CMSSM: Seminars

October 27, 2009 (Tuesday)
"Size-dependent Mechanics"
Pradeep Sharma, University of Houston, (Dr. Stelios Kyriakides)

December 2, 2009 (Thursday)
"TITLE - TBD "
Rob Phillips, Cal-Tech, (Dr. Ravi-Chandar)

March 31, 2009 (Tuesday)
"Kink-Folds by Optimization "
Yves M. Leroy and G. Kampfer, Laboratorie de Geologie, CNRS, Ecole Normale Superieure, Paris, (Dr. Stelios Kyriakides)

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)

"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.

"Compressive Failure Mechanisms in Layered Materials"
by
Dr. Kim D. Sorensen
Aalborg University, Denmark

Two important failure modes in layered materials occur under loading conditions dominated by compression in the layer direction. These two distinctly different failure modes are buckling driven delamination and failure by formation of kink bands.

In the first part of this presentation the effects of system geometry on buckling driven delamination is investigated. Attention is directed towards double curved substrates, which introduces a new non-dimensional combination of geometrical parameters. It is shown for a wide range of parameters that by choosing the two non-dimensional parameters suitably, one of them plays a very insignificant role. In some cases, the local curvatures of the system is so high compared to the extent of the delamination that it may be better modelled as a sharp corner. The effects of sharp corners on buckling driven delamination have been studied and are shown to have a significant effect on the fracture mechanical parameters.

"Nanoscale Growth Twins in Sputtered Metallic Thin Films"
by
Dr. Xinghang Zhang
Department of Mechanical Engineering
Texas A&M University

Nanoscale growth twins enable unique properties in metallic materials. We have recently studied nanoscale growth twins in sputtered austenitic stainless steels and Cu films. These growth twins have an average twin spacing of a few to tens of nanometers, with {111} twin interface oriented preferentially perpendicular to growth direction. The mechanisms of formation of growth twins during sputtering are discussed. The average twin spacing can be varied by tailoring deposition parameters. The mechanical properties of nanotwinned metals have clear size dependence on the average twin spacing. Molecular dynamics simulations indicate that twin interfaces can significantly block the transmission of single dislocations. Furthermore nanoscale growth twins seem to possess much better thermal stability than high angle grain boundaries in nanocrystalline metals. Our latest studies show that high strength and high conductivity can be achieved simultaneously in epitaxial nanotwinned Cu films. These films may have wide applications in electronic devices.

"Modeling of Nonlinear Magnetomechanical Coupling in Magnetic Shape Memory Alloys"
by
Dimitris C. Lagoudas
Department of Aerospace Engineering
Texas A&M University

Multifunctional or active materials have been successfully used as embedded sensors and actuators in engineering structures with the purpose of integrating the thermomechanical with electromagnetic or additional functionalities. Shape Memory Alloys (SMAs) and Magnetic shape memory alloys (MSMAs) have been the subject of much research in recent years as potential high actuation energy multifunctional materials. Some of the recent advances in the modeling and characterization of the magnetomechanical response of MSMAs will be discussed in this presentation. The modeling approach will address key thermodynamical considerations with internal state variables, motivated by microstructural observations of martensitic variant formation, reorientation and magnetic domain evolution. Maxwell’s equations of the magnetostatic problem will be studied in their direct coupling with the conservation of linear and angular momentum, through body forces, body couples and the nonlinear constitutive behavior. Observed localization zones of the field variables will be analyzed by considering the character of the field equations and the conditions under which loss of ellipticity occurs.

"The Mechanics of the Cytoskeleton and Cell Adhesions "
by
Robert M. McMeeking
University of California at Santa Barbara

The mechanical characteristics of eukaryotic cells arise largely due to the cytoskeleton, which assembles and dissociates in response to biochemical signals. Actin protein chains form the most significant structural element in the cytoskeleton, with myosin motor proteins endowing such stress-fibers with contractility. It is notable that tensile stress seems to stabilize stress fibers against depolymerization, so that there is an intimate coupling between the mechanics of the cytoskeleton and its biochemistry. In addition, stress-fibers interact with integrins that are the principal proteins in the adhesions transducting contractile force to the cell’s substrate or extracellular matrix. A model is presented for the processes of cytoskeleton stress-fiber formation, dissociation, contractility and their interaction with focal adhesions and mobile integrins. In the model, the polymerization of the stress fiber network is driven by a signal that rises quickly and then decays exponentially, causing the stress-fiber formation process to be self-limited. Subsequent depolymerization of the stress-fibers occurs spontaneously, unless it is inhibited by tension, which is generated by the constrained contractility of the actomyosin fibers. Consequently, the stress-fiber component of the cytoskeleton is most robust when it is able to generate tensile stress by contracting against an external constraint. Examples characterizing the model are given, illustrating the mechanical features and behavior of eukaryotic cells. The model is able to simulate the cell’s differing behavior when attached to a stiff substrate compared to its activity on a compliant substrate. In addition, a cell’s response to a measurement protocol can be predicted with the model, such as when a bead is attached to the cell and displaced outwards by an external force, causing a reconfiguration of the cytoskeleton. Additional phenomena that are successfully simulated with use of the model include the orientation of the stress-fibers in cyclically strained cells, and their orientations when a cell is adhered to a shaped pattern of fibronectin. The model has also been extended to simulate tissue constructs grown as sheets of cells suspended between posts.

"Bioengineering and Stem Cells in Articular Cartilage Regeneration"
by
Professor Kyriacos Athanasiou
Rice University

Articular cartilage is arguably the tissue most pivotal for motion and overall function. This soft, white tissue that covers the ends of our long bones cannot heal by itself. Indeed, articular cartilage is notorious for its degenerative progression to osteoarthritis following an injury. The demanding biomechanical milieu of a joint, plus cartilage’s relative lack of cells and blood supply, renders this tissue almost unique in its inability to repair adequately. This presentation will describe our group's efforts toward helping joint cartilages, such as hyaline tissue, knee meniscus, and the TMJ disc, repair themselves via tissue engineering approaches. Central to our efforts is understanding the biomechanical relationships at multiple dimensional levels. Also shown will be some of our latest results using various stem cell sources that indicate that cartilage regeneration is inexorably becoming a tractable problem.

"Survivability Materials Research: Shortening Research to Production; FCS Challenge "
by
Kyu Cho, Chian Yen, and Ernest S.C. Chin
U.S. Army Research Laboratory
Aberdeen Proving Ground, MD

The Future Combat Systems (FCS, Brigade Combat Team) is the cornerstone of Army modernization. FCS provides soldiers and leaders with leading-edge technologies and capabilities allowing them to dominate in asymmetric ground warfare while allowing the Army to build a force that can sustain itself in remote areas. The FCS program consists of eight new Manned Ground Vehicles (MGVs), a family of unmanned air and ground vehicles, the Non Line of Sight-Launch System, and advanced tactical and urban sensors that are all connected by a state-of-the-art network. Working together, these systems will help Soldiers share real-time information across the battlefield. Overall, FCS will provide Soldiers vastly increased situational awareness, survivability, and lethality, thus ensuring they can take the fight to the enemy before the enemy has time to react.

The main challenges to the FCS-MGVs systems, survivability related in particular, are meeting performance criteria of the survivability systems with weight per space requirements, effectively and seamlessly integrating multiple novel technologies, and meeting the low rate initial production (LRIP) schedule. The FCS-MGVs survivability Science and Technology (S&T) and Manufacturing Technology (ManTech) have adapted a three-tier technical strategy from which survivability subsystems execution strategies are mapped to the sets of combined technical readiness level (TRL) and manufacturing readiness level (MRL). A three tier strategy consists of tier one system assembly and integration, tier two survivability materials integration, and tier three survivability materials research and production.

The current presentation summarizes challenges to the FCS-MGVs survivability materials research and addresses strategies in shortening execution time to meet the LRIP. The presentation also presents examples of survivability materials integration and the survivability materials research and production programs. Detailed summaries of tri-modal aluminum composites and material modeling to expedite design and development of survivability applications are elaborated as well.

"Elastodynamics of Cracks with Interacting Faces "
by
Professor Igor A. Guz
University of Aberdeen, Scotland

The presence of cracks and delaminations considerably decreases the strength and the lifetime of engineering structures as well as significantly increases the cost of exploitation. In particular, localized damage in form of micro-cracks or delaminations often occurs at the bonding surfaces between two materials of the same or different mechanical properties. The elastodynamic response of intra- and inter-component cracks to an elastic wave is a topic of long standing interest. The overwhelming majority of studies focus on the wave diffraction problems for a crack in homogeneous materials. They also neglect the contact interaction of crack faces in spite of the evident significance of this factor. Only the recent works consider the non-linear effects due to the crack closure under dynamic loading. However, elastodynamic investigation of inter-component cracks received considerably less attention than the case of cracks in homogeneous materials.

The talk gives several examples of modelling elastodynamic response of 3-D and plane cracks in homogeneous and composite materials with the focus on the effect of crack closure leading to contact interaction of the opposite faces. In particular, application of boundary integral equations to the problem of interface microcracks between two dissimilar elastic half-spaces is studied in detail. The distributions of the displacements and tractions at the interface and the crack surface are obtained and analysed. The stress intensity factors (opening and shear modes) are computed for different values of the wave frequency and different properties of the half-spaces, and the effect of the distance between cracks, the friction between the crack faces and the direction of the applied harmonic loading is investigated.

"Fink-Folds by Optimization "
by
Yves M. Leroy and G. Kampfer
Laboratorie de Geologie, CNRS, Ecole Normale Superieure, Paris, France

The objective of this seminar is to present a simple, analytical construction of the onset and the development of kink-folds in laminated rocks, by application and extension of the kinematics approach of limit analysis. The structure is a composite with competent beds separated by weak interfaces which maximum strength is described by the Coulomb criterion. The competent beds are also composed of a cohesive and frictional material but with an additional potential to fail in the compressive stress domain to depict the action of compacting deformation mechanisms. The kink is defined by two parallel hinges bounding the kink band in which slip along the weak interfaces takes place. No appeal is made to the elastic properties nor to the possible buckling of the structure. Concerning the onset of failure, a comparison is made between three modes, compactions band, shear band (reverse fault) and kink-fold, in terms of the confining pressure (burial depth) and the mis-orientation (dip angle) of the competent layers with respect to the direction of compression, seen as an imperfection. The respective domain of dominance of each mode is presented in failure-mechanism maps, in the space spanned by the dip angle and the burial depth. The domain of the compaction band is at greater depths than the reverse faults with a boundary varying almost linearly with the imperfection angle. The kink-fold domain is for the larger imperfection angles regardless of the depth. The boundary with the two other domains of dominance is rather sensitive to the number of beds as well as to the friction of these weak interfaces. During the early development of the kink, with compaction band dominant conditions, it resembles to a slip-enhanced compaction band due to the weak interfaces activation and the compaction along the parallel hinges. This hybrid mode migrates through the laminated structure from the deepest towards the shallowest region and develops as a kink fold, after a negligible amount of shortening. The development occurs then in two phases: rapid rotation of the hinges followed by a widening of the kink band. It is found that the force for this development is decreasing first, sustains a minimum and then increases again. The orientation of the weak interfaces at this minimum force is a function of the burial depth and of the increase in potential energy of the competent layer.

"Negative Mass and Dynamic Cracks in Inhomogeneous Lattices"
by
Alexander Movchan
University of Liverpool

This lecture is based on the results of the joint work with L. Slepyan and G. Mishuris. The talk addresses modeling of defects in inhomogeneous dynamic lattice structures. The emphasis is on localization, within certain frequency range, around impurities and crack-like defects. Sometimes, this is interpreted as a "negative mass" in the corresponding homogenization approximations, as described in a recent paper by G.W. Milton and J.R. Willis. Our model involves a band gap dynamic lattice Green’s function, which will be discussed for different lattice structures in the lecture. Applications involve modeling of dynamic cracks in lattices, where Green’s function is incorporated via the kernel of the corresponding functional equation. A crack is understood as a partial fracture of a lattice by a propagating fault, which breaks the elastic bonds. For a semi-infinite fault, the mathematical problem is reduced to the functional equation of the Wiener-Hopf type, which is solved analytically. This is followed by the evaluation of the energy release for different values of the crack speed, which also has implications on the stability analysis for the crack propagating within the lattice. We also evaluate the dissipation rate, which is found to be strongly dependent on the crack speed.

"Cohesive-Zone Models for Fracture of Interfaces and Adhesive Joints "
by
Michael Thouless
University of Michigan

Historically, the strengths of interfaces and adhesive joints have been modeled by two different approaches - strength-based criteria and energy-based criteria. Cohesive-zone models form a natural and self-consistent approach to bridge these two approaches within a single framework. In laminated systems with elastic adherends, these models allow many of the well-known concepts of mixed-mode linear-elastic interfacial fracture mechanics to evolve under appropriate conditions, even in the absence of singular crack-tip stress fields. In laminated systems with plastically-deforming adherends, cohesive-zone models provide a quantitative approach for failure analyses in regimes that cannot be addressed by traditional fracture mechanics. Strategies associated with determining the appropriate fracture parameters will be discussed, with a particular interest in automotive applications where methodologies for designing adhesive joints for energy-management during crashes need to be developed.

DISSERTATION DEFENSE

"Wrinkling of Elastic Thin Films on Compliant Substrates"
by
Se Hyuk Im
The University of Texas At Austin

Complex wrinkle patterns have been observed in various thin film systems, typically with integrated hard and soft materials for various technological applications as well as in nature. The underlying mechanism of wrinkling has been generally understood as a stress-driven instability, similar to buckling of an elastic column under compression. On an elastic substrate, equilibrium and energetics set the critical condition and select the wrinkle wavelength and amplitude. On a viscous substrate, wrinkle grows over time and the kinetics selects the fastest growing wavelength. More generally, on a viscoelastic substrate, both energetics and kinetics play important roles in determining the critical condition, the growth rate, and wrinkle patterns. The dynamics of wrinkling, while analogous to other phase ordering phenomena, is rich and distinct under the effects of a variety of stress conditions and nonlinear film-substrate interactions.
In this study, mathematical models are developed for wrinkling of isotropic and anisotropic elastic films on viscoelastic substrates. Analytic solutions are obtained by linear perturbation analysis and a nonlinear energy minimization method, which predict the kinetics of wrinkle growth at the initial stage and the equilibrium states at the long-time limit, respectively. In between, a power-law coarsening of the wrinkle wavelength is predicted by a scaling analysis. Numerical simulations confirm the analytical predictions and show diverse wrinkle patterns under various stress conditions. For isotropic elastic films, a transition from parallel wrinkles to zigzag patterns is predicted under biaxial stresses. For cubic crystal films, the anisotropic elastic property leads to formation of orthogonal wrinkle patterns under equi-biaxial stresses. In general, the competition between the stress anisotropy and the material anisotropy controls the evolution of wrinkle patterns. Based on the mathematical model, two potential applications of the wrinkling phenomenon are explored, one for surface patterning and the other for measuring viscoelastic properties of thin polymer films. The theoretical and numerical results from this study are compared with experimental observations that are available in literature and through collaborations with experimental groups.

"Interfaces: In Fluid Mechanics and Across Disciplines"
by
Howard Stone
Harvard University

Interfaces play an important role in many fluid mechanics problems. Here we recognize (i) coating flows where interfacial tension and viscous effects regulate film thickness, (ii) interfacial rheology, which links various problems from complex fluids to transport processes in biological membranes, and (iii) interfaces between traditional disciplines that so often are the source of new questions in mechanics.

This lecture is part of the Southwest Mechanics Lecture Series
For Further Information Please Contact
K. Ravi-Chandar (471-4213)

"Tensile Strength and Fracture Toughness of Brittle Materials Considering and Connecting Microstructure and Atomicity "
by
Francisco G. Emmerich
Federal University of Espirito Santo, Brazil

We address the fracture properties of brittle materials under tension by using a force-atomistic approach: we analyze the forces that act in the solid down to the smallest dimensions between the atoms, unit cells, or grains, observing the minimum characteristic length scale around the point where the fracture begins, and discussing from fundamental principles the criterion for brittle fracture initiation. We take into account the forces due to the applied stress and the material cohesion forces, particularly at the crack tip, where the local hyperelasticity of the material plays a governing role. We connect microstructure and atomicity by using the concept of a total stress concentration factor, equivalent to a local resultant force, which can be obtained through the interaction of two multiplicative terms. By using an experimentally proved maximum tensile-stress criterion of bond rupture, based on the satisfaction of the static equilibrium condition given by Newton’s second law up to the beginning of the rupture, we obtain a general expression for the tensile strength, which can be simplified through an effective local cohesive stress. By using the approximation of the stress concentration factor obtained from the concept of equivalent ellipse, we obtain expressions for the tensile strength and fracture toughness. Thus, we explain in a unified framework from fundamental principles a set of established experimental results of brittle fracture of materials under tension, including the dependence of the tensile strength on the crack tip radius of curvature, and some scatter in reported values of fracture toughness and cleavage surface energy. This work can be useful to make more realistic predictions of fracture properties of brittle materials taking into account microstructure and atomicity.

"Microstructural Aspects in the Fracture of Human Teeth from Occlusal Loading"
by
Herzl Chai
Tel AvivUniversity

Mammalian teeth are brittle, yet capable of sustaining considerable deformation and damage from biting forces before ultimately failing. Of particular interest in this study is the fracture behavior of the enamel coat, the object which comes in direct contact with external loading. Enamel is a highly smart composite, integrating a woven-like hierarchical rod structure and a cathedral-like geometry to impart unique damage tolerance. This study examines the fundamental nature of enamel resilience via in-situ fracture experiments coupled with post-mortem morphological examination on extracted human molar teeth encased in epoxy base for support.
Several failure modes are encountered, including median cracks originating from the contact site and propagating downward and channel cracks propagating upward from the cervical base to the occlusal surface, both of which are confined to the enamel coat by the tougher dentin underneath. The morphological study reveals new information on the interaction between cracks and enamel’s microstructural entities. Key elements in the tooth’s resilience appear to be tufts – intrinsic crack-like defects emanating from the enamel-dentin junction, and Hunter-Schreger bands (HSB) – undulated enamel rods running in different directions within the enamel coat. We found stabilization in the evolution of these defects, by stress shielding from neighbors or by inhibition of ensuing crack extension from prism interweaving (decussation). Combined with a crack healing capability, these features work interactively to slow crack propagation in the enamel and enhance teeth survivability.

Transverse sections of a molar tooth after occlusal loading at 450N 4.4 mm below cusp

"Modeling Resonses of Composites with Field Couling and Time Effects"
by
Anastasia Muliana
Texas A&M University

Composite systems can be customized to meet the desired performance for many aerospace, automobile, and civil engineering applications that cannot be achieved by a single material. Recent efforts have been made to develop composites that are able to withstand extreme environments, sustain complex mechanical loading, and incorporate actuators, sensors and electronic devices. One of the most important issues concerning these composites is changes in the properties of constituents due to environmental conditions.

Our group is currently investigating the effects of coupled heat conduction, moisture diffusion, and time-dependent deformation on the responses of particle and fiber reinforced composites. For these purposes, we are developing a multi-scale framework, based on simplified microstructures of composites, for field coupling and time-dependent analyses of composites. The multi-scale framework is based on a synthesis of three major components: constitutive models with field coupling and time-dependent effects at the constituent level (fiber, particle, and matrix), hierarchical micromechanical models for several composite reinforcements, and layered structural elements that can incorporate through-thickness material variability. The framework is compatible with general finite element (FE) code. Some examples are: 1) analyzing heat conduction and deformation of particulate composites, functionally graded materials, and fiber reinforced laminated composites; 2) predicting responses of piezocomposites comprising of ferroelectric fibers and polymer matrix; 3) using the piezocomposites to control deformation in host structures during the transient moisture diffusion and heat conduction process.

"Engineering Challenges and Opportunities in the Army"
by
Jill Smith

Director, SES, Weapons and Materials Research Directorate
U.S. Army

The Army Research Laboratory (ARL) is the Army's corporate basic and applied research laboratory. Our mission is to provide innovative science, technology, and analysis to enable full-spectrum operations. ARL consists of the Army Research Office (ARO) and six Directorates—Weapons and Materials, Sensors and Electron Devices, Human Research and Engineering, Computational and Information Sciences, Vehicle Technology, and Survivability and Lethality Analysis. The Army relies on this ARL Team for scientific discoveries, technologic advances, and analyses to provide warfighters with capabilities to succeed on the battlefield.

Classical mechanics is intrinsically size-independent and as such does not distinguish between structures that are self-similarly scaled from miles to nanometers. In this presentation, we discuss the various physical causes that may render the elastic state size-dependent and elucidate the “length scales at which classical continuum elasticity breaks down for various materials”. In particular we discuss the role of surface energy, nonlocal interactions, quantum confinement and electrical polarization. We also briefly touch upon the oft-debated question on the (ir)-relevancy of nonlocal elasticity for nanotechnologies. Finally, focusing on electro-mechanical coupling related size-effects, we argue the tantalizing possibility of creating “apparently piezoelectric” nano-composites without piezoelectric constituents, emergence of “giant” piezoelectricity in nanostructures and resolution of the dead-layer bottleneck in nanocapacitors for energy storage.

"Quantitative In Situ Characterization of Metallic Nanowires"
by
Jun Lou
Department of Mechanical Engineering and Materials Science
Rice University

Metallic nanowires are of great technological importance due to their current and potential applications in miniaturized electronic, optical, thermal and electromechanical systems. In addition, one-dimensional metallic materials provide a unique opportunity to investigate fundamental mechanisms in materials science governing the origin and transitions of size dependent mechanical behavior for metals. This talk presents some of our recent efforts to study the size dependent mechanical behaviors of metallic nanowires. We have developed a simple micro-device that allows in situ quantitative mechanical characterization of metallic nanowires, in scanning electron microscope (SEM) or transmission electron microscope (TEM) chamber equipped with a quantitative nanoindenter. The unique design of this device makes it possible to convert compression from nanoindentation to uni-axial tension at the sample stages. Fabrication of the micro-device is successfully demonstrated using established micro-fabrication processes. Finite element analysis (FEA) is employed to model the device behavior under mechanical loading and compared with experiments. Finally, some in situ results on deformation and fracture behavior of Ni and Au nanowires will be discussed. Also in this work, we will demonstrate that at near room temperature, individual <111> single crystalline gold nanowires with 7-10 nanometers in diameter can be welded together within seconds by only making mechanical contact without any local heating process. Subsequent quantitative in situ tensile and electrical measurements confirmed that the strength of the as-welded nanowire was very close to that of the original nanowire, and the electrical properties of as-welded nanowires had little change for each successful welding. The implication of the “cold welding” for bottom-up assembly at the nanoscale will be discussed.