7,185 research outputs found
Repeatable pre-cracking preparation for fracture testing of polymeric materials
Currently, in order to create a sharp pre-crack in a polymeric material for fracture toughness
testing, a hand-held razor blade is used. This technique produces cracks of varying
angle, length, and crack-front shape. Additionally, there are considerable safety concerns
regarding the handling of sharp razor blades. A repeatable, safe method of producing a
consistent, sharp pre-crack of a specified length, orientation and crack front is presented
here, by use of a simple custom fixture. The specimen preparation procedure has wide
applicability for fracture analysis of many brittle materials
Velocity weakening and possibility of aftershocks in nanofriction experiments
We study the frictional behavior of small contacts as those realized in the
atomic force microscope and other experimental setups, in the framework of
generalized Prandtl-Tomlinson models. Particular attention is paid to
mechanisms that generate velocity weakening, namely a decreasing average
friction force with the relative sliding velocity.The mechanisms studied model
the possibility of viscous relaxation, or aging effects in the contact. It is
found that, in addition to producing velocity weakening, these mechanisms can
also produce aftershocks at sufficiently low sliding velocities. This provides
a remarkable analogy at the microscale, of friction properties at the
macroscale, where aftershocks and velocity weakening are two fundamental
features of seismic phenomena.Comment: 8 pages, 7 figure
Optical Conductivity in a Two-Band Superconductor: Pb
We demonstrate the effect of bandstructure on the superconducting properties
of Pb by calculating the strong-coupling features in the optical conductivity,
, due to the electron-phonon interaction. The importance of
momentum dependence in the calculation of the properties of superconductors has
previously been raised for MgB. Pb resembles MgB in that it is a two
band superconductor in which the bands' contributions to the Fermi surface have
very different topologies. We calculate by calculating a
memory function which has been recently used to analyze of
BiSrCaCuO. In our calculations the two components of
the Fermi surface are described by parameterizations of de Haas--van Alphen
data. We use a phonon spectrum which is a fit to neutron scattering data. By
including the momentum dependence of the Fermi surface good agreement is found
with the experimentally determined strong-coupling features which can be
described by a broad peak at around 4.5 meV and a narrower higher peak around 8
meV of equal height. The calculated features are found to be dominated by
scattering between states within the third band. By contrast scattering between
states in the second band leads to strong-coupling features in which the height
of the high energy peak is reduced by compared to that of the low
energy peak. This result is similar to that in the conventional isotropic
(momentum independent) treatment of superconductivity. Our results show that it
is important to use realistic models of the bandstructure and phonons, and to
avoid using momentum averaged quantities, in calculations in order to get
quantitatively accurate results
Place-Based Industrial and Regional Strategy – Levelling the Playing Field
Over the past decade there has been renewed interest in the role of industrial strategy in enhancing innovation, productivity, and competitiveness within and across firms, sectors and regions with an eye to fostering more balanced regional economic growth. This special issue explores how policymakers could adopt place-based industrial policy measures to foster regional catch up and a more balanced and cohesive regional growth. In doing so, the papers are a mix of contributions that develop theory, provide evidence and highlight ‘state of the art’ or good practice that can inform a level playing field fostering regional industrial strategy.</p
Dynamics of Phononic Dissipation at the Atomic Scale: Dependence on Internal Degrees of Freedom
Dynamics of dissipation of a local phonon distribution to the substrate is a
key issue in friction between sliding surfaces as well as in boundary
lubrication. We consider a model system consisting of an excited nano-particle
which is weakly coupled with a substrate. Using three different methods we
solve the dynamics of energy dissipation for different types of coupling
between the nano-particle and the substrate, where different types of
dimensionality and phonon densities of states were also considered for the
substrate. In this paper, we present our analysis of transient properties of
energy dissipation via phonon discharge in the microscopic level towards the
substrate. Our theoretical analysis can be extended to treat realistic
lubricant molecules or asperities, and also substrates with more complex
densities of states. We found that the decay rate of the nano-particle phonons
increases as the square of the interaction constant in the harmonic
approximation.Comment: 10 pages, 6 figures, submitted to Phys. Rev.
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