User login

You are here

research

Jie Wang's picture

Phase field simulations of polarization switching-induced toughening in ferroelectric ceramics

Polarization switching-induced shielding or anti-shielding of an electrically permeable crack in a mono-domain ferroelectric material with the original polarization direction perpendicular to the crack is simulated by a phase field model based on the time-dependent Ginzburg-Landau equation. The domain wall energy and the long-range mechanical and electrical interactions between polarizations are taken into account. The phase field simulations exhibit a wing-shape- switched zone backwards the crack tip.

Ashkan Vaziri's picture

"Wrinkled hard skins on polymers created by Focused Ion Beam", PNAS , January 2007

A stiff skin forms on surface areas of a flat polydimethylsiloxane (PDMS) upon exposure to focused ion beam (FIB) leading to ordered surface wrinkles. By controlling the FIB fluence and area of exposure of the PDMS, one can create a variety of patterns in the wavelengths in the micrometer to submicrometer range, from simple one-dimensional wrinkles to peculiar and complex hierarchical nested wrinkles. Examination of the chemical composition of the exposed PDMS reveals that the stiff skin resembles amorphous silica. Moreover, upon formation, the stiff skin tends to expand in the direction perpendicular to the direction of ion beam irradiation. The consequent mismatch strain between the stiff skin and the PDMS substrate buckles the skin, forming the wrinkle patterns. The induced strains in the stiff skin are estimated by measuring the surface length in the buckled state. Estimates of the thickness and stiffness of the stiffened surface layer are estimated by using the theory for buckled films on compliant substrates. The method provides an effective and inexpensive technique to create wrinkled hard skin patterns on surfaces of polymers for various applications. Click here for access to the full article. See also the press release: Applied scientists create wrinkled 'skin' on polymers

Call for papers: Mahalanobis-Taguchi System Analysis

Call for papers: Mahalanobis-Taguchi System Analysis.  A special issue of the International Journal of Industrial and Systems Engineering (IJISE).

With rapid advances in technology, use of automated data collection methods is on a steep rise. Situations that call for decision-making with voluminous datasets involving several variables are being encountered in an ever-increasing number of fields. Mahalanobis-Taguchi System (MTS) analysis provides an effective decision-making methodology in such situations. It is being successfully used by engineers in companies such as Nissan, Ford, Delphi, Xerox, and Yamaha, to name but a few.  This special issue invites submission of papers that could be state-of-the-art, new contributions, technical notes, review papers, or case studies in the area of Mahalanobis-Taguchi System analysis. For more information, please see the Journal Call for Papers website.

International Journal for Computational Vision and Biomechanics

International Journal for Computation Vision and Biomechanics - Announcement and First Call for papers

ISSN: 0973-6778

Subject: Computational Vision and Biomechanics

Frequency: 2 issues per year

Start date: First trimester of 2007

Dear Colleague,

It is a pleasure to announce the new International Journal for Computation Vision and Biomechanics (IJCV&B) and its first call for papers.

Lift weight using less energy

Choose a channel featured in the header of iMechanica: 

As shown in figure(energyefficiency1.jpg) sliding plates can slide over fixed plates. Stationary plate is simply supported horizontaly on sliding plates. Lubrication is provided at contact surfaces of stationary plate and sliding plates. Weight or load or force (mg) is applied at center of stationary plate. This load is equally devided and applied on each sliding plate in vertical downward direction (mg/2). This mg/2 cos(alpha) helps  sliding plate to slide in nearly downward direction.

Energy problem is about to solve

The figure is in cross section. as shown in figure, there are two cases. in each case there are two pulleys of same diameter. each pulley is of exactly circular shape.In first case 50 k.g weight is fixed with each pulley as shown in figure. The center of each pulley is fixed. Between these two pulleys there is a stationary plate. plate will remain stationary, while rotating the pulleys because centers of pulleys are fixed. 100 k.g weight is put on this plate. The force or weight of plate is applied on these two pulleys in vertical downward direction.

Taxonomy upgrade extras: 

Rotate pulleys using less energy

As shown in figure (energyefficiency1.jpg) sliding plates can slide over fixed plates. Stationary plate is simply supported horizontaly on sliding plates. Lubrication is provided at contact surfaces of stationary plate and sliding plates. Weight or load or force (mg) is applied at center of stationary plate. This load is equally devided and applied on each sliding plate in vertical downward direction (mg/2). This mg/2 cos(alpha) helps sliding plate to slide in nearly downward direction.

Arash_Yavari's picture

On Spatial and Material Covariant Balance Laws in Elasticity

This paper presents some developments related to the idea of covariance in elasticity. The geometric point of view in continuum mechanics is briefly reviewed. Building on this, regarding the reference configuration and the ambient space as Riemannian manifolds with their own metrics, a Lagrangian field theory of elastic bodies with evolving reference configurations is developed. It is shown that even in this general setting, the Euler-Lagrange equations resulting from horizontal (referential) variations are equivalent to those resulting from vertical (spatial) variations. The classical Green-Naghdi-Rivilin theorem is revisited and a material version of it is discussed. It is shown that energy balance, in general, cannot be invariant under isometries of the reference configuration, which in this case is identified with a subset of R^3. Transformation properties of balance of energy under rigid translations and rotations of the reference configuration is obtained. The spatial covariant theory of elasticity is also revisited. The transformation of balance of energy under an arbitrary diffeomorphism of the reference configuration is obtained and it is shown that some nonstandard terms appear in the transformed balance of energy. Then conditions under which energy balance is materially covariant are obtained. It is seen that material covariance of energy balance is equivalent to conservation of mass, isotropy, material Doyle-Ericksen formula and an extra condition that we call ‘configurational inviscidity’. In the last part of the paper, the connection between Noether’s theorem and covariance is investigated. It is shown that the Doyle-Ericksen formula can be obtained as a consequence of spatial covariance of Lagrangian density. Similarly, it is shown that the material Doyle-Ericksen formula can be obtained from material covariance of Lagrangian density.

Julia R. Greer's picture

Effective Use of Focused Ion Beam (FIB) in Investigating Fundamental Mechanical Properties of Metals at the Sub-Micron Scale

I would like to share some of our more recent findings on nano-pillar compression, namely the role of the surface treatment in plastic deformation at the nano-scale. Recent advances in the 2-beam focused ion beams technology (FIB) have enabled researchers to not only perform high-precision nanolithography and micro-machining, but also to apply these novel fabrication techniques to investigating a broad range of materials' properties at the sub-micron and nano-scales. In our work, the FIB is utilized in manufacturing of sub-micron cylinders, or nano-pillars, as well as of TEM cross-sections to directly investigate plasticity of metals at these small length scales. Single crystal nano-pillars, ranging in diameter between 300 nm and 870 nm, were fabricated in the FIB from epitaxial gold films on MgO substrates and subsequently compressed using a Nanoindenter fitted with a custom-fabricated diamond flat punch. We show convincingly that flow stresses strongly depend on the sample size, as some of our smaller specimens were found to plastically deform in uniaxial compression at stresses as high as 600 MPa, a value ~25 times higher than for bulk gold. We believe that these high strengths are hardened by dislocation starvation. In this mechanism, once the sample is small enough, the mobile dislocations have a higher probability of annihilating at a nearby free surface than of multiplying and being pinned by other dislocations. Contrary to this, if the dislocations are trapped inside the specimen by a coating, the strengthening mechanism is expected to be different. Here we present for the first time the comparison of plastic deformation of passivated and unpassivated single crystal specimens at the sub-micron scale. The role of free surfaces is investigated by comparing stress results of both as-FIB'd, annealed, and alumina-passivated pillars. Preliminary results show that ALD-coated pillars exhibit much higher flow stresses at equivalent sizes and strains compared with the uncoated samples. We also found that while FIB damage during pillar fabrication might account for a small portion of the strength increase, it is not the major contributor.

ASYMPTOTIC ELASTIC STRESS FIELDS AT SINGULAR POINTS

Singular elastic stress fields are generally developed at sharp re-entrant corners and at the end of bonded interfaces between dissimilar elastic materials. This behaviour can present difficulties in both analytical and numerical solution of such problems. For example, excessive mesh refinement might be needed in a finite element solution.

Arash_Yavari's picture

A Theory of Anharmonic Lattice Statics for Analysis of Defective Crystals

This paper develops a theory of anharmonic lattice statics for the analysis of defective complex lattices. This theory differs from the classical treatments of defects in lattice statics in that it does not rely on harmonic and homogeneous force constants. Instead, it starts with an interatomic potential, possibly with in¯nite range as appropriate for situations with electrostatics, and calculates the equilibrium states of defects. In particular, the present theory accounts for the differences in the force constants near defects and in the bulk. The present formulation reduces the analysis of defective crystals to the solution of a system of nonlinear difference equations with appropriate boundary conditions. A harmonic problem is obtained by linearizing the nonlinear equations, and a method for obtaining analytical solutions is described in situations where one can exploit symmetry. It is then extended to the anharmonic problem using modified Newton-Raphson iteration. The method is demonstrated for model problems motivated by domain walls in ferroelectric materials.

Surface Roughness and Electrical Contact Resistance

J.R.Barber The contact of rough surfaces Surfaces are rough on the microscopic scale, so contact is restricted to a few `actual contact areas'. If a current flows between two contacting bodies, it has to pass through these areas, causing an electrical contact resistance. The problem can be seen as analogous to a large number of people trying to get out of a hall through a small number of doors.

Classical treatments of the problem are mostly based on the approximation of the surfaces as a set of `asperities' of idealized shape. The real surfaces are represented as a statistical distribution of such asperities with height above some datum surface. However, modern measurement techniques have shown surfaces have multiscale, quasi-fractal characteristics over a wide range of length scales. This makes it difficult to decide on what scale to define the asperities.

Ken P. Chong's picture

Call for proposals on advanced high strength steel

The Division of Civil, Mechanical and Manufacturing Innovation (CMMI) of NSF and the DOE Office of Freedom CAR and Vehicle Technologies intend to co-sponsor proposals addressing fundamental research issues in advanced high strength steels (AHSS). Specifically, proposals focused on

  1. AHSS materials development and characterization,
  2. predictive modeling that integrates AHSS material structure and product performance, and
  3. fundamental research in the area of processing and manufacturing of AHSS, are of interest. This collaborative effort is a direct outcome of the Advanced High Strength Steel Workshop.

Interested PIs should consider submitting an unsolicited proposal to the core programs of the CMMI Division namely, (1) Materials Processing & Manufacturing (MPM), (2) Materials Design & Surface Engineering (MDSE), (3) Applications & Structural Mechanics, or (4) Mechanics & Structures of Materials (MSM), during the January 15, 2007 to February 15, 2007 submission window. Unsolicited proposals in response to this letter should have titles beginning with "AHSS:".  Proposals from the March-April 2007 panel review will be eligible for co-funding, pending availability of funds.

MichelleLOyen's picture

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

Choose a channel featured in the header of iMechanica: 

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)

sem image

Taxonomy upgrade extras: 
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

Choose a channel featured in the header of iMechanica: 

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

Choose a channel featured in the header of iMechanica: 

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.

Pages

Subscribe to RSS - research

Recent comments

More comments

Syndicate

Subscribe to Syndicate