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CONTENTS
Volume 5, Number 4, December 2016
 

Abstract
The effects of large rotations and p-delta on the dynamic response of a structure subjected to seismic loading and local uplift of its foundation were analyzed in this work. The structure was modeled by an equivalent flexible mat mounted on a rigid foundation that is supported either by a Winkler soil type or a rigid soil. The equations of motion of the system were derived by taking into account the equilibrium of the coupled foundation-mat system where the structure was idealized as a single-degree-of-freedom. The obtained nonlinear coupled system of ordinary differential equations was integrated by using an adequate numerical scheme. A parametric study was performed then in order to evaluate the maximum response of the system as function of the intensity of the earthquake, the slenderness of the structure, the ratio of the mass of the foundation to the mass of the structure. Three cases were considered: (i) local uplift of foundation under large rotation with the p-delta effect, (ii) local uplift of foundation under large rotation without including the p-delta effect, (iii) local uplift of foundation under small rotation. It was found that, in the considered ranges of parameters and for moderate earthquakes, assuming small rotation of foundation under seismic loading can yield more adverse structural response, while the p-delta effect has almost no effect.

Key Words
seismic response; Winkler foundation; rigid soil; local uplift; large rotations; p-delta effect

Address
Nadia El Abbas and Mohammed Bezzazi: Department of Physics, Laboratory of Civil Engineering and Mechanics, Faculty of Science and Techniques, Tangier, Morocco
Abdellatif Khamlichi: Department of STIC, National School of Applied Sciences, Tetouan, Morocco

Abstract
We utilize first-principles calculations within density-functional theory to investigate the possibility of strain engineering in the tuning of the band structure of two-dimensional MoS2. We find that the band structure of MoS2 monolayers transits from direct to indirect when mechanical strain is applied. In addition, we discuss the change in the band gap energy and the critical stains for the direct-to-indirect transition under various strains such as uniaxial, biaxial, and pure shear. Biaxial strain causes a larger change, and the pure shear stain causes a small change in the electronic band structure of the MoS2 monolayer. We observe that the change in the interaction between molecular orbitals due to the mechanical strain alters the band gap type and energy.

Key Words
molybdenum disulfide; density functional theory; mechanical strain; band gap engineering

Address
Soon-Dong Park: School of Materials Science and Engineering, Ulsan National Institute of Science and Technology, 50 UNIST-gil, Ulju-gun, Ulsan 689-798, Republic of Korea
Sung Youb Kim: Department of Mechanical Engineering, Ulsan National Institute of Science and Technology,
50 UNIST-gil, Ulju-gun, Ulsan 689-798, Republic of Korea

Abstract
Undergoing large volumetric changes upon incremental environmental stimulation, hydrogels are interesting materials which hold immense potentials for utilization in a wide array of applications in diverse industries. Owing to the large magnitudes of deformation it undergoes, swelling induced instability is a commonly observed sight in all types of gels. In this work, we investigate the instability of photo-thermal sensitive hydrogels, produced by impregnating light absorbing nano-particles into the polymer network of a temperature sensitive hydrogel, such as PNIPAM. Earlier works have shown that by using lights of different intensities, these hydrogels follow different swelling trends. We investigate the possibility of utilizing this fact for remote switching applications. The analysis is built on a thermodynamic framework of inhomogeneous large deformation of hydrogels and implemented via commercial finite element software, ABAQUS. Various examples of swelling induced instabilities, and its corresponding dependence on light intensity, will be investigated. We show that the instabilities that arise have their morphologies dependent on the light intensity.

Key Words
dual-sensitive; photo-thermal sensitive hydrogel; bifurcation; buckling; instability

Address
William Toh and Teng Yong Ng: School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore
Zhiwei Ding: Institute of High Performance Computing, Fusionopolis Way, #16-16 Connexis, Singapore 138632, Singapore
Zishun Liu: International Center for Applied Mechanics, State Key Laboratory for Strength and Vibration of Mechanical Structures, Xi

Abstract
The trigonal shape memory alloys (SMAs) have a great potential to be utilized as the applications with special purposes, such as actuators with high operation frequency. Most studies on the trigonal microstructures typically focus on the well-known classic herringbone pattern, but many other patterns are also possible, such as non-classic herringbone, toothbrush and checkerboard patterns. In the current work, a systematic procedure is developed to find all possible laminate twin microstructures by using geometrically linear compatibility theory. The procedure is verified by SEM images with the information of crystallographic axes of unitcells obtained by EBSD, showing good agreement. Many interesting trigonal R-phase patterns are found in the specimen. Then, their incompatibility are analyzed with nonlinear compatibility theory. The relationship between such incompatibility and the likelihood of occurrence of the microstructures is revealed. The current procedure is rapid, computationally efficient and sufficiently general to allow further extension to other crystal systems and materials.

Key Words
shape memory alloys; compatibility; laminate twins

Address
Tzu-Cheng Liu and Nien-Ti Tsou: Department of Materials Science and Engineering, NCTU, Ta Hsueh Road, HsinChu 300, Taiwan

Abstract
Manufactures of multi-crystalline silicon ingots by means of the directional solidification system (DSS) is important to the solar photovoltaic (PV) cell industry. The quality of the ingots, including the grain size and morphology, is highly related to the shape of the crystal-melt interface during the crystal growth process. We performed numerical simulations to analyze the thermo-fluid field and the shape of the crystal-melt interface both for steady conditions and transient processes. The steady simulations are first validated and then applied to improve the hot zone design in the furnace. The numerical results reveal that, an additional guiding plate weakens the strength of vortex and improves the desired profile of the crystal-melt interface. Based on the steady solutions at an early stage, detailed transient processes of crystal growth can be simulated. Accuracy of the results is supported by comparing the evolutions of crystal heights with the experimental measurements. The excellent agreements demonstrate the applicability of the present numerical methods in simulating a practical and complex system of directional solidification system.

Key Words
directional solidification system; multi-crystalline silicon; crystal growth; solar photovoltaic cell

Address
Ting-Kang Lin, Chung-Hao Lin and Ching-Yao Chen: Department of Mechanical Engineering, National Chiao Tung University, Taiwan, R.O.C.

Abstract
Changes in chemical structure have profound effects on the physical properties of epoxy-based materials, and eventually affect the durability of the entire system. Microscopic structural voids generally existing in the epoxy cross-linked networks have a detrimental influence on the epoxy mechanical properties, but the relation remains elusive, which is hindered by the complex structure of epoxy-based materials. In this paper, we investigate the effect of structural voids on the epoxy-based materials by using our developed mesoscale model equipped with the concept of multiscale modeling, and SU-8 photoresist is used as a representative of epoxy-based materials. Developed from the results of full atomistic simulations, the mesoscopic model is validated against experimental measurements, which is suitable to describe the elastic deformation of epoxy-based materials over several orders of magnitude in time- and length scales. After that, a certain quantity of the structure voids is incorporated in the mesoscale model. It is found that the existence of structural voids reduces the tensile stiffness of the mesoscale epoxy network, when compared with the case without any voids in the model. In addition, it is noticed that a certain number of the structural voids have an insignificant effect on the epoxy elastic properties, and the mesoscale model containing structural voids is close to those found in real systems.

Key Words
multiscale modeling; molecular dynamics; mesoscale mechanics; epoxy-based materials; structural voids

Address
Lik-ho Tam: Department of Architecture and Civil Engineering, City University of Hong Kong, Hong Kong, China
Denvid Lau:
1) Department of Architecture and Civil Engineering, City University of Hong Kong, Hong Kong, China
2) Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge,
Massachusetts 02139, USA

Abstract
We propose a heat jet approach for a two-dimensional square lattice with nearest neighbouring harmonic interaction. First, we design a two-way matching boundary condition that linearly relates the displacement and velocity at atoms near the boundary, and a suitable input in terms of given incoming wave modes. Then a phonon representation for finite temperature lattice motion is adopted. The proposed approach is simple and compact. Numerical tests validate the effectiveness of the boundary condition in reflection suppression for outgoing waves. It maintains target temperature for the lattice, with expected kinetic energy distribution and heat flux. Moreover, its linear nature facilitates reliable finite temperature atomic simulations with a correct description for non-thermal motions.

Key Words
heat jet approach; atomic simulations; finite temperature; square lattice

Address
Baiyili Liu and Shaoqiang Tang: HEDPS, CAPT, and LTCS, College of Engineering, Peking University, Beijing 100871, P.R. China


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