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MichelleLOyen's picture

Journal Club Theme of January 2007: Biomechanics and Non-Affine Kinematics

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Biological materials are frequently constructed of hydrated biopolymer networks. Examples include fibrous collagen in the extracellular matrix and actin within the cell's cytoskeleton. There are differences in the molecular composition of the biopolymer subunits as well as differences in the network density and organization. Images can be seen here and here for dense collagen networks and for portions of actin networks look at images here and here.

Call for papers: Computer Applications in Research and Development of Complex Mechanical Systems

http://www.inderscience.com/browse/callpaper.php?callID=579

Call for papers: Computer Applications in Research and Development of Complex Mechanical Systems

A special issue of the International Journal of Computer Applications in Technology  (IJCAT)

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Luis Dorfmann's picture

Nonlinear Electroelastic Deformations

Electro-sensitive (ES) elastomers form a class of smart materials whose mechanical properties can be changed rapidly by the application of an electric field. These materials have attracted considerable interest recently because of their potential for providing relatively cheap and light replacements for mechanical devices, such as actuators, and also for the development of artificial muscles. In this paper we are concerned with a theoretical framework for the analysis of boundary-value problems that underpin the applications of the associated electromechanical interactions. We confine attention to the static situation and first summarize the governing equations for a solid material capable of large electroelastic deformations. The general constitutive laws for the Cauchy stress tensor and the electric field vectors for an isotropic electroelastic material are developed in a compact form following recent work by the authors. The equations are then applied, in the case of an incompressible material, to the solution of a number of representative boundary-value problems. Specifically, we consider the influence of a radial electric field on the azimuthal shear response of a thick-walled circular cylindrical tube, the extension and inflation characteristics of the same tube under either a radial or an axial electric field (or both fields combined), and the effect of a radial field on the deformation of an internally pressurized spherical shell.

thermoPhoresis

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Hi every body:

I'm searching for some one that can help me about the boundary conditions on the wall for a based fluid including nanoparticles. The most dominant phenomena is the thermophoresis effect, and also close to wall the Brownain effect seems be important.

Thanks/farid

Nonlinear elasticity of biological gels

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I would like to propose the recent papers by Janmey, P.A., and coworkers on the nonlinear elasticity behavior of biopolymer gels for "biomechanics" issue in J Club. In their original work, they proposed the biopolymer network model composed of semi-flexible polymers that behave like a worm-like-chain (WLC) model. Their models surprisingly capture the mechanical response of biopolymer gels such as neuro-filaments. The details of their work are as follows:

Teng Li's picture

Delocalizing Strain in a Thin Metal Film on a Polymer Substrate

Teng Li, Zhenyu Huang, Zhichen Xi, Stephanie P. Lacour, Sigurd Wagner, Zhigang Suo, Mechanics of Materials, 37, 261-273 (2005).

Under tension, a freestanding thin metal film usually ruptures at a smaller strain than its bulk counterpart. Often this apparent brittleness does not result from cleavage, but from strain localization, such as necking. By volume conservation, necking causes local elongation. This elongation is much smaller than the film length, and adds little to the overall strain. The film ruptures when the overall strain just exceeds the necking initiation strain, εN , which for a weakly hardening film is not far beyond its elastic limit. Now consider a weakly hardening metal film on a steeply hardening polymer substrate. If the metal film is fully bonded to the polymer substrate, the substrate suppresses large local elongation in the film, so that the metal film may deform uniformly far beyond εN. If the metal film debonds from the substrate, however, the film becomes freestanding and ruptures at a smaller strain than the fully bonded film; the polymer substrate remains intact. We study strain delocalization in the metal film on the polymer substrate by analyzing incipient and large-amplitude nonuniform deformation, as well as debond-assisted necking. The theoretical considerations call for further experiments to clarify the rupture behavior of the metal-on-polymer laminates.

Related posts and discussions

Tension of Cu film on Pi substrate
Local thinning of Cu film
High ductility of a metal film adherent on a polymer substrate

Intergranular fracture

Microcracks form by a mixture of local thinning and intergranular fracture in a 170-nm-thick Cu film that is well bonded to a polyimide substrate and is stretched to a strain of 30%. Details can be found in this paper. A related forum topic can be found

High ductility of a metal film adherent on a polymer substrate

In recent development of deformable electronics, it has been noticed that thin metal films often rupture at small tensile strains. Here we report experiments with Cu films deposited on polymeric substrates, and show that the rupture strains of the metal films are sensitive to their adhesion to the substrates. Well-bonded Cu films can sustain strains up to 10% without appreciable cracks, and up to 30% with discontinuous microcracks. By contrast, poorly bonded Cu films form channel cracks at strains about 2%. The cracks form by a mixture of strain localization and intergranular fracture.

Teng Li's picture

Mechanics of flexible macroelectronics

The following entry was first posted in www.macroelectronics.org on 8 May 2006.

Flat-panel displays are rapidly replacing cathode-ray tubes as the monitors of choice for computers and televisions, a commercial success that has opened the era of macroelectronics, in which transistors and other micro-components are integrated over large areas. In addition to the flat-panel displays, other macroelectronic products include x-ray imagers, thin-film solar cells, and thin-film antennas.
Like a microelectronic product, a macroelectronic product consists of many thin-film components of small features. While microelectronics advances by miniaturizing features, macroelectronics does so by enlarging systems. Macroelectronic products today are mostly fabricated on substrates of glass or silicon; they are expensive, fragile and not readily portable when their areas are large. To reduce cost and enhance portability, future innovation will come from new choice of materials and of manufacturing processes. For example, thin-film devices on thin polymer substrates lend themselves to roll-to-roll fabrication, resulting in lightweight, rugged and flexible products. These macroelectronic products will have diverse architectures, hybrid materials, and small features. Their mechanical behavior during manufacturing and use poses significant challenges to the creation of the new technologies.

A recent review paper by Suo et al. describes ongoing work in the emerging field of research – mechanics of flexible macroelectronics, with emphasis on the mechanical behavior at the scale of individual features, and over a long time. The following topics have been discussed in the paper:

Konstantin Volokh's picture

Griffith controversy

Using the Griffith energy method for analysis of cavitation under hydrostatic tension we conclude that the critical tension tends to infinity when the cavity radius approaches zero (IJSS, 2006, doi: 10.1016/j.ijsolstr.2006.12.022). The conclusion is physically meaningless, of course. Moreover, if we assume that the failure process occurs at the edge of the cavity then the critical tension should be length-independent for small but finite cavities while the Griffith analysis always exhibits length-dependence. The main Griffith idea - introduction of the surface energy - is controversial because it sets up the characteristic length, say, surface energy over volume energy. By no means is this approach in peace with the length-independent classical continuum mechanics.

The Ranking of Mechanics Related Journals (2004)

Based on a survey from Journal Citation Report (JCR), we listed below the 2004 Journal Impact Factors (IF) for some mechanics, material science, and solid state physics related scientific journals. Our list and information may not be complete. We welcome readers' input, comments, and information. We also caution readers that using IF as the sole criterion to rank scientific journals' academic reputation may not be objective nor true to a journal's actual scientific merits.

Which are the benchmark problems for a numerical method ?

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Hello mechanicians,

Please help me with some following questions.

Which are the benchmark problems must be tested for a new numerical method in 1D, 2D, and 3D ?

If there are two methods, with what criterions we can say one is better than other in such a particular problem? in all problems ?

And who supposed above rules ?

I look forward to your replies.

Happy new year to all !

Tienchong Chang's picture

Nonlinear stick-spiral model for predicting mechanical behavior of single-walled carbon nanotubes

(PRB,74,245428,2006)  Based on a molecular mechanics concept, a nonlinear stick-spiral model is developed to investigate the mechanical behavior of single walled carbon nanotubes (SWCNTs). The model is capable of predicting not only the initial elastic properties (e.g., Young’s modulus) but also the stress-strain relations of a SWCNT under axial, radial, and torsion conditions. The elastic properties, ultimate stress, and failure strain under various loading conditions are discussed and special attentions have been paid to the effects of the tube chirality and tube size. Some unique mechanical behaviors of chiral SWCNTs, such as axial strain-induced torsion, circumferential strain-induced torsion, and shear strain-induced extension are also studied. The predicted results from the present model are in good agreement with existing data, but very little computational cost is needed to yield them.

A "cool" way to remove hydrogen...and possibly a faster way to grow better crystals?

In growth of essentially every compound material such as GaN, one element always diffuses faster than the other(s) at the growth front. To grow good-quality materials, even the most sluggish element has to be sufficiently mobile, forcing materials growers to go to higher growth temperatures.

Multi-phase hyperelasticity with interface energy effect

Recently, J. Wang, L. Sun and I have formulated some ideas about the effective properties of heterogeneous materials with surface/interface energy effect, which are shown in the attached file.

Papers in the attached file can be viewed as a two-part paper, called “Multi-phase hyperelasticity with interface energy effect” if it is standalone. Part one of this topic is covered in “A theory of hyperelasticity of multi-phase media with surface/interface energy effect”, which provides theoretical background. Part two is covered in “Size-dependent effective properties of a heterogeneous material with interface energy effect: from finite deformation theory to infinitesimal strain analysis”, with more emphasis on application.

Young's modulus of single-walled carbon nanotubes

We report in detail that unlike other materials, carbon nanotubes are so small that changes in structure can affect the Young's modulus. The variation in modulus is attributed to differences in torsional strain, which is the dominant component of the total strain energy. Torsional strain, and correspondingly Young's modulus, increases significantly with decreasing tube diameter and increases slightly with decreasing tube helicity.  Journal of Applied Physics 84, 1939 (1998).

Force response and actin remodeling (agglomeration) in fibroblasts due to lateral indentation

We report the loading and unloading force response of single living adherent fibroblasts due to large lateral indentation obtained by a two-component microelectromechanical systems (MEMS) force sensor. Strong hysteretic force response is observed for all the tested cells. For the loading process, the force response is linear (often with small initial non-linearity) to a deformation scale comparable to the undeformed cell size, followed by plastic yielding. In situ visualization of actin fibers (GFP) reveals that during the indentation process, actin network depolymerizes irreversibly at discrete locations to form well-defined circular actin agglomerates all over the cell, which explains the irreversibility of the force response. Similar agglomeration is observed when the cell is compressed laterally by a micro plate. The distribution pattern of the agglomerates strongly correlates with the arrangement of the actin fibers of the pre-indented cell. The size of the agglomerates increases with time as ta  with a= 2~3 initially,   followed by a=.5~1. The higher growth rate suggests influx of actin into the agglomerates. The slower rate suggests a diffusive spreading, but the diffusion constant is two orders of magnitude lower than that of an actin monomer through the cytoplasm. Actin agglomeration has previously been observed due to biochemical treatment, gamma-radiation, and ischemic injury, and has been identified as a precursor to cell death. We believe, this is the first evidence of actin agglomeration due to mechanical stimuli. The study demonstrates that living cells may initiate similar functionalities in response to dissimilar mechanical and biochemical stimuli.

Axial-Strain-Induced Torsion in Single-Walled Carbon Nanotubes

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Using classical molecular dynamics and empirical potentials, we show that the axial deformation of single-walled carbon nanotubes is coupled to their torsion. The axial-strain-induced torsion is limited to chiral nanotubes—graphite sheets rolled around an axis that breaks its symmetry. Small strain behavior is consistent with chirality and curvature-induced elastic anisotropy (CCIEA)—carbon nanotube rotation is equal and opposite in tension and compression, and decreases with curvature and chirality. The largestrain compressive response is remarkably different.

Dhirendra Kubair's picture

Functionally Graded Materials

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Dear Fellow Mechanicians,

My group is looking into some aspects of fracture in "functionally graded materials". I was curious to know if there are groups (on imechanica) interested or actively pursuing research on functionally graded materials or nanocomposites.

kubair@aero.iisc.ernet.in

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