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MichelleLOyen's picture

Introductory Biomechanics Courses

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In the early days of biomechanics, there probably were not many dedicated biomechanics courses and instead a regular mechanics curriculum was studied by people interested in tissues and biosystems. However, now that there are so many dedicated bioengineering programs at Universities throughout the world, it seems as though it is more likely that much of students' basic mechanics knowledge comes through dedicated biomechanics courses. This then in turn raises the interesting question of what is taught in these courses?

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

Jonathan Zimmerman's picture

Summer Internship at Sandia National Labs, Livermore, CA

Sandia National Laboratories, California has established the Engineering Sciences Summer Institute (ESSI) program, in which applied mechanics, structural analysis and mechanical engineering graduate students are invited to spend a summer at SNL/CA performing research that would jointly benefit the students and Sandia. The program is nine years old and a description of the program is attached. Because of the funding base for this program, we can only consider students having U.S.

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Zhigang Suo's picture

2007 ASME Congress, 12-15 November, Seattle, Washington

Deadline for submitting an abstract: 5 March 2007.

Responding to the wishes of members, the ASME Congress will change to a new format, starting this year. Sessions will not be allocated to Divisions, but will be allocated to symposiums after abstracts are reviewed. Thus, your action item is to submit an abstract to a symposium. Here are terms as used in the 2007 Congress:

Session. Technical sessions will be scheduled for four days, Monday-Thursday. Each session will last 90 minutes, and consists of 4-6 talks. There will be 23 parallell sessions at a given time, 5 time slots for sessions per day, and a total of 23x5x4 = 460 sessions for the entire congress.

Mogadalai Gururajan's picture

The SIAM 100-digit challenge of Bronemann et al: A review

Suppose if somebody asked you the following question, and more importantly, wanted the answer to an accuracy of 100-digits:

  • Problem A: A particle at the center of a 10 x 1 rectangle undergoes Brownian motion (i.e., two-dimensional random walk with infinitesimal step lengths) until it hits the boundary. What is the probability that it hits at one of the ends rather than at one of the sides?

Or, this question (again, demanding the answer to an accuracy of 100-digits):

Nicolas Cordero's picture

Channel cracks in a hermetic coating consisting of organic and inorganic layers

Abstract: Flexible electronic devices often require hermetic coatings that can withstand applied strains. This paper calculates the critical strains for various configurations of channel cracks in a coating consisting of organic and inorganic layers. We show that the coating can sustain the largest strain when the organic layer is of some intermediate thicknesses.

Flexible electronics are promising for diverse applications, such as rollable displays, conformal sensors, and printable solar cells. These systems are thin, rugged, and lightweight. They can be manufactured at low costs, for example, by roll-to-roll printing. The development of flexible electronics has raised many issues concerning the mechanical behavior of materials. This paper examines a particular issue: channel cracks in hermetic coatings.

Electronic devices (e.g., organic light-emitting devices, OLEDs) often degrade when exposed to air. Developing hermetic coatings has been a significant challenge. Organic films are permeable to gases, and inorganic films inevitably contain processing flaws, so that neither by themselves are effective gas barriers. These considerations have led to the development of multilayer coatings consisting of alternating organic and inorganic films. To be used in flexible electronics, these coatings must also withstand applied strains without forming channel cracks...

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.

Faculty Position at NC State University

The Department of Mechanical and Aerospace Engineering at North Carolina State University invites applications for a tenure-track faculty position in the general area of mechanical sciences. Candidates must have an earned doctorate in Mechanical or Aerospace Engineering or closely-related field. Successful candidates will be expected to teach at the undergraduate and graduate levels, to advise graduate students, and to establish a high quality, nationally-visible externally funded research program.

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.

Robert Gracie's picture

A new finite element method for dislocations based on interior discontinuities

Comments and feedback of the following paper would be appreciated.

Abstract:

A new technique for the modelling of multiple dislocations based on introducing interior discontinuities is presented. In contrast to existing methods, the superposition of infinite domain solutions is avoided; interior discontinuities are specified on the dislocation slip surfaces and the resulting boundary value problem is solved by a finite element method. The accuracy of the proposed method is verified and its efficiency for multi-dislocation problems is illustrated. Bounded core energies are incorporated into the method through regularization of the discontinuities at their edges. Though the method is applied to edge dislocations here, its extension to other types of dislocations is straightforward.

Luis Dorfmann's picture

New Directions in Large Deformation Solid Mechanics

We write to invite your participation in the symposium on New Directions in Large Deformation Solid Mechanics to be held at Texas A&M University campus in College Station as part of the 44th Annual Technical Meeting of the Society of Engineering Science, October 21–24, 2007. The purpose of this symposium is to address and outline new directions in large deformation solid mechanics and to furnish a forum for discussions on a wide range of research in all fields comprising modern mechanics.

Luis Dorfmann, Ray Ogden, Alan Wineman

5 PhD positions in solid mechanics

There are 5 vacant PhD positions with good stipend in the field of solid mechanics, constitutive modelling and numerical simulations. We are looking for very competent, creative students all over the world. It is a rare opportunity for a young scientist to develop his versatile skills and this is your chance. As some people know, SIMLab (Structural Impact Laboratory) is internationally reputed group working on problems related to Crashworthiness and Structural Impact. Our group recently got a Centre for Research based Innovation (CRI). All these positions are filling under CRI. Please find the details in the links here:

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

J-Club participation: advice for students

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Now that the Journal Club is getting started, I wanted to post some advice for students who may not have participated in such a forum in the past, either online or in person. The fun of a J-club includes the opportunity to broaden your understanding of the literature of a broad field (like mechanics) by reading carefully-selected groups of papers on small topics of interest to different sub-communities. The challenge in participating in the subsequent discussion is ensuring that you understand the papers sufficiently to participate confidently in the ensuing discussion. This will also prepare you to be a good reviewer for technical journals some day.

Alexander A. Spector's picture

Magnetic Twisting Cytometry and Cell Mechanical Propertries

Some time ago (12-19-06), Daniel Isabey posted an interesting comment on mechanical responses of cells obtained via magnetic twisting cytometry. While the comment was about the nonlinearity of the bead angular displacement, a broader question is how adequately the bead moment/angle relationship represents the complex cell mechanics. There are different patterns of actin bundles at the whole-cell level.

George Herrmann passed away

(A message from Dave Barnett) George Herrmann passed away yesterday in Switzerland -- quickly, quietly, and peacefully.

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.

The Energy Blog

I just jointed iMechanica. Great blog site! I thought to bring to your attention another blog that I enjoy, run by a retired engineer, on renewable energy issues. Here is the link: http://thefraserdomain.typepad.com/

Zhigang Suo's picture

Josiah Willard Gibbs and his two shorter papers on thermodynamics

There seems to be tremendous enthusiasm among young mechanicians to master thermodynamics. I have found no better source for enlightenment than Gibbs's own writings on the subject, collected in a paperback, still in print. By common consensus, his masterpiece on the subject is the 300-page paper entitled "On the Equilibrium of Heterogeneous Substances". Although I have returned to the long paper many times for illumination, my own favorites are his two shorter papers, written in 1873, before the long one. In many ways, I think, the longer paper is an elaboration of the ideas in the two shorter ones. The title of the short papers are

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