• Structural Engineering and Mechanics
• /
• v.77 no.4
• /
• pp.535-547
• /
• 2021

This paper concerns with free torsional vibration analysis of size dependent circular nanobars with von kármán type nonlinearity. Although review of the literature suggests several studies employing nonlocal elasticity theory to investigate linear torsional behavior, linear/nonlinear transverse vibration and buckling of the nanoscale structures, so far, no study on the nonlinear torsional behavior of the nanobars, considering the size effect, has been reported. This study employs nonlocal elasticity theory along with a variational approach to derive nonlinear equation of motion of the nanobar. Then, the nonlinear equation is solved using the elliptic functions to extract the natural frequencies of the structure under fixed-fixed and fixed-free end conditions. Finally, the natural frequencies of the nanobar under different nanobar lengths, diameters, nonlocal parameters, and amplitudes of vibration are reported to illustrate the effect of these parameters on the vibration characteristics of the nanobars. In addition, the phase plane diagrams of the nanobar for various cases are reported.

• Transactions of Materials Processing
• /
• v.22 no.7
• /
• pp.394-400
• /
• 2013

Microforming is a very efficient and economical technology to fabricate very small metallic parts in various applications. In order to extend the use of this forming technology for the production of microparts, the size effect, which occurs with the reduction of part size and affects the forming process significantly, must be thoroughly investigated. In this study, the tribological size effect in microforming was studied using modeling and scaled ring compression experiments. A micro-scale friction approach based on the slip-line field theory and lubricant pocket model was used to understand the friction mechanism and explain the tribological size effect. Ring compression tests were performed to analyze the interfacial friction condition from the deformation characteristics of the ring specimens. In addition, finite element analysis results were utilized to quantitatively determine the size-dependent frictional behavior of materials in various process conditions. By comparing theoretical results and experimental measurements for different size factors, the accuracy and reliability of the model were verified.

• Advances in nano research
• /
• v.14 no.1
• /
• pp.45-65
• /
• 2023

This paper aimed to investigate the nonclassical size dependent free vibration behavior of regularly squared cutout viscoelastic nanobeams. The nonlocal strain gradient elasticity theory is modified and adopted to incorporate the viscoelasticity effect. The Kelvin Voigt viscoelastic model is adopted to model the linear viscoelastic constitutive response. To explore the influence of shear deformation effect due to cutout, both Euler Bernoulli and Timoshenko beams theories are considered. The Hamilton principle is utilized to derive the dynamic equations of motion incorporating viscoelasticity and size dependent effects. Closed form solutions for the resonant frequencies for both perforated Euler Bernoulli nanobeams (PEBNB) and perforated Timoshenko nanobeams (PTNB) are derived considering different boundary conditions. The developed procedure is verified by comparing the obtained results with the available results in the literature. Parametric studies are conducted to show the influence of the material damping, the perforation, the material and the geometrical parameters as well as the boundary and loading conditions on the dynamic behavior of viscoelastic perforated nanobeams. The proposed procedure and the obtained results are supportive in the analysis and design of perforated viscoelastic NEMS structures.

• Advances in nano research
• /
• v.4 no.3
• /
• pp.197-228
• /
• 2016

In the present study, thermo-electro-mechanical vibration characteristics of functionally graded piezoelectric (FGP) Timoshenko nanobeams subjected to in-plane thermal loads and applied electric voltage are carried out by presenting a Navier type solution for the first time. Three kinds of thermal loading, namely, uniform, linear and non-linear temperature rises through the thickness direction are considered. Thermo-electro-mechanical properties of FGP nanobeam are supposed to vary smoothly and continuously throughout the thickness based on power-law model. Eringen's nonlocal elasticity theory is exploited to describe the size dependency of nanobeam. Using Hamilton's principle, the nonlocal equations of motion together with corresponding boundary conditions based on Timoshenko beam theory are obtained for the free vibration analysis of graded piezoelectric nanobeams including size effect and they are solved applying analytical solution. According to the numerical results, it is revealed that the proposed modeling can provide accurate frequency results of the FGP nanobeams as compared to some cases in the literature. In following a parametric study is accompanied to examine the effects of several parameters such as various temperature distributions, external electric voltage, power-law index, nonlocal parameter and mode number on the natural frequencies of the size-dependent FGP nanobeams in detail. It is found that the small scale effect and thermo-electrical loading have a significant effect on natural frequencies of FGP nanobeams.

• Journal of the Korean Ceramic Society
• /
• v.29 no.1
• /
• pp.65-73
• /
• 1992

The effect of grain size on the time-dependent piezoelectrice degradation of a poled PZT of MPB composition Pb0.988Sr0.012 (Zr0.52Ti0.48)O3 with 2.4 mol% of Nb2O5 was studied, and the degradation mechanism was discussed. Changes in the internal bias field and the internal stress both responsible for the time-dependent degradation of poled PZT were examined by the polarization reveral technique, XRD and Vickers indentation, respectively. The piezoelectric degradation increased with increasing time and grain size, and the internal bias field due to space charge diffusion decreased with increasing grain size of poled PZT. The internal bias field, however, was almost insensitive to the degradation time regardless of the grain size. On the other hand, both the x-ray diffraction peak intensity ratio of (002) to (200) and the fracture behavior including the crack propagation support that the ferroelectric domain rearrangement of larger grain size showed rapid relaxation of the internal stress compared with smaller one, which is thought the origin of the larger piezoelectric degradation in the former. In conclusion, the contribution of space charge diffusion on the piezoelectric degradation of PZT is strongly dependent on both the grain size and the composition. Thus, the relaxation of internal stress due to the ferroelectric domain rearrangement as well as the amount and time-dependence of the internal bias field due to space charge diffusion should be considered simultaneously in the degradation mechanism of PZT.

• Structural Engineering and Mechanics
• /
• v.71 no.1
• /
• pp.37-43
• /
• 2019

This paper studies application of modified couple stress theory and first order shear deformation theory to magneto-electro-mechanical vibration analysis of three-layered size-dependent curved beam. The curved beam is resting on Pasternak's foundation and is subjected to mechanical, magnetic and electrical loads. Size dependency is accounted by employing a small scale parameter based on modified couple stress theory. The magneto-electro-mechanical preloads are accounted in governing equations to obtain natural frequencies in terms of initial magneto-electro-mechanical loads. The analytical approach is applied to investigate the effect of some important parameters such as opening angle, initial electric and magnetic potentials, small scale parameter, and some geometric dimensionless parameters and direct and shear parameters of elastic foundation on the magneto-electro-elastic vibration responses.

• Advances in nano research
• /
• v.7 no.4
• /
• pp.277-292
• /
• 2019

In this present paper, a new two dimensional (2D) and quasi three dimensional (quasi-3D) nonlocal shear deformation theories are formulated for free vibration analysis of size-dependent functionally graded (FG) nanoplates. The developed theories is based on new description of displacement field which includes undetermined integral terms, the issues in using this new proposition are to reduce the number of unknowns and governing equations and exploring the effects of both thickness stretching and size-dependency on free vibration analysis of functionally graded (FG) nanoplates. The nonlocal elasticity theory of Eringen is adopted to study the size effects of FG nanoplates. Governing equations are derived from Hamilton's principle. By using Navier's method, analytical solutions for free vibration analysis are obtained through the results of eigenvalue problem. Several numerical examples are presented and compared with those predicted by other theories, to demonstrate the accuracy and efficiency of developed theories and to investigate the size effects on predicting fundamental frequencies of size-dependent functionally graded (FG) nanoplates.

• Structural Engineering and Mechanics
• /
• v.48 no.3
• /
• pp.415-434
• /
• 2013

This work is concerned with transverse vibrations of axially traveling nanobeams including strain gradient and thermal effects. The strain gradient elasticity theory and the temperature field are taken into consideration. A new higher-order differential equation of motion is derived from the variational principle and the corresponding higher-order non-classical boundary conditions including simple, clamped, cantilevered supports and their higher-order "offspring" are established. Effects of strain gradient nanoscale parameter, temperature change, shape parameter and axial traction on the natural frequencies are presented and discussed through some numerical examples. It is concluded that the factors mentioned above significantly influence the dynamic behaviors of an axially traveling nanobeam. In particular, the strain gradient effect tends to induce higher vibration frequencies as compared to an axially traveling macro beams based on the classical vibration theory without strain gradient effect.

• Advances in nano research
• /
• v.12 no.1
• /
• pp.101-115
• /
• 2022

Some researchers pointed out that the nonlocal cantilever models do not predict the dynamic softening behavior for nanostructures (including nanobeams) with clamped-free (CF) ends. In contrast, some indicate that the nonlocal cantilever models can capture the stiffness softening characteristics. There are substantial differences on this issue between them. The vibration analysis of porosity-dependent functionally graded nanoscale tubes with variable boundary conditions is investigated in this study. Using a modified power-law model, the tube's porosity-dependent material coefficients are graded in the radial direction. The theory of nonlocal strain gradients is used. Hamilton's principle is used to derive the size-dependent governing equations for simply-supported (S), clamped (C) and clamped-simply supported (CS). Following the solution of these equations by the extended differential quadrature technique, the effect of various factors on vibration issues was investigated further. It can be shown that these factors have a considerable effect on the vibration characteristics. It also can be found that our numerical results can capture the unexpected softening phenomena for cantilever tubes.

• Journal of the Korea Academia-Industrial cooperation Society
• /
• v.20 no.4
• /
• pp.214-223
• /
• 2019

This study is a meta-analysis based on results of empirical studies related to work-life balance(WLB), and the relationships between WLB and other variables. In order to achieve this objective, articles published in domestic journals prior to December 2018 were collected. Data was collected using an online database provided by the Korea Educational and Scientific Information Service, and a total of 27 studies and 126 sub data were coded. Data was analyzed using CMA (comprehensive meta-analysis) 3.0 program. Results of this study are as follows. First, the overall mean effect size of WLB was 0.365, indicating a small effect size. Second, the effect sizes of dependent variables influenced by WLB included immersion, innovation, and performance in order. Third, the effect size of organizational focus variables was more than twice as big as that of individual focus variables. Fourth, the negative theoretical background dependent variables of WLB, such as sacrifice, job stress, and turnover showed -0.254 effect size, and the positive theoretical background dependent variables, such as job satisfaction and emotional commitment have mid-size effect (0.576). Fifth, the effect size of independent variables were in the order of work-development balance, work-home balance, and work-leisure balance.