iMechanica

I am a senior research engineer at Medical Instill Technologies, New Milford, CT. Our company is currently focused on designing and manufacturing innovative dispensing systems for pharmaceutical and nutritional products, and also their filling systems. You may visit our website (http://www.medinstill.com) for more information.

We are currently exploring elastomer-based dispensing systems for fluids of different viscosities. In relation to this project, we are interested in hiring somebody with a doctoral level education and who has exposure to modeling elastomers, particularly elastomer solid mechanics, adhesion between elastomers, and flow of fluids over elastomers.

Dear Colleagues:

The 13th International Conference on Experimental Mechanics (ICEM13, http://www.icem13.gr) will be held on July 1-6, 2007 in Alexandroupolis, Greece. It is our pleasure to announce that the Conference will include a special symposium organized by us entitled, “Plasticity, Fracture and Fatigue at the Micro and Nano Scales,” which will focus on recent developments in this area within the larger scope of assessing research needs in a variety of applications of interest.

These notes are part of my notes on thermodynamics.

Update on 14 December 2019. By now I have taught undergraduate thermodynamics three times at Harvard. I have written up my lecture notes as a book, and posted the book online.

Here are sections that I have now:

A student pointed me to a recent article on physicsweb. This article discusses a new (scientific) ranking system developed by a German student (Michael Banks) in Max Planck Institute of Solid State Physics to characterize the "hotness" of the scientific subject. If, after reading the popular physicsweb article linked above, you are interested in more details you may wish to read the attached original article posted by Banks. "Carbon nanotubes" emerges at the top of the list.

The nanotechnology of the future demands controlled fabrication of nanostructures. Much success has been made in the last decade in fabricating nanostructures on surface with desirable size and shape, either in serial using scanned-probe techniques or in parallel using self-assembly/self-organization processes sometimes combined with lithographic patterning techniques. However, controlled fabrication of nanostructures remains in general a formidable challenge. For example, despite the enormous success we have so far enjoyed with carbon nanotubes (CNTs), it is still very difficult (if not impossible) to synthesize CNTs with a degree of control that we would like in terms of their size and chirality. Fabrication of nanostructures in many other forms and with other materials is even less developed. There exists a strong need for the development of nanofabrication techniques with higher degree of control. Here, we demonstrate the general design principles of an emerging nanofabrication approach based on nanomechanical architecture of strained bi-layer thin films, which allows fabrication of a variety of nanostructures, such as nanotubes, nanorings, nanodrills, and nanocoils, with an unprecedented level of control.

The nomination of colleagues for awards is one of the most important and gratifying aspects of participating in the scientific community. Help celebrate the contributions of your colleagues by submitting a nomination for The National Medal of Science.

The National Medal of Science was established in 1959 as a Presidential Award to be given to individuals "deserving of special recognition by reason of their outstanding contributions to knowledge in the physical, biological, mathematical, or engineering sciences." In 1980 Congress expanded this recognition to include the social and behavioral sciences. The National Medal of Science is the highest honor the President bestows on scientists. A Committee of 12 scientists and engineers is appointed by the President to evaluate the nominees for the Award. Since its establishment, the National Medal of Science has been awarded to 425 distinguished scientists and engineers whose careers spanned decades of research and development.

Teng Li, Zhigang Suo, Stephanie P. Lacour, Sigurd Wagner

Journal of Materials Research, 20, 3274-3277 (2005)

When a mechanical engineer and a material scientist were asked for the root cause of an in-vivo fracture. Mechanical engineer pointed to the loading and the material scientist pointed to the processing. While they both are correct, they both also missed the real ROOT cause, the design.

It is very common that medical device design engineers are so focused on the device functionality that often the very basic mechanics is overlooked. Lack of knowledge on the in-vivo environment (Design Requirements) is another subject to blame. However, it is common that even technology driven companies have gaps between design department and duarability deparment. Up front design engineers do not necessarily keep up with the fast paces of material advances. On the other hand, downstram subject matter experts, device tesing teams or often the R&D departments are not informed of design changes before the design is fixed. The problem is worse often in industrial leaders than in start-ups, but the sympton is the same, problem found in animal studies and/or clinical trials before they reached industrial subject matter experts.

OSTEONECROSIS is the death of bone that results in the collapse of the bony structure, leading to joint pain, bone destruction, and loss of function. Destruction of the bone frequently is severe enough to require joint replacement surgery. Osteonecrosis is a common disorder and accounts for 10% or more of the 500,000 total joint replacement procedures performed annually in the United States. Approximately 75% of patients with osteonecrosis are between 30 and 60 years of age.

From the point of view of mechanics, osteonecrosis means deterioration of mechanical properties of the bone. Decrease of the magnitude of the elastic modulus of the bone leads to its inability to bear the external load and culminates in bone damage and fracturing. For a couple of decades the engineers were trying to estimate the critical stress-strain state of the femoral head using the available data on the osteonecrotic bone properties, finite element analysis based on 3D elasticity, and Von Mises stress as a criticality condition. The fact that the cortical shell of the femoral head is significantly stiffer than the underlying cancellous bone did not attract much attention yet. However, from the solid mechanics point of view the difference in the stiffness of the cortical and cancellous parts of the femoral head under both normal and necrotic conditions is important. This difference allows for considering the femoral head as an elastic cortical shell on an elastic cancellous foundation. This, in its turn, suggests the buckling of the cortical shell as a possible starting point of the overall head collapse. The purpose of the study, described here, was to assess the cortical shell buckling scenario as a possible mechanism of the femoral head collapse at the various stages of osteonecrosis.

Recent results on fluid-structure interaction for plates subject to high intensity air shocks are employed to assess the performance of all-metal sandwich plates compared to monolithic solid plates of the same material and mass per area. For a planar shock wave striking the plate, the new results enable the structural analysis to be decoupled from an analysis of shock propagation in the air. The study complements prior work on the role of fluid-structure interaction in the design and assessment of sandwich plates subject to water shocks. Square honeycomb and folded plate core topologies are considered. Fluid-structure interaction enhances the performance of sandwich plates relative to solid plates under intense air shocks, but not as significantly as for water blasts. The paper investigates two methods for applying the loading to the sandwich plate-responses are contrasted for loads applied as a time-dependent pressure history versus imposition of an initial velocity. Click here for the full paper.

Selective oxidation induced void growth is observed in thermal barrier coating (TBC) systems used in gas turbines. These voids occur at the interface between the bond coat and the thermally grown oxide layer. In this article we develop the modeling framework to simulate microvoid growth due to coupled diffusion and creeping in binary alloys. We have implemented the modeling framework into an existing finite element program. The developed modeling framework and program is used to simulate microvoid growth driven by selective oxidation in a binary beta-NiAl alloy. Axisymmetric void growth due to the combined action of interdiffusion and creeping is simulated. The sharpness of the void and direction of creeping are considered as parameters in our study. Our simulations show that the voids dilate without any change in shape when creeping is equally likely in all the directions (isotropic). Void growth patterns similar to those observed in experiments are predicted when the creeping is restricted to occur only along the radial and tangential directions. A hemispherical void grows faster compared to a sharp void. The sharpness increases in the case of a sharp void and could lead to interactions with the neighboring voids leading to spallation of the thermally grown oxide layer as observed in experiments.

As an indenter penetrates an elastoplastic material, the indentation load P can be measured as a continuous function of the indentation displacement δ, to obtain the so-called P-δ curve. A primary goal of the indentation analysis is to relate the material elastoplastic properties (such as the Young's modulus, yield stress, and work-hardening exponent) with the indentation response (i.e. the shape factors of the P-δ curve, including its curvature, unloading stiffness, loading work, unloading work, maximum penetration, residual penetration, maximum load, etc.). The sharp indenters (e.g.

Rui Huang and Se Hyuk Im, Physical Review E 74, 026214 (2006).

A stressed thin film on a soft substrate can develop complex wrinkle patterns. The onset of wrinkling and initial growth is well described by a linear perturbation analysis, and the equilibrium wrinkles can be analyzed using an energy approach. In between, the wrinkle pattern undergoes a coarsening process with a peculiar dynamics. By using a proper scaling and two-dimensional numerical simulations, this paper develops a quantitative understanding of the wrinkling dynamics from initial growth through coarsening till equilibrium. It is found that, during the initial growth, a stress-dependent wavelength is selected and the wrinkle amplitude grows exponentially over time. During coarsening, both the wrinkle wavelength and amplitude increases, following a simple scaling law under uniaxial compression. Slightly different dynamics is observed under equi-biaxial stresses, which starts with a faster coarsening rate before asymptotically approaching the same scaling under uniaxial stresses. At equilibrium, a parallel stripe pattern is obtained under uniaxial stresses and a labyrinth pattern under equi-biaxial stresses. Both have the same wavelength, independent of the initial stress. On the other hand, the wrinkle amplitude depends on the initial stress state, which is higher under an equi-biaxial stress than that under a uniaxial stress of the same magnitude.

See an accompanying powerpoint presentation: The aged-rejuvenation-glue-liquid (ARGL) shear band model has been proposed for bulk metallic glasses (Acta Mater. 54 (2006) 4293), based on small-scale molecular dynamics simulations and thermomechanical analysis. The model predicts the existence of a critical lengthscale ~100 nm and timescale ~100 ps, above which melting occurs in shear-alienated glass. Large-scale molecular dynamics simulations with up to 5 million atoms have directly verified these predictions. When the applied stress exceeds the glue traction (computed separately before), we indeed observe maturation of the shear band embryo into bona fide shear crack, accompanied by melting.

The deadline of October 1, 2006 for my program of Mechanics & Structures of Materials was inadvertently omitted in our website. However, at the beginning of our CMS home page there are 2 deadlines listed for all programs. In the meantime any unsolicited proposals for my program, please put in GPG 04-23 as the Program Announcement [1st box]. In the 2nd box put in my program name [Mechanics & Structures of Materials].

A recent discussion here about the effect of surface stress on vibrations of microcantilever has gained some interest from our members. A few years ago, Zhigang and I looked at surface effect on buckling of a thin elastic film on a viscous layer (Huang and Suo, Thin Solid Films 429, 273-281, 2003). Although the physical phenomena (buckling vs vibrations) are different, the conclusion is quite consistent with Wei Hong and Pradeep's comments toward the end of the discussion. That is, surface stress only contributes as a residual stress and thus does not affect the buckling wavelength (frequency in space in analogy to frequency in time for vibrations).

1. iMechanica is free for all to use. iMechanica is hosted on a server at the School of Engineering and Applied Sciences, of Harvard University, and is managed by a team of volunteers -- mechanicians just like you. You pay nothing to post, and readers pay nothing to read. The limit of each upload file is 50MB, and each user is given 1GB server space.

One of the most common forms of cohesive failure observed in brittle thin film subjected to a tensile residual stress is channel cracking, a fracture mode in which through-film cracks propagate in the film. The crack growth rate depends on intrinsic film properties, residual stress, the presence of reactive species in the environments, and the precise film stack.

It is well established that the growth of microscopic voids near a crack tip plays a fundamental role in establishing the fracture behavior of ductile metals. Mechanics analyses of plastic void growth have typically assumed the plastic properties of the surrounding metal to be isotropic. However voids are typically of the order of magnitude of one micron so that they exist within individual grains of the metal, or along grain boundaries, at least at the initial growth stage. For that reason, the plastic properties of the material surrounding the void are most properly treated as being anisotropic, rather than isotropic.

In the uploaded preprint, the stress state and deformation state are derived around a cylindrical void in a hexagonal close packed single crystal. The orientation of the cylindrical void and the loading state relative to the crystal are chosen so that the deformation state is one of plane strain. The active slip systems reduce to a total of three slip systems which act within the plane of plane strain. The solution shows that the deformation state consists of angular sectors around the void within which only one slip system is active. Further, it is shown that the stress state and deformation state exhibit self-similarity both radially and circumferentially, as well as periodicity along certain logarithmic spirals which emanate from the void surface.

A possible explanation of the variety of fingerprints comes from the consideration of the mechanics of tissue growth. Formation of fingerprints can be a result of the surface buckling of the growing skin. Remarkably, the surface bifurcation enjoys infinite multiplicity. The latter can be a reason for the variety of fingerprints. Tissue morphogenesis with the surface buckling mechanism and the growth theory underlying this mechanism are presented in the attached notes.

M.A. Hussein and Jun He (Intel Corporation)

IEEE Transactions on Semiconductor Manufacturing, vol. 18, No. 1, p.69-85, 2005

In this work, we explain how the manufacturing technology and reliability for advanced interconnects is impacted by the choice of metallization and interlayer dielectric (ILD) materials. The replacement of aluminum alloys by copper, as the metal of choice at the 130nm technology node, mandated notable changes in integration, metallization, and patterning technologies. Those changes directly impacted the reliability performance of the interconnect system. Although further improvement in interconnect performance is being pursued through utilizing progressively lower dielectric constant (low-k) ILD materials from one technology node to another, the inherent weak mechanical strength of low-k ILDs and the potential for degradation in the dielectric constant during processing, pose serious challenges to the implementation of such materials in high volume manufacturing. We will consider the cases of two ILD materials; carbon-doped silicon dioxide (CDO) and low-k spin-on-polymer to illustrate the impact of ILD choice on the process technology and reliability of copper interconnects. preprint pdf 2.49 MB

Flip-chip plastic ball grid array (FC-PBGA) packages are widely used in high performance components. However, its die back is normally under tensile stress at low temperatures. This paper presents a probabilistic mechanics approach to predict the die failure rate in the FC-PBGA qualification process. The methodology consists of three parts:

Ting Y. Tsui, Andrew J. McKerrow, and Joost J. Vlassak

Published in the Journal of The Mechanics and Physics of Solids, 54 (5), 887-903 (2006)

Abstract – Organosilicate glass (OSG) is a material that is used as a dielectric in advanced integrated circuits. It has a network structure similar to that of amorphous silica where a fraction of the Si-O bonds has been replaced by organic groups. It is well known from prior work that OSG is sensitive to subcritical crack growth as water molecules in the environment are transported to the crack tip and assist in rupturing Si-O bonds at the crack tip. In this study, we demonstrate that exposure of an OSG containing film stack to water prior to fracture results in degradation of the adhesion of the film stack. This degradation is the result of the diffusion of water into the film stack. We propose a quantitative model to predict adhesion degradation as a function of exposure time by coupling the results of independent subcritical crack growth measurements with diffusion concentration profiles. The model agrees well with experimental data and provides a novel method for measuring the water diffusion coefficient in film stacks that contain OSG. This study has important implications for the reliability of advanced integrated circuits.