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Sy, Ngoc Nguyen,Lee, Jaehun,Cho, Maenghyo The Korean Society for Aeronautical and Space Scie 2012 International Journal of Aeronautical and Space Sc Vol.13 No.4
In this study, the linear viscoelastic response of a rectangular laminated plate is investigated. The viscoelastic properties, expressed by two basic spring-dashpot models, that is Kelvin and Maxwell models, is assumed in the range to investigate the influence of viscoelastic coefficients to mechanical behavior. In the present study, viscoelastic responses are performed for two popular equivalent single-layered theories, such as the first-order shear deformation theory (FSDT) and third-order shear deformation theory (TSDT). Compliance and relaxation modulus of time-dependent viscoelastic behavior are approximately determined by Prony series. The constitutive equation for linear viscoelastic material as the Boltzmann superposition integral equation is simplified by the convolution theorem of Laplace transformation to avoid direct time integration as well as to improve both accuracy and computational efficiency. The viscoelastic responses of composite laminates in the real time domain are obtained by applying the inverse Laplace transformation. The numerical results of viscoelastic phenomena such as creep, cyclic creep and recovery creep are presented.
Ngoc Nguyen Sy,Jaehun Lee,Maenghyo Cho 한국항공우주학회 2012 International Journal of Aeronautical and Space Sc Vol.13 No.4
In this study, the linear viscoelastic response of a rectangular laminated plate is investigated. The viscoelastic properties, expressed by two basic spring-dashpot models, that is Kelvin and Maxwell models, is assumed in the range to investigate the influence of viscoelastic coefficients to mechanical behavior. In the present study, viscoelastic responses are performed for two popular equivalent single-layered theories, such as the first-order shear deformation theory (FSDT) and third-order shear deformation theory (TSDT). Compliance and relaxation modulus of time-dependent viscoelastic behavior are approximately determined by Prony series. The constitutive equation for linear viscoelastic material as the Boltzmann superposition integral equation is simplified by the convolution theorem of Laplace transformation to avoid direct time integration as well as to improve both accuracy and computational efficiency. The viscoelastic responses of composite laminates in the real time domain are obtained by applying the inverse Laplace transformation. The numerical results of viscoelastic phenomena such as creep, cyclic creep and recovery creep are presented.
Viscoelastic behavior of Naghdi shell model based on efficient higher-order zig-zag theory
Nguyen, Sy-Ngoc,Lee, Jaehun,Cho, Maenghyo Elsevier 2017 Composite structures Vol.164 No.-
<P><B>Abstract</B></P> <P>This paper proposes a method based on efficient higher-order zig-zag theory to analyze the viscoelastic response of doubly-curved laminated shell structures. In the general curvilinear coordinates, displacement fields are obtained by imposing a varying cubic displacement field on a varying linear zig-zag field. Then, the transverse shear stress-free condition at the top and bottom surfaces and the continuity condition at the interfaces are employed to reduce the number of unknown variables. The Laplace transformation is then used to simplify the integral-formed constitutive equation for viscoelastic material in the real time domain into a linear system equation in the Laplace domain so that all computations can be carried out in the Laplace domain. Therefore, the equilibrium equation for general viscoelastic Naghdi shell model can be obtained by converting the virtual work principle into the Laplace domain. Finally, solutions for the long-term viscoelastic properties in the real-time domain are obtained by using numerical inverse Laplace techniques. To simplify the formulation and conveniently evaluate the method proposed in the present study and to compare its outcomes with those of an elastic laminated composite shell, several numerical examples for a cylindrical shallow shell model are investigated.</P>
Sy-Ngoc Nguyen,Chien Truong-Quoc,Jang-woo Han,Sunyoung Im,Maenghyo Cho 대한기계학회 2021 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.35 No.10
Recurrent neural network (RNN)-based accelerated prediction was achieved for the long-term time-dependent behavior of viscoelastic composite laminated Mindlin plates subjected to arbitrary mechanical and hygrothermal loading. Time-integrated constitutive stressstrain relation was simplified via Laplace transform to a linear system to reduce the computational storage. A fast converging smooth finite element method named cell-based smoothed discrete shear gap was employed to enhance the data generation procedure for straining RNNs with a sparse mesh. This technique is applicable under varying hygrothermal conditions for real engineering structure problems with fluctuating temperature and moisture. Hence, accurate RNN-based long-term deformation prediction for laminated structures was realized using the history of environmental temperature and moisture condition.
Hong Khanh Dieu Nguyen,Toan Dang Nguyen,Dung Ngoc Hoang,Duc Sy Dao,Thao Tien Nguyen,Limphirat Wanwisa,Lan Linh Hoang 한국화학공학회 2017 Korean Journal of Chemical Engineering Vol.34 No.2
Hydrotalcite-like compound containing metal cations such as Mg2+, Al3+ and Ni2+ was characterized using Ni K-edge EXAFS and in situ Ni K-edge XANES techniques for clarifying its bonding environment around Ni2+ sites and structure changes during calcination from room temperature to 550 oC, respectively. At the fixed molar ratio of Mg/Ni/Al of 2/1/1, the results obtained from EXAFS analysis showed a slight blue shift before and after the calcination at 550 oC and a reduction in white line peak; the best fits of the two samples revealed tiny change in coordination number about 7 for Ni-O path but considerable difference for Ni-Mg(Al) path from about 4.5 to 9.5, confirming a modification from brucite like to mixed oxide structure. On the other hand, bond distances of the Ni-O and Ni-Mg paths nearly fixed at about 2.06 Å to 3.0 Å reflected stability of the cationic bond order on each plane, but partial collapse and decomposition of the interlayer formed by water molecules and anion CO3 2− after the calcination. Linear combination fit extracted from the in situ Ni K-edge XANES also confirmed the changes along with the calcination such as slow and fast decreases of brucite fraction at 150 oC and 330 oC, respectively, in corresponding to the mixed oxide fraction increases. The achieved bonding structures were also applied to explain acid-base occurrence of the hydrotalcite-like material, especially the acid sites generated by different static charges along with the bonds. The explanation was illustrated by NH3-TPD method.
Han, Jang-Woo,Kim, Jun-Sik,Nguyen, Sy-Ngoc,Cho, Maenghyo AMERICAN SOCIETY MECHANICAL ENGINEERS 2016 Journal of applied mechanics Vol.83 No.3
<P>An enhanced first-order shear deformation theory (EFSDT) is developed for linear viscoelastic analysis of laminated composite and sandwich plates. Improved strain energy expression of the conventional Reissner/Mindlin first-order shear deformation theory (FSDT) through strain energy transformation is derived in the Laplace domain by minimizing the strain energy difference between FSDT and an efficient higher-order zigzag theory (EHOPT). The convolution theorem of Laplace transformation is applied to circumvent the complexity of dealing with linear viscoelastic materials. The present EFSDT with the Laplace domain approach has the same computational advantage of the conventional FSDT while improving upon the accuracy of the viscoelastic response by utilizing the postprocess recovery procedure. The accuracy and efficiency of the proposed theory are demonstrated through the numerical results obtained herein by comparing to those available in the open literature.</P>