Brown University

About: Brown University is a education organization based out in Providence, Rhode Island, United States. It is known for research contribution in the topics: Population & Poison control. The organization has 35778 authors who have published 90896 publications receiving 4471489 citations. The organization is also known as: brown.edu & Brown.

An experimental study of the formation process of adiabatic shear bands in a structural steel

Abstract: A series of experiments is described in which the local temperature and local strain are measured during the formation of an adiabatic shear band in a low alloy structural steel (HY-100). The specimen employed consists of a short thin-walled tube and the required rapid deformation rates are imposed by loading the specimen in a torsional Kolsky bar (split-Hopkinson bar). The local temperature is determined by measuring the infrared radiation emanating at twelve neighboring points on the specimen's surface, including the shear band area. Indium-antimonide elements are employed for this purpose to give the temperature history during deformation. In addition, high speed photographs are made of a grid pattern deposited on the specimen's surface, thus providing a measure of the strain distribution at various stages during shear band formation. By testing a number of specimens, it is possible to form a picture of the developing strain localization process, of the temperature history within the forming shear band, and of the consequent loss in the load carrying capacity of the steel. It appears that plastic deformation follows a three stage process which begins with a homogeneous strain state, followed by a generally inhomogeneous strain distribution, and finally by a narrowing of the localization into a fine shear band. It is estimated that the shear band propagates at a speed of about 510 m/s in the material tested. Results also include data on the stress-strain behavior of HY-100 steel over the temperature range —190°C to 250°C and at quasi-static as well as dynamic strain rates.

First-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Parameter Estimation Methodology

Abstract: We describe our methodology for comparing the Wilkinson Microwave Anisotropy Probe (WMAP) measurements of the cosmic microwave background (CMB) and other complementary data sets to theoretical models. The unprecedented quality of the WMAP data and the tight constraints on cosmological parameters that are derived require a rigorous analysis so that the approximations made in the modeling do not lead to significant biases. We describe our use of the likelihood function to characterize the statistical properties of the microwave background sky. We outline the use of the Monte Carlo Markov Chains to explore the likelihood of the data given a model to determine the best-fit cosmological parameters and their uncertainties. We add to the WMAP data the l 700 Cosmic Background Imager (CBI) and Arcminute Cosmology Bolometer Array Receiver (ACBAR) measurements of the CMB, the galaxy power spectrum at z ~ 0 obtained from the Two-Degree Field Galaxy Redshift Survey (2dFGRS), and the matter power spectrum at z ~ 3 as measured with the Lyα forest. These last two data sets complement the CMB measurements by probing the matter power spectrum of the nearby universe. Combining CMB and 2dFGRS requires that we include in our analysis a model for galaxy bias, redshift distortions, and the nonlinear growth of structure. We show how the statistical and systematic uncertainties in the model and the data are propagated through the full analysis.

Optical Properties and Band Structure of SrTi O 3 and BaTi O 3

Abstract: The normal-incidence reflectance spectra of SrTi${\mathrm{O}}_{3}$ and BaTi${\mathrm{O}}_{3}$ and the optical constants derived by the Kramers-Kronig method are reported. These spectra are very similar to those of rutile. The strongest optical transitions ($A$) occur at about 4.4 eV: They are responsible for the dispersion observed below the fundamental absorption edge (3.2 eV). A tentative interpretation of these spectra in terms of the energy band structure of the materials is offered. The $A$ peak seems due to transitions between $2p$ levels of oxygen and $3s$ levels of titanium at the $X$ point of the Brillouin zone. The splitting of the $A$ peak is assigned to the splitting of the $2p$ levels of the oxygen in the cubic field. Transitions between $2p$ oxygen and $4s$ titanium levels, between $2s$ oxygen and $3d$ titanium levels, and between $2s$ oxygen and $5s$ titanium or $6s$ barium levels are also observed. Plasma oscillations are seen around 21 eV.