• Steel and Composite Structures
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• v.26 no.4
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• pp.421-437
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• 2018

Free vibration analysis of a three-layered microbeam including an elastic micro-core and two piezo-magnetic face-sheets resting on Pasternak's foundation are studied in this paper. Strain gradient theory is used for size-dependent modeling of microbeam. In addition, three-unknown shear and normal deformations theory is employed for description of displacement field. Hamilton's principle is used for derivation of the governing equations of motion in electro-magneto-mechanical loads. Three micro-length-scale parameters based on strain gradient theory are employed for prediction of vibrational characteristics of structure in micro-scale. The results show that increase of three micro-length-scale parameters leads to significant increase of three natural frequencies especially for increase of second micro-length-scale parameter. This result is according to this fact that stiffness of a micro-scale structure is increased with increase of micro-length-scale parameters.

• Advances in nano research
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• v.8 no.1
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• pp.49-58
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• 2020

This paper investigates the vibration characteristics of flexoelectric nanobeams resting on viscoelastic foundation and subjected to magneto-electro-viscoelastic-hygro-thermal (MEVHT) loading. In this regard, the Nonlocal strain gradient elasticity theory (NSGET) is employed. The proposed formulation accommodates the nonlocal stress and strain gradient parameter along with the flexoelectric coefficient to accurately predict the frequencies. Further, with the aid of Hamilton's principle the governing differential equations are derived which are then solved through Galerkin-based approach. The variation of the natural frequency of MEVHT nanobeams under the influence of various parameters such as the nonlocal strain gradient parameter, different field loads, power-law exponent and slenderness ratio are also investigated.

• Structural Engineering and Mechanics
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• v.71 no.3
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• pp.257-269
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• 2019

This paper develops a nonlocal strain gradient plate model for damping vibration analysis of smart magneto-electro-viscoelastic nanoplates resting on visco-Pasternak medium. For more accurate analysis of nanoplate, the proposed theory contains two scale parameters related to the nonlocal and strain gradient effects. Viscoelastic effect which is neglected in all previous papers on magneto-electro-viscoelastic nanoplates is considered based on Kelvin-Voigt model. Governing equations of a nonlocal strain gradient smart nanoplate on viscoelastic substrate are derived via Hamilton's principle. Galerkin's method is implemented to solve the governing equations. Effects of different factors such as viscoelasticity, nonlocal parameter, length scale parameter, applied voltage and magnetic potential on damping vibration characteristics of a nanoplate are studied.

• Journal of information and communication convergence engineering
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• v.12 no.1
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• pp.60-65
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• 2014

This paper describes an efficient framework for the extraction of a brain tumor from magnetic resonance (MR) images. Before the segmentation process, a median filter is used to filter the image. Then, the morphological gradient is computed and added to the filtered image for intensity enhancement. After the enhancement process, the thresholding value is calculated using the mean and the standard deviation of the image. This thresholding value is used to binarize the image followed by the morphological operations. Moreover, the combination of these morphological operations allows to compute the local thresholding image supported by a flood-fill algorithm and a pixel replacement process to extract the tumor from the brain. Thus, this framework provides a new source of evidence in the field of segmentation that the specialist can aggregate with the segmentation results in order to soften his/her own decision.

• Journal of Biomedical Engineering Research
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• v.21 no.2
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• pp.129-136
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• 2000

중재적 시술을 위한 자기공명영상(MRI)용 주자석은 수직 자계를 가지는 경우가 대부분인데 본 논문에서는 수직 자계를 발생하는 주자석에 장착할 수 있는 볼록형 경사자계코일을 소개하였다. 중재적 시술에 필요한 고속 촬영을 하기 위해서는 강한 경사자계 및 낮은 코일 인덕턱스가 필요한데 본 논문에서는 이를 효율적으로 실현하기 위해 경사자계코일을 볼록 곡면 위에 실현하였다. 기존 방법에서처럼 평면 위에 경사자계코일을 실현하지 않고 볼록 곡면 위에 실현함으로써 경사자계코일의 자계 강도 특성 및 코일 인덕턱스 특성을 향상시킬 수 있을 뿐만 아니라 중재적 시술을 위한 경사자계코일 내 공간을 충분히 확보할 수 있다. Prolate spheroid 좌표계에서 표현되는 경사자계코일 면을 정의하였고, 유한요소법을 이용한 볼록형 경사자계코일 설계 방법을 기술하였다. 또한 경사자계코일 면의 곡률에 따라 경사자계코일의 성능이 어떻게 변화하는지에 대한 결과를 제시하였다.

• Proceedings of the KIEE Conference
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• 1998.07a
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• pp.143-145
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• 1998

This paper presents the shape optimization algorithm of magnetic systems with, state variable constraints using the Finite Element Method. In the design' of electromagnetic systems, sometimes we have to consider the state variables when they seriously affect the performance of electromagnetic systems. So we should define that some design problems have the constraints of the state variables. We use the gradient of constraints and sensitivity analysis in order to consider the state variable constraints and obtain an optimal shape. The optimal shape must be satisfied constraints, so we take the gradient projection method as a kind of optimization methods. In this paper a numerical example with state variable constraints uses the superconducting electromagnet that has another constraint which the volume of the superconductor should be constant.

• Journal of Institute of Control, Robotics and Systems
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• v.8 no.2
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• pp.97-103
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• 2002

This paper deals with a PWM(Pulse Width Modulation) current amplifier using digital controller in order to generate a gradient magnetic field far the MRI(Magnetic Resonance Image) system. Because of tolerance of discrete devices, it is difficult to set accurate values of the control parameters and to make an analog-controlling circuit. However, using digital controller, it is possible to set exact control parameters and to adopt a modern control techniques. It is shown that the digital controller will highly enhance the output current response and it will improve the quality of the MRI.

• Journal of Magnetics
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• v.18 no.3
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• pp.225-229
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• 2013

We report a measurement for the anomalous Nernst effects induced by a temperature gradient in [CoSiB/Pt] multilayer films with perpendicular magnetic anisotropy. The Nernst voltage shows a characteristic hysteresis which reflects the magnetization of the film as in the case of the anomalous Hall effects. With a local heating geometry, we also measure the dependence of the anomalous Nernst voltage on the distance d from the heating element. It is roughly proportional to $1/d^{1.3}$, which can be conjectured from the expected temperature gradient along the sample from the heat equation.

• Steel and Composite Structures
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• v.26 no.4
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• pp.513-531
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• 2018

In this article, static, buckling and free vibration analyses of a sinusoidal micro composite beam reinforced by single-walled carbon nanotubes (SWCNTs) with considering temperature-dependent material properties embedded in an elastic medium in the presence of magnetic field under transverse uniform load are presented. This system is used at micro or sub micro scales to enhance the stiffness of micro composite structures such as bar, beam, plate and shell. In the present work, the size dependent effects based on surface stress effect and modified strain gradient theory (MSGT) are considered. The generalized rule of mixture is employed to predict temperature-dependent mechanical and thermal properties of micro composite beam. Then, the governing equations of motions are derived using Hamilton's principle and energy method. Numerical results are presented to investigate the influences of material length scale parameters, elastic foundation, composite fiber angle, magnetic intensity, temperature changes and carbon nanotubes volume fraction on the bending, buckling and free vibration behaviors of micro composite beam. There is a good agreement between the obtained results by this research and the literature results. The obtained results of this study demonstrate that the magnetic intensity, temperature changes, and two parameters elastic foundations have important effects on micro composite stiffness, while the magnetic field has greater effects on the bending, buckling and free vibration responses of micro composite beams. Moreover, it is shown that the effects of surface layers are important, and observed that the changes of carbon nanotubes volume fraction, beam length-to-thickness ratio and material length scale parameter have noticeable effects on the maximum deflection, critical buckling load and natural frequencies of micro composite beams.

• Smart Structures and Systems
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• v.19 no.3
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• pp.309-322
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• 2017

In this paper, the buckling, and free vibration analysis of tapered functionally graded carbon nanotube reinforced composite (FG-CNTRC) micro Reddy beam under longitudinal magnetic field using finite element method (FEM) is investigated. It is noted that the material properties of matrix is considered as Poly methyl methacrylate (PMMA). Using Hamilton's principle, the governing equations of motion are derived by applying a modified strain gradient theory and the rule of mixture approach for micro-composite beam. Micro-composite beam are subjected to longitudinal magnetic field. Then, using the FEM, the critical buckling load, and natural frequency of micro-composite Reddy beam is solved. Also, the influences of various parameters including ${\alpha}$ and ${\beta}$ (the constant coefficients to control the thickness), three material length scale parameters, aspect ratio, different boundary conditions, and various distributions of CNT such as uniform distribution (UD), unsymmetrical functionally graded distribution of CNT (USFG) and symmetrically linear distribution of CNT (SFG) on the critical buckling load and non-dimensional natural frequency are obtained. It can be seen that the non-dimensional natural frequency and critical buckling load decreases with increasing of ${\beta}$ for UD, USFG and SFG micro-composite beam and vice versa for ${\alpha}$. Also, it is shown that at the specified value of ${\alpha}$ and ${\beta}$, the dimensionless natural frequency and critical buckling load for SGT beam is more than for the other state. Moreover, it can be observed from the results that employing magnetic field in longitudinal direction of the micro-composite beam increases the natural frequency and critical buckling load. On the other hands, by increasing the imposed magnetic field significantly increases the stability of the system that can behave as an actuator.