10,415 research outputs found
Wavet, a Custom Device Able to Measure Viscoelastic Properties of Wood Under Water-Saturated Conditions Up to 140 deg C (WAVET : Environmental Vibration Analyser for Wood)
This work presents an innovative experimental device conceived to
characterize the time-dependent behavior of hygroscopic materials, as wood, at
controlled moisture content and temperature. This device, the WAVET, permits
the determination of the viscoelastic properties of samples, using harmonic
tests at frequencies varying from 5.10-3 Hz to 10 Hz. Conceived to work up to 5
bar, it allows tests in dry or water-saturated conditions to be performed over
the temperature range 0\degree C to 140\degree C. In spite of these severe
working conditions, the careful device design, together with a rigorous data
analysis, allows a rigorous determination of the storage and loss modulus and
the loss factor. The results collected for several species of wood emphasize
the ability of the WAVET to underline the influence of numerous parameters,
namely specie, material direction, anatomical and macromolecular structure, on
the rheological properties and notably on the softening temperature. The WAVET
also establishes a efficient tool to follow the modifications of the
constitutive amorphous polymers of materials submitted to hydrothermal
treatments, in relation to the evolution of their viscoelastic properties.Comment: 3
Viscoelastic properties of wood across the grain measured under water-saturated conditions up to 135\degree C: evidence of thermal degradation
In this paper, the viscoelastic properties of wood under water-saturated
conditions are investigated from 10\degree C to 135\degree C using the WAVET*
apparatus. Experiments were performed via harmonic tests at two frequencies
(0.1 Hz and 1 Hz) for several hours. Four species of wood were tested in the
radial and tangential material directions: oak (Quercus sessiliflora), beech
(Fagus sylvatica), spruce (Picea abies) and fir (Abies pectinata). When the
treatment is applied for several hours, a reduction of the wood rigidity is
significant from temperature values as low as 80-90\degree C, and increases
rapidly with the temperature level. The storage modulus of oak wood is divided
by a factor two after three hours of exposure at 135\degree C. This marked
reduction in rigidity is attributed to the hydrolysis of hemicelluloses. The
softening temperature of wood is also noticeably affected by hygrothermal
treatment. After three short successive treatments up to 135\degree C, the
softening temperature of oak shifted from 79\degree C to 103\degree C, at a
frequency of 1 Hz. This reduction in mobility of wood polymers is consistent
with the condensation of lignins observed by many authors at this temperature
level. In the same conditions, fir exhibited a softening temperature decreasing
of about 4\degree C. In any case, the internal friction clearly raises
Late Conversion: The Impact of Professionalism on European Rugby Union
Rugby union only went professional in 1995, much later than other major team sports. League structures and arrangements regarding revenue sharing and salary caps differ between the three main European leagues. We consider the impact of these differences on competitive balance. In addition, unlike soccer, rugby does not require leagues to be organised along national lines, which has enabled the smaller rugby playing countries to establish a joint league. This has prevented a migration of all the best players to larger country leagues as has happened in soccer and resulted in a greater degree of competitive balance in European rugby competitions.Rugby, Economics
Collaborative Beamforming for Distributed Wireless Ad Hoc Sensor Networks
The performance of collaborative beamforming is analyzed using the theory of
random arrays. The statistical average and distribution of the beampattern of
randomly generated phased arrays is derived in the framework of wireless ad hoc
sensor networks. Each sensor node is assumed to have a single isotropic antenna
and nodes in the cluster collaboratively transmit the signal such that the
signal in the target direction is coherently added in the far- eld region. It
is shown that with N sensor nodes uniformly distributed over a disk, the
directivity can approach N, provided that the nodes are located sparsely
enough. The distribution of the maximum sidelobe peak is also studied. With the
application to ad hoc networks in mind, two scenarios, closed-loop and
open-loop, are considered. Associated with these scenarios, the effects of
phase jitter and location estimation errors on the average beampattern are also
analyzed.Comment: To appear in the IEEE Transactions on Signal Processin
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