. Primary, super-harmonic and sub-harmonic... 

Traumatic brain injury (TBI) has long been known as one of the most anonymous reasons for death around the world. This phenomenon has been under study for many years and yet it remains a question due to physiological, geometrical and computational complexity. Although the modeling facilities for soft tissue have improved, the precise CT-imaging of human head has revealed novel details of the brain, skull and meninges. In this study a 3D human head including the brain, skull, and meninges is modeled using CT-scan and MRI data of a 30-year old human. This model is named "Sharif University of Technology Head Trauma Model (SUTHTM)". By validating SUTHTM, the model is then used to study the... 

The biomechanical behavior of brain tissue is needed for predicting the traumatic brain injury (TBI). Each year over 1.5 million people sustain a TBI in the United States. The appropriate coefficients for modeling the injury prediction can be evaluated using experimental data. In the present paper, using an experimental setup on bovine brain tissue, unconfined compression tests at quasi-static strain rates of ϵ 0.0004s-1, 0.008s-1 and 0.4s-1 combined with a stress relaxation test under unconfined uniaxial compression with ϵ 0.67s-1 ramp rate are performed. The fitted viscohyperelastic parameters were utilized by using obtained stressstrain curves. The finite element analysis (FEA) is... 

In this study, dynamic modeling of an elbow axially functionally graded (AFG) macro/micro-tube carrying magnetic flow with different cross-sections is considered. Parametric optimization is performed for vibration suppression of such fluid-interaction systems. Implementing computer simulations, passive vibration control procedures, along with the effect of AFG materials and magnetic properties of the fluid as well as precisely manufactured geometry of the system, is investigated. It is assumed that the material characteristics of the system vary in the longitudinal direction based on exponential and power-law distribution profiles. Influence of the downstream inclination angle and... 

This study proposes a new method for the detection of a weak scatterer among strong scatterers using prior-information ultrasound (US) imaging. A perfect application of this approach is in vivo cell detection in the bloodstream, where red blood cells (RBCs) serve as identifiable strong scatterers. In vivo cell detection can help diagnose cancer at its earliest stages, increasing the chances of survival for patients. This work combines time-domain US with frequency-domain compressive US imaging to detect a 20-μ MCF-7 circulating tumor cell (CTC) among a number of RBCs within a simulated venule inside the mouth. The 2D image reconstructed from the time-domain US is employed to simulate the... 

In this study, dynamic modeling of an elbow axially functionally graded (AFG) macro/micro-tube carrying magnetic flow with different cross-sections is considered. Parametric optimization is performed for vibration suppression of such fluid-interaction systems. Implementing computer simulations, passive vibration control procedures, along with the effect of AFG materials and magnetic properties of the fluid as well as precisely manufactured geometry of the system, is investigated. It is assumed that the material characteristics of the system vary in the longitudinal direction based on exponential and power-law distribution profiles. Influence of the downstream inclination angle and... 

Within the compressive sensing (CS) framework, one effective way to increase the likelihood of successful signal reconstruction is to employ random processes in the construction of the sensing matrix. This study presents a 3D holey cavity, with diverse frequency modes, to spectrally code, that is, randomize, the ultrasound wave fields. The simulated results show that the use of such a cavity enables imaging simple or complex targets, such as spheres or the letter E, by only a single transceiver—something that is not possible without the use of a coding structure like the cavity. The effect of noise on imaging results and the size of the targets on the first-order Born approximation (BA) are...