CMSSM: Seminars

October 10 , 2008 (Friday)
"Virtual Fracture Testing of Composites: From Materials to Components"
Javier LLorca, Departamento de Ciencia de Materiales, Universidad Politécnica de Madrid &
Instituto Madrileño de Estudios Avanzados en Materiales (IMDEA-Materiales), (Dr. Stelios Kyriakides)

“Photonic Micro-electro-mechanical Systems for In Vivo Cell Manipulation,
Force Microscopy and Endoscopic Sub-cellular Confocal Imaging”
by
John X.J. Zhang
Biomedical Engineering
UT-Austin

Translational biomedical engineering plays an important role in assimilating the advancement of device engineering towards developing innovative diagnosis, treatment and prevention of important human diseases at genetic, molecular and cellular levels. The micro-nano scale tools for cell manipulation, dynamic culturing and in vivo microscopy contribute to the continuity of investigations across the biological hierarchy of multiple scales, culminating in the understanding of whole body functions in health and disease. In this talk, we focus on the integration of photonic science and engineering with microelectromechanical systems (MEMS) on silicon to: (1) characterize cell and embryo mechanics. Cell mechanics is important for efficient RNA interference (RNAi) microinjections. RNAi is widely employed to systematically study gene functions and to understand new molecular mechanisms that are important for development and disease. Our goal is to identify, through high-throughput microinjection on self-assembled Drosophila embryos, the functions of the new proteins that have been inferred from the genome sequence. In-vivo force characterization is critical for the miniaturized microinjection and self-assembly tools. The intrinsic sensitivity of micro-nano photonic interrogation devices enables accurate force measurement through displacement sensing with sub-wavelength resolution. (2) develop miniaturized in vivo single fiber optic confocal endoscopes using reflective silicon MEMS scanners for non-invasive, early detection of epithelial cancer at sub-cellular resolution. The MEMS scanner replaces the resonance and galvanometer scanning mirrors in our current confocal systems. We developed MEMS imaging scanners with imaging speed and field-of-view suitable for endoscopic applications. We present 500x700µm metal-coated scanning micromirrors fabricated from bonded SOI-Si wafers with ~90% reflectivity at 633nm. Confocal images with 1µm resolution were generated using single axis actuation of ±2.5° at 1.87 KHz.

“Buckling of Thin Pressurized Cylinders Under Bending ”
by
Dr. Ali Limam
Unite de Recherche en Genie Civil
Institut National des Sciences Appiquees
INSA Lyon, France

The buckling design of the cryogenic tank of the European launchers ARIANE 3, 4 and 5 is based on the American methodology given by the NASA SP8007 rule. We are interested in particular in the section dealing with combined loads such as internal pressure, axial compression and bending. The design methodology dating from 1968 uses the so-called lower bound design curves. These methods do not differentiate between badly and well manufactured shell structures. On one hand, the evolution of fabrication and control methods has led to an improvement in the manufacture of the structures. On the other hand, the majority of tests on which the knock down factor was based, were carried out on mylar specimens rather than on metal ones. Since, for high internal pressure, the effect of local plasticity on geometrical imperfections or near the boundaries is to precipitate instability, it is questioned whether the conservativeness of NASA SP-8007 applies. The aim of the present work, based on experimental and numerical approaches, is to give some design recommendations for thin cylindrical shells subjected to combined bending and internal pressure.

Many authors have supposed that global bending and axial compression of a short and thin cylindrical shell are indistinguishable. Here, a careful examination of the differences of behaviour between these two loading configurations is presented, which identifies the key features in the elastic regime. More than 200 experimental results are obtained on electroplated shells with a radius-to-thickness ratio varying between 600 and 1400. The conducted study provides a clear explanation on the reason why internal pressure can have such differing effects on the bearing capacity of shells with different forms of imperfections and the difference between axial compression and bending load configurations are highlighted. Finally, the post-bifurcation response is examined, first looking at observations from experiments and analyses of the experiments, then with simulations conducted with FE codes (STANLAX, CASTEM2000, ABAQUS). Considering these experimental results and a large parametric numerical study, the design recommendations formulated in two codes (NASA SP8007 and Eurocode 3) are examined, and a new design method is proposed.

“Computational Materials Science: Is It Ready to Bridge
Fundamental Physics and Engineering Applications ”
by
H. Eliot Fang
Sandia National Laboratories

For many years, material scientists, chemists and physicists have longed to have computer simulations that predict the behavior of materials and track the evolution of their microstructures from the atomic to the engineering scales. With the staggering advances in computer speed and the continued improvements in numerical techniques in recent years, computational materials science has become a powerful and, in many cases, a predictive tool. As the field of computational materials science develops and matures, the notion that modeling efforts should be an integral part of interdisciplinary materials research and must include experimental validation is taking hold in the community.

At Sandia National Laboratories, scientists develop computational models to predict the performance and behavior of complex materials, such as metals, ceramics, and polymers, across all relevant length and time scales. Promising progresses have been made in modeling material structures at interfaces and in bulk, simulating microstructural evolution during processing, and predicting the property degradation during service and aging. However, significant challenges in theory and numerical algorithm developments still remain to be overcome.

In this presentation, recent efforts of materials modeling and simulation at Sandia to support national security and industry applications will be reviewed, highlighting their successes and challenges. General issues existing in the computational modeling community will be discussed. Lessons learned from bridging physics at different length scales and coupling different simulation codes will also be shared.

“Constitutive Modeling for Ratcheting-Fatigue Response ”
by
Tasnim Hassan
North Carolina State University

Ratcheting-fatigue is a low-cycle fatigue failure mechanism which is yet to be understood clearly for rationally incorporating it into design. Modeling of ratcheting-fatigue is considered one of the most difficult problems in solid mechanics. Lack of experimental data needed for developing ratcheting simulation models is another primary reason for the slow progress in this area. A research program at NC State University is developing experimental data and simulation models for ratcheting responses. The experimental responses at the materials and structures level will be presented in order to demonstrate the significance of the ratcheting-fatigue failure. Performance of several constitutive models in simulating ratcheting responses will be discussed, thus the progresses made and challenges to overcome in simulating ratcheting response will be demonstrated.

“Linear Stress Transformations and Anisotropic Plasticity ”
by
F. Barlat
Alloy Technology and Materials Research Division
Alcoa Technical Center

In order to develop anisotropic yield functions, several methods are available. Among them, the theory of representation of tensor functions is general and rigorous. A special case of this general theory is the approach based on linear transformations applied to a stress tensor. This special case is of practical importance because convex formulations, which ensure stability in numerical simulations, can be easily developed in this framework. The derivation of anisotropic yield functions based on the approach of linear transformations of a stress tensor is investigated for general and plane stress states. The number of coefficients available for the description of plastic anisotropy is discussed. A few specific yield functions are described to illustrate the concept for FCC, BCC and HCP metals. Application examples of finite element (FE) simulations of material performance with this type of constitutive descriptions are presented.

“Mechanics of Stretchable Electronics ”
by
Yonggang Huang
University of Illinois at Urbana-Champaign

Stretchable electronics is important in the development of next-generation electronics since it has many applications such as portable electronics, flexible display, small optical sensor and compact digital camera, sensors and drive electronics for artificial muscles, structural monitors wrapped around aircraft wings, and surgeon’s gloves studded with stretchable sensors that can monitor a patient’s vital signs. However, silicon is an intrinsically brittle material and is not stretchable. We have produced a stretchable form of silicon that consists of sub-micrometer single crystal elements structured into shapes with microscale periodic, wave-like geometries (Science, v 311, pp 208-212, 2006)*. When supported by an elastomeric substrate, this wavy silicon can be reversibly stretched and compressed to large strains without damaging the silicon. The amplitudes and periods of the waves change to accommodate these deformations, thereby avoiding significant strains in the silicon itself. Dielectrics, patterns of dopants, electrodes and other elements directly integrated with the silicon yield fully formed, high performance wavy metal oxide semiconductor field effect transistors, pn diodes and other devices for electronic circuits that can be stretched or compressed to similarly large levels of strain. Mechanics plays an important role in the development of stretchable electronics.

In his lecture, Professor Huang will present several key mechanics problems related to stretchable electronics.

*This work has been selected by MIT Technology Review as "One of the Ten Technologies That Will Change the World" in 2006.
It is also on the museum display in The Tech Museum of Innovation, San Jose, CA, since October 1, 2006.

"Structure, Stiffness, and Strength: Continuum Mechanics Modeling of Clay-Reinforced Polymer Nanocomposites "
by
David Parks
Massachusetts Institute of Technology

In recent years, polymer matrix nanocomposites have received considerable research and commercial attention, owing to their attractive combinations of properties and processability.

We review major features of the structure and properties of clay-reinforced polymer nanocomposites, focusing on montmorillonite/polyamide (MMT/PA) systems. Key features influencing macroscopic mechanical properties include clay particle structure (intercalated/exfoliated), aspect ratio, orientation, volume fraction, and interface chemistry, along with the resulting molecular 'state' of the matrix polymer. The nanometer scale of the silicate reinforcement motivates special procedures for assigning effective particle geometry and properties for continuum-based modeling.

A multidisciplinary demonstration of the efficacy and consistency of continuum modeling quantitatively predicts experimentally-measured shifts in clay FTIRpeaks with nanocomposite strain in terms of MD-calculated, strain-induced shifts in silicate lattice vibration spectrum, connecting these results using Eshelby theory and finite element analysis.

Comparisons of model predictions with extensive literature data for nanoclay/PA indicate that traditional continuum models of particle-based reinforcement of an isotropic linear elastic (neat) polymer matrix broadly account for observed stiffness enhancement, but often under-predict observed composite yield strength. In these semi-crystalline matrix polymers, preferred lamellar orientation of matrix crystallization nucleated adjacent to processing-aligned clay nanoparticles renders the matrix flow strength strongly anisotropic, providing a second, indirect mechanism for composite strengthening.

"Mechanotransduction and the Clustering of Integrins"
by
Wolfgang Frey
Biomedical Engineering, UT-Austin

For adherent cells, such as endothelial cells or fibroblasts, the binding of integrins to the extracellular matrix and of integrins and other adhesion molecules to neighboring cells is crucial for survival and regulates many cellular processes ranging from motility and proliferation to the establishment of a specific phenotype. While the importance of soluble chemical triggers, such as growth factors, has been established for some time, the role of mechanical parameters, such as mechanical stress resulting form asymmetric interactions, spreading restrictions, integrin clustering, and the elasticity of the substrate, has only recently been recognized to be of equal importance. Cellular phenotype control is an important goal in advanced biomaterials design for tissue engineering, for instance to address the long-standing problem of EC adhesion to synthetic and tissue-engineered vascular grafts.

We have developed techniques to create nanopatterned surfaces with well-defined biochemical functionality over cm2 areas, allowing us to direct cellular adhesion sites (focal adhesions) to controlled nanoareas, and thereby vary the number of integrins within individual focal adhesion sites. These changes in the number of bonds per focal adhesion cluster affect the density and total number of focal adhesions and the cell mobility. The found results agree with studies on compliant substrates and indicate that restricting integrin clustering similarly affects the forces that act on each individual anchor, which ultimately influences cellular phenotype. We will present results of the influence of integrin clustering and attempts to measure the force exerted on a single adhesion site.

"Designing, Measuring and Controlling Molecular- and Supramolecular-Scale Properties for Molecular Devices "
by
Paul S. Weiss
Department of Chemistry and Physics
The Pennsylvania State University

"Polymer Nanocomposites Characterization by a Stochatic Finite Elements Representation"
by
Antonios Kontsos
Mechanical Engineering & Materials Science
Rice University

This talk presents a novel multiscale stochastic finite element method (MSFEM) for determining mechanical properties of polymer nanocomposites (PNC) that consist of polymers reinforced with single-walled carbon nanotubes (SWCNT). The proposed method identifies the non-uniform distribution of SWCNT in polymers as a source of uncertainty in the macroscopic mechanical behavior of PNC. Consequently, the method uses a multiscale homogenization approach to link the structural variability at the nano-/micro-scales with the local constitutive behavior. Subsequently, the method formulates a Monte Carlo finite element scheme to determine overall mechanical properties. The derived numerical results agree with pertinent experimental tensile test findings. The method is further applied in modeling of nanoidentation data and of the buckling behavior of PNC structures.

"Adhesion, Friction and Failure of Surfaces: Nano- and Micro-Scale Studies on the Transition from Liquid-Like to Solid-Like Behavior "
by
Jacob Israelachvili
Dept. of Chemical Engineering
University of California - Santa Barbara

Recent experiments using the Surface Forces Apparatus and various surface imaging techniques have been conducted to study the adhesion, friction and fracture processes of various polymer and material surfaces and films. The aim was to investigate the transition between pure liquid/ductile and solid/brittle behavior. Both cross-linked and uncross-linked polymers were studied over a large range of molecular weights and viscoelastic properties, and sugars over a range of temperatures, thereby spanning the purely liquid- or ductile-like (T > glass transition temperature, Tg) to the glassy, elastomeric and brittle regimes (T < Tg). We investigated how the deformations and pathways of failing junctions change on passing through Tg, starting with liquid-like rupture at high T (determined by surface tension and viscous forces) and ending up with fracture via micro-cracks, determined by completely different material properties.

"Biological Structures Mitigate Catastrophic Fracture Through Various Strategies "
by
Roberto Ballarini
Dept. of Civil Engineering
University of Minnesota

"Stress- and Temperature-Induced Martensitic Transformations in Perfect Bi-Atomic Crystals "
by
Nick Triantafyllidis
University of Michigan

Solid-to-solid martensitic phase transformations are technologically important phenomena that result in unique macroscopic material properties such as the shape memory effect, ferromagnetism, and ferroelectric behavior. In shape memory alloys, such as CuAlNi and NiTi, the martensitic transformation, i.e. the change of the alloy’s crystal structure, can result from a change in temperature or the application of stress. In fact, both temperature-induced and stress-induced transformations are essential for the existence of shape memory behavior.

A model for bi-atomic shape memory alloys, based on a set of temperature-dependent atomic potentials, is presented. The equilibrium solutions of the governing nonlinear equations are found using bifurcation techniques and symmetry arguments. To check if a given equilibrium path is observable, its stability against perturbations of arbitrary (with respect to inter-atomic distance) wavelengths is investigated, which requires continuum energy calculations as well as phonon spectra analysis. Our work predicts the existence of a hysteretic two-step temperature-induced proper martensitic transformation from the high-temperature B2 cubic austenite phase, to an intermediate orthorhombic phase, to a final B19 orthorhombic martensitic phase. The application of a uniaxial stress on the B2 cubic austenitic phase at different orientations results in a large number of stable paths. Stress-induced, reversible martensitic transformations are also found in this case with the help of the Erickesn-Pitteri neighborhood concept. The existence of both temperature- and stress-induced transformations indicates the possibility for shape memory behavior. Finally, the predicted transformation parameters show good correspondence with experimental values for the shape memory alloys CuAlNi and AuCd.

This work is done jointly with Prof. R. Elliott, Aerospace Engineering & Mechanics, The University of Minnesota and Prof. J. Shaw, Aerospace Engineering, The University of Michigan.

"Deformation-Induced Anisotropy in Porous Metals: Constitute Modeling and Computational Issues "
by
Nikolaos Aravas
Dept. of Mechanical and Industrial Engineering
University of Thessaly, Greece

A constitutive model for a porous metal subjected to general three-dimensional finite deformations is presented. The model takes into account the evolution of porosity and the development of anisotropy due to changes in the shape and the orientation of the voids during deformation. The pores are initially spherical and distributed randomly in an elastic-plastic matrix (metal). Under finite plastic deformation, the voids are assumed to become ellipsoids, and to change their volume, shape and orientation. At every point in the homogenized continuum, a “representative” ellipsoid is considered with principal axes defined by the unit vectors n⁽¹⁾, n⁽²⁾, n⁽³⁾ = n⁽¹⁾ x n⁽²⁾ and corresponding principal lengths a, b and c. The homogenized continuum is locally orthotropic, with the local axes of orthotropy coinciding with the principal axes of the representative ellipsoid. The basic “internal variables” characterizing the state of the microstructure at every point in the homogenized continuum are given by the local equivalent plastic strain bar ε^p, the local void volume fraction or porosity f, the two aspect ratios of the local representative ellipsoid (w₁= c/a and w₂= c/b) and the orientation of the principal axes of the ellipsoid (n⁽¹⁾, n⁽²⁾, n⁽³⁾). A methodology for the numerical integration of the elastoplastic constitutive model is developed. The problem of ductile fracture near the tip of a blunt crack is studied by using the finite element method and comparisons with traditional constitutive models that assume isotropic behavior are made.

"Microstructure of Multi-functional Materials Visualization to Simulation Journey "
by
Osden Ochoa
Dept. of Mechanical Engineering
Texas A&M University

Carbon foams offer great promise as a porous foundation to be infiltrated, coated and/or co-foamed with different materials to enable desired multifunctionality in broad range of applications. Our current efforts are focused on two specific venues; (i) thermal-mechanical-electrical properties for thermal management via copper coating and (ii) mechanical-biocompatible-bioactive features for orthopedic devices to prevent stress-protection atrophy. The actual three dimensional geometry of the microstructure is captured with micro-CT X-ray scans and converted to solid models for comprehensive computational studies which incorporate material anisotropy, coatings and coupled loads. The details of our unique approach will be discussed through multi-scale models of copper coated carbon foam.

In this dissertation, the results from carefully controlled experiments aimed at investigating fracture and frictional sliding under dynamic loading conditions are presented. An electromagnetic loading device is used to generate a compressive stress wave and dynamic photoelasticity and high-speed photography are used for a diagnostic tool as full-field optical techniques.

For dynamic fracture problems, shear cracks in homogeneous materials by introducing a groove in the specimen and trapping the crack to grow within it are examined. Such shear induced cracks growing at speeds in the intersonic regime are demonstrated. Furthermore, it is shown that the main mechanism of the shear crack growth is the sequential nucleation, growth and coalescence of echelon cracks. The spacing and angle relative to the groove plane of the echelon cracks are measured directly from the experimental specimen. Numerical simulation shows that the echelon cracks are well aligned perpendicular the maximum principal (tensile) stress generated in this specimen. The spacing is interpreted as an intrinsic characteristic of the failure process. These experiments also enable the determination of the dynamic failure stress at which microcracks are nucleated.

For frictional sliding along interface, a novel apparatus has been constructed for the understanding of the nature of dynamic friction by examining the slip pulse propagation under such extremely high rates of loading. When slip occurs across the interface, it is forced to run along the interface at some speed regimes. Several interesting results for a slip pulse due to frictional sliding are presented. Accumulation of fringes near the slip pulse and the orientation of the Mach lines suggest the slip pulse propagates at a speed close to the dilatational wave speed.

"Surface Evolution and Self Assembly of Epitaxial Thin Films: Nonlinear and Anisotropic Effects"

A strained epitaxial film can undergo surface instability and self assemble into discrete islands. The unique physical features of these islands make self-assembly an enabling technique for advanced device technology while control of the island size, shape, and alignment is critical. During the process of self-assembly, the stress field and the interface interaction have profound effects on the dynamics of surface evolution. In this dissertation, a continuum model is developed to study the nonlinear dynamics of surface pattern evolution and self assembly in epitaxial thin films. Within the framework of non-equilibrium thermodynamics, a nonlinear evolution equation is developed, and a spectral method is implemented for numerical simulations. The effects of stress and wetting are examined. It is found that, without wetting, the nonlinear stress field induces a “blow-up” instability. With wetting, the thin film self assembles into an array of discrete islands lying on a thin wetting layer. The dynamics of island formation and coarsening over a long time and a large area is well captured by the interplay of the nonlinear stress field and the wetting effect in the present model.

For single-crystal epitaxy, the anisotropic material properties in the bulk and surface play important roles in the process of self assembly and pattern formation. In particular, this study investigates the effects of anisotropic mismatch stress and generally anisotropic elasticity. First, under an anisotropic mismatch stress, a bifurcation of surface pattern is predicted. The effect of anisotropic elasticity on pattern evolution is then investigated for two specific systems, one for SiGe films on Si substrates with different surface orientations, and the other for hexagonal silicides on Si substrates. It is shown that the consideration of elastic anisotropy reveals a much richer dynamics of surface pattern evolution as opposed to isotropic models. Based on the theoretical and numerical results from the present study, experimental approaches may be developed to control the size and organization of self assembled surface patterns in epitaxial systems.

"The Path from Cell Biology to Mechanical Function of Dental Tissues "
by
Brian Cox
Teledyne Scientific Co.
Thousand Oaks, CA

Rather than describing the mechanical performance of biological hard tissues as primarily determined by their hierarchical nature, we take up the view that the structures are the result of specific actions of creation executed by cells (osteoblasts, osteoclasts, ameloblasts, odontoblasts, …), which have resulted in morphological characteristics that are in various ways optimal. In particular, those morphological characteristics that are commensurate with the cells that created them are known to be very important factors, if not dominant factors, in the mechanical performance of dental enamel, dentin, cortical bone, and trabecular bone. The creation of a tissue morphology can be posed as a purely mathematical problem, in which simple rules of generation govern the evolution of the structure from a set of initial conditions and result in a certain geometrical outcome. As a purely mathematical construct, such a rule-based generator can be an efficient method of creating a computational mesh that represents the morphology. However, the rules can also be interpreted in terms of the response functions of individual cells, providing a pathway from cell function to morphology and thence to mechanical performance. We illustrate these ideas by some preliminary and speculative work on dental enamel. We also argue the merits of embedding models of morphology in a top-down, rather than bottom-up, model of the overall mechanical behaviour of the tissue.

"Finding the Mechanisms of Plastic Deformation in Amorphous
and Nanocrystalline Silicon by Atomistic Simulation"
by
Dr. Michael J. Demkowicz
Los Alamos National Laboratory

Unlike crystalline materials, disordered solids such as amorphous silicon (a-Si) do not undergo plastic deformation by the motion of dislocations. Atomic-level simulations, however, allow the specific mechanisms that govern plasticity in a-Si to be found. I model a-Si using the Stillinger-Weber potential and show that two distinct types of atomic environments can exist in it: "solidlike'' and "liquidlike'' ones. The latter act as plasticity "carriers'' in a-Si, i.e. configurations containing a higher mass fraction of liquidlike material are more amenable to plastic flow. This insight helps to understand the deformation of a-Si to large plastic stain as well as the role of grain boundaries in plastic deformation of nanocrystalline silicon (nc-Si). Consequences of this work for understanding plasticity in metallic and polymeric glasses will be discussed.

"Research Opportunities in Mechanics and Nano/Biosciences at the Army Research Office"
by
Dr. A.M. Rajendran
Chief Scientist (Engineering Sciences)
Army Research Office

This talk will include discussions on the various research opportunities at the U.S. Army Research Laboratory, especially at the Army Research Office at Research Triangle Park, North Carolina. To meet Army’s transformation requirements, the future vehicle and soldier systems have to be ultra lightweight. There are challenging scientific hurdles to overcome in order to meet the technological requirements in the area of applied mechanics, and Nano/Biosciences. The short and long term research focuses are on latest developments in computational mechanics, nanomechanics, biomechanics, rotor dynamics, smart structures and materials, heterogeneous materials, etc. Research opportunities in programs, such as SBIR, STTR, MURI, and other Army initiatives are also addressed.

"Advances in Surface Wrinkling as a Measurement Tool"
by
Christopher Stafford
National Institute of Standards and Technology

Recently, elastic instabilities have received considerable attention in the fields of surface patterning, flexible electronics, and alignment of colloidal particles. At NIST, we pioneered the use of elastic wrinkling to measure the physical properties of thin polymer films, which cannot be accurately measured using conventional techniques due inadequate sensitivity and resolution. In this presentation, I will first briefly review the ability of surface wrinkling to characterize the physical, thermal, and mechanical properties of nanoscale polymer films. Then, I will discuss a new method, based on the critical strain to induce wrinkling, for determining the residual stresses in polymeric thin films that arise as a natural consequence of film formation. In quantifying the amount of residual stress in these films, we also use our approach to provide new insights into two widely used strategies for the dissipation of residual stress in polymer thin films: thermal annealing and plasticizer addition. Furthermore, we demonstrate that the residual stress in these films decreases as the film thickness decreases below a critical thickness ~ 100 nm. Our robust and simple route to measure residual stress adds a key component to the understanding of polymer thin film behavior and will enable more effective processing routes that mitigate the effects of residual stress.

"Influences of Indenter Geometry on the Indentation Size Effect "
by
George Pharr
The University of Tennessee, Department of Materials Science & Engineering
and
Oak Ridge National Laboratory, Materials Science and Technology Division

The indentation size effect has most often been studied using sharp, geometrically self-similar indenters like the Vickers square-based pyramid used in microhardness testing and the Berkovich triangular-based pyramid employed in nanoindentation testing. For these indenters, the size effect is manifested as an increase in hardness with decreasing depth of penetration, with the effect becoming apparent only at depths of less than a few microns. Recent studies have revealed, however, that the nature of the size effect depends critically on the geometry of the indenter. For example, the size effect for spherical indenters is manifested not through the depth of the penetration but rather through the size of the indenter itself, i.e., significant increases in hardness are observed as the radius of the indenter is decreased. In this presentation, the influence of the indenter geometry on the size effect is examined by experiments conducted in a variety of materials with a variety of spherical and pyramidal indenters in order to critically evaluate prevailing mechanistic models.

"Mechanics of Carbon Nanotubes, Diamond and Amorphous Carbon by Atomistic and Multiscale Modeling "
by
Qiang Lu
ASE/EM, UT-Austin

Atomistic modeling and multiscale methods are developed to study mechanical behavior of a group of carbon-based nanomaterials. Of particular interest is the failure of diamond, tetrahedral amorphous carbon (ta-C), and interfaces between carbon nanotube (CNT) and amorphous carbon. The random nature of ta-C structure introduces special problems such as mixed hybridizations modeling and multiple local minimum energy points. These problems are solved by using a bridging domain coupling method, which combines explicit Finite Element at the continuum level and molecular dynamics with an Environmental Dependent Interatomic Potential (EDIP) at the atomic level. Interesting findings include: (i) ta-C nano-specimen fracture is gradual rather than brittle; and (ii) ta-C fracture is insensitive to nano-scale cracks, in contradiction to macro-scale observations. Comparisons of the failure modes among ta-C, abstract random networks and crystalline materials (diamond, graphene sheet, CNT) imply that the nano-crack insensitivity behavior of ta-C is due to its amorphous structure. As an important failure mode in CNT tensile tests, the failure of CNT/ta-C interface is studied by molecular dynamics simulations. It is found that the failure is mainly due to stress concentration and local changes of hybridizations.

"Mechanical and Chemical Effects in the Adhesion of Thin Structures "
by
John Bassani
Dept of Mechanical Engineering and Applied Mechanics
University of Pennsylvania

The adhered state of thin films used in electronic applications and of biological cells in both in vivo and in vitro microenvironments are strongly influenced by a variety of mechanical factors, which include chemistry-dependent adhesive interactions at interfaces. Transitions between bistable snapped-in and snapped-out configurations are predicted from a model that includes nonlinear shell kinematics coupled with elastic material response and an adhesion law. Non-uniform energy and traction fields are a general signature of adhered states. Coupling between these spatially non-uniform fields can result in segregation of chemical species that directly affects equilibrium states. One example occurring in biology is the enhanced adhesion of closed vesicles (e.g., cells) via integrin segregation in membranes. A second example is impurity driven failure of film-substrate, wafer-substrate, and wafer-wafer interface (e.g., due to moisture). Surface topography is found to have a strong influence on the equilibrium configurations including the distributions of adhesive species and tractions.

"Electronic Structure Calculations at Macroscopic Scales "
by
Kaushik Bhattacharya
California Institute of Technology

This talk will describe a method for seamless integration of orbital-free density functional theory calculations with continuum mechanics. Density functional theory, a formulation of quantum mechanics, has provided insights into various materials properties in the recent decade. However, the computational complexity of this approach has made other aspect, especially those involving defects, beyond reach. We describe a method that enables the study of a multi-million atom cluster using orbital free density functional theory with no spurious physics or restrictions on geometry. The key idea is a systematic means of adaptive coarse-graining retaining full resolution where it is necessary and coarsening with no patches, assumptions or structure. We demonstrate the method, its accuracy under modest computational cost and the physical insights it offers using various examples motivated by the radiative damage of structural materials.

"Technical Challenges in the Offshore Oil and Gas Industry"
by
Rahul Pakal
ExxonMobil Upstream Research Company

"Adsorption-Generated Surface Stresses and Implications for
Chemo-mechanical Biochemical Sensors "
by
Matthew Begley
Depts. of Mechanical and Aerospace Engineering
and
Materials Science and Engineering
University of Virginia

Selective adsorption of molecules on the surfaces of compliant structures often leads to coupling between chemical and mechanical behaviors; this coupling widely utilized in nature (e.g. lipid bilayers comprising cell walls) and is being exploited to develop microfabricated biochemical sensors. A key challenge in understanding and capitalizing on this behavior is the ability to translate molecular characteristics into continuum frameworks useful in describing microscale structural behavior. This talk will describe a multi-scale modeling framework that translates molecular interactions into continuum properties: this allows quantitative connections between binding energy, adsorption density, persistence length, etc. and the mechanical properties of adsorbed layers, such as surface stress and effective moduli . A key advantage of the framework is that it effectively decouples molecular inputs from structural deformation, such that one can easily analyze the effects of adsorption on any arbitrary geometry, such as cantilevers, clamped beams, membranes, etc. The utility of the framework will be demonstrated in terms of the adsorption density and surface stresses generated by DNA, using gold-thiol adsorption experiments and experimentally-calibrated pair potentials developed for nematically-ordered semi-flexible molecules. These examples will be used to discuss the potential of microfabricated structures to: (a) extract molecular information (such as parameters utilized in intermolecular pair potentials) from microscale observations of deformation, and (b) develop on-chip sensing to replace fluorescent spectroscopy in micro-Total Analysis Systems. The talk will conclude with a brief discussion of our on-going development of highly flexible elastomer sensors, which utilize gold nanoparticles and nano-porous gold layers to facilitate surface functionalization without comprising mechanical sensitivity.

"Scaling Properties of Fracture Surfaces"
by
Elisabeth Bouchaud
Fracture Group, Div. of Physics & Chemistry of Surfaces and Interfaces
CEA-Saclay, France

The quantitative study of fracture surfaces has revealed interesting scale invariance properties. Two and sometimes even three self-affine regimes characterized by universal exponents and material-dependent length scales can be observed on these surfaces.

At large length scales, i.e. at scales much larger than the material Process Zone (PZ) size where damage such as plasticity, cavity formation, micro-cracking, etc… occur, the material can be considered as linear elastic. We show that a stochastic model derived from Linear Elastic Fracture Mechanics can actually predict what is observed. In this model, the microstructure of the material is described as an array of randomly distributed obstacles likely to induce local shear.

At smaller length scales, where the fracture surfaces of most materials are analyzed, a different regime arises, characterized by two different roughness exponents, one along the direction of crack propagation, and one in the direction perpendicular to it. Although no model is able to predict these observations, we argue that in this regime, any kind of damage (plasticity, cavity or micro-cracks nucleation and growth, according to the material) has the neat effect to screen out long range elastic interactions.

A third regime may arise at even smaller scales of observation in the case of metallic alloys or metallic glasses. In this regime, at the scale of a single, or a few damage cavities, any notion of a crack front has been lost. These isotropic fracture surfaces are shown to be the result of the coalescence of fractal damage cavities

"Virtual Fracture Testing of Composites: From Materials to Components"
by
Javier LLorca
Departamento de Ciencia de Materiales, Universidad Politécnica de Madrid &
Instituto Madrileño de Estudios Avanzados en Materiales (IMDEA-Materiales

The burden of testing to prove the safety of structures upon whose integrity human lives depend is immense: a typical large airframe, for example, currently requires ≈ 104 tests of material specimens, along with tests of components and structures up to entire tails, wing boxes, and fuselages, to achieve safety certification. This cost has to date been unavoidable: while computational stress analyses provide good predictions in the elastic regime, they have not achieved predictive accuracy in the presence of damage and fracture. This limitation is starting to be overcome by new modeling strategies, advances in simulation tools, and the increased power of digital computers, which are making possible “virtual” tests in which the mechanical behavior of a structure up to ultimate failure is computed through simulations of the physical processes involved at the atomic, microscopic and structural scales.

Virtual testing is rapidly emerging as a key technology in the area of structural composites, which will help to reduce dramatically design time, facilitate optimization and cut down the cost of certification. A successful multiscale approach to implement this strategy for structural composites is presented in this talk. The methodology encompasses three different analysis levels. In the first one, accurate predictions for the onset and propagation of damage at the lamina level are obtained through the numerical simulation of a representative volume element of the composite microstructure, which takes into account the fibers, matrix and interfaces in the lamina. The actual fracture mechanisms experimentally observed in the matrix, fibers and interfaces are included in the simulations through the appropriate constitutive equations. The second level simulates the deformation of laminates in which intralaminar failure as well as decohesion between plies are explicitly considered. Laminate performance is addressed through numerical simulations in which the mechanical behavior of material points in each ply is controlled by a continuum damage model. Damage accumulation is taken into account by reducing progressively the elastic constants at each point according to a set of internal damage variables which depend on the various failure mechanisms. Interlaminar failure is introduced by means of a cohesive crack model, which provides very good results to predict the progression of damage by interface decohesion. Finally, the laminate properties obtained in these simulations are used to determine the mechanical behavior until fracture of structural elements in the third level.

Various examples of application of this methodology at the three levels are presented. They include the determination of the failure locus a fiber-reinforced composite lamina subjected to transverse compression and longitudinal shear, the simulation of the low speed impact by a rigid body on C/epoxy laminates, and the analysis of a bird impact on the leading edge of the horizontal tail plane of the Airbus A350. Finally, the potential and the limitations of this multiscale strategy are discussed.