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Taejeon, South Korea

Choi J.,Machinery | Song O.,Chungnam National University | Kim S.-K.,Korea Atomic Energy Research Institute
Acta Mechanica | Year: 2013

Both divergence (static) and flutter (dynamic) instabilities of carbon nanotubes conveying fluid and modeled as thin-walled beams are investigated in this paper. The effects of boundary conditions, geometric nonlinearity, non-classical transverse shear, and rotary inertia on the static and dynamic instability characteristics are studied. Governing equations and various boundary conditions are derived simultaneously via extended Hamilton's principle. Numerical analysis is performed using the extended Galerkin method which enables us to obtain solutions even when it is not feasible to find trial functions satisfying all the boundary conditions. The variations in critical flow velocity with both geometric parameters and three different boundary conditions of carbon nanotubes are investigated and compared with those of a linear system, and pertinent conclusions are outlined. © 2013 Springer-Verlag Wien.

Jee K.K.,Korea Institute of Science and Technology | Choi W.I.,Chosun University | Kim K.H.,Machinery | Jang W.Y.,Chosun University
Advanced Materials Research | Year: 2012

The effect of thermal cycling on the martensitic transformation and response characteristics has been studied in bias-SMA spring actuators aged at 350 °C ∼ 500 °C. As the number of thermal cycling increases, the M s temperature of the SMA spring showing only B2→B19′ martensitic transformation by aging at 350 or 500 °C goes down. However, SMA spring in which B2→R→B19′ martensitic transformation taken place by aging at 400 or 450 °C shows multi-step transformations i.e., M1 and M2 transformations with thermal cycling. The heat flow of M2 transformation increases while the transformation temperature difference between M1 and M2 transformations is enlarged as the number of cycling increases. The recovery displacement and force of bias-SMA spring actuator increase up to the 10th cycle and the extent of increase is the largest at aging temperature of 500 °C. However, both recovery properties decrease after the 103 or 104 cycles, while the decreasing tendency becomes larger with a rise in aging temperature. Such a change in recovery characteristics of bias-SMA spring actuator with thermal cycling is discussed in connection with training effect or lattice defects introduced during thermal cycling. © (2012) Trans Tech Publications.

Lee S.-H.,Machinery | Kim H.,University of California at San Diego | Hang S.,University of California at San Diego | Cheong S.-K.,Seoul National University of Science and Technology
Composites Science and Technology | Year: 2012

A nonwoven carbon tissue (NWCT), composed of conventional carbon fibers, was coated with carbon nanotubes (CNT) to compose a multi-scale reinforcing interleave layer which can be directly incorporated into the composite laminate layup process. This CNT-enhanced NWCT layer was found to significantly improve the Modes I and II interlaminar fracture toughness of a carbon fiber reinforced polymer (CFRP) laminate, as measured by double-cantilever-beam (DCB) and end-notched-flexure (ENF) tests. G IC and G IIC of the CNT-enhanced NWCT specimens were compared with baseline CFRP specimens having no interleave, and also specimens having NWCT interleave with no CNT. The mean G IC of the CNT-enhanced NWCT specimens was measured to be 353% higher than the CFRP specimens, a great improvement over the NWCT specimens which showed a 5% reduction relative to CFRP. The mean G IIC of the CNT-enhanced NWCT specimens was 246% higher than the CFRP specimens, which is a significant improvement over the 194% increase measured for the NWCT specimens. Increased G IIC was mainly achieved via crack bridging provided by the NWCT carbon fiber morphology. Additional improvement in G IIC, and the increase in G IC (rather than decrease) of the CNT-enhanced NWCT specimens was due to local CNT reinforcement of the resin near the NWCT fiber surfaces. Scanning electron microscopy shows evidence of improved resin-to-NWCT interface adhesion, short fiber breakage, and CNT pullout at the fracture surfaces. © 2012 Elsevier Ltd.

Lee S.-J.,Korea Atomic Energy Research Institute | Cho B.-G.,Machinery | Lee I.,KAIST
Computers and Fluids | Year: 2012

Compressible aerodynamics is analyzed by the CIP/CCUP (constraint interpolation profile/CIP-combined unified procedure) method. The CIP method is feasible for an analysis of various phase change problems, those associated with compressible and incompressible flow areas. For aeroelastic problems based on CFD (computational fluid dynamics), the use of Lagrangian body-fitted grids is problematic because these grids increase the skewness of a mesh, or can be broken - even during structural motions. Therefore, a method embedding a physical boundary in a fixed Eulerian grid is appropriate compared to Lagrangian grids. In this paper, the CCUP method based on a pressure-based algorithm in which the pressure Poisson equation is modified to deal with compressible flows. A collocated grid system with the velocity components on the cell face obtained from the CIP interpolations is applied. Boundary condition for arbitrary surfaces are newly derived using a simple algebraic relationship based on the immersed boundary method. Far-field boundary condition is replaced with sponge layers. Several numerical benchmarking problems, in this case a wedge or a bump under transonic and supersonic flows, are tested to verify the code. Unsteady motions are directly applied to embedded solid areas. Finally, the steady and unsteady aerodynamics of airfoil sections are calculated and compared to reference data to validate the proposed method. © 2011 Elsevier Ltd.

Kang D.-H.,Machinery | Jang C.,Machinery | Park Y.-S.,National University of Civil Engineering | Han S.-Y.,National University of Civil Engineering | Kim J.H.,Yeungnam University
Advances in Mechanical Engineering | Year: 2012

The fatigue reliability of a steel member in a bridge is estimated by using the probabilistic stress-life method. The stress history of a member is defined as the loading block when a truck passes over a bridge, and the stress range frequency distribution of the stress history is obtained by a stress range frequency analysis. A probabilistic method is applied to the stress range frequency distribution, and the parameters of the probability distribution for the stress range frequency distribution are used in a numerical simulation. To obtain the probability of failure of a member under a loading block, Monte Carlo simulation is performed in conjunction with Miner's rule, the modified Miner's rule, and Haibach's rule for fatigue damage evaluation. Through these analyses procedures, we obtain an evaluation method for fatigue reliability that can predict the block number of the failure load and residual fatigue life. © 2012 Dae-Hung Kang et al.

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