256 research outputs found
Deformation and Failure of Amorphous Solidlike Materials
Since the 1970's, theories of deformation and failure of amorphous, solidlike
materials have started with models in which stress-driven, molecular
rearrangements occur at localized flow defects via "shear transformations".
This picture is the basis for the modern theory of "shear transformation zones"
(STZ's), which is the focus of this review. We begin by describing the
structure of the theory in general terms and by showing several applications,
specifically: interpretation of stress-strain measurements for a bulk metallic
glass, analysis of numerical simulations of shear banding, and the use of the
STZ equations of motion in free-boundary calculations. In the second half of
this article, we focus for simplicity on what we call an "athermal" model of
amorphous plasticity, and use that model to illustrate how the STZ theory
emerges within a systematic formulation of nonequilibrium thermodynamics.Comment: 28 pages, 4 figures, submitted to Annual Reviews of Condensed Matter
Physic
Nonequilibrium Thermodynamics of Amorphous Materials I: Internal Degrees of Freedom and Volume Deformation
This is the first of three papers devoted to the nonequilibrium
thermodynamics of amorphous materials. Our focus here is on the role of
internal degrees of freedom in determining the dynamics of such systems. For
illustrative purposes, we study a solid whose internal degrees of freedom are
vacancies that govern irreversible volume changes. Using this model, we compare
a thermodynamic theory based on the Clausius-Duhem inequality to a statistical
analysis based directly on the law of increase of entropy. The statistical
theory is used first to derive the the Clausius-Duhem inequality. We then use
the theory to go beyond those results and obtain detailed equations of motion,
including a rate factor that is enhanced by deformation-induced noisy
fluctuations. The statistical analysis points to the need for understanding how
both energy and entropy are shared by the vacancies and their environments.Comment: 7 pages. First of a three-part serie
Free-Boundary Dynamics in Elasto-plastic Amorphous Solids: The Circular Hole Problem
We develop an athermal shear-transformation-zone (STZ) theory of plastic
deformation in spatially inhomogeneous, amorphous solids. Our ultimate goal is
to describe the dynamics of the boundaries of voids or cracks in such systems
when they are subjected to remote, time-dependent tractions. The theory is
illustrated here for the case of a circular hole in an infinite two-dimensional
plate, a highly symmetric situation that allows us to solve much of the problem
analytically. In spite of its special symmetry, this example contains many
general features of systems in which stress is concentrated near free
boundaries and deforms them irreversibly. We depart from conventional
treatments of such problems in two ways. First, the STZ analysis allows us to
keep track of spatially heterogeneous, internal state variables such as the
effective disorder temperature, which determines plastic response to subsequent
loading. Second, we subject the system to stress pulses of finite duration, and
therefore are able to observe elasto-plastic response during both loading and
unloading. We compute the final deformations and residual stresses produced by
these stress pulses. Looking toward more general applications of these results,
we examine the possibility of constructing a boundary-layer theory that might
be useful in less symmetric situations.Comment: 30 pages (preprint format), 9 figure
Assessing biological condition in small streams of the Puget Sound lowlands through collaborative regional monitoring
In 2015, the condition of Puget Sound Lowland streams was evaluated by collecting data for stream invertebrates, algae, water and sediment quality, and instream and riparian habitat. The study was designed and implemented as part of the Stormwater Action Monitoring program, a collaborative, regional stormwater monitoring program funded by more than 90 Western Washington cities and counties, the ports of Seattle and Tacoma, and the Washington State Department of Transportation. The goal of this long term program is to inform stakeholders on the status and trends of small streams within the Puget Lowlands and to track whether stream condition improves as a result of stormwater management practices in the region. A comparable number of sites were randomly selected inside and outside the Urban Growth Area (UGA). Benthic invertebrate taxa were used to calculate the benthic multi-metric index (B-IBI) and three stressor index scores for each of the 104 sites. All sites showed that sites within UGAs had poorer invertebrate condition compared to sites outside the UGA. Similar patterns were shown for algae, with the Trophic Diatom Index (TDI) indicating elevated nutrients inside the UGA compared to outside the UGA. We used boosted regression trees and a relative risk/attributable risk analysis to determine the most important human and natural factors influencing biological condition in the region. For the B-IBI, the most important factors influencing scores were December precipitation, watershed percent urban development, percent of watershed and riparian canopy cover, and stream substrate. For the TDI, the most important factors influencing condition were mean summer total phosphorus and nitrogen concentrations and watershed percent urban development. The intent is to use this status year of data to refine the sample design, and begin trend monitoring in the coming years with the goal to determine if streams are getting better or worse over time
Dynamics of Large-Scale Plastic Deformation and the Necking Instability in Amorphous Solids
We use the shear transformation zone (STZ) theory of dynamic plasticity to
study the necking instability in a two-dimensional strip of amorphous solid.
Our Eulerian description of large-scale deformation allows us to follow the
instability far into the nonlinear regime. We find a strong rate dependence;
the higher the applied strain rate, the further the strip extends before the
onset of instability. The material hardens outside the necking region, but the
description of plastic flow within the neck is distinctly different from that
of conventional time-independent theories of plasticity.Comment: 4 pages, 3 figures (eps), revtex4, added references, changed and
added content, resubmitted to PR
Microstructural Shear Localization in Plastic Deformation of Amorphous Solids
The shear-transformation-zone (STZ) theory of plastic deformation predicts
that sufficiently soft, non-crystalline solids are linearly unstable against
forming periodic arrays of microstructural shear bands. A limited nonlinear
analysis indicates that this instability may be the mechanism responsible for
strain softening in both constant-stress and constant-strain-rate experiments.
The analysis presented here pertains only to one-dimensional banding patterns
in two-dimensional systems, and only to very low temperatures. It uses the
rudimentary form of the STZ theory in which there is only a single kind of zone
rather than a distribution of them with a range of transformation rates.
Nevertheless, the results are in qualitative agreement with essential features
of the available experimental data. The nonlinear theory also implies that
harder materials, which do not undergo a microstructural instability, may form
isolated shear bands in weak regions or, perhaps, at points of concentrated
stress.Comment: 32 pages, 6 figure
A robotic crawler exploiting directional frictional interactions: Experiments, numerics and derivation of a reduced model
We present experimental and numerical results for a model crawler which is able to extract net positional changes fromreciprocal shape changes, i.e. 'breathinglike' deformations, thanks to directional, frictional interactions with a textured solid substrate, mediated by flexible inclined feet. We also present a simple reduced model that captures the essential features of the kinematics and energetics of the gait, and compare its predictions with the results from experiments and from numerical simulations
Logarithmic spin, logarithmic rate and material frame-indifferent generalized plasticity
In this work we present a new rate type formulation of large deformation generalized plasticity which is based on the consistent use of the logarithmic rate concept. For this purpose, the basic constitutive equations are initially established in a local rotationally neutralized configuration which is defined by the logarithmic spin. These are then rephrased in their spatial form, by employing some standard concepts from the tensor analysis on manifolds. Such an approach, besides being compatible with the notion of (hyper)elasticity, offers three basic advantages, namely:(i) The principle of material frame-indifference is trivially satisfied ; (ii) The structure of the infinitesimal theory remains essentially unaltered ; (iii) The formulation does not preclude anisotropic response. A general integration scheme for the computational implementation of generalized plasticity models which are based on the logarithmic rate is also discussed. The performance of the scheme is tested by two representative numerical examples
Structural Performance of Reinforced Concrete Flat Plate Buildings Subjected to Fire
El siguiente artĂculo se propone estudiar la poesĂa de Luis Hernández a partir de los problemas que surgen al intentar estudiar su obra reunida. Antes que ser valorada como una poesĂa “inacabada”, la deliberada asistematicidad de su poĂ©tica debe ser entendida como el resultado de un calculado y consciente ejercicio artĂstico, cuyas fuentes filosĂłficas tradicionalmente han pugnado por una ontologĂa del movimiento frente a una metafĂsica de la permanencia. Bajo esta perspectiva, la obra de Hernández se revela como un “plano de inmanencia”, desde el cual acontece el sentido de su poesĂa en el quehacer de la escritura.The article aims to study Luis Hernández’ poetry from the point of view of the problems that emerge when trying to analyze his Complete Works. Rather than being assessed as “unfinished”, the deliberate and unsystematic appearance of his poetic, should rather be understood as the result of a calculated and conscious artistic practice that stems from philosophical trends which traditionally have fostered an ontology of movement against a metaphysics of permanence. Under this view, Hernández work shows up as “plane of immanence” from where sense becomes an event for the creative writing process.El segĂĽent article es proposa estudiar la poesia de Luis Hernández a partir dels problemes que sorgeixen al tractar d’estudiar la seva obra reunida. Abans de ser valorada com una poesia “incabada”, la deliberada asistematicitat de la seva poètica deu ser entesa com el resultat d’un calculat i conscient exercici artĂstic, fonts filosòfiques de les quals han pugnat tradicionalment per una ontologia del moviment davant una metafĂsica de la permanència. Sota aquesta perspectiva, l’obra d’HernĂ ndez es revela com un “pla d’inmanència”, des del qual esdevĂ© el sentit de la seva poesia en el afer de l’escriptura
- …