6,724 research outputs found
Quality-weld parameters for microwelding techniques and equipment
Limited-amplitude, controlled-decay process improves the reliability of microwelding. The system consists in building a capacitor-discharge welder for control of the shape of the weld pulse. Standard welders may be modified
Quality weld parameters for microwelding techniques and equipment Final report
Amplitude controlled welding process for electronic circuit module
Cotton culture in Mississippi : in areas infested with the Mexican cotton boll weevil
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Soils of Mississippi: texture and water conditions
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Modeling the buckling and delamination of thin films
I study numerically the problem of delamination of a thin film elastically
attached to a rigid substrate. A nominally flat elastic thin film is modeled
using a two-dimensional triangular mesh. Both compression and bending
rigidities are included to simulate compression and bending of the film. The
film can buckle (i.e., abandon its flat configuration) when enough compressive
strain is applied. The possible buckled configurations of a piece of film with
stripe geometry are investigated as a function of the compressive strain. It is
found that the stable configuration depends strongly on the applied strain and
the Poisson ratio of the film. Next, the film is considered to be attached to a
rigid substrate by springs that can break when the detaching force exceeds a
threshold value, producing the partial delamination of the film. Delamination
is induced by a mismatch of the relaxed configurations of film and substrate.
The morphology of the delaminated film can be followed and compared with
available experimental results as a function of model parameters.
`Telephone-cord', polygonal, and `brain-like' patterns qualitatively similar to
experimentally observed configurations are obtained in different parameter
regions. The main control parameters that select the different patterns are the
mismatch between film and substrate and the degree of in-plane relaxation
within the unbuckled regions.Comment: 8 pages, 10 figure
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Cohesive Traction-Separation Laws for Tearing of Ductile Metal Plates
The failure process ahead of a mode I crack advancing in a ductile thin metal plate or sheet produces plastic dissipation through a sequence of deformation steps that include necking well ahead of the crack tip and shear localization followed by a slant fracture in the necked region somewhat closer to the tip. The objective of this paper is to analyze this sequential process to characterize the traction–separation behavior and the associated effective cohesive fracture energy of the entire failure process. The emphasis is on what is often described as plane stress behavior taking place after the crack tip has advanced a distance of one or two plate thicknesses. Traction–separation laws are an essential component of finite element methods currently under development for analyzing fracture of large scale plate or shell structures. The present study resolves the sequence of failure details using the Gurson constitutive law based on the micromechanics of the ductile fracture process, including a recent extension that accounts for damage growth in shear. The fracture process in front of an advancing crack, subject to overall mode I loading, is approximated by a 2D plane strain finite element model, which allows for an intensive study of the parameters influencing local necking, shear localization and the final slant failure. The deformation history relevant to a cohesive zone for a large scale model is identified and the traction–separation relation is determined, including the dissipated energy. For ductile structural materials, the dissipation generated during necking prior to the onset of shear localization is the dominant contribution; it scales with the plate thickness and is mesh-independent in the present numerical model. The energy associated with the shear localization and fracture is secondary; it scales with the width of the shear band, and inherits the finite element mesh dependency of the Gurson model. The cohesive traction–separation laws have been characterized for various material conditions.Engineering and Applied Science
Forces on a spherical conducting particle in E x B fields
The forces acting on a spherical conducting particle in a transversely flowing magnetized plasma are calculated in the entire range of magnetization and Debye length, using the particle code SCEPTIC3D (Patacchini and Hutchinson 2010 Plasma Phys. Control. Fusion 52 035005, 2011 Plasma Phys. Control. Fusion 53 025005). In short Debye length (i.e. high density) plasmas, both the ion-drag and Lorentz force arising from currents circulating inside the dust show strong components antiparallel to the convective electric field, suggesting that a free dust particle should gyrate faster than what predicted by its Larmor frequency. In intermediate to large Debye length conditions, by a downstream depletion effect already reported in unmagnetized strongly collisional regimes, the ion-drag in the direction of transverse flow can become negative. The internal Lorentz force, however, remains in the flow direction, and large enough in magnitude so that no spontaneous dust motion should occur.National Science Foundation (U.S.)United States. Dept. of Energy (grant DE-FG02-06ER54891
Fertilizers for Cotton
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Optimal query complexity for estimating the trace of a matrix
Given an implicit matrix with oracle access for any
, we study the query complexity of randomized algorithms for
estimating the trace of the matrix. This problem has many applications in
quantum physics, machine learning, and pattern matching. Two metrics are
commonly used for evaluating the estimators: i) variance; ii) a high
probability multiplicative-approximation guarantee. Almost all the known
estimators are of the form for being i.i.d. for some special distribution.
Our main results are summarized as follows. We give an exact characterization
of the minimum variance unbiased estimator in the broad class of linear
nonadaptive estimators (which subsumes all the existing known estimators). We
also consider the query complexity lower bounds for any (possibly nonlinear and
adaptive) estimators: (1) We show that any estimator requires
queries to have a guarantee of variance at most
. (2) We show that any estimator requires
queries to achieve a
-multiplicative approximation guarantee with probability at
least . Both above lower bounds are asymptotically tight.
As a corollary, we also resolve a conjecture in the seminal work of Avron and
Toledo (Journal of the ACM 2011) regarding the sample complexity of the
Gaussian Estimator.Comment: full version of the paper in ICALP 201
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