Browsing by Author "Lee, C. R."

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Free-vibration analysis and material constants identification of laminated composite sandwich plates
(2007-08-01)
Lee, C. R.
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Kam, T. Y.
;
Sun, S. J.
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機械工程學系
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Department of Mechanical Engineering
Free vibration of symmetrically laminated composite sandwich plates with elastic edge restraints is studied via the Rayleigh-Ritz approach. The proposed Rayleigh-Ritz method is constructed on the basis of the layer-wise linear displacement theory. The accuracy of the method in predicting natural frequencies of composite sandwich plates with different boundary conditions is verified by the results reported in the literature or the experimental data obtained in this study. The proposed method is then applied to the material constant identification of free composite sandwich plates using the first six theoretical natural frequencies of the plates. In the identification process, trial material constants are used in the present method to predict the theoretical natural frequencies, a frequency discrepancy function is established to measure the sum of the squared differences between the experimental and theoretical natural frequencies, and a stochastic global minimization algorithm is used to search for the best estimates of the material constants by making the frequency discrepancy function a global minimum. Applications of the material constant identification technique are demonstrated by means of several examples.
Identification of mechanical properties of elastically restrained laminated composite plates using vibration data
(2006-08-22)
Lee, C. R.
;
Kam, T. Y.
;
機械工程學系
;
Department of Mechanical Engineering
A nondestructive evaluation method using vibration data to determine mechanical properties (material and spring constants) of elastically restrained laminated composite plates is presented. The Rayleigh-Ritz method in which a set of Legendre's polynomials is adopted to approximate the plate deflection is used to determine the theoretical natural frequencies of the elastically restrained laminated composite plates. A number of natural frequencies extracted from the impulse vibration test data of the laminated composite plates supported by elastic restraints at both the edges and centers of the plates are used in the present method to determine the mechanical properties of the plates. The sum of the squared differences function which measures the differences between the experimentally and theoretically predicted natural frequencies of the elastically restrained laminated composite plates is established. The identification of the plate mechanical properties is then formulated as a constrained minimization problem in which the mechanical properties are determined by making the sum of the squared differences function a global minimum. The feasibility and accuracy of the proposed method are studied by means of several numerical examples on the mechanical properties identification of elastically restrained laminated composite plates with different layups made of various composite materials. Experimental investigation of the mechanical properties identification of several elastically restrained laminated composite plates has been performed to illustrate the applications of the present method. It has been shown that the present method can produce good estimates of the mechanical properties of the elastically restrained laminated composite plates in an efficient and effective way. (c) 2006 Elsevier Ltd. All rights reserved.
System identification of partially restrained composite plates using measured natural frequencies
(2006-08-01)
Lee, C. R.
;
Kam, T. Y.
;
機械工程學系
;
Department of Mechanical Engineering
A nondestructive evaluation technique established on the basis of a global minimization method is presented for the system identification of laminated composite plates partially restrained by elastic edge supports. Six natural frequencies extracted from the vibration data of the flexibly restrained plate are used to identify the system parameters of the plate. In the identification process, the trial system parameters are used in the Rayleigh-Ritz method to predict the theoretical natural frequencies of the plate, an error function is established to measure the sum of the differences between the experimental and theoretical predictions of the natural frequencies, and the global minimization method is used to search for the best estimates of the parameters by making the error function a global minimum. The accuracy and efficiency of the proposed technique in identifying the parameters of several flexibly supported plates made of different composite materials are studied via both theoretical and experimental approaches. The excellent results obtained in this study have validated the applicability of the proposed technique.
System parameters evaluation of flexibly supported laminated composite sandwich plates
(2007-09-01)
Lee, C. R.
;
Sun, S. J.
;
Kam, T. Y.
;
機械工程學系
;
Department of Mechanical Engineering
A nondestructive evaluation technique established on the basis of a global minimization method is presented for the system parameters identification of flexibly supported rectangular laminated composite sandwich plates using measured natural frequencies of the sandwich plates. In this study, the first eight natural frequencies extracted from impulsive vibration testing data are used to identify the system parameters of the flexibly supported sandwich plates. In the identification process, the trial system parameters are used in the Rayleigh-Ritz method to predict the theoretical natural frequencies of the sandwich plate, a frequency discrepancy function is established to measure the sum of the squared differences between the experimental and theoretical predictions of the natural frequencies, and the global minimization method is used to search for the best estimates of the system parameters by making the frequency discrepancy function a global minimum. The accuracy and efficiency of the proposed technique in identifying the system parameters of several flexibly supported sandwich plates made of different face and core materials are studied via both theoretical and experimental approaches. The reasonably good results obtained in this study have demonstrated the applicability of the proposed technique.
Twin-GD: A new twin gated-diode measurement for the interface characterization of ultra-thin gate oxide MOSFET's with EOT down to 1nm
(2006)
Lee, G. D.
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Chung, S. S.
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Mao, A. Y.
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Lin, W. M.
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Yang, C. W.
In this paper, a new twin gated-diode (T-GD) method has been greatly improved for the oxide interface characterization of MOS devices with gate oxide as thin as 1nm (EOT). With the scaling of gate oxide thickness into 1mn regime, reported GD measurement can not give correct measurement due to gate tunneling leakage current. Here, we provide a simple method to remove this limitation. This method has been demonstrated successfully for the ultra-thin (EOT = 10.2 angstrom) gate oxide nMOSFET devices. Application of the method to the PBTI effects of high-k gate dielectric devices has been demonstrated. It was found that high-K device has worse gate oxide quality, but its interface damage is less than that of control oxide.