147 research outputs found
Impact resistance of spar-shell composite fan blades
Composite spar-shell fan blades for a 1.83 meter (6 feet) diameter fan stage were fabricated and tested in a whirling arm facility to evaluate foreign object damage (FOD) resistance. The blades were made by adhesively bonding boron-epoxy shells on titanium spars and then adhesively bonding an Inconel 625 sheath on the leading edge. The rotating blades were individually tested at a tip speed of 800 feet per second. Impacting media used were gravel, rivets, bolt, nut, ice balls, simulated birds, and a real bird. Incidence angles were typical of those which might be experienced by STOL aircraft. The tests showed that blades of the design tested in this program have satisfactory impact resistance to small objects such as gravel, rivets, nuts, bolts, and two inch diameter ice balls. The blades suffered nominal damage when impacted with one-pound birds (9 to 10 ounce slice size). However, the shell was removed from the spar for a larger slice size
Monte Carlo Study of Short-Range Order and Displacement Effects in Disordered CuAu
The correlation between local chemical environment and atomic displacements
in disordered CuAu alloy has been studied using Monte Carlo simulations based
on the effective medium theory (EMT) of metallic cohesion. These simulations
correctly reproduce the chemically-specific nearest-neighbor distances in the
random alloy across the entire Cu\$_x\$Au\$_{1-x}\$ concentration range. In the
random equiatomic CuAu alloy, the chemically specific pair distances depend
strongly on the local atomic environment (i.e. fraction of like/unlike nearest
neighbors). In CuAu alloy with short-range order, the relationship between
local environment and displacements remains qualitatively similar. However the
increase in short-range order causes the average Cu-Au distance to decrease
below the average Cu-Cu distance, as it does in the ordered CuAuI phase. Many
of these trends can be understood qualitatively from the different neutral
sphere radii and compressibilities of the Cu and Au atoms.Comment: 9 pages, 5 figures, 2 table
A simple model of epitaxial growth
A discrete solid-on-solid model of epitaxial growth is introduced which, in a
simple manner, takes into account the effect of an Ehrlich-Schwoebel barrier at
step edges as well as the local relaxation of incoming particles. Furthermore a
fast step edge diffusion is included in 2+1 dimensions. The model exhibits the
formation of pyramid-like structures with a well-defined constant inclination
angle. Two regimes can be distinguished clearly: in an initial phase (I) a
definite slope is selected while the number of pyramids remains unchanged. Then
a coarsening process (II) is observed which decreases the number of islands
according to a power law in time. Simulations support self-affine scaling of
the growing surface in both regimes. The roughness exponent is alpha =1 in all
cases. For growth in 1+1 dimensions we obtain dynamic exponents z = 2 (I) and z
= 3 (II). Simulations for d=2+1 seem to be consistent with z= 2 (I) and z= 2.3
(II) respectively.Comment: 8 pages Latex2e, 4 Postscript figures included, uses packages
a4wide,epsfig,psfig,amsfonts,latexsy
Diffusion processes and growth on stepped metal surfaces
We study the dynamics of adatoms in a model of vicinal (11m) fcc metal
surfaces. We examine the role of different diffusion mechanisms and their
implications to surface growth. In particular, we study the effect of steps and
kinks on adatom dynamics. We show that the existence of kinks is crucially
important for adatom motion along and across steps. Our results are in
agreement with recent experiments on Cu(100) and Cu(1,1,19) surfaces. The
results also suggest that for some metals exotic diffusion mechanisms may be
important for mass transport across the steps.Comment: 3 pages, revtex, complete file available from
ftp://rock.helsinki.fi/pub/preprints/tft/ or at
http://www.physics.helsinki.fi/tft/tft_preprints.html (to appear in Phys.
Rev. B Rapid Comm.
Dislocation Kinks in Copper: Widths, Barriers, Effective Masses, and Quantum Tunneling
We calculate the widths, migration barriers, effective masses, and quantum
tunneling rates of kinks and jogs in extended screw dislocations in copper,
using an effective medium theory interatomic potential. The energy barriers and
effective masses for moving a unit jog one lattice constant are close to
typical atomic energies and masses: tunneling will be rare. The energy barriers
and effective masses for the motion of kinks are unexpectedly small due to the
spreading of the kinks over a large number of atoms. The effective masses of
the kinks are so small that quantum fluctuations will be important. We discuss
implications for quantum creep, kink--based tunneling centers, and Kondo
resonances
Self-diffusion of adatoms, dimers, and vacancies on Cu(100)
We use ab initio static relaxation methods and semi-empirical
molecular-dynamics simulations to investigate the energetics and dynamics of
the diffusion of adatoms, dimers, and vacancies on Cu(100). It is found that
the dynamical energy barriers for diffusion are well approximated by the
static, 0 K barriers and that prefactors do not depend sensitively on the
species undergoing diffusion. The ab initio barriers are observed to be
significantly lower when calculated within the generalized-gradient
approximation (GGA) rather than in the local-density approximation (LDA). Our
calculations predict that surface diffusion should proceed primarily via the
diffusion of vacancies. Adatoms are found to migrate most easily via a jump
mechanism. This is the case, also, of dimers, even though the corresponding
barrier is slightly larger than it is for adatoms. We observe, further, that
dimers diffuse more readily than they can dissociate. Our results are discussed
in the context of recent submonolayer growth experiments of Cu(100).Comment: Submitted to the Physical Review B; 15 pages including postscript
figures; see also http://www.centrcn.umontreal.ca/~lewi
Is there a contraction of the interatomic distance in small metal particles?
A theoretical analysis is made of the bond lengths of small (100–1000 atoms) Cu particles at various temperatures. The interatomic interactions are calculated using the effective-medium theory and the finite-temperature properties obtained from a molecular-dynamics simulation. We find only very small changes in bond length with particle size, but the motion in the small particles is very anharmonic. We use this observation to resolve the current experimental controversy about the existence of bond contraction for small metal particles
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