iMechanica

Anyone interested in the history of mechanical technology might find interesting the series that I have published in Mechanical Engineering magazine.

Galileo’s Telescope Lenses

http://www.memagazine.org/oct06/features/clearas/clearas.html

Atmospheric Railway

http://www.memagazine.org/backissues/feb06 /features/tallyho/tallyho.html

As you know, the volumetric expansion by 9% during the water-to-ice transition can generate tremendous pressure in a confined space is a common sense. As a result, one may expect freezing water to also fracture rocks.

However, in a recent article in Science, Bernard Hallet explains the power of the 9% water-to-ice expansion in confined spaces is undeniable, but it may rarely be significant for rocks under natural conditions, because it requires a tight orchestration of unusual conditions. Unless the rocks are essentially saturated with water and frozen from all sides, the expansion can simply be accommodated by the flow of water into empty pores, or out of the rock through its unfrozen side.

I think it may be of interest to mechanics. Read more
I hope to hear opinions from people who know about the breaking mechanics of rocks.

This is a paper we recently published in JMPS on a study of the mechanical properties on thin films comparing experimental results with discrete dislocation simulations. It provides insight in the strengthening that occurs in thin metal films when surface or interface effects become important.

The abstract is below; the full paper can be downloaded from here

Abstract - Experimental measurements and computational results for the evolution of plastic deformation in freestanding thin films are compared. In the experiments, the stress–strain response of two sets of Cu films is determined in the plane-strain bulge test. One set of samples consists of electroplated Cu films, while the other set is sputter-deposited. Unpassivated films, films passivated on one side and films passivated on both sides are considered. The calculations are carried out within a two-dimensional plane strain framework with the dislocations modeled as line singularities in an isotropic elastic solid. The film is modeled by a unit cell consisting of eight grains, each of which has three slip systems. The film is initially free of dislocations which then nucleate from a specified distribution of Frank–Read sources. The grain boundaries and any film-passivation layer interfaces are taken to be impenetrable to dislocations. Both the experiments and the computations show: (i) a flow strength for the passivated films that is greater than for the unpassivated films and (ii) hysteresis and a Bauschinger effect that increases with increasing pre-strain for passivated films, while for unpassivated films hysteresis and a Bauschinger effect are small or absent. Furthermore, the experimental measurements and computational results for the 0.2% offset yield strength stress, and the evolution of hysteresis and of the Bauschinger effect are in good quantitative agreement.

It is well-recognized that MEMS switches, compared to their more traditional solid state counterparts, have several important advantages for wireless communications.  These include superior linearity, low insertion loss and high isolation.  Indeed, many potential applications have been investigated such as Tx/Rx antenna switching, frequency band selection, tunable matching networks for PA and antenna, tunable filters, and antenna reconfiguration. 

However, none of these applications have been materialized in high volume products to a large extent because of reliability concerns, particularly those related to the metal contacts.  The subject of the metal contact in a switch was studied extensively in the history of developing miniaturized switches, such as the reed switches for telecommunication applications.  While such studies are highly relevant, they do not address the issues encountered in the sub 100mN, low contact force regime in which most MEMS switches operate.  At such low forces, the contact resistance is extremely sensitive to even a trace amount of contamination on the contact surfaces.  Significant work was done to develop wafer cleaning processes and storage techniques for maintaining the cleanliness.  To preserve contact cleanliness over the switch service lifetime, several hermetic packaging technologies were developed and their effectiveness in protecting the contacts from contamination was examined.  

Stresses inevitably arise in a microelectronic device due to mismatch in coefficients of thermal expansion, mismatch in lattice constants, and growth of materials. Moreover, in the technology of strained silicon devices, stresses have been deliberately introduced to increase carrier mobility. A device usually contains sharp features like edges and corners, which may intensify stresses, inject dislocations into silicon, and fail the device.

In a recent article in Physical Review Letters, Alain Goriely and Sébastien Neukirch offer a mechanical model of how the free tip of a twining plant can hold onto a smooth support, allowing the plant to grow upward. The model also explains why these vines cannot grow on supports of too large a diameter. Read more.

The mechanics involves large deflection and bifurcation of a rod. I hope to hear opinions from people who know about the mechanics of plants.

Applications are invited from suitably qualified candidates for a University Lectureship in Solid Mechanics, which falls within the Mechanics, Materials and Design Division of the Engineering Department. The successful candidate will take up the appointment as soon as possible.

The lectureship has recently been endowed by David and Susan Hibbitt, and the aim is to attract a high calibre researcher with a record of scholarship and research in experimental, computational and/or theoretical solid mechanics. Expertise is required in the mechanics of materials (structural, biological or energy materials, for example) and the successful candidate is expected to make a significant contribution to the Department’s teaching and research activities and to build a strong, externally funded research programme. The activity will fit within the Cambridge Centre for Micromechanics, which is an inter-departmental, inter-disciplinary research group housed within the Engineering Department.

Biomechanics is a reasonably well-developed field of study, with a modern history usually linked to the pioneering work of Prof. Y.C. Fung in the 1960s. There are a number of dedicated biomechanics journals (including but not limited to the Journal of Biomechanics and the Journal of Biomechanical Engineering). The field is well-enough established to have several generations of researchers working on the subject at universities across the world.

Symposium DD at the upcoming Materials Research Society Annual Meeting (Nov. 26-Dec. 1, Boston, MA) will be the latest in a series of MRS symposia on the mechanics of biological materials and materials designed following natural principles ("biomimetic" or "bio-inspired").   The full program is available at the MRS website (www.mrs.org).  This topic was also the subject of the August, 2006 focus issue of the Journal of Materials Research, which contained over 30 articles on the subject.

The electromigration lifetime is measured for a large number of copper lines encapsulated in an organosilicate glass low-permittivity dielectric. Three testing variables are used: the line length, the electric current density, and the temperature. A copper line fails if a void near the upstream via grows to a critical volume that blocks the electric current. The critical volume varies from line to line, depending on line-end designs and chance variations in the microstructure. However, the statistical distribution of the critical volume (DCV) is expected to be independent of the testing variables. By contrast, the distribution of the lifetime (DLT) strongly depends on the testing variables. For a void to grow a substantial volume, the diffusion process averages over many grains along the line. Consequently, the void volume as a function of time, V(t), is insensitive to chance variations in the microstructure. As a simplification, we assume that the function V(t) is deterministic, and calculate this function using a transient model. We use the function V(t) to convert the experimentally measured DLT to the DCV. The same DCV predicts the DLT under untested conditions.

A viscous-elastic-plastic indentation model was used to assess the local variability of properties in healing porcine bone. Constant loading- and unloading-rate depth-sensing indentation tests were performed and properties were computed from nonlinear curve-fits of the unloading displacement-time data. Three properties were obtained from the fit: modulus (the coefficient of an elastic reversible process), hardness (the coefficient of a nonreversible, time-independent process) and viscosity (the coefficient of a nonreversible, time-dependent process). The region adjacent to the dental implant interface demonstrated a slightly depressed elastic modulus along with an increase in local time-dependence (lower viscosity); there was no clear trend in bone hardness with respect to the implant interface.

Jeannette M. Jacques, Ting Y. Tsui, Andrew J. McKerrow, and Robert Kraft

To improve capacitance delay performance of the advanced back-end-of-line (BEOL) structures, low dielectric constant organosilicate glass (OSG) has emerged as the predominant choice for intermetal insulator. The material has a characteristic tensile residual stress and low fracture toughness. A potential failure mechanism for this class of low-k dielectric films is catastrophic fracture due to channel cracking. During fabrication, channel cracks can also form in a time-dependent manner due to exposure to a particular environmental condition, commonly known as stress-corrosion cracking. Within this work, the environmental impacts of pressure, ambient, temperature, solution pH, and solvents upon the channel cracking of OSG thin films are characterized. Storage under high vacuum conditions and exposure to flowing dry nitrogen gas can significantly lower crack propagation rates. Cracking rates experience little fluctuation as a function of solution pH; however, exposure to aqueous solutions can increase the growth rate by three orders of magnitude.

A (001) surface of silicon consists of terraces of two variants, which have an identical atomic structure, except for a 90° rotation. We formulate a model to evolve the terraces under the combined action of electric current and applied strain. The electric current motivates adatoms to diffuse by a wind force, while the applied strain motivates adatoms to diffuse by changing the concentration of adatoms in equilibrium with each step. To promote one variant of terraces over the other, the wind force acts on the anisotropy in diffusivity, and the applied strain acts on the anisotropy in surface stress. Our model reproduces experimental observations of stationary states, in which the relative width of the two variants becomes independent of time. Our model also predicts a new instability, in which a small change in experimental variables (e.g., the applied strain and the electric current) may cause a large change in the relative width of the two variants.

Xi Wang, Yves Bellouard, Joost J. Vlassak

Published in Acta Materialia 53 (2005) p4955-4961.

Abstract — We present the results of a crystallization study on NiTi shape memory thin films in which amorphous films are annealed by a scanning laser. This technique has the advantage that shape memory properties can be spatially distributed as required by the application. A kinetics study shows that nucleation of the crystalline phase occurs homogenously in the films. Consequently, the laser annealing process produces polycrystalline films with a random crystallographic texture. The crystallized films have a uniform microstructure across the annealed areas. The material in the crystalline regions transforms reversibly to martensite on cooling from elevated temperature and stress measurements show that a significant recovery stress is achieved in the films upon transformation.

Cells function based on a complex set of interactions that control pathways resulting in ultimate cell fates including proliferation, differentiation, and apoptosis. The interworkings of his immensely dense network of intracellular molecules are influenced by more than random protein and nucleic acid distribution where their interactions culminate in distinct cellular function.

Low dielectric constant (low-k) is achieved often at the cost of degraded mechanical properties, making it difficult to integrate the dielectric in the back end of line (BEOL) and to package low-k chips. Development of low-k technology becomes costly and time-consuming. Therefore, more frequently than before, people resort to modeling to understand mechanical issues and avoid failures. In this paper we present three multilevel patterned film models to examine channel cracking in low-k BEOL. The effects of copper features, caps and multilevel interconnects are investigated and their implications to BEOL fabrication are discussed.

Low-k BEOL Mechanical Modeling
Liu, Xiao Hu; Lane, Michael W; Shaw, Thomas M; Liniger, Eric G; Rosenberg, Robert R; Edelstein, Daniel C
Advanced Metallization Conference 2004 (AMC 2004); San Diego, CA and Tokyo; USa and Japan; 19-21 Oct. 2004 and 28-29 Sept. 2004. pp. 361-367. 2005

DEPARTMENT OF MECHANICAL ENGINEERING AND MATERIALS SCIENCE

PRATT SCHOOL OF ENGINEERING

The Department of Mechanical Engineering and Materials Science invites applications for tenure-track faculty positions. Two tenure-track appointments are anticipated and are open to all ranks, Assistant, Associate and Full Professor level. Applications are invited from candidates with research interests in autonomous vehicles and robotic systems, conventional and alternative energy technology, and MEMS/NEMS devices. Applications will also be accepted for allied mechanical engineering disciplines such as nonlinear dynamics and control, sensor technology, small and micro-scale propulsion systems, aerodynamics and aeroelasticity, thermal sciences, and vehicle dynamics.

Successful candidates are expected to establish a vibrant research program, obtain competitive external research funding, and participate actively in teaching at both the undergraduate and graduate levels. Applicants should submit a cover letter describing their research interests and qualifications, a curriculum vitae, and the names and addresses of three references. Please submit your application to mems-search@mems.duke.edu as a PDF (preferred) or Word file attached to your email. Duke University is an Affirmative Action/Equal Opportunity Employer.

Experiments with a multidimensional nano-contact system (Lucas, Hay, and Oliver, J. Mater. Res. 2004) have shown that, prior to kinetic frictional sliding, there is a significant reduction of the tangential contact stiffness relative to the elastic prediction. The reduction occurs at contact sizes below about 50~200nm for aluminum single crystals and several other materials. Using a cohesive interface model, we find that this reduction corresponds to a transition from a small-scale-slip to large-scale-slip condition of the interface.

The effect of long-range (LR) elastic interactions on the toroidal moment of polarization in a two-dimensional ferroelectric particle is investigated using a phase field model. The phase field simulations exhibit vortex patterns with purely toroidal moments of polarization and negligible macroscopic polarization when the spontaneous strains are low and the simulated ferroelectric size is small. However, a monodomain structure with a zero toroidal moment of polarization is formed when the spontaneous strains are high in small simulated ferroelectrics, indicating that, because of the LR elastic interactions, high values of spontaneous strains hinder the formation of polarization vortices in ferroelectric particles. Applied Physics Letters 88, 182904 (2006)

The Multi-Physics Modeling and Simulation Department at Sandia National Laboratories, California, is seeking a technical staff member to develop finite element-based simulation codes for linear and nonlinear solid mechanics and/or to perform solid mechanics and structural dynamics modeling and simulation. Typical departmental programs include: detailed analyses of weapon systems; design guidance of weapon components through analysis; development of forging and welding modeling capabilities; pressure vessel analysis including aging and failure; penetration modeling; ground shock and hydrodynamics modeling and simulations; failure model development and implementation (metals and composites); thermal and dynamic analysis of artillery projectiles; and electromagnetics and EM wave propagation analysis.

The Mechanics of Materials Department performs experimental and analytical studies to understand the mechanical behavior of materials. Our experimental work covers the entire discoverycharacterization-
validation spectrum. Motivated by observations, we develop models to simulate material responses under various loading and environmental conditions. The fidelity of our models and simulations vary from atomic to continuum scales corresponding to the requirement of Sandia applications. Accuracy of the models for specific applications is validated by experimental data. Numerical codes are developed to allow implementation of the
material models for high performance computing simulations.

The highly motivated scientist or engineer with expertise in experimental mechanics will work as a part of a diverse team in our state-of-the-art laboratories. The applicant is expected to develop and apply experimental research methods in one or more of several research areas, including: material model development, failure
model development, rate-dependent material effects and advanced experimental methods/diagnostic technique development.

Introduction:

This is the Fifteenth Conference in Mechanical Engineering series that started in 1992. The conference is concerned with the latest in theoretical, mathematical and scientific developments in Mechanical Engineering as well as application of established techniques to new domains. Following tradition of conference, ISME 2007 will provide an international technical forum for experts and researches from both the academia and industry to meet and exchange new ideas and present their findings of ongoing research in various Mechanical engineering disciplines.

Although nanostructures, such as nanoparticles, nanotubes, nanowires, nanobelts, and nanometer thick films, nanostructured materials and nanocomposites have been synthesized and fabricated by various techniques, their mechanical properties have not been well explored. These nanostructures are being used as structural and functional building blocks to construct micro/nanodevices. Some nanostructured materials exhibit the breakdown of Hall-Petch behavior. The failure of conventional reinforcing models has been found in nanocomposites. The extremely small dimensions of nanomaterials and micro/nanodevices impose tremendous challenges to many existing experimental techniques and modeling tools. An in-depth understanding of mechanics at the nanoscale is greatly needed. Development of mechanical testing, and manipulation instruments and techniques, is also a technological necessity. This symposium will focus on research on mechanical properties of nanostructures, nanostructured materials and nanocomposites, and reliability testing of micro/nanodevices.