• Journal of the Korean Society for Aeronautical & Space Sciences
• /
• v.32 no.7
• /
• pp.43-50
• /
• 2004

For the simultaneous measurement of strain and damage signal a fiber Bragg grating sensor system with a dual demodulator was proposed. The dual demodulator is composed of a demodulator using a tunable Fabry-Perot filter measuring the low-frequency signal with large magnitude such as strain and the other using a passive Mach-Zehnder interferometer detecting the high-frequency signal with small amplitude such as impact or damage signal. Using the proposed fiber Bragg grating sensor system, both the strain and damage signals of a cross-ply laminated composite beam under tensile loading were simultaneously measured. The strain and damage signals detected by single fiber Bragg grating sensor showed that sudden strain shifts were accompanied with vibration at a maximum frequency of several hundreds of kilohertz at the instant of matrix crack propagation in the 90 degree layer in composite beam.

• Journal of the Computational Structural Engineering Institute of Korea
• /
• v.27 no.5
• /
• pp.393-401
• /
• 2014

In this paper, we propose an evaluation scheme of the coefficients of thermal expansion (CTE) of constituents in composite materials using an inverse analysis. The size of constituents typically is about a few micrometers, which makes the identification of material properties difficult as well as the measurement results inaccurate. The proposed inverse analysis scheme, which is combined with the Mori-Tanaka method for predicting an equivalent CTE of composite materials, provides the CTE of the constituents in a straightforward manner by minimizing the cost function defined in lamina scale with the steepest descent method. To demonstrate the effectiveness and accuracy of the proposed scheme, the CTEs of several fibers (glass fiber, P75, P100, and M55J) embedded in matrix are evaluated and compared with experimental results. Furthermore, we discuss the effects of uncertainty of laminar and matrix properties on the prediction of fiber properties.

• Korean Journal of Materials Research
• /
• v.12 no.4
• /
• pp.296-303
• /
• 2002

For potential application to quantum mechanical devices, nano-composite thin films, consisting of GaAs quantum dots dispersed in SiO$_2$ glass matrix, were fabricated and studied in terms of structural, chemical, and optical properties. In order to form crystalline GaAs quantum dots at room temperature, uniformly dispersed in $SiO_2$matrix, the composite films were made to consist of alternating layers of GaAs and $SiO_2$in the manner of a superlattice using RF magnetron sputter deposition. Among different film samples, nominal thickness of an individual GaAs layer was varied with a total GaAs volume fraction fixed. From images of High Resolution Transmission Electron Microscopy (HRTEM), the formation of GaAs quantum dots on SiO$_2$was shown to depend on GaAs nominal thickness. GaAs deposits were crystalline and GaAs compound-like chemically according to HRTEM and XPS analysis, respectively. From measurement of optical absorbance using a spectrophotometer, absorption edges were determined and compared among composite films of varying GaAs nominal thicknesses. A progressively larger shift of absorption edge was noticed toward a blue wavelength with decreasing GaAs nominal thickness, i.e. quantum dots size. Band gaps of the composite films were also determined from Tauc plots as well as from PL measurements, displaying a linear decrease with increasing GaAs nominal thickness.

• Structural Engineering and Mechanics
• /
• v.84 no.4
• /
• pp.489-502
• /
• 2022

This paper is aimed at developing an optimization-based Finite Element model updating approach for structural damage identification and quantification. A modal flexibility-based error function is introduced, which uses modal assurance criterion to formulate the updating problem as an optimization problem. Because of the inexplicit input/output relationship between the candidate solutions and the error function's output, a robust and efficient optimization algorithm should be employed to evaluate the solution domain and find the global extremum with high speed and accuracy. This paper proposes a new multi-stage Selective Particle Swarm Optimization (SPSO) algorithm to solve the optimization problem. The proposed multi-stage strategy not only fixes the premature convergence of the original Particle Swarm Optimization (PSO) algorithm, but also increases the speed of the search stage and reduces the corresponding computational costs, without changing or adding extra terms to the algorithm's formulation. Solving the introduced objective function with the proposed multi-stage SPSO leads to a smart feedback-wise and self-adjusting damage detection method, which can effectively assess the health of the structural systems. The performance and precision of the proposed method are verified and benchmarked against the original PSO and some of its most popular variants, including SPSO, DPSO, APSO, and MSPSO. For this purpose, two numerical examples of complex civil engineering structures under different damage patterns are studied. Comparative studies are also carried out to evaluate the performance of the proposed method in the presence of measurement errors. Moreover, the robustness and accuracy of the method are validated by assessing the health of a six-story shear-type building structure tested on a shake table. The obtained results introduced the proposed method as an effective and robust damage detection method even if the first few vibration modes are utilized to form the objective function.

• Structural Engineering and Mechanics
• /
• v.63 no.6
• /
• pp.743-750
• /
• 2017

Modal parameter identification plays a key role in the structural health monitoring (SHM) for civil engineering. Eigensystem realization algorithm (ERA) is one of the most popular identification methods. However, the complex environment around civil structures can introduce the noises into the measurement from SHM system. The spurious modes would be generated due to the noises during ERA process, which are usually ignored and be recognized as physical modes. This paper proposes an improved stabilization diagram method in ERA to distinguish the spurious modes. First, it is proved that the ERA can be performed by any two Hankel matrices with one time step shift. The effect of noises on the eigenvalues of structure is illustrated when the choice of two Hankel matrices with one time step shift is different. Then, a moving data diagram is proposed to combine the traditional stabilization diagram to form the improved stabilization diagram method. The moving data diagram shows the mode variation along the different choice of Hankel matrices, which indicates whether the mode is spurious or not. The traditional stabilization diagram helps to determine the concerned truncated order before moving data diagram is implemented. Finally, the proposed method is proved through a numerical example. The results show that the proposed method can distinguish the spurious modes.

• Smart Structures and Systems
• /
• v.10 no.4_5
• /
• pp.331-351
• /
• 2012

In this study an output-only system identification technique for civil structures under ambient vibrations is carried out, mainly focused on using the Stochastic Subspace Identification (SSI) based algorithms. A newly developed signal processing technique, called Singular Spectrum Analysis (SSA), capable to smooth a noisy signal, is adopted for preprocessing the measurement data. An SSA-based SSI algorithm with the aim of finding accurate and true modal parameters is developed through stabilization diagram which is constructed by plotting the identified system poles with increasing the size of data matrix. First, comparative study between different approaches, with and without using SSA to pre-process the data, on determining the model order and selecting the true system poles is examined in this study through numerical simulation. Finally, application of the proposed system identification task to the real large scale structure: Canton Tower, a benchmark problem for structural health monitoring of high-rise slender structures, using SSA-based SSI algorithm is carried out to extract the dynamic characteristics of the tower from output-only measurements.

• Health Policy and Management
• /
• v.11 no.2
• /
• pp.123-140
• /
• 2001

Hypotheses In health services research are becoming increasingly more complex and specific. As a result, health services research studies often include multiple independent, intervening, and dependent variables in a single hypothesis. Nevertheless, the statistical models adopted by health services researchers have failed to keep pace with the increasing complexity and specificity of hypotheses and research designs. This article introduces a statistical model well suited for complex and specific hypotheses tests in health services research studies. The covariance structure modeling(CSM) methodology is especially applied to regional relevance indices(RIs) to assess the impact of health resources and healthcare utilization. Data on secondary statistics and health insurance claims were collected by each catchment area. The model for RI was justified by direct and indirect effects of three latent variables measured by seven observed variables, using ten structural equations. The resulting structural model revealed significant direct effects of the structure of health resources but indirect effects of the quantity on RIs, and explained 82% of correlation matrix of measurement variables. Two variables, the number of beds and the portion of specialists among medical doctors, became to have significant effects on RIs by being analyzed using the CSM methodology, while they were insignificant in the regression model. Recommendations for the CSM methodology on health service research data are provided.

• Wind and Structures
• /
• v.12 no.5
• /
• pp.401-412
• /
• 2009

A method is applied for the estimation of structural damage of tall slender structures using natural frequency and displacements measurements by GPS. The relationship between the variation in the global stiffness matrix (or in the stiffness of each finite element) and the change in the natural frequencies of the structure is given. In engineering practice the number of frequencies which can be derived by GPS measurement of long-period structures will be equal to one, two or three first natural frequencies. This allows us in initial studies to detect damage with frequency changes based on forward methods in which the measured frequencies are compared with the predicted analytical data. This idea, of health monitoring from possible changes to natural frequencies, or from a statement of excessive displacements is applied to the Stuttgart TV Tower.

• Proceedings of the KIEE Conference
• /
• 2005.07d
• /
• pp.2477-2479
• /
• 2005

A robust bending frequency tracker is proposed to design the adaptive notch filter which removes the time-varying missile structural modes from the sensor measurements. To do this, the state-space form of a bending frequency model is derived under the assumption that the bending signal could be described as the lightly damped sinusoid. Since the resultant bending frequency model contains the parametric uncertainties in the measurement matrix, the design problem of bending frequency tracker is tackled by applying the robust Kalman filter to the model. This technique could be easily expanded to the multiple frequencies case because it newly illuminates the bending frequency tracking problem in view of general state estimation.

• Journal of the Korean institute of surface engineering
• /
• v.45 no.1
• /
• pp.31-36
• /
• 2012

In order to improve the mechanical roperties of the Gangjin celadon $BaTiO_3$ was added into the raw materials of celadon matrix. Through SEM and XRD analysis the structural changes were observed and the hardness values were measured. We could confirm that the mechanical strength considerably increased in the $BaTiO_3$ added celadon through the measurement of hardness values. The increase of mechanical strength values in the celadon may result from the compositional change in the microstructure such as grain boundary area through EDAX analysis. We might suggest a fundamental idea to improve the mechanical intensity of the celadon.