Dr. C.W. Lim
Department of Building and Construction, City University of Hong Kong,
时间：6月1日星期三上午 10:30-11:30
地点：安中大楼A322
Abstract
Three critical but overlooked issues in the physics of nonlocal elastic stress field theory for nanobeams are discussed: (i) why does the presence of increasing nonlocal effects induce reduced nanostructural stiffness in many, but not consistently for all, cases of study, ie. increasing static deflection, decreasing natural frequency and decreasing buckling load, in virtually all previously published works in this subject (a total of 93 papers have been identified since 2003) although intuition in physics tells otherwise? (ii) the intriguing conclusion that nanoscale effects are missing in the solutions in many exemplary cases of study for bending of nanobeams, and (iii) the missing of additional boundary conditions required in the governing higher-order differential equations. Applying the nonlocal elasticity field theory in nanomechanics and an exact variational principal approach, the exact equilibrium conditions, domain governing differential equation and boundary conditions for bending of nanobeams are derived for the first time. These new equations and conditions involve essential higher-order terms which are missing in virtually all nonlocal models and analyses in previously published works in statics and dynamics of nonlocal nanostructures. Such negligence of higher-order terms results in misleading nanoscale effects which predicts completely reverse trends with respect to what the conclusion of this work tells.
The speaker will also discuss briefly on the strange behaviour of Young’s Modulus for tensile analysis of a nanorod/nanotube based on nonlocal elasticity theory. It has been a known fact in classical mechanics of materials that Young’s modulus is an indicator of material stiffness and materials with higher Young’s modulus are stiffer. At the nanoscale, within the scope and under specific circumstances described here, however, a nanorod (or a nanotube) with a smaller Young’s modulus (smaller stress-strain rate) is stiffer. In such a scenario, Young’s modulus is no longer a stiffness indicator for nanostructures. Furthermore, the nonlocal stress-strain rate is dependent on types of load, boundary conditions and location. This is likely to be one of the many possible reasons why numerous experiments in the past obtained significantly varying values of Young’s modulus for a seemingly identical nanotube, i.e. because the types of loading and/or boundary conditions in the experiments were different, as well as at which point the property was measured.
About the Speaker
Currently an associate professor and a registered professional engineer (RPE), Ir Dr Lim received a first degree from Univ. of Technology of Malaysia, a Master’s Degree and PhD from National Univ. of Singapore and Nanyang Technological Univ., respectively. Prior to joining CityU, he was a post-doctoral research fellow at Department of Civil Engineering, The University of Queensland and Department of Mechanical Engineering, The University of Hong Kong. Dr. Lim has expertise in vibration of plate and shell structures, dynamics of smart piezoelectric structures, nanomechanics and symplectic elasticity. He is the Associate Editor (Asia-Pacific Region) for Advances in Vibration Eng., Guest Associate Editor for Int. J. of Bifurcation and Chaos, both are SCI-listed international journals, and also on the editorial board of a few international journals. He has published a best-selling title “Symplectic Elasticity” in Engineering Mechanics as recorded in April 2010 by the publisher, World Scientific; more than 180 research papers in peer reviewed international journals; more than 100 papers at international and local conferences; and accumulated more than 1600 self-excluded independent citations. In addition, he has attracted 25 research projects as the principal investigator and participated in another 14 research projects. He has also conducted multiple consultancy projects.