28,296 research outputs found
Fracture toughness of SiC/Al metal matrix composite
An experimental study was conducted to evaluate fracture toughness of SiC/Al metal matrix composite (MMC). The material was a 12.7 mm thick extrusion of 6061-T6 aluminum alloy with 40 v/o SiC particulates. Specimen configuration and test procedure conformed to ASTM E399 Standard for compact specimens. It was found that special procedures were necessary to obtain fatigue cracks of controlled lengths in the preparation of precracked specimens for the MMC material. Fatigue loading with both minimum and maximum loads in compression was used to start the precrack. The initial precracking would stop by self-arrest. Afterwards, the precrack could be safely extended to the desired length by additional cyclic tensile loading. Test results met practically all the E399 criteria for the calculation of plane strain fracture toughness of the material. A valid K sub IC value of the SiC/Al composite was established as K sub IC = 8.9 MPa square root of m. The threshold stress intensity under which crack would cease to grow in the material was estimated as delta K sub th = 2MPa square root of m for R = 0.09 using the fatigue precracking data. Fractographic examinations show that failure occurred by the micromechanism involved with plastic deformation although the specimens broke by brittle fracture. The effect of precracking by cyclic loading in compression on fracture toughness is included in the discussion
All-optical switching in a two-channel waveguide with cubic-quintic nonlinearity
We consider dynamics of spatial beams in a dual-channel waveguide with
competing cubic and quintic (CQ) nonlinearities. Gradually increasing the power
in the input channel, we identify four different regimes of the pulses coupling
into the cross channel, which alternate three times between full pass and full
stop, thus suggesting three realizations of switching between the channels. As
in the case of the Kerr (solely cubic) nonlinearity, the first two regimes are
the linear one, and one dominated by the self-focusing nonlinearity, with the
beam which, respectively, periodically couples between the channels, or stays
in the input channel. Further increase of the power reveals two novel
transmission regimes, one characterized by balance between the competing
nonlinearities, which again allows full coupling between the channels, and a
final regime dominated by the self-defocusing quintic nonlinearity. In the
latter case, the situation resembles that known for a self-repulsive
Bose-Einstein condensate trapped in a double-well potential, which is
characterized by strong symmetry breaking; accordingly, the beam again abides
in the input channel, contrary to an intuitive expectation that the
self-defocusing nonlinearity would push it into the cross channel. The
numerical results are qualitatively explained by a simple analytical model
based on the variational approximation.Comment: Journal of Physics B (in press
Equilibrium orbit analysis in a free-electron laser with a coaxial wiggler
An analysis of single-electron orbits in combined coaxial wiggler and axial
guide magnetic fields is presented. Solutions of the equations of motion are
developed in a form convenient for computing orbital velocity components and
trajectories in the radially dependent wiggler. Simple analytical solutions are
obtained in the radially-uniform-wiggler approximation and a formula for the
derivative of the axial velocity with respect to Lorentz factor
is derived. Results of numerical computations are presented and the
characteristics of the equilibrium orbits are discussed. The third spatial
harmonic of the coaxial wiggler field gives rise to group orbits which
are characterized by a strong negative mass regime.Comment: 13 pages, 9 figures, to appear in phys. rev.
Pressure-Temperature Phase Diagram of Multiferroic
The pressure-temperature phase diagram of multiferroic is
investigated for hydrostatic pressures up to 2 GPa. The stability range of the
ferroelectric phase associated with the incommensurate helical spin order is
reduced by pressure and ferroelectricity is completely suppressed at the
critical pressure of 1.64 GPa at 6.2 K. Thermal expansion measurements at
ambient pressure show strong step-like anomalies of the lattice parameters
associated with the lock-in transition into the commensurate paraelectric
phase. The expansion anomalies are highly anisotropic, the related volume
change is consistent with the high-pressure phase diagram
Simulating Star Formation and Feedback in Galactic Disk Models
We use a high-resolution grid-based hydrodynamics method to simulate the
multi-phase interstellar medium in a Milky Way-size quiescent disk galaxy. The
models are global and three-dimensional, and include a treatment of star
formation and feedback. We examine the formation of gravitational instabilities
and show that a form of the Toomre instability criterion can successfully
predict where star formation will occur. Two common prescriptions for star
formation are investigated. The first is based on cosmological simulations and
has a relatively low threshold for star formation, but also enforces a
comparatively low efficiency. The second only permits star formation above a
number density of 1000 cm^-3 but adopts a high efficiency. We show that both
methods can reproduce the observed slope of the relationship between star
formation and gas surface density (although at too high a rate for our adopted
parameters). A run which includes feedback from type II supernovae is
successful at driving gas out of the plane, most of which falls back onto the
disk. This feedback also substantially reduces the star formation rate.
Finally, we examine the density and pressure distribution of the ISM, and show
that there is a rough pressure equilibrium in the disk, but with a wide range
of pressures at a given location (and even wider for the case including
feedbackComment: 14 pages, 12 figures, accepted to Astrophysical Journa
Magnetic Helicity in Sphaleron Debris
We develop an analytical technique to evaluate the magnetic helicity in the
debris from sphaleron decay. We show that baryon number production leads to
left-handed magnetic fields, and that the magnetic helicity is conserved at
late times. Our analysis explicitly demonstrates the connection between
sphaleron-mediated cosmic baryogenesis and cosmic magnetogenesis.Comment: 9 pages, 1 figure. v2: Minor revisions; matches published version in
Physical Review
Temperature dependence of the ohmic conductivity and activation energy of Pb1+y(Zr0.3Ti0.7)O3 thin films
The ohmic conductivity of the sol-gel derived Pb1+y(Zr0.3Ti0.7)O3 thin films
(with the excess lead y=0.0 to 0.4) are investigated using low frequency small
signal alternate current (AC) and direct current (DC) methods. Its temperature
dependence shows two activation energies of 0.26 and 0.12 eV depending on
temperature range and excess Pb levels. The former is associated with Pb3+
acceptor centers, while the latter could be due to a different defect level yet
to be identified.Comment: 13 pages, 3 figures, PostScript. Submitted to Applied Physics Letter
SAM-2 ground-truth plan: Correlative measurements for the Stratospheric Aerosol Measurement-2 (SAM 2) sensor on the Nimbus G satellite
The SAM-2 will fly aboard the Nimbus-G satellite for launch in the fall of 1978 and measure stratospheric vertical profiles of aerosol extinction in high latitude bands. The plan gives details of the location and times for the simultaneous satellite/correlative measurements for the nominal launch time, the rationale and choice of the correlative sensors, their characteristics and expected accuracies, and the conversion of their data to extinction profiles. The SAM-2 expected instrument performance and data inversion results are presented. Various atmospheric models representative of polar stratospheric aerosols are used in the SAM-2 and correlative sensor analyses
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