Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session B24: Focus Session: Friction, Fracture, and Deformation I |
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Sponsoring Units: GSNP DMP Chair: Martin Muser, Western Ontario Room: LACC 411 |
Monday, March 21, 2005 11:15AM - 11:27AM |
B24.00001: Simulation studies of ductile yield at finite temperature Robin Selinger, Matthew Davidson, Julie Kaufman At finite temperature, a ductile crack under even a small applied load can emit dislocations via thermal activation, but the relevant activation energy and its dependence on the stress state of the crack tip are not well understood. We investigate this process via a simulation study of crack-tip plasticity using an idealized model in two dimensions based on the XY rotor model. A subcritical shear load and prescribed temperature are applied to a sample containing a single crack which emits screw dislocations; these glide across the sample and annihilate on the opposite free surface. The activation energy is determined from an Arrhenius plot of the resulting strain rate vs temperature. We find that the activation energy is highly sensitive to the stress state of the crack tip and doubles when the stress intensity factor is reduced by 20{\%}. We also find super-Arrhenius behavior at high temperature and identify its onset with activation of a secondary dislocation source away from the crack tip. These results, if replicated in three dimensional simulations, provide useful insight for construction of mesoscale models of ductile yield at finite temperature. [Preview Abstract] |
Monday, March 21, 2005 11:27AM - 11:39AM |
B24.00002: Analytical Study of Crack-Tip Plasticity Sergio Picozzi, Robin Selinger The mechanical response of a crack in a ductile solid depends on both the temperature and the stress state of the crack tip. At finite temperature, a crack under a subcritical applied load may emit dislocations via thermal activation, a process associated with creep. However it is not clear how to estimate the relevant activation energy or its dependence on the stress state of the crack tip. To address these issues, we consider a classical problem in elastostatics: the interaction of a screw dislocation with a finite crack under a shear load in an isotropic elastic solid. In the simplest geometry, this boundary value problem can be framed in two dimensions. Using conformal mapping techniques, we analytically solve for the displacement field, which in turn yields the relevant component of the stress field and, finally, the energy of the dislocation as a function of distance from the crack tip. This technique provides a means to explore the nature of the energy barrier and its variation with applied load in the continuum limit. We compare these findings with results from computer simulation studies of thermally activated ductile yield at finite temperature. [Preview Abstract] |
Monday, March 21, 2005 11:39AM - 11:51AM |
B24.00003: Hierarchical Modeling of Failure Mechanisms and Grain-Boundary Effects in Nanocrystalline Aggregates Omid Rezvanian, Waeil Ashmawi, Toshirhio Kameda, Mohammed Zikry, Donald Brenner, A. Rajendran New hierarchical computational methodologies have been developed to predict dominant material behavior and mechanisms at scales ranging from the nano to the macro. Physically based scaling relations have been developed to characterize mechanisms and grain-boundary effects in nanocrystalline materials. These scaling relations have been used to link molecular dynamic and microstructural finite-element techniques to delineate the interrelated effects of grain boundary orientation and structure, grain- boundary sliding and friction, and dislocation transmission, absorption, and blockage through GBs, such that dominant failure mechanisms can be accurately identified and predicted from initiation to unstable growth. . [Preview Abstract] |
Monday, March 21, 2005 11:51AM - 12:03PM |
B24.00004: MD simulation of Deformation and Fracture in Nanocrystalline Ag and Nano-composite AgNi Yue Qi, Yang-Tse Cheng The deformation and fracture mechanisms of columnar nano-crystalline Ag and nano-composite AgNi have been studied using molecular dynamics. In addition to dislocation-mediated plasticity at an early stage of deformation, we found grain-rotation induced grain growth and crack formation at larger deformation. The crack nucleation at the grain boundaries and the linkage of such cracks will finally lead to the fracture of the material. However, the ductility of the nanocrystals is largely controlled by the competition between grain growth and crack nucleation. As a result, lower temperature, larger grain size and introduction of a second phase tend to accelerate crack formation and reduce the fracture strain, such decrease the ductility of the nanocrystals. [Preview Abstract] |
Monday, March 21, 2005 12:03PM - 12:15PM |
B24.00005: Study of dislacation Ni-Cu interface interaction with Peierls-Nabarro model Wei Xiao, Nick Kioussis, Gang Lu, Nasr Ghoniem Metallic multi-layered structures have received increasingly interest in the past few years because of their unusual and interesting mechanical properties and high strength-to-weight ratio. The mechanical properties of an interface are determined, in large part, by the nature of the chemical bonding at the interface, which may be significantly different from that within either of the materials meeting at the interface. The resistance of interfaces to dislocation transmission is a fundamental quantity that often serves to control strength in plastically deforming multiphase materials. We have generalized an multi-scale approach based on semi-discrete variational generalized Peierls-Nabarro (SVGPN) for the pure Cu, Ni, and the (001) Ni-Cu interface have calculated from ab initio calculations. Various dislocation properties, such as the core width, energy, the Peierls stress, the dissociation behavior are investigated and compared to those of pure Cu and Ni hosts. Supported by US Air Force grant 0205GDD417. [Preview Abstract] |
Monday, March 21, 2005 12:15PM - 12:27PM |
B24.00006: Numerical study of shock fronts in tin and silicon J. Matthew D. Lane, Michael P. Marder Shock waves in solid materials exhibit wave front structure which depends on the time-dependent deformation response and failure properties of the material. I will discuss our study of the structure and transitions within the shock fronts found in single crystal tin and silicon. Our unique computation technique concentrates on calculating properties of the front, by employing a moving window. This improved method, based on theoretically derived criteria, allows for long-time steady state simulations of shock fronts using the Modified Embedded Atom Method (MEAM) potentials. We used a non-equilibrium molecular dynamics simulation code developed at the University of Texas at Austin for fracture simulation. Our method has general applicability to studies of shock fronts. Application to studies of elastic-plastic transitions will be discussed. [Preview Abstract] |
Monday, March 21, 2005 12:27PM - 1:03PM |
B24.00007: Multiscale Modeling of Fracture Invited Speaker: Fracture in non-brittle crystalline materials involves material separation at the atomistic scale and dislocation plasticity, with its associated dissipation, occurring over much larger scales. Small-scale plasticity effects associated with dislocation interactions and ordering preclude the application of standard continuum plasticity models. Thus, multiscale models are required to handle the atomistic and mesoscale aspects of the deformation and fracture. Here, we first present the Coupled Atomistic Discrete Dislocation (CADD) model, which is a seamless method for addressing fracture while including the multiscale phenomena, and its extension to finite temperature dynamic modeling. The method is applied to investigate finite-temperature rate-dependent crack growth and dislocation emission in Al, Ni, and Fe. We then present a purely discrete dislocation model, wherein the atomistic region is replaced by a cohesive surface representation, to predict bimaterial interface fracture nucleation and propagation in a variety of geometries. [Preview Abstract] |
Monday, March 21, 2005 1:03PM - 1:15PM |
B24.00008: Fracture Strength of Multi-wall Carbon Nanotubes Weiqiang Ding Arc-grown MWCNTs were studied with tensile loading experiments inside an SEM with a home-built nanomanipulator. A newly developed and rapid electron beam induced deposition method was used to make the clamps. HR-SEM images were acquired at each loading step, and two independent methods of analysis of each image were used to obtain the corresponding tensile load. The MWCNT diameters were measured by TEM after the tests. The stress \textit{vs.} strain, Young's modulus, and tensile strength of the MWCNTs were obtained through data analysis. The measurements strongly suggest the presence of defects in the tested nanotubes. Assuming defects like clusters of adjacent vacancies (e.g., atomistic blunt cracks) the experimental evidence is rationalized by applying Quantized Fracture Mechanics. \textit{We gratefully acknowledge the grant support from the NSF ({\#}020079; {\#}030450); ONR {\#}N000140210870 (support for W. Ding), and NASA BIMat URETI {\#} NCC-1-02037(support for X. Chen). } [Preview Abstract] |
Monday, March 21, 2005 1:15PM - 1:27PM |
B24.00009: In-situ testing of templated carbon nanotubes using an integrated MEMS testing stage Shaoning Lu, Dmitriy Dikin, Jaehyun Chung, Rodney S. Ruoff We present in-situ mechanics studies of nanostructures inside an SEM conducted using a MEMS testing system; it has a stage based on a deep reactive ion-etched high aspect ratio MEMS device with integrated actuator and force sensing structures. The device provides nanoscale displacement and force resolution for tensile testing. Novel approaches have been developed for mounting and clamping nanostructures onto the stage. A new approach, with combined use of electric field and a special drying method, has been developed to mount templated carbon nanotubes (T-CNTs) present in a liquid dispersion onto the device without chemical or mechanical damage. This method also can control the location and number of deposited T-CNTs. Tensile testing has been done on the T-CNTs and the device provides sufficient force to break them. The deformation of the T-CNTs has also been imaged during loading. We appreciate the support from the Naval Research Laboratory (grant No. N00173-04-2-C003), the ONR (grant No. N000140210870) and the NSF (CMS-0304506). This work was performed in part at the Cornell Nano-Scale Science {\&} Technology Facility which is supported by the NSF under Grant ECS-9731293, its users, Cornell University and Industrial Affiliates. [Preview Abstract] |
Monday, March 21, 2005 1:27PM - 1:39PM |
B24.00010: Mechanics of Nanoscale Clamps W. Ding Proper sample clamping is crucial for nanostructure mechanics studies. We pioneered using electron beam induced deposition (EBID) for \textit{in situ} clamp fabrication inside an SEM, and here report the chemical composition, atomic structure, and stiffness and hardness of such clamps as measured by HRTEM, EELS, SIMS, Raman, and nanoindentation. A strong clamp means tensile loading to break without slippage or failure, and achieving, for mechanical resonance testing, the boundary conditions appropriate for simple beam theory. Here we report nanoscale pullout tests on 1-d nanostructures to evaluate EBID clamp strength. Several analytic models were used to analyze the pullout results. FIB sectioning was used to study the 3-d cross section of clamps previously used in tensile loading. FEA was used to study the stress distribution in the clamp. The clamp influence on resonance measurements of cantilevered nanostructures will also be presented. \textit{This work was funded by ONR {\#}~N000140210870. Also: E. Z. NSF {\#}~0200797; D. D. and X. C. NSF {\#}0304506 and by the NASA BIMat URETI {\#} NCC-1-02037. X. W. and X. Li: NSF EPS-0296165, the SC} \textit{Space Grant Consortium-NASA, and USC NanoCenter Seed Grant. } [Preview Abstract] |
Monday, March 21, 2005 1:39PM - 1:51PM |
B24.00011: Dynamics of Cutting Viscoelastic Materials Stephan Koehler, W. R. Matson Mechanical cutting of visco-elastic polymers is experimentally investigated using sharp knives. The knife is aligned orthogonally to the substrate's surface, and is plunged directly into the substrate. As the knife moves into the sample, the sample deforms viscoelastically and is cut (i.e. new surface is created). The rates of viscoelastic deformation \& cutting depend on the plunging rate, geometry of the knife and substrate, as well as the material properties of the substrate. A simple model based upon viscoelastic rheology that includes cutting \& surface friction is discussed. [Preview Abstract] |
Monday, March 21, 2005 1:51PM - 2:03PM |
B24.00012: Instabilities and the intersonic nature of fracture in thin latex sheets Paul Petersan, Robert Deegan, Michael Marder, Harry Swinney The instability of crack running in a popped balloon provides a novel test bed to study fundamental questions in fracture mechanics like crack path and velocity selection. Cracks in stretched latex sheets exhibit a transition from straight to oscillating paths as the amount of biaxiallity in the sheet is increased. It has also been recently observed that the cracks run at intersonic speeds, between the longitudinal and shear wave speeds, in the stretched material. These experiments studying the path and velocity of a fast running crack in biaxial stretched thin latex sheets will be described, as will recent and promising analytical and numerical investigations of fracture in highly extensible materials. [Preview Abstract] |
Monday, March 21, 2005 2:03PM - 2:15PM |
B24.00013: Breakdown of disordered media by surface loads Jakob Knudsen, Ali Massih An interface layer connecting two parts of a solid body is modeled by N parallel elastic springs connecting two rigid blocks. We load this system by a shear force acting on the top side. The springs are assumed to have equal stiffness but are ruptured randomly when the load reaches a critical value. For this system, we calculate the shear modulus as a function of the order parameter, describing the state of damage, and also the worn material size distribution. In particular, we evaluate the relation between the damage parameter and the applied force and explore the behavior in the vicinity of material breakdown. Using this simple model for material breakdown, we show that damage caused by applied shear forces is analogous to a first-order phase transition. The scaling behavior of the shear modulus with the order parameter is explored analytically and numerically close in the vicinity of the critical order parameter when the shear load is close but below the threshold force that causes material breakdown. Our model calculation represents a first approximation of a system subject to wear induced loads. [Preview Abstract] |
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B24.00014: Delayed fracture of porous media Daniel Bonn The fracture of porous media subjected to a constant load is studied. Contrary to homogeneous solids in which fracture usually happens instantaneously at a well-defined breaking strength, the fracture of a porous medium can occur with a delay, allowing to quantify the probability of material failure. Here we show that the fracture probability, a key property for risk analysis in civil engineering, is given by the probability of nucleating the first crack. The nucleation process can be understood quantitatively by calculating the activation energy for crack nucleation, taking into account the porosity of the medium. [Preview Abstract] |
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B24.00015: A new diffuse interface approach to brittle fracture Veronica I. Marconi, Eduardo Jagla We present a continuum model for the propagation of cracks and fractures in brittle materials where {\it the full set of variables are the components of the strain tensor $\varepsilon$}. The evolution equations are based on a free energy that reduces to that of linear elasticity for small $\varepsilon$, and accounts for cracks through energy saturation at large values of $\varepsilon$. We regularize the model including terms dependent on gradients of $\varepsilon$ in the free energy. No additional fields are introduced, and then the whole dynamics is perfectly defined. We show that the model is able to reproduce basic facts in fracture physics, like the Griffith's criterion. In addition, regularization makes the results insensitive to the numerical mesh used, something not at all trivial in crack modeling. This new technique presents a broad spectrum of possibilities. We will show a non trivial applications for quasistatic crack propagation in $2D$, the prediction of the growth and curving of cracks. The model could be used to study the dynamics of cracks, as sound emission, cracks instabilities and bifurcation, and it also could be extended to study problems in 3D. [Preview Abstract] |
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B24.00016: Mechanics of Crystalline Boron Nanowires Weiqiang Ding, Lorenzo Calabri, Xinqi Chen, Rodney S. Ruoff* 1-d nanostructures such as nanowires (NWs) have attracted attention in part due to their promise in sensing, materials reinforcement, and nanoelectronics. Crystalline boron NWs have been synthesized by the CVD method with preformed metal catalyst particles. They have $p$-type semiconductor behavior, and show rectification. We report two mechanical properties of these B NWs, which were studied with the \textit{mechanical resonance method} and \textit{tensile testing}. The mechanical resonances of cantilevered B NWs were excited and the resonance peak frequencies were used to obtain the \textit{Young's modulus} according to simple beam theory. A parallax method was used to obtain the correct 3-d representation of the NW based on two SEM images acquired at different tilt angles. The influences of curvature and nonideal boundary conditions on the NW resonance frequencies will be presented. Tensile loading measurements were performed to obtain the \textit{tensile strength} and \textit{Young's modulus}; the latter could be compared with that obtained from mechanical resonance. \textit{This work was funded by NSF EEC-0210120, and in part by ONR {\#}~N000140210870 (partial support, W. Ding) and by the NASA BIMat URETI {\#} NCC-1-02037 (support for X Chen).} [Preview Abstract] |
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