Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session R33: Focus Session: Friction, Fracture and Deformation IV |
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Sponsoring Units: DMP GSNP Chair: Stefan Koehler, Emory University Room: Baltimore Convention Center 336 |
Wednesday, March 15, 2006 2:30PM - 3:06PM |
R33.00001: Scanning Probe Applications to the Adhesive, Tribological and Rheological Properties of Materials Invited Speaker: Scanning probes are finding expanding application to the local analysis of a broad range of materials properties. I will discuss studies of adhesion, tribology and rheology applied to a range of materials using Interfacial Force Microscopy (IFM), a scanning force-probe technique distinguished by its use of a quantitative and mechanically stable force-feedback sensor. This unique sensor enables the force to be recorded as two interfaces approach, make contact, deform and separate making possible an accurate evaluation of the development of the adhesive bond and its failure. Thus, the nature of the adhesive bond, e.g., van der Waals, electrostatic, covalent, etc., can be established and the total adhesive energy measured quantitatively. Lateral forces can also be measured enabling direct observation of the interfacial friction force as a function of the normal force. These advantages will be illustrated with examples involving: (1) the interaction of a tip and substrate functionalized with self-assembled monolayer films having various combinations of chemically distinct end groups, as well as the interaction involving polymer surfaces, (2) the viscous properties of adventitious water adsorbed on various tip and substrate materials and (3) a quantitative, local rheological analysis of an extreme example of a viscoelastic material. These examples clearly demonstrate the intimate relationship between interfacial bond strength and the mechanical properties of the contact in determining overall adhesive strength. In addition, they dramatically demonstrate the weak correlation between the maximum adhesive force upon contact separation (the so called ``pull-off force'') and the quantitative work of adhesion. The assumptions required to make this correlation strong are seldom valid in ``real'' contact situations. Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U.S. Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000. [Preview Abstract] |
Wednesday, March 15, 2006 3:06PM - 3:18PM |
R33.00002: Sub-micron void structure during spallation fracture James Belak, John Kinney, Mukul Kumar, J. Ilavsky, Lyle Levine Samples of single and poly-crystal aluminum were shocked to incipient spallation fracture and recovered on the LLNL light gas gun. Previously, we analyzed the void structure in these samples using 3D x-ray tomography. Here, we extend this analysis to sub-micron length scales using ultra-small-angle scattering (USAXS and USANS). The USANS data overlaps in length-scale with the tomography data. The data displays novel power law scaling and a Guinier region suggesting a mean size for sub-micron voids of 60nm. These results will be compared to direct numerical simulation using molecular dynamics. [Preview Abstract] |
Wednesday, March 15, 2006 3:18PM - 3:30PM |
R33.00003: Dislocation Compression and Recovery Mechanisms in Shock-Recovered Al Single Crystals from White Beam Diffraction* R. Barabash, G. Ice, J. Belak, M. Kumar, J. Illlavsky A spatially resolved diffraction method with a sub-micron beam is applied to characterize both the dislocation structure and strain gradients in shock-recovered samples of Al (123) single crystal. Complementary OIM and SEM analysis were performed. The microbeam-Laue diffraction reveals several distinct zones located at different depths under the shock front. Pronounced streaking of Laue images are observed in the zones close to the front and back surface, consistent with a single slip mode. The portion of geometrically necessary dislocations reduces with depth while the portion of statistically stored dislocations increases. The Al sample was shocked to incipient spallation fracture and the Laue diffraction in the region of void formation shows a peculiarly complicated shape. To get a better understanding of the reasons for such a complex shape, 3D depth resolved measurements were performed. These measurements showed that in the central region alternating local lattice rotation takes place. This is due to the inhomogeneous plastic deformation surrounding each void. The density and organization of dislocations is presented as a function of depth under the shock front as well as comparison to SEM on the same samples. * This work was performed by DOE, at LLNL under Contract W-7405-Eng-48, at ORNL under the Contract DE-AC05-00OR22725 and at APS under Contract No. W-31-109-ENG-38. [Preview Abstract] |
Wednesday, March 15, 2006 3:30PM - 3:42PM |
R33.00004: Multiscale Simulations of High-Temperature Fracture in Silicon Noam Bernstein We simulate the effects of temperature on the dynamic fracture process in silicon. We use a dynamically coupled method that combines tight-binding at the crack tip with empirical potentials far from the tip in a molecular dynamics (MD) simulation.$^1$ This method has been shown to give brittle fracture at low temperatures, in agreement with experiment.$^1$. Here we present simulations of fracture at high temperatures, about 1000~K. These simulations are carried out in a strip geometry, and are relatively long and well equilibrated. There is no evidence that the velocity gap closes at these temperatures, at least not on the MD time scale. Fluctuations manifest themselves in the spontaneous nucleation of defects at the crack tip. We discuss the nature and evolution of the defects, and their possible relation to the sharp brittle-to-ductile transition observed in silicon. \\ $^1$ N. Bernstein and D. Hess, MRS Proc. vol. 653 (2001); -- Phys. Rev. Lett. vol. 91 (2003). [Preview Abstract] |
Wednesday, March 15, 2006 3:42PM - 3:54PM |
R33.00005: Fracture scaling in columnar cornstarch Lucas Goehring, Stephen Morris We have studied fracture spacing in desiccated cornstarch slurries, which exhibit columnar jointing. This fracture process creates long hexagonal pillars, and is famous for causing spectacular geologic features such as the Giant’s Causeway. The columnar pattern is formed as a planar network of cracks pass through a cooling or drying body. Even in simple 2D shrinkage fracture experiments it can be difficult to explain the spacing between cracks, however, in this case it is generally believed that the crack spacing depends on the average crack advance rate. Using computerized feedback, we controlled the desiccation rate of starch slurries. Continuous measurements of sample mass were converted into estimates of crack position and crack advance rate. After drying, direct measurements of crack spacing were made throughout the sample by cutting up the colonnade. With a constant crack advance rate, the jointing selects a particular scale after a transient coarsening. The selected scale does not uniquely depend on the final crack advance rate, but rather shows a type of memory inherited from its transient initiation. We present our investigations into this scaling, and how it depends on the fracture advance rate. [Preview Abstract] |
Wednesday, March 15, 2006 3:54PM - 4:06PM |
R33.00006: Reversible Rupture of a Two-Dimensional Alkane Crystal Shishir Prasad, Ali Dhinojwala We present the first study of the rupture of a surface frozen monolayer of alkane (nonadecane) by oscillating an air bubble in its disordered melt. The two-dimensional (2D) crystal breaks abruptly at the start of every expansion and contraction cycle and recovers rapidly back to its original structure in a fraction of a second. This is unlike our experience of watching glass sheets or solids break due to the fast recovery times after the rupture of 2D crystals. The strength of this 2D crystal is determined by the presence of defects or grain boundaries and it is a strong function of temperature and rate of change in (surface) area. These results have important consequences in understanding the role of defect-mediated mechanical properties in crystals, colloids, glasses, granular materials, and fluids in confined geometries. [Preview Abstract] |
Wednesday, March 15, 2006 4:06PM - 4:18PM |
R33.00007: Spontaneous curvature cancellation in forced thin sheets Tao Liang, Thomas Witten We report numerically observed spontaneous vanishing of mean curvature on a developable cone made by pushing a thin elastic sheet into a circular rim [1]. The mean curvature is seen to drop by nearly two orders of magnitude in a narrow zone near this rim, independent of thickness of the sheet, the supporting radius and the amount of deflection. Several variants of developable cone are studied to examine the necessary conditions that lead to the vanishing of mean curvature. It is found that the presence of appropriate amount of radial stress is necessary. The d-cone geometry somehow produces the right amount of radial stress to induce just enough radial curvature to cancel the conical azimuthal curvature. In addition, the circular symmetry of supporting rim plays an important role. When the supporting ring is elliptical, the radial curvature overcompensates the azimuthal curvature near the minor axis and undercompensates near the major axis. Our numerical finding is verified by a crude experiment using reflective plastic sheets. We expect this finding to have broad importance in describing the general geometrical properties of forced crumpling of thin sheets. [1] Cerda et al, Nature 401, 46 (1999) [Preview Abstract] |
Wednesday, March 15, 2006 4:18PM - 4:30PM |
R33.00008: Elasticity and gravity competing in a 2D system P. Mellado, A. Concha We propose a minimal model for studying the consequences of the competition between elasticity and gravity on a surface. The model describes a cylindrically symmetric membrane composed of an elastic substrate and a stiff crust that experience a gravitational field directed radially inward. Under certain conditions an instability that breaks the initial symmetry can appear. When a stretching energy for the crust is introduced and an appropriate geometrical constraint on the system is imposed, an exotic periodic pattern forms. We provide an analytical solution to the model and show that while the stretching energy is responsible for the periodic pattern, faults in the stiff crust emerge generically when elasticity and gravity are competing. Possible implications in geology are discussed. [Preview Abstract] |
Wednesday, March 15, 2006 4:30PM - 4:42PM |
R33.00009: Development of First and Second Generation Fractures Martin Ferer, Duane Smith In fractured reservoirs, one often finds a first generation of nearly parallel fractures and a second generation of fractures approximately perpendicular to the first-generation fractures. We have developed a simple blocks and springs model to study how the first generation fractures affect the development of the second generation. In the model, a layer of squares is connected to a substrate by spring-like shear forces, and each square is connected to its neighbors by intra-layer spring-like shear and tensile forces, of randomly chosen strengths. First the substrate gradually expands in the x direction generating stresses on the layer, which cause the failure of some of the intra-layer springs. Stopping this expansion at a maximum value X, we have the first generation of fractures. Then the substrate gradually expands in the y direction to a maximum value, Y, producing the second generation of fractures. During these expansions, we determine the length distribution of the first and the second generation fractures. Comparing the two distributions shows how the first-generation fractures affect the development of the second-generation fractures. After expansion has stopped, we determine the size (number of broken bonds) of each fracture, the maximum linear extent of each fracture, and the distributions of each. [Preview Abstract] |
Wednesday, March 15, 2006 4:42PM - 4:54PM |
R33.00010: Growing Fractures Using Iterated Conformal Maps. H.G.E. Hentschel We will describe how iterated conformal mapping techniques can be used to grow fractures computationally and investigate their geometric characteristics as well as stress distributions. We will describe the similarities and differences between Mode I, II, and III fracturing patterns. Iterated conformal mapping techniques allows an efficient and accurate solution of the Lame equations without resorting to lattice models and for calculating the fracture roughness exponent. [Preview Abstract] |
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