Nanoindentation and incipient plasticity

This paper presents a large-scale atomic resolution simulation of nanoindentation into a thin aluminum film using the recently introduced quasicontinuum method. The purpose of the simulation was to study the initial stages of plastic deformation under the action of an indenter. Two different crystallographic orientations of the film and two different indenter geometries (a rectangular prism and a cylinder) were studied. We obtained both macroscopic load versus indentation depth curves, as well as microscopic quantities, such as the Peierls stress and density of geometrically necessary dislocations beneath the indenter. In addition, we obtain detailed information regarding the atomistic mechanisms responsible for the macroscopic curves. A strong dependence on geometry and orientation is observed. Two different microscopic mechanisms are observed to accommodate the applied loading: (i) nucleation and subsequent propagation into the bulk of edge dislocation dipoles and (ii) deformation twinning

Quasicontinuum simulation of fracture at the atomic scale

We study the problem of atomic scale fracture using the recently developed quasicontinuum method in which there is a systematic thinning of the atomic-level degrees of freedom in regions where they are not needed. Fracture is considered in two distinct settings. First, a study is made of cracks in single crystals, and second, we consider a crack advancing towards a grain boundary (GB) in its path. In the investigation of single crystal fracture, we evaluate the competition between simple cleavage and crack-tip dislocation emission. In addition, we examine the ability of analytic models to correctly predict fracture behaviour, and find that the existing analytical treatments are too restrictive in their treatment of nonlinearity near the crack tip. In the study of GB-crack interactions, we have found a number of interesting deformation mechanisms which attend the advance of the crack. These include the migration of the GB, the emission of dislocations from the GB, and deflection of the crack front along the GB itself. In each case, these mechanisms are rationalized on the basis of continuum mechanics arguments

Finite-Temperature Quasicontinuum: Molecular Dynamics without All the Atoms

Using a combination of statistical mechanics and finite-element interpolation, we develop a coarse-grained (CG) alternative to molecular dynamics (MD) for crystalline solids at constant temperature. The new approach is significantly more efficient than MD and generalizes earlier work on the quasicontinuum method. The method is validated by recovering equilibrium properties of single crystal Ni as a function of temperature. CG dynamical simulations of nanoindentation reveal a strong dependence on temperature of the critical stress to nucleate dislocations under the indenter

Quasicontinuum Models of Interfacial Structure and Deformation

Microscopic models of the interaction between grain boundaries (GBs) and both dislocations and cracks are of importance in understanding the role of microstructure in altering the mechanical properties of a material. A recently developed mixed atomistic and continuum method is extended to examine the interaction between GBs, dislocations and cracks. These calculations elucidate plausible microscopic mechanisms for these defect interactions and allow for the quantitative evaluation of critical parameters such as the stress to nucleate a dislocation at a step on a GB and the force needed to induce GB migration.Comment: RevTex, 4 pages, 4 figure

Probing Individual Environmental Bacteria for Viruses by Using Microfluidic Digital PCR

Viruses may very well be the most abundant biological entities on the planet. Yet neither metagenomic studies nor classical phage isolation techniques have shed much light on the identity of the hosts of most viruses. We used a microfluidic digital polymerase chain reaction (PCR) approach to physically link single bacterial cells harvested from a natural environment with a viral marker gene. When we implemented this technique on the microbial community residing in the termite hindgut, we found genus-wide infection patterns displaying remarkable intragenus selectivity. Viral marker allelic diversity revealed restricted mixing of alleles between hosts, indicating limited lateral gene transfer of these alleles despite host proximity. Our approach does not require culturing hosts or viruses and provides a method for examining virus-bacterium interactions in many environments

Moments of spectral functions: Monte Carlo evaluation and verification

The subject of the present study is the Monte Carlo path-integral evaluation of the moments of spectral functions. Such moments can be computed by formal differentiation of certain estimating functionals that are infinitely-differentiable against time whenever the potential function is arbitrarily smooth. Here, I demonstrate that the numerical differentiation of the estimating functionals can be more successfully implemented by means of pseudospectral methods (e.g., exact differentiation of a Chebyshev polynomial interpolant), which utilize information from the entire interval $(-\beta \hbar / 2, \beta \hbar/2)$. The algorithmic detail that leads to robust numerical approximations is the fact that the path integral action and not the actual estimating functional are interpolated. Although the resulting approximation to the estimating functional is non-linear, the derivatives can be computed from it in a fast and stable way by contour integration in the complex plane, with the help of the Cauchy integral formula (e.g., by Lyness' method). An interesting aspect of the present development is that Hamburger's conditions for a finite sequence of numbers to be a moment sequence provide the necessary and sufficient criteria for the computed data to be compatible with the existence of an inversion algorithm. Finally, the issue of appearance of the sign problem in the computation of moments, albeit in a milder form than for other quantities, is addressed.Comment: 13 pages, 2 figure

Control of a Semi-Circular Planform Wing in a "Gusting" Unsteady Free Stream Flow II: Modeling and Feedback Design

Active flow control has been demonstrated in Part I of this article to modify the lift, drag and pitching moments on a semi-circular wing during "gusting" flow conditions. The low aspect ratio wing, AR = 2.54, is mounted on a captive trajectory system that responds to the instantaneous lift force and pitching moment and the "gusting" flow is simulated by a 0.2 Hz oscillation of the free stream speed of the wind tunnel. The mean chord Reynolds number of the wing is 70,600. Active flow control occurs along the leading edge of the airfoil, which contains 16 spatially localized micro-valve actuators. Details of the experimental setup, a quasi steady state lift model and results involving open-loop proof of concept validation are provided in Part I of this paper. Here we outline principles and considerations associated with close loop design that will be discussed in our talk

Matching Conditions in Atomistic-Continuum Modeling of Materials

A new class of matching condition between the atomistic and continuum regions is presented for the multi-scale modeling of crystals. They ensure the accurate passage of large scale information between the atomistic and continuum regions and at the same time minimize the reflection of phonons at the interface. These matching conditions can be made adaptive if we choose appropriate weight functions. Applications to dislocation dynamics and friction between two-dimensional atomically flat crystal surfaces are described.Comment: 6 pages, 4 figure

Lift Response of a Stalled Wing to Pulsatile Disturbances

The transient lift response of a low-Reynolds-number wing subjected to small amplitude pulsatile disturbances is investigated. The wing has a small aspect ratio and a semicircular planform, and it is fully stalled at a 20 deg angle of attack. Microvalve actuators distributed along the leading edge of the wing produce the transient disturbance. It is shown that the lift response to a single pulse increases with increasing actuator supply pressure and that the lift response curves are similar to each other when scaled by the total impulse. Furthermore, for fixed actuator supply pressure, the amplitude and total impulse of the transient lift response curve increases with increasing external flow speed. In this case, the lift response curves are similar when scaled by the dynamic pressure. The lift response to a single pulse can be treated as a filter kernel, and it can be used to predict the lift time history for the arbitrary actuator input signals. The kernel is similar in shape to transient measurements obtained by other investigators on two-dimensional wings and flaps. Comparisons between the model predictions and the experiments using multiple pulse inputs and square-wave modulated input signals at low frequencies are presented