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Xiaodong Li's picture

Nanostructured Metals Reveal Their Secret Strengthening Mechanisms

It is well known that metals are hardened by deformation and soften by annealing. How about nanostructured metals? Can we reply on conventional metal-working lore?

Intracellular CalciumWaves in Bone Cell Networks Under Single Cell Nanoindentation

In this study, bone cells were successfully cultured into a micropatterned network with dimensions close to that of in vivo osteocyte networks using microcontact printing and self-assembled monolyers (SAMs). The optimal geometric parameters for the formation of these networks were determined in terms of circle diameters and line widths. Bone cells patterned in these networks were also able to form gap junctions with each other, shown by immunofluorescent staining for the gap junction protein connexin 43, as well as the transfer of gap-junction permeable calcein-AM dye.

No need to worry about gravity at the atomic-/nano-scale

When a metal is grown onto a substrate of itself (homoepitaxy), the growth front is typically smooth, or at most is roughened by the formation of shallow hills (called surface mounds). The underlying reason for the roughening has been recognized to be of kinetic nature: Atoms landed on an upper terrace do not have enough time to overcome the "road blocks" provided by the steps and fill all the valleys (known as the Villian instability).

Molecular and Cellular Biomechanics Journal

A new journal dedicated to the field of Molecular and Cellular Biomechanics has been formed for about a year. Many members in this community (such as Ning Wang, Cheng Zhu, Phil LeDuc) are on the board of editors. You may want to check it out....

Micro cantilever pre-stress

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Dear all,

Im a PhD student in Cambridge Uni, UK working in the field of MEMS, and as part of my work, of late Ive been looking at deriving materials properties of MEMS thin film materials by means of resonant testing. The basic outline of the experiment is first creating free standing rectangular cantilevers of the material under test, evaporate with gold to increase reflectivity (when needed), then (under reasonable vacuum) applying a base excitation using a chirp signal into a piezo actuator and logging the cantilever tip response using a laser/photodetector setup. The frequency response is then calculated and the modal frequencies noted.

To determine the materials' properties, both an analytical model (with bending/torsion modes) and finite element model (using 2D mindlin) are created with similar geometry as the sample, and by minimising the squared relative error between the measured modes and those from the models, the value of Young's modulus (known density) and poissons ratio may be determined iteratively. These yield fairly consistent results.

To take the work further I now feel I should also include the effects of residual stress in the cantilevers. The method Ive been looking at is by using finite element (via COMSOL) - the beam geometry is created and loaded with the stress model ('surface stress' as a force tangential to the top boundary, and gradient stress as a 'tangential' force that varies from +F to -F from top to bottom boundary of the cantilever). The model is solved statically, and the deformed shape is then saved as the linearisation point for the next model, which then computes the eigenfrequencies. Btw I can only do this in 3D FE, which makes computation times quite long hence using iteration to quantify this stress highly unlikely.

In any case, is there an analytical model I can use to model the effect of this stress on multiple modes of a cantilever. Id like to verify whether the FE is giving me anything close to ball park numbers before I work out a means to compare them with experimental results. I was thinking of using the Rayleigh method by representing the effect of prestress as an additional term in the potential energy. The original mode shapes, with some modification will be used to evaluate the two energy integrals. The potential energy due to stress is worked out by measuring the static deflected shape using a zygo inteferometer - some rough model is used, with the beam curvature and peak deflection as input to work out the amount of this energy. Not having much experience in mechanics (i was an electronics undergrad!), Im not sure how good an estimate this would be, if at all its a useable or even possible one. Will the extra energy factor in to the torsion modes just the same?

The iMechanica Journal Club (iMech jClub)

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2017 Themes and Discussion Leaders

Mass sensing by using a resonating microcantilever

We recently reported the mass sensing by using resonating microcantilevers. The characterization of mass-sensing and its related sensitivity was suggested on the basis of elasticity theory.

Model Reduction of Large Proteins for Normal Mode Studies

Recently, I reported the model reduction method for large proteins for understanding large protein dynamics based on low-frequency normal modes. This work was pubslihed at Journal of Computational Chemistry (click here).

Coarse-Graining of protein structures for the normal mode studies

Abstracts 

Quantum Stability of Metallic Thin Films and Nanostructures

When a metal system shrinks its dimension(s), the conduction electrons inside the metal feel the squeezing, and are forced into (discrete) quantum states. Such confined motion of the conduction electrons may influence the global or local stability of the low dimensional systems, and in the case of a thin film on a foreign substrate this "quantum energy" of electronic origin can easily overwhelm the strain effects in definging the film stability, thereby severely influencing the preferred growth mode (see, e.g., Suo and Zhang, Phys. Rev. B 58, 5116 (1998)).

Ashkan Vaziri's picture

Mechanics and deformation of the nucleus in micropipette aspiration experiment

Robust biomechanical models are essential for studying the nuclear mechanics and can help shed light on the underlying mechanisms of stress transition in nuclear elements. Here, we develop a computational model for an isolated nucleus undergoing micropipette aspiration. Our model includes distinct components representing the nucleoplasm and the nuclear envelope. The nuclear envelope itself comprises three layers: inner and outer nuclear membranes and one thicker layer representing the nuclear lamina.

Atomistic simulations for the evolution of a U-shaped dislocation in fcc Al

We show, through MD simulations, a new evolution pattern of the U-shaped dislocation in fcc Al that would enrich the FR mechanism. Direct atomistic investigation indicates that a U-shaped dislocation may behave in different manners when it emits the first dislocation loop by bowing out of an extended dislocation. One manner is that the glissile dislocation segment always bows in the original glide plane, as the conventional FR mechanism. Another is that non-coplanar composite dislocations appear owing to conservative motion of polar dislocation segments, and then bow out along each slip plane, creating a closed helical loop. The motion of these segments involves a cross-slip mechanism by which a dislocation with screw component moves from one slip plane into another. Ultimately, such non-coplanar evolution results in the formation of a FR source.

Mogadalai Gururajan's picture

Some numerical mechanics software

Recently, during one of my net searches, I came across this page of RPI, where I learnt about a couple of numerical mechanics software which might be of interest to some of you.

FMDB:

As for the effort toward the scalable engineering simulations on distributed environements, we addressed this challenge by developing a distributed mesh data management infrastructure that satisfies the needs of distributed domain of applications.

A structure-based sliding-rebinding mechanism for catch bonds

This is a paper by Jizhong Lou and myself, which is in press in Biophysical Journal.

Abstract.  Catch bonds, whose lifetimes are prolonged by force, have been observed in selectin-ligand interactions and other systems. Several biophysical models have been proposed to explain this counter-intuitive phenomenon, but none was based on the structure of the interacting molecules and the noncovalent interactions at the binding interface. Here we used molecular dynamics simulations to study changes in structure and atomic-level interactions during forced unbinding of P-selectin from P-selectin glycoprotein ligand-1. A mechanistic model for catch bonds was developed based on these observations. In the model, "catch" results from forced opening of an interdomain hinge that tilts the binding interface to allow two sides of the contact to slide against each other. Sliding promotes formation of new interactions and even rebinding to the original state, thereby slowing dissociation and prolonging bond lifetimes. Properties of this sliding-rebinding mechanism were explored using a pseudo-atom representation and Monte Carlo simulations. The model has been supported by its ability to fit experimental data and can be related to previously proposed two-pathway models.

How can we obtain more information from protein structure?

We know - or believe - protein function is determined by structure. Crystallographic and NMR studies can provide protein structures with atomic-level details at equilibrium. MD simulations can follow protein conformational changes in time with fs temporal resolution in the absence or presence of a bias mechanism, e.g., applied force, used to induce such changes.

Dhirendra Kubair's picture

Mode-3 spontaneous crack propagation along functionally graded bimaterial interfaces

This is a paper that has been accepted for publication in the Journal of the Mechanics and Physics of Solids from our group. The paper describes the combined effect of material inertia and inhomogeneous material property variation on spontaneous cohesive-crack propagation in functionally graded materials. The preprint is attached as a PDF.

Abstract- The effects of combining functionally graded materials of different inhomogeneous property gradients on the mode-3 propagation characteristics of an interfacial crack are numerically investigated. Spontaneous interfacial crack propagation simulations were performed using the newly developed spectral scheme. The numerical scheme derived and implemented in the present work can efficiently simulate planar crack propagation along functionally graded bimaterial interfaces. The material property inhomogeneity was assumed to be in the direction normal to the interface. Various bimaterial combinations were simulated by varying the material property inhomogeneity length scale. Our parametric study showed that the inclusion of a softening type functionally graded material in the bimaterial system leads to a reduction in the fracture resistance indicated by the increase in crack propagation velocity and power absorbed. An opposite trend of increased fracture resistance was predicted when a hardening material was included in the bimaterial system. The cohesive tractions and crack opening displacements were altered due to the material property inhomogeneity, but the stresses ahead of the cohesive zone remained unaffected.

Semiflexible polymer chain under sustained tension as a model of cytoskeletal rheology

This is a model of a single semiflexible polymer chain under sustained tension. The model captures two key features of the cytoskeletal rheology: a) the power-law behavior; and b) the dependence of the power-law on mechanical prestress. The model also reveals the underlying mechanisms.

Alexander A. Spector's picture

Mechanics vs. Biochemistry in Adhesions-Cytoskeleton-Nucleus Signal Transduction in Cells

The essence of mechanobiology is, probably, the interrelation between mechanical and biochemical factors.  An exciting example of such phenomenon is signaling associated with the interaction between the cell and extracellular matrix (EM).  While some purely biochemical pathways initiated in the area of contact of the cell and EM are known, there are interesting ideas how the mechanical forces, stresses and strains can be involved too. This view goes back to works of Donald Ingber's group in the 90s that showed how perturbations of the adhesion area as a whole and of an individual integrin result in deformation of the cell nucleus. Interestingly, a distinguished oncologist at Johns Hopkins, Donald Coffey, published similar experimental results about the same time, and he also demonstrated that the observed cytoskeleton/nucleus interaction is different in tumor cells. There are several separate pieces of the puzzle that have been resolved: mechanical forces are generated at focal adhesions, the cytoskeleton is involved, nucleus deforms, gene expression changes as a result of perturbation of the adhesions, however, the whole picture of the interrelated mechanical and biochemical factors has yet to be understood. We recently published some results on this topic in the Journal of Biomechanical Engineering (Jean et al., 2004 and 2005). I was glad to find an interest in the same problem from some participants of this website (e.g., N. Wang, Z. Suo,   Long-distance propagation of forces in a cell, 2005 and P.R. LeDuc and R.M. Bellin, Nanoscale Intracellular Organization and Functional Architecture Mediating Cellular Behavior, 2006). This aspect of mechanotransduction is important for many areas beyond mechanics such as cancer, wound healing, cell adhesion and motility, effect of surface micro- and nanopatterning, etc.

In Quest of Virtual Tests for Structural Composites

Listed below is a recent publication of mine in Science for your possible interest and critics. This is a review article focusing on the multiscale simulation issues in strucutral composites. I will be more than happy to discuss with those of you who are interested. The following is the abstract.

The difficult challenge of simulating diffuse and complex fracture patterns in tough structural composites is at last beginning to yield to conceptual and computational advances in fracture modeling. Contributing successes include the refinement of cohesive models of fracture and the formulation of hybrid stress-strain and traction-displacement models that combine continuum (spatially averaged) and discrete damage representations in a single calculation. Emerging hierarchical formulations add the potential of tracing the damage mechanisms down through all scales to the atomic. As the models near the fidelity required for their use as virtual experiments, opportunities arise for reducing the number of costly tests needed to certify safety and extending the design space to include material configurations that are too complex to certify by purely empirical methods.

Vlado A. Lubarda's picture

A Variable Core Model and the Peierls Stress for the Mixed Dislocation

A variable core model of a moving crystal dislocation is proposed and used to derive an expression for the Peierls stress. The dislocation width varies periodically as a dislocation moves through the lattice, which leads to an expression for the Peierls stress in terms of the difference of the total energies in the crystal corresponding to stable and unstable equilibrium configurations of the dislocation, rather than the difference in the misfit energies alone. Results for both edge and mixed dislocations are given and proposed to be used in conjunction with ab initio calculations.

Terra Preta Soil Technology

Please look at this low cost alternative CO2 Sequestration system.The integrated energy strategy offered by Terra Preta Soil technology may
provide the only path to sustain our agricultural and fossil fueled power
structure without climate degradation, other than nuclear power.

I feel we should push for this Terra Preta Soils CO2 sequestration strategy as not only a global warming remedy for the first world, but to solve fertilization and transport issues for the third world. This information needs to be shared with all the state programs.
 
The economics look good, and truly great if we had CO2 cap & trade in place: 
 
These are processes where you can have your Bio-fuel and fertility too.Terra Preta' soils I feel has great possibilities to revolutionize sustainable agriculture into a major CO2 sequestration strategy.

Superplastic carbon nanotubes

Nature 439, 281 (2006)

The theoretical maximum tensile strain — that is, elongation — of a single-walled carbon nanotube is almost 20%, but in practice only 6% is achieved. Here we show that, at high temperatures, individual single-walled carbon nanotubes can undergo superplastic deformation, becoming nearly 280% longer and 15 times narrower before breaking. This superplastic deformation is the result of the nucleation and motion of kinks in the structure, and could prove useful in helping to strengthen and toughen ceramics and other nanocomposites at high temperatures.

Xin Zhang's picture

Size-dependent creep behavior of plasma-enhanced chemical vapor deposited silicon oxide films

The time-dependent plastic deformation (creep) behaviors of both the as-deposited and annealed plasma-enhanced chemical vapor deposited (PECVD) silicon oxide (SiOx) films were probed by nanoindentation load relaxation tests at room temperature. Our experiments found a strong size effect in the creep responses of the as-deposited PECVD SiOx thin films, which was much reduced after rapid thermal annealing (RTA). Based on the experimental results, the deformation mechanism is depicted by the "shear transformation zone" (STZ) based amorphous plasticity theories. The physical origin of the STZ is elucidated and linked with the shear banding dynamics. It is postulated that the high strain gradient at shallow indentation depths may be responsible for the reduction in the stress exponent n=∂log(strain rate)/∂log(stress), characteristic of a more homogenous flow behavior.

N. Sukumar's picture

2. Is a mesh required in meshfree methods?

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In meshfree (this is more in vogue than the term meshless) methods, two key steps need to be mentioned: (A) construction of the trial and test approximations; and (B) numerical evaluation of the weak form (Galerkin or Rayleigh-Ritz procedure) integrals, which lead to a linear system of equations (Kd = f). In meshfree Galerkin methods, the main departure from FEM is in (A): meshfree approximation schemes (linear combination of basis functions) are constructed independent of an underlying mesh (union of elements).

However, since a Galerkin method is typically used in solid mechanics applications, (B) arises and the weak form integrals need to be evaluated. Three main directions have been pursued to evaluate these integrals:

splacour's picture

Mechanisms of reversible stretchability of thin metal films on elastomeric substrates

Gold films on an elastomeric substrate can be stretched and relaxed reversibly by tens of percents. The films initially form in two different structures, one continuous and the other containing tri-branched microcracks. We have identified the mechanism of elastic stretchability in the films with microcracks. The metal, which is much stiffer than the elastomer, forms a percolating network.

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