• Smart Structures and Systems
• /
• v.33 no.1
• /
• pp.41-53
• /
• 2024

Grouting sleeves are an essential connecting component of prefabricated components, and the quality of grouting has a significant influence on structural integrity and seismic performance. The embedded grouting sleeve (EGS)'s grouting defects are highly undetectable and random, and no effective monitoring method exists. This paper proposes an ultrasonic guided wave method and provides a set of guidelines for selecting the optimal frequency and suitable period for the EGS. The optimal frequency was determined by considering the group velocity, wave structure, and wave attenuation of the selected mode. Guided waves are prone to multi-modality, modal conversion, energy leakage, and dispersion in the EGS, which is a multi-layer structure. Therefore, a time-reversal (TR)-based multi-mode focusing and dispersion automatic compensation technology is introduced to eliminate the multi-mode phase difference in the EGS. First, the influence of defects on guided waves is analyzed according to the TR coefficient. Second, two major types of damage indicators, namely, the time domain and the wavelet packet energy, are constructed according to the influence method. The constructed wavelet packet energy indicator is more sensitive to the changes of defecting than the conventional time-domain similarity indicator. Both numerical and experimental results show that the proposed method is feasible and beneficial for the detection and quantitative estimation of the grouting defects of the EGS.

• Structural Engineering and Mechanics
• /
• v.84 no.6
• /
• pp.813-822
• /
• 2022

In this article, the buckling and free vibration of multi-directional FGM sandwich plates are investigated. The material properties of FGM sandwich plates are assumed to be varying continuously in the in the longitudinal, transverse and thickness directions. The material properties are evaluated based on Voigt's micro-mechanical model considering power law distribution method with arbitrary power index. Equations of motion for the buckling and vibration analysis of multi-directional FGM sandwich plate are obtained based on refined shear deformation theory. Analytical solution for simply supported multidirectional FGM sandwich plate is carried out using Navier's solution technique. The FGM sandwich plate considered in this work has a homogeneous ceramic core and two functionally graded face sheets. Influence of volume fraction index in the longitudinal, transverse and thickness direction, layer thickness, and geometrical parameter over natural frequency and critical buckling load of multi-directional FGM sandwich plate is investigated. The finding shows a multi-directional functionally graded structures perform better compared to uni-directional gradation. Hence, critical grading parameters have been identified which will guide researchers in selecting fabrication routes for improving the performance of such structures.

• Smart Structures and Systems
• /
• v.8 no.4
• /
• pp.413-431
• /
• 2011

Under the external shearing stress, the external radial stress and the electric potential simultaneously, the piezoelectric hollow cylinder transducer is studied. With the Airy stress function method, the analytical solutions of this transducer are obtained based on the theory of piezo-elasticity. The solutions are compared with the finite element results of Ansys and a good agreement is found. Inherent properties of this piezoelectric cylinder transducer are presented and discussed. It is very helpful for the design of the bearing controllers.

• Proceedings of the IEEK Conference
• /
• 2000.11b
• /
• pp.341-344
• /
• 2000

Several new via structures in printed circuit boards are proposed, fabricated and characterized in RF regime. The new structure with a larger inductance component in the bottom layer shows 3㏈ improvement over the conventional structure. The ADS simulation with model parameters extracted from 3D fie]d solver matches with the characterization of these vias

• Journal of the Earthquake Engineering Society of Korea
• /
• v.1 no.4
• /
• pp.59-68
• /
• 1997

In analysis of underground structures, the effects of artificial boundary conditions are considered as one of the major reasons for differences from experimental results. These phenomena can be overcome by using the boundary elements which satisfy the multi-layered half space conditions. The fundamental solutions of multi-layered half-space for boundary element method is formulated satisfying the transmission and reflection of waves at each layer interface and radiation conditions at bottom layer. The governing equations can be obtained from the displacements at each layer which are expressed in terms of harmonic functions. All types of waves can be included using the complete response from semi-infinite integrals with respect to horizontal wavenumbers using expansion of Fourier series and Hankel transformation. Two dimensional Green's functions are derived from cylindrical Navier equations and potentials performing infinite integration in y-direction. In this case, it is effective to transform into two dimensional problem using semi-analytical integration and sinusoidal Bessel function. Some verifications are given to show the accuracy and efficiency of the developed method, and numerical examples to demonstrate the dynamic behavior of underground with various properties.

• Steel and Composite Structures
• /
• v.44 no.2
• /
• pp.241-254
• /
• 2022

The cables in a cable-stayed bridge are critical load-carrying parts. The potential damage to cables should be identified early to prevent disasters. In this study, an efficient deep learning model is proposed for the damage identification of cables using both a multi-layer perceptron (MLP) and a graph neural network (GNN). Datasets are first generated using the practical advanced analysis program (PAAP), which is a robust program for modeling and analyzing bridge structures with low computational costs. The model based on the MLP and GNN can capture complex nonlinear correlations between the vibration characteristics in the input data and the cable system damage in the output data. Multiple hidden layers with an activation function are used in the MLP to expand the original input vector of the limited measurement data to obtain a complete output data vector that preserves sufficient information for constructing the graph in the GNN. Using the gated recurrent unit and set2set model, the GNN maps the formed graph feature to the output cable damage through several updating times and provides the damage results to both the classification and regression outputs. The model is fine-tuned with the original input data using Adam optimization for the final objective function. A case study of an actual cable-stayed bridge was considered to evaluate the model performance. The results demonstrate that the proposed model provides high accuracy (over 90%) in classification and satisfactory correlation coefficients (over 0.98) in regression and is a robust approach to obtain effective identification results with a limited quantity of input data.

• Journal of the Optical Society of Korea
• /
• v.18 no.1
• /
• pp.71-77
• /
• 2014

We present design and fabrication of a multi-focusing microlens array (MLA) using a thermal reflow method. To obtain multi-focusing properties with different numerical apertures at the elemental lens of the MLA, double-cylinder photoresist (PR) structures with different diameters were made within the guiding pattern with both photolithographic and partial developing processes. Due to the base PR layer supporting the thermal reflow process and the guiding structure, the thermally reflowed PR structure had different radii of curvatures with lens shapes that could be precisely modeled by the initial volume of the double-cylinder PR structures. Using the PR template, the hexagonally packed multi-focusing MLA was made via the replica molding method, which showed four different focal lengths of 0.9 mm, 1.1 mm, 1.6 mm, and 2.5 mm, and four different numerical apertures of 0.1799, 0.2783, 0.3973, and 0.4775.

• Journal of Welding and Joining
• /
• v.30 no.6
• /
• pp.120-125
• /
• 2012

Because ships and offshore structures have very large dimensions and complicated shapes, it is difficult to determine the deformation or internal stress in the structure by simple lab tests. Thus, a rigorous analysis by using the computer simulation technology is essential for obtaining their distortions by considering the entire production process characteristics. The rapid development of computer technology made it possible to analyze the heat transfer phenomena, deformation and phase transformation in the welded joint. For large shell structures, shell elements modeling contributed primarily to this development. But if a welding is done by multi-pass, shell elements whose thickness are unchangeable can hard to describe the local situation. Recently, it was researched how to introduce the imaginary temperature for V grooved multi-layer butt welding in strain-boundary method (a kind of shrinkage methodologies). In the present study, we formulated the imaginary temperature for the double bevel and double V groove by considering the thickness change of each pass through the bead and the thickness directions simultaneously and also demonstrated the feasibility of the formula by applying it to the thermal distortion analysis of the erection process of crane pedestal.

• Proceedings of the Korean Vacuum Society Conference
• /
• 2012.02a
• /
• pp.531-531
• /
• 2012

So many groups have been researching the green quantum dots such as InP, InP/ZnS for overcoming the semiconductor nanoparticles composed with heavy metals like as Cd and Pb so on. In spite of much effort to keep up CdSe quantum dots, it does not reach the good properties compared with CdSe/ZnS quantum dots. This quantum dot has improved its properties through the generation of core/shell CdSe/ZnS structure or core/multi-shell structures like as CdSe/CdS/ZnS and CdSe/CdS/ CdZnS/ZnS. In this research, we try to synthesize the InP multi-shell structure by the successiveion layer absorption reaction (SILAR) in the one pot. The synthesized multi-shell structure has improved quantum yield and photo-stability. To generate white light, highly luminescent InP multi-shell quantum dots were mixed with yellow phosphor and integrated on the blue LED chip. This InP multi-shell improved red region of the LEDs and generated high CRI.

• Korean Journal of Materials Research
• /
• v.20 no.4
• /
• pp.187-193
• /
• 2010

Chromium nitride (CrN) samples with two different layer structures (multilayer and single layer) were coated on bipolar plates of polymer electrolyte membrane fuel cells (PEMFC) using the reactive sputtering method. The effects with respect to layer structure on corrosion resistance and overall cell performance were investigated. A continuous and thin chromium nitride layer ($Cr_{0.48}\;N_{0.52}$) was formed on the surface of the SUS 316L when the nitrogen flow rate was 10 sccm. The electrochemical stability of the coated layers was examined using the potentiodynamic and potentiostatic methods in the simulated corrosive circumstances of the PEMFC under $80^{\circ}C$. Interfacial contact resistance (ICR) between the CrN coated sample and the gas diffusion layer was measured by using Wang's method. A single cell performance test was also conducted. The test results showed that CrN coated SUS316L with multilayer structure had excellent corrosion resistance compared to single layer structures and single cell performance results with $25\;cm^2$ in effective area also showed the same tendency. The difference of the electrochemical properties between the single and multilayer samples was attributed to the Cr interlayer layer, which improved the corrosion resistance. Because the coating layer was damaged by pinholes, the Cr layer prevented the penetration of corrosive media into the substrate. Therefore, the CrN with a multilayer structure is an effective coating method to increase the corrosion resistance and to decrease the ICR for metallic bipolar plates in PEMFC.