115,724 research outputs found
Contact mechanics of and Reynolds flow through saddle points: On the coalescence of contact patches and the leakage rate through near-critical constrictions
We study numerically local models for the mechanical contact between two
solids with rough surfaces. When the solids softly touch either through
adhesion or by a small normal load , contact only forms at isolated patches
and fluids can pass through the interface. When the load surpasses a threshold
value, , adjacent patches coalesce at a critical constriction, i.e., near
points where the interfacial separation between the undeformed surfaces forms a
saddle point. This process is continuous without adhesion and the interfacial
separation near percolation is fully defined by scaling factors and the sign of
. The scaling factors lead to a Reynolds flow resistance which diverges
as with . Contact merging and destruction near
saddle points becomes discontinuous when either short-range adhesion or
specific short-range repulsion are added to the hard-wall repulsion. These
results imply that coalescence and break-up of contact patches can contribute
to Coulomb friction and contact aging.Comment: 6 pages, 6 figures, submitted to Euro. Phys. Let
Towards time-dependent, non-equilibrium charge-transfer force fields: Contact electrification and history-dependent dissociation limits
Force fields uniquely assign interatomic forces for a given set of atomic
coordinates. The underlying assumption is that electrons are in their
quantum-mechanical ground state or in thermal equilibrium. However, there is an
abundance of cases where this is unjustified because the system is only locally
in equilibrium. In particular, the fractional charges of atoms, clusters, or
solids tend to not only depend on atomic positions but also on how the system
reached its state. For example, the charge of an isolated solid -- and thus the
forces between atoms in that solid -- usually depends on the counterbody with
which it has last formed contact. Similarly, the charge of an atom, resulting
from the dissociation of a molecule, can differ for different solvents in which
the dissociation took place. In this paper we demonstrate that such
charge-transfer history effects can be accounted for by assigning discrete
oxidation states to atoms. With our method, an atom can donate an integer
charge to another, nearby atom to change its oxidation state as in a redox
reaction. In addition to integer charges, atoms can exchange "partial charges"
which are determined with the split charge equilibration method.Comment: 11 pages, 7 figure
Characterization of Mode 1 and Mode 2 delamination growth and thresholds in graphite/peek composites
Composite materials often fail by delamination. The onset and growth of delamination in AS4/PEEK, a tough thermoplastic matrix composite, was characterized for mode 1 and mode 2 loadings, using the Double Cantilever Beam (DCB) and the End Notched Flexure (ENF) test specimens. Delamination growth per fatigue cycle, da/dN, was related to strain energy release rate, G, by means of a power law. However, the exponents of these power laws were too large for them to be adequately used as a life prediction tool. A small error in the estimated applied loads could lead to large errors in the delamination growth rates. Hence strain energy release rate thresholds, G sub th, below which no delamination would occur were also measured. Mode 1 and 2 threshold G values for no delamination growth were found by monitoring the number of cycles to delamination onset in the DCB and ENF specimens. The maximum applied G for which no delamination growth had occurred until at least 1,000,000 cycles was considered the threshold strain energy release rate. Comments are given on how testing effects, facial interference or delamination front damage, may invalidate the experimental determination of the constants in the expression
Systematic analysis of Persson's contact mechanics theory of randomly rough elastic surfaces
We systematically check explicit and implicit assumptions of Persson's
contact mechanics theory. It casts the evolution of the pressure distribution
with increasing resolution of surface roughness as a diffusive
process, in which resolution plays the role of time. The tested key assumptions
of the theory are: (a) the diffusion coefficient is independent of pressure
, (b) the diffusion process is drift-free at any value of , (c) the point
acts as an absorbing barrier, i.e., once a point falls out of contact, it
never reenters again, (d) the Fourier component of the elastic energy is only
populated if the appropriate wave vector is resolved, and (e) it no longer
changes when even smaller wavelengths are resolved. Using high-resolution
numerical simulations, we quantify deviations from these approximations and
find quite significant discrepancies in some cases. For example, the drift
becomes substantial for small values of , which typically represent points
in real space close to a contact line. On the other hand, there is a
significant flux of points reentering contact. These and other identified
deviations cancel each other to a large degree, resulting in an overall
excellent description for contact area, contact geometry, and gap distribution
functions. Similar fortuitous error cancellations cannot be guaranteed under
different circumstances, for instance when investigating rubber friction. The
results of the simulations may provide guidelines for a systematic improvement
of the theory.Comment: 27 pages, 16 figures, accepted for publication by Journal of Physics:
Condensed Matte
Cryogenic seal remains leaktight during thermal displacement
Cryogenic seals protect the surfaces of a plastic member in a low-pressure system subjected to extreme temperature changes. The outer seal is an aluminum expansion ring bonded to the lens outer surface and the inner seal consists of a resin-filled aluminum U-ring bonded to the inner surface
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