56,717 research outputs found
Analysis of crack propagation as an energy absorption mechanism in metal matrix composites
The crack initiation and crack propagation capability was extended to the previously developed generalized plane strain, finite element micromechanics analysis. Also, an axisymmetric analysis was developed, which contains all of the general features of the plane analysis, including elastoplastic material behavior, temperature-dependent material properties, and crack propagation. These analyses were used to generate various example problems demonstrating the inelastic response of, and crack initiation and propagation in, a boron/aluminum composite
Energy absorption mechanisms during crack propagation in metal matrix composites
The stress distributions around individual fibers in a unidirectional boron/aluminum composite material subjected to axial and transverse loadings are being studied utilizing a generalized plane strain finite element analysis. This micromechanics analysis was modified to permit the analysis of longitudinal sections, and also to incorporate crack initiation and propagation. The analysis fully models the elastoplastic response of the aluminum matrix, as well as temperature dependent material properties and thermal stress effects. The micromechanics analysis modifications are described, and numerical results are given for both longitudinal and transverse models loaded into the inelastic range, to first failure. Included are initially cracked fiber models
Super-hard Superconductivity
We present a study of the magnetic response of Type-II superconductivity in
the extreme pinning limit, where screening currents within an order of
magnitude of the Ginzburg-Landau depairing critical current density develop
upon the application of a magnetic field. We show that this "super-hard" limit
is well approximated in highly disordered, cold drawn, Nb and V wires whose
magnetization response is characterized by a cascade of Meissner-like phases,
each terminated by a catastrophic collapse of the magnetization. Direct
magneto-optic measurements of the flux penetration depth in the virgin
magnetization branch are in excellent agreement with the exponential model in
which J_c(B)=J_co exp(-B/B_o), where J_co~5x10^6 A/cm^2 for Nb. The
implications for the fundamental limiting hardness of a superconductor are
discussed.Comment: corrected Fig.
A non-monotonic constitutive model is not necessary to obtain shear banding phenomena in entangled polymer solutions
In 1975 Doi and Edwards predicted that entangled polymer melts and solutions
can have a constitutive instability, signified by a decreasing stress for shear
rates greater than the inverse of the reptation time. Experiments did not
support this, and more sophisticated theories incorporated Marrucci's idea
(1996) of removing constraints by advection; this produced a monotonically
increasing stress and thus stable constitutive behavior. Recent experiments
have suggested that entangled polymer solutions may possess a constitutive
instability after all, and have led some workers to question the validity of
existing constitutive models. In this Letter we use a simple modern
constitutive model for entangled polymers, the non-stretching Rolie-Poly model
with an added solvent viscosity, and show that (1) instability and shear
banding is captured within this simple class of models; (2) shear banding
phenomena is observable for weakly stable fluids in flow geometries that impose
a sufficiently inhomogeneous total shear stress; (3) transient phenomena can
possess inhomogeneities that resemble shear banding, even for weakly stable
fluids. Many of these results are model-independent.Comment: 5 figure
Parameters for Twisted Representations
The study of Hermitian forms on a real reductive group gives rise, in the
unequal rank case, to a new class of Kazhdan-Lusztig-Vogan polynomials. These
are associated with an outer automorphism of , and are related to
representations of the extended group . These polynomials were
defined geometrically by Lusztig and Vogan in "Quasisplit Hecke Algebras and
Symmetric Spaces", Duke Math. J. 163 (2014), 983--1034. In order to use their
results to compute the polynomials, one needs to describe explicitly the
extension of representations to the extended group. This paper analyzes these
extensions, and thereby gives a complete algorithm for computing the
polynomials. This algorithm is being implemented in the Atlas of Lie Groups and
Representations software
Low Temperature Susceptibility of the Noncentrosymmetric Superconductor CePt_3Si
We report ac susceptibility measurements of polycrystalline CePt_3Si down to
60 mK and in applied fields up to 9 T. In zero field, a full Meissner state
emerges at temperatures T/Tc < 0.3, where Tc=0.65 K is the onset transition
temperature. Though transport measurements show a relatively high upper
critical field Bc2 ~ 4-5 T, the low temperature susceptibility, \chi', is quite
fragile to applied field, with \chi' diminishing rapidly in fields of a few kG.
Interestingly, the field dependence of \chi' is well described by the power
law, 4\pi\chi'=(B/B_c)^{1/2}, where Bc is the field at which the onset of
resistance is observed in transport measurements.Comment: 5 figure
The Influence of Federal Laboratory R&D on Industrial Research
Over the past 60 years the United States has created the world's largest system of government laboratories. The impact of the laboratories on the private economy has been little studied though their research accounts for 14% of total U.S. R&D, more than the R&D of all colleges and universities combined. In this paper we study the influence of federal laboratory R&D on industrial research using a sample of industrial laboratories. In head-to-head comparisons with alternative measures, we find that Cooperative Research and Development Agreements or CRADAs, are the primary channel by which federal laboratories increase the patenting and R&D of industrial laboratories. With a CRADA industrial laboratories patent more, spend more on company-financed R&D and spend more of their own money on federal laboratories. Without a CRADA patenting stays about the same and only federally funded R&D increases, mostly because of direct subsidies by government. These results are consistent with the literature on endogenous R&D spillovers, which emphasizes that knowledge spills over when recipients work at making it spill over. CRADAs are legal agreements between federal laboratories and firms to work together on joint research. They are backed by real budgets and accompanied by cost sharing that could bind the parties together in joint research. Moreover, the CRADA instrument is the main form of such agreements. Thus, both in theory and in fact CRADAs may be more beneficial to firms than other public- private interactions, precisely because of the mutual effort that they require of firms and government laboratories.
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