2,109 research outputs found
An overview of laser surface modification of die steels
In recent years, surface modification using advanced heat source like laser has been
replacing the conventional methods to produce amorphous microstructure via rapid
solidification. Due to the benefits of laser to enhance the tribological and mechanical properties
of materialsâ surface, several laser surface processing were developed including laser surface
modification, namely laser alloying, transformation hardening, surface amorphization, shock
hardening and glazing. In high temperature applications, the laser surface modification technique
is beneficial to prolong the die life cycle, and also to improve the surface roughness of thermal
barrier coatings (TBC). To produce the amorphous layer at a particular depth, laser parameter
such as irradiance, frequency, and exposure time are controlled. Variations of parameter may
result in modified microhardness properties of heat affected zone and transition zone.
Nevertheless, works on laser glazing of bearings, railroad rails and TBC had proven the surface
properties were enhanced through laser glazing to cope with excessive load, wear, fatigue,
bending and friction demand
Thermal stability of laser treated die material for semi-solid metal forming
This paper presents laser surface modification work performed to improve the lifetime of die materials. Die material AISI H13, with typical hardness in the range of 42 to 48 HRC, offers high wear and corrosion resistance. However the cyclic high temperature conditions along with exposure to high viscosity molten metal in semi-solid forming cause the die to wear and crack with resultant shortened die lifetime. In this study, the thermal stability of die material at elevated temperature was investigated through micro-hardness testing and a metallographic study. AISI H13 samples were laser glazed using CO2 continuous wave mode laser with 10.6 ÎŒm wavelength. Samples were attached to a specially designed rotating chuck to enable it to be rotated at speeds up to 1500 rpm and allow flat surface glazing to take place. The micro-hardness was measured for as-glazed samples and annealed samples which were held at temperatures ranging from 550oC to 800oC with 50oC intervals. The metallographic study conducted examined the formation of three zones at different depths which were the glazed zone, the heat affected zone and the substrate. As a result of rapid heating and cooling from the laser glazing process, a metallic glass layer was developed which exhibited an average micro-hardness of 900 HV when exposed to 3.34E+10 W/m2 laser irradiance within a range of 0.0011 to 0.0018 s exposure time. Crystallization in glazed zone increased as the annealing temperature increased. As the annealing temperature reached above approximately 600oC, the micro-hardness decreased to approximately 600 HV (equivalent to approx. 54 HRC) due to local crystallization. These findings show potential direct application of glazed dies for non-ferrous semi-solid forming and the requirement for thermal barrier protection for application at higher temperatures
Effect of nearest neighbor repulsion on the low frequency phase diagram of a quarter-filled Hubbard-Holstein chain
We have studied the influence of nearest-neighbor (NN) repulsion on the low
frequency phase diagram of a quarter-filled Hubbard-Holstein chain. The NN
repulsion term induces the apparition of two new long range ordered phases (one
CDW for positive and one CDW for
negative ) that did not exist in the V=0 phase diagram. These results
are put into perspective with the newly observed charge ordered phases in
organic conductors and an interpretation of their origin in terms of
electron-molecular vibration coupling is suggested.Comment: 10 pages, 10 figure
Skew-Unfolding the Skorokhod Reflection of a Continuous Semimartingale
The Skorokhod reflection of a continuous semimartingale is unfolded, in a
possibly skewed manner, into another continuous semimartingale on an enlarged
probability space according to the excursion-theoretic methodology of Prokaj
(2009). This is done in terms of a skew version of the Tanaka equation, whose
properties are studied in some detail. The result is used to construct a system
of two diffusive particles with rank-based characteristics and skew-elastic
collisions. Unfoldings of conventional reflections are also discussed, as are
examples involving skew Brownian Motions and skew Bessel processes.Comment: 20 pages. typos corrected, added a remark after Proposition 2.3,
simplified the last part of Example 2.
Automatic eduction and statistical analysis of coherent structures in the wall region of a confine plane
This paper describes a vortex detection algorithm used to expose and statistically characterize the
coherent flow patterns observable in the velocity vector fields measured by Particle Image
Velocimetry (PIV) in the impingement region of air curtains. The philosophy and the architecture of
this algorithm are presented. Its strengths and weaknesses are discussed. The results of a
parametrical analysis performed to assess the variability of the response of our algorithm to the 3
user-specified parameters in our eduction scheme are reviewed. The technique is illustrated in the
case of a plane turbulent impinging twin-jet with an opening ratio of 10. The corresponding jet
Reynolds number, based on the initial mean flow velocity U0 and the jet width e, is 14000. The
results of a statistical analysis of the size, shape, spatial distribution and energetic content of the
coherent eddy structures detected in the impingement region of this test flow are provided.
Although many questions remain open, new insights into the way these structures might form,
organize and evolve are given. Relevant results provide an original picture of the plane turbulent
impinging jet
Reading ecosystem services at the local scale through a territorial approach: The case of peri-urban agriculture in the thau Lagoon, Southern France
© 2015 by the author(s). In recent years, the ecosystem services (ES) concept has become a major paradigm for natural resource management. While policy-makers demand âhardâ monetary evidence that nature conservation would be worth investing in, ongoing attempts are being made to formalize the concept as a scientifically robust âone size fits allâ analytical framework. These attempts have highlighted several major limitations of the ES concept. First, to date, the concept has paid little attention to the role of humans in the production of ES. Second, the ongoing formalization of the ES concept is turning it into a âtechnology of globalization,â thereby increasingly ignoring the socio-cultural context and history within which ecosystems emerge. Third, economic valuation has been shown to limit local stakeholders in expressing their daily and immediate ways of interacting with their environment over and beyond extrinsic motivation provided by financial gains. We address these three limitations by analyzing a social evaluation of the roles of peri-urban farmland from a territorial perspective. Our case study is the Thau lagoon in southern France. We conducted in-depth interviews with a broad range of stakeholders and ran two participatory workshops. Using a territorial meta-model that distinguishes three levelsâ physical, logical, and existentialâstakeholder data were analyzed to unravel the interplay of territorial elements at these three levels that gives rise to ES in two broad categories: food production and aesthetic landscape. The coupling of ES and territory concepts opens up several novel analytical perspectives. It allows partitioning of ES in a manner that âre-contextualizesâ them and gives insight about both their physical constituents and their meaning at the territorial level. Additional research should incorporate the dynamics of service demand and supply, and further investigate options for implementation
A Comparison of Accuracy of Image- versus Hardware-based Tracking Technologies in 3D Fusion in Aortic Endografting
OBJECTIVES: Fusion of three-dimensional (3D) computed tomography and intraoperative two-dimensional imaging in endovascular surgery relies on manual rigid co-registration of bony landmarks and tracking of hardware to provide a 3D overlay (hardware-based tracking, HWT). An alternative technique (image-based tracking, IMT) uses image recognition to register and place the fusion mask. We present preliminary experience with an agnostic fusion technology that uses IMT, with the aim of comparing the accuracy of overlay for this technology with HWT. METHOD: Data were collected prospectively for 12 patients. All devices were deployed using both IMT and HWT fusion assistance concurrently. Postoperative analysis of both systems was performed by three blinded expert observers, from selected time-points during the procedures, using the displacement of fusion rings, the overlay of vascular markings and the true ostia of renal arteries. The Mean overlay error and the deviation from mean error was derived using image analysis software. Comparison of the mean overlay error was made between IMT and HWT. The validity of the point-picking technique was assessed. RESULTS: IMT was successful in all of the first 12 cases, whereas technical learning curve challenges thwarted HWT in four cases. When independent operators assessed the degree of accuracy of the overlay, the median error for IMT was 3.9 mm (IQR 2.89-6.24, max 9.5) versus 8.64 mm (IQR 6.1-16.8, max 24.5) for HWT (p = .001). Variance per observer was 0.69 mm(2) and 95% limit of agreement ±1.63. CONCLUSION: In this preliminary study, the error of magnitude of displacement from the "true anatomy" during image overlay in IMT was less than for HWT. This confirms that ongoing manual re-registration, as recommended by the manufacturer, should be performed for HWT systems to maintain accuracy. The error in position of the fusion markers for IMT was consistent, thus may be considered predictable
Creep stability of the proposed AIDA mission target 65803 Didymos: I. Discrete cohesionless granular physics model
As the target of the proposed Asteroid Impact & Deflection Assessment (AIDA)
mission, the near-Earth binary asteroid 65803 Didymos represents a special
class of binary asteroids, those whose primaries are at risk of rotational
disruption. To gain a better understanding of these binary systems and to
support the AIDA mission, this paper investigates the creep stability of the
Didymos primary by representing it as a cohesionless self-gravitating granular
aggregate subject to rotational acceleration. To achieve this goal, a
soft-sphere discrete element model (SSDEM) capable of simulating granular
systems in quasi-static states is implemented and a quasi-static spin-up
procedure is carried out. We devise three critical spin limits for the
simulated aggregates to indicate their critical states triggered by reshaping
and surface shedding, internal structural deformation, and shear failure,
respectively. The failure condition and mode, and shear strength of an
aggregate can all be inferred from the three critical spin limits. The effects
of arrangement and size distribution of constituent particles, bulk density,
spin-up path, and interparticle friction are numerically explored. The results
show that the shear strength of a spinning self-gravitating aggregate depends
strongly on both its internal configuration and material parameters, while its
failure mode and mechanism are mainly affected by its internal configuration.
Additionally, this study provides some constraints on the possible physical
properties of the Didymos primary based on observational data and proposes a
plausible formation mechanism for this binary system. With a bulk density
consistent with observational uncertainty and close to the maximum density
allowed for the asteroid, the Didymos primary in certain configurations can
remain geo-statically stable without including cohesion.Comment: 66 pages, 24 figures, submitted to Icarus on 25/Aug/201
Compiling Linguistic Constraints into Finite State Automata
International audienceThis paper deals with linguistic constraints encoded in the form of (binary) tables, generally called lexicon-grammar tables. We describe a unified method to compile sets of tables of linguistic constraints into Finite State Automata. This method has been practically implemented in the linguistic platform Unitex
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