Browsing by Author "Chen, C. M."

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A two-level optimization procedure for material characterization of composites using two symmetric angle-ply beams
(2007-10-01)
Chen, C. M.
;
Kam, T. Y.
;
機械工程學系
;
Department of Mechanical Engineering
A two-level optimization procedure for determining elastic constants E-1, E-2, G(12), and v(12) of laminated composite materials using measured axial and lateral strains of two symmetric angle-ply beams with different fiber angles subjected to three-point-bending testing is presented. In the first-level optimization process, the theoretically and experimentally predicted axial and lateral strains of a [(45 degrees/-45 degrees)(6)](s) beam are used to construct the strain discrepancy function which is a measure of the sum of the squared differences between the experimental and theoretical predictions of the axial and lateral strains. The identification of the material constants is then formulated as a constrained minimization problem in which the best estimates of shear modulus and Poisson's ratio of the beam are determined to make the strain discrepancy function a global minimum. In the second-level optimization process, shear modulus and Poisson's ratio determined in the first level of optimization are kept constant and Young's moduli of the second angle-ply beam with fiber angles different from 45 degrees are identified by minimizing the strain discrepancy function established t this level of optimization. The suitability of the proposed procedure for material characterization of composite materials has been demonstrated by means of a number of examples. (c) 2007 Elsevier Ltd. All rights reserved.
A two-level optimization procedure for material characterization of composites using two symmetric angle-ply beams
(2007-10-01)
Chen, C. M.
;
Kam, T. Y.
;
機械工程學系
;
Department of Mechanical Engineering
A two-level optimization procedure for determining elastic constants E-1, E-2, G(12), and v(12) of laminated composite materials using measured axial and lateral strains of two symmetric angle-ply beams with different fiber angles subjected to three-point-bending testing is presented. In the first-level optimization process, the theoretically and experimentally predicted axial and lateral strains of a [(45 degrees/-45 degrees)(6)](s) beam are used to construct the strain discrepancy function which is a measure of the sum of the squared differences between the experimental and theoretical predictions of the axial and lateral strains. The identification of the material constants is then formulated as a constrained minimization problem in which the best estimates of shear modulus and Poisson's ratio of the beam are determined to make the strain discrepancy function a global minimum. In the second-level optimization process, shear modulus and Poisson's ratio determined in the first level of optimization are kept constant and Young's moduli of the second angle-ply beam with fiber angles different from 45 degrees are identified by minimizing the strain discrepancy function established t this level of optimization. The suitability of the proposed procedure for material characterization of composite materials has been demonstrated by means of a number of examples. (c) 2007 Elsevier Ltd. All rights reserved.
Compact efficient high-power triple-color Nd:YVO4 yellow-lime-green self-Raman lasers
(2020-03-01)
Liu, Y. C.
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Chen, C. M.
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Hsiao, J. Q.
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Pan, Y. Y.
;
Tsou, C. H.
A novel, to the best of our knowledge, approach is developed to realize a high-power compact efficient yellow-lime-green triple-color Nd : YVO4 self-Raman laser. The 588 nm yellow laser, the 559 nm lime laser, and the 532 nm green laser are converted from the 1064 nm fundamental wave and the 1176 nm Stokes Raman field. The simultaneous three-color operation is accomplished with three stages to step-by-step generate the 588 nm, 559 nm, and 532 nm lasers by using three different lithium triborate (LBO) crystals. By tuning the temperature of each individual LBO crystal, the 588 nm, 559 nm, and 532 nm output powers can be nearly the same and concurrently up to 2.4W at the incident pump power of 30 W, corresponding to a conversion efficiency of 24% for the total output power. (C) 2020 Optical Society of America
Criterion for optimizing high-power acousto-optically Q-switched self-Raman yellow lasers with repetition rates up to 500 kHz
(2020-04-01)
Chen, Y. F.
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Chen, K. Y.
;
Liu, Y. C.
;
Chen, C. M.
;
Tsou, C. H.
The criterion for optimizing the high-power acousto-optically Q-switched self-Raman yellow laser is originally explored for the repetition rate within 100-500 kHz. The minimum allowed value for the gate-open time is experimentally verified to be determined by the pulse buildup time. By using the minimum allowed gate-open time, the highest conversion efficiency can be achieved to raise the output power by approximately 20% in comparison with the conventional results. At a repetition rate of 200 kHz, the maximum output power at 588 nm can be up to 8.8 W at an incident pump power of 26 W. Furthermore, a practical formula is developed to accurately calculate the threshold pump power as a function of the gate-open time for a given repetition rate. (C) 2020 Optical Society of America
Effect of supercritical fluids on field emission from carbon nanotubes
(2006)
Liu, P. T.
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Tsai, C. T.
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Kin, K. T.
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Chang, P. L.
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Chen, C. M.
This paper proposes a novel method to enhance the emission characteristics of carbon nanotubes (CNTs). It is extremely possible for CNTs to adsorb moisture and other contaminants during the fabrication processes, leading to the degraded field emission characteristics. In this work, CNT emitters are activated with commonly used heating process and supercritical carbon dioxide (SCCO2) fluids technology for removing adsorbed residue moisture. Experimental results have demonstrated that the electrical stability and field emission enhancement of CNT emitters are effectively achieved by the SCCO2 fluids treatment compared to the heating process, due to the minimization of residuary moisture in CNTs.
Elastic constants identification of composite materials using single angle-ply laminate
(2006-11-01)
Chen, C. M.
;
Kam, T. Y.
;
機械工程學系
;
Department of Mechanical Engineering
A method is presented for identification of elastic constants for composite materials using three measured strains of a single angle-ply laminate subjected to tensile testing. In the proposed method, the trial material constants of the angle-ply laminate are used to predict the corresponding strains in the laminate. An error function is established to measure the difference between the experimental and theoretical predictions of the strains. The identification of material constants is then formulated as a constrained minimization problem in which the material constants are determined to make the error function a global minimum. The accuracy and capability of the proposed method are demonstrated by means of a number of examples of the identification of material constants of angle-ply laminates with different lay-ups. Experimental data obtained from static tensile tests of several angle-ply laminates are used to identify the material constants of the laminates. The excellent results obtained in the experimental investigation have validated the applicability of the proposed method.
Elastic constants identification of symmetric angle-ply laminates via a two-level optimization approach
(2007-03-01)
Chen, C. M.
;
Kam, T. Y.
;
機械工程學系
;
Department of Mechanical Engineering
A two-level optimization method for elastic constants identification of symmetric angle-ply laminates is presented. Measured axial and lateral strains of two symmetric angle-ply laminates with different fiber angles are used in the proposed method to identify four elastic constants of the composite laminates. In the first-level optimization process, the theoretically and experimentally predicted axial and lateral strains of a [(45 degrees/-45 degrees)(2)](s) laminate are used to construct the error function which is a measure of the differences between the experimental and theoretical predictions of the axial and lateral strains. The identification of the material constants is then formulated as a constrained minimization problem in which the best estimates of the shear modulus and Poisson's ratio of the laminate are determined by making the error function a global minimum. The problem of this level of optimization is then solved using a multi-start global minimization algorithm. In the second-level optimization process, the shear modulus and Poisson's ratio determined in the previous level of optimization are kept constant while the Young's moduli of the second angle-ply laminate with fiber angles other than 45 degrees are identified using the same minimization technique that has been used in the previous level. The accuracy of the proposed method are studied by means of a number of numerical examples on the material constants identification of symmetric angle-ply laminates made of different composite materials. Finally, static tensile tests of [(45 degrees/-45 degrees)(2)](s) and [(30 degrees/-30 degrees)(2)](s) laminates made of Gr/ep composite material are performed to measure the strains of the laminates. The experimental data are then used to identify the elastic constants of the laminates. The excellent results obtained in the experimental investigation have demonstrated the feasibility and applications of the proposed method. (c) 2006 Elsevier Ltd. All rights reserved.
High-temperature magnetoresistance study of a magnetic tunnel junction
(2006-09-01)
Chen, D. C.
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Yao, Y. D.
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Chen, C. M.
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Hung, James
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Chen, Y. S.
The thermal stability and the spin transportation phenomenon at room temperature and 140 degrees C of a series of magnetic tunneling junctions with the structure of bottom electrode/PtMn/Pinned layer/ AlO(x)/CoFe/NiFe/top electrode have been investigated. The MR ratio decreases from 33.5% at room temperature to 29% at 140 degrees C. The MR ratio at room temperature increases roughly 0.8% after thermal treatment at temperatures above 60 degrees C. This is related to the thermal relaxation of the strains existing in the samples. (C) 2006 Elsevier B.V. All rights reserved.
Material characterization of laminated composite materials using a three-point-bending technique
(2009-05-01)
Kam, T. Y.
;
Chen, C. M.
;
Yang, S. H.
;
機械工程學系
;
Department of Mechanical Engineering
A three-point-bending technique is presented for identifying the elastic constants of laminated composite materials. In the proposed technique, three strains in the axial, lateral, and 45 directions on the bottom surface at the mid-span of a symmetric angle-ply beam subjected to three-point-bending testing are measured for elastic constants identification. The narrow beam theory together with the trial elastic constants is used to predict the theoretical strains of the beam. The theoretical and experimental strains of the beams are then used in a stochastic optimization method to identify the elastic constants of the beam. The accuracy and applications of the proposed technique are demonstrated by means of a number of examples on the elastic constants identification of graphite/epoxy (Gr/ep) or glass/epoxy (Gl/ep) laminated composite materials. The effects of specimen aspect ratio and thickness on the accuracy of the proposed method are investigated. (C) 2008 Elsevier Ltd. All rights reserved.