Continuum Mechanics

Vibration and symmetry-breaking of boron-nitride nanotubes

by Rajib Chowdhury

Nanotechnology, 2010

The unique features of axial, torsional, transverse and radial breathing vibrations are captured for armchair and... more The unique features of axial, torsional, transverse and radial breathing vibrations are captured for armchair and zigzag singlewalled boron nitride nanotubes (BNNTs) based on molecular mechanics simulations and continuum mechanics theories. Equivalent Young's modulus 1TPa and shear modulus 0.4TPa are obtained independent of the chirality of BNNTs. In particular, a distorted optimized structure is observed for the first time for BNNTs with sufficiently large diameter and length. It is found that the deformed structures result in the behaviours of BNNTs deviating from those of classical columns/beams. Such symmetry breaking could also exert significant impacts on the structural instability (buckling) and the electronic properties of BNNTs that are sensitive to the structural symmetry.

Vibration of ZnO nanotubes: a molecular mechanics approach

by Rajib Chowdhury

Applied Physics A: Materials Science & Processing, 2010

We investigate the vibrational properties of two kinds of single-wall ZnO nanotubes. The simulations are carried out... more We investigate the vibrational properties of two kinds of single-wall ZnO nanotubes. The simulations are carried out for three types of zigzag nanotubes (5,0), (8,0), (10,0) and armchair nanotubes (3,3), (4,4), (6,6). The natural frequencies are determined by means of the molecular mechanics approach with the universal force field potential. The first four natural frequencies are obtained for length/diameter ratio of about 5–20. The vibration modes associated with these frequencies have been computed. Closed-form analytical expressions have been derived using the continuum shell theory for the physical explanations of the simulations results. We observe that the natural frequencies decrease as the aspect ratios increase. The results follow similar trends with results of previous studies for carbon nanotubes (CNT). However, the magnitudes of the frequencies are lower from the corresponding CNT counterparts, indicating that ZnO nanotubes are comparatively less stiff.

Bio-inspired Compliant Robotic Fish: Design and Experiments

by Maarja Kruusmaa

H. El Daou, T. Salumäe, G. Toming, M. Kruusmaa, "Bio-inspired Compliant Robotic Fish: Design and Experiments", IEEE International Conference on Robotics and Automation, St. Paul, USA, May 14-18, 2012, to appear

This paper studies the modelling, design and fabrication of a bio-inspired fish-like robot propelled by a compliant... more This paper studies the modelling, design and fabrication of a bio-inspired fish-like robot propelled by a compliant body. The key to the design is the use of a single motor to actuate the compliant body and to generate thrust. The robot has the same geometrical properties of a subcarangiform swimmer with the same length. The design is based on rigid head and fin linked together with a compliant body. The flexible part is modelled as a non-uniform cantilever beam actuated by a concentrated moment. The dynamics of the compliant body are studied and a relationship between the applied moment and the resulting motion is derived. A prototype that implements the proposed approach is built. Experiments on the prototype are done to identify the model parameters and to validate the theoretical modelling

A thermodynamic formulation of the equations of motion and buoyancy frequency for Earth's fluid outer core

by Klaus Joehnk

Jöhnk, K.D., Svendsen, B., 1996. A thermodynamic formulation of the equations of motion and buoyancy frequency for Earth's fluid outer core. Continuum Mechanics and Thermodynamics 8: 75-101.

Steady state gravity flow of a binary mixture down an inclined plane

by Klaus Joehnk

Jöhnk, K.D., Hutter, K., Svendsen, B., 1993. Steady state gravity flow of a binary mixture down an inclined plane. Recent Advances in Mechanics of Structured Continua, AMD-Vol. 160/MD-Vol. 41: 55-63.

On interfacial transition conditions in two phase gravity flow

by Klaus Joehnk

Hutter, K., Jöhnk, K.D., Svendsen, B., 1994. On interfacial transition conditions in two phase gravity flow. Zeitschrift für Angewandte Mathematik und Physik 45: 746-762.

On the role of mechanical interactions in the steady-state gravity flow of a two-constituent mixture down an inclined plane

by Klaus Joehnk

Svendsen, B., Wu, T., Jöhnk, K.D., Hutter, K.,1996. On the role of mechanical interactions in the steady-state gravity flow of a two-constituent mixture down an inclined plane. Proceedings of the Royal Society of London A 452: 1189-1205.

On the gravity-driven shear flow of an ice-till Mixture

by Klaus Joehnk

Wu, T., Jöhnk, K.D., Svendsen, B., Hutter, K., 1996. On the gravity-driven shear flow of an ice-till mixture. Annals of Glaciology 23: 124-128.

Effective elastic moduli evaluation of single walled carbon nanotubes using flexural vibrations

by José Viriato Araújo dos Santos

J. V. Araújo dos Santos, Mechanics of Advanced Materials and Structures, 18(4), 262-271, 2011

Two new methods for effective elastic moduli evaluation of single walled carbon nanotubes (SWCNT) are presented. These... more Two new methods for effective elastic moduli evaluation of single walled carbon nanotubes (SWCNT) are presented. These methods rely on the Euler--Bernoulli and the Timoshenko beam theories, the modification of finite elements based on these theories, and an optimization technique. The proposed modification of the standard Euler-Bernoulli and Timoshenko finite elements assumes that the SWCNT diameter is much larger than its thickness. It is shown that the method based on the Timoshenko finite element presents better elastic moduli evaluations of short nanotubes than the ones based on the Euler-Bernoulli finite element.

Compaction behavior of Al6061 powder in the semi-solid state

by Yufeng Wu

published at Powder Technology 2011

Semi-solid powder processing involves compaction of metallic alloy powders in temperature ranges when both solid and... more Semi-solid powder processing involves compaction of metallic alloy powders in temperature ranges when both solid and liquid phases coexist. However, modeling of such process has been very scarce. In this study, compaction behavior of Al6061 powder in the presence of liquid phase was investigated. The relationship between compaction pressure and relative density of powder in the semi-solid state was modeled and verified against experimental measurements. The results showed that the powder compaction behavior can be accurately predicted by Shima-Oyane model when the liquid fraction was below 20%. The normalized pressure–relative density curves merged into a single one and can be expressed with a simple exponential expression. When the liquid fraction was higher than 20%, the model failed to predict the compaction behavior because of the non-uniform distribution of liquid phase. Liquid squeeze out and interlocking of irregular arm solids was speculated to occur during the compaction process.

Modeling Sliding Contact of Rough Surfaces with Molecularly Thin Lubricants

by Antonis Vakis

The sliding contact between two rough surfaces in the presence of a molecularly thin lubricant layer is investigated.... more The sliding contact between two rough surfaces in the presence of a molecularly thin lubricant layer is investigated. Under very high shear rates, the lubricant is treated as a semi-solid layer with normal and lateral shear-dependent stiffness components obtained from experimental data. The adhesive force in the presence of lubricant is also adapted from the Sub-boundary lubrication model and improved to account for variation in surface energy with penetration into the lubricant layer. A model is then proposed, based on the Improved sub-boundary lubrication model, which accounts for lubricant contact and adhesion and its validity is discussed. The model is in good agreement with published experimental measurements of friction in the presence of molecularly thin lubricant layers and suggests that a molecularly thin lubricant bearing could be successfully used to reduce solid substrate damage at the interface.

Analytical and Experimental Comparisons of Electromechanical Vibration Response of a Piezoelectric Bimorph Beam for Power Harvesting

by Mikail Lumentut

Special Issue (In Press)

The analysis of a piezoelectric bimorph beam with two input base motions for power harvesting

by Mikail Lumentut

Modeling bearing and shear forces in molecularly thin lubricants

by Antonis Vakis

Under the effects of high shear rate and confinement between solid surfaces, the behavior of a thin lubricant film... more Under the effects of high shear rate and confinement between solid surfaces, the behavior of a thin lubricant film deviates from that of the bulk, resulting in significant increases of lubricant viscosity and interfacial slip. A semi-empirical model accounting for the breakdown of continuum theory at the nanoscale is proposed—based on film morphology and chemistry from available experimental and molecular dynamics simulation data—to describe lubricant behavior under shear. Viscosity stiffening and interfacial slip models are introduced into the formulations of the normal (bearing) and shear forces acting on a sphere that moves within a thin lubricant film parallel to a rigid plane. The experimentally measured ‘apparent’ viscosity confounding the effects of both stiffening and slip is used to predict the hydrodynamic forces acting on a fully or partially submerged sphere for the purposes of describing lubricant contact in magnetic storage. The proposed sphere-on-flat model forms the basis of a future, dynamic contact with friction model that will account for lubricant contact in the context of molecularly thin lubricated rough surface contact.

Analytical Modelling of Self-Powered Electromechanical Piezoelectric Bimorph Beams with Multidirectional Excitation

by Mikail Lumentut