5,491 research outputs found
Dusty plasma cavities: probe-induced and natural
A comprehensive exploration of regional dust evacuation in complex plasma
crystals is presented. Voids created in 3D crystals on the International Space
Station have provided a rich foundation for experiments, but cavities in dust
crystals formed in ground-based experiments have not received as much
attention. Inside a modified GEC RF cell, a powered vertical probe was used to
clear the central area of a dust crystal, producing a cavity with high
cylindrical symmetry. Cavities generated by three mechanisms are examined.
First, repulsion of micrometer-sized particles by a negatively charged probe is
investigated. A model of this effect developed for a DC plasma is modified and
applied to explain new experimental data in RF plasma. Second, the formation of
natural cavities is surveyed; a radial ion drag proposed to occur due to a
curved sheath is considered in conjunction with thermophoresis and a flattened
confinement potential above the center of the electrode. Finally, cavity
formation unexpectedly occurs upon increasing the probe potential above the
plasma floating potential. The cavities produced by these methods appear
similar, but each are shown to be facilitated by fundamentally different
processes.Comment: 10 pages, 12 figure
On the interpretation of results from small punch creep tests
The small punch creep testing method is highly complex and involves interactions between a number of non-linear processes. The deformed shapes that are produced from such tests are related to the punch and specimen dimensions and to the elastic, plastic, and creep behaviour of the test material, under contact and large deformation conditions, at elevated temperature. Owing to its complex nature, it is difficult to interpret the small punch test creep data in relation to the corresponding uniaxial creep behaviour of the material. One of the aims of this paper is to identify the important characteristics of the creep deformation resulting from ‘localized’ deformations and from the ‘overall’ deformation of the specimen. Following this, the results of approximate analytical and detailed finite element analyses of small punch tests are investigated. It is shown that the regions of the uniaxial creep test curves dominated by primary, secondary, and tertiary creep are not those that are immediately apparent from the displacement versus time records produced during a small punch test. On the basis of the interpretation of the finite element results presented, a method based on a reference stress approach is proposed for interpreting the results of small punch test experimental data. Future work planned for the interpretation of small punch tests data is briefly addressed
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Detection of human influence on a new, validated 1500-Year temperature reconstruction
Climate records over the last millennium place the twentieth-century warming in a longer historical context. Reconstructions of millennial temperatures show a wide range of variability, raising questions about the reliability of currently available reconstruction techniques and the uniqueness of late-twentieth-century warming. A calibration method is suggested that avoids the loss of low-frequency variance. A new reconstruction using this method shows substantial variability over the last 1500 yr. This record is consistent with independent temperature change estimates from borehole geothermal records, compared over the same spatial and temporal domain. The record is also broadly consistent with other recent reconstructions that attempt to fully recover low-frequency climate variability in their central estimate. High variability in reconstructions does not hamper the detection of greenhouse gas-induced climate change, since a substantial fraction of the variance in these reconstructions from the beginning of the analysis in the late thirteenth century to the end of the records can be attributed to external forcing. Results from a detection and attribution analysis show that greenhouse warming is detectable in all analyzed high-variance reconstructions (with the possible exception of one ending in 1925), and that about a third of the warming in the first half of the twentieth century can be attributed to anthropogenic greenhouse gas emissions. The estimated magnitude of the anthropogenic signal is consistent with most of the warming in the second half of the twentieth century being anthropogenic
Cyclic thermomechanical testing of 316 stainless steel
Materials used for components such as power plant steam pipes, gas turbines discs and die forming machinery can be subject to combinations of extreme loading and temperature conditions. In addition, the materials can contain or develop cracks. Once a crack has initiated, the conditions under which the components operate can cause these cracks to propagate. This paper is concerned with the experimental testing of 316 stainless steel, corner cracked samples under thermomechanical fatigue conditions, and the measurement of the crack propagation during testing using alternating current potential difference readings. Fracture mechanics (Paris Law) methods have been used in the processing of the experimental data
Use of small specimen creep data in component life management: a review
Small specimen creep testing techniques are novel mechanical test techniques that have been developed over the past 25 years. They mainly include the sub-size uniaxial test, the small punch creep test, the impression creep test, the small ring creep test and the two-bar creep test. This paper outlines the current methods in practice for data interpretation as well as the state-of-the-art procedures for conducting the tests. Case studies for the use of impression creep testing and material strength ranking of creep resistant steels are reviewed along with the requirement for the standardisation of the impression creep test method. A database of small specimen creep testing is required to prove the validity of the tests
Phase Transitions in a Dusty Plasma with Two Distinct Particle Sizes
In semiconductor manufacturing, contamination due to particulates
significantly decreases the yield and quality of device fabrication, therefore
increasing the cost of production. Dust particle clouds can be found in almost
all plasma processing environments including both plasma etching devices and in
plasma deposition processes. Dust particles suspended within such plasmas will
acquire an electric charge from collisions with free electrons in the plasma.
If the ratio of inter-particle potential energy to the average kinetic energy
is sufficient, the particles will form either a liquid structure with short
range ordering or a crystalline structure with long range ordering. Otherwise,
the dust particle system will remain in a gaseous state. Many experiments have
been conducted over the past decade on such colloidal plasmas to discover the
character of the systems formed, but more work is needed to fully understand
these structures. The preponderance of previous experiments used monodisperse
spheres to form complex plasma systems
Biodiversity studies of fungi on monocotyledonous plants in the tropics
Abstractpublished_or_final_versionThe Joint Annual Meeting of the Mycological Society of America and the American Bryological and Lichenological Society, San Juan, Puerto Rico, USA, 11-16 June 1998, In Inoculum, 1998, v. 49 n. 2, p. 2
Dispersion Relations for Thermally Excited Waves in Plasma Crystals
Thermally excited waves in a Plasma crystal were numerically simulated using
a Box_Tree code. The code is a Barnes_Hut tree code proven effective in
modeling systems composed of large numbers of particles. Interaction between
individual particles was assumed to conform to a Yukawa potential. Particle
charge, mass, density, Debye length and output data intervals are all
adjustable parameters in the code. Employing a Fourier transform on the output
data, dispersion relations for both longitudinal and transverse wave modes were
determined. These were compared with the dispersion relations obtained from
experiment as well as a theory based on a harmonic approximation to the
potential. They were found to agree over a range of 0.9<k<5, where k is the
shielding parameter, defined by the ratio between interparticle distance a and
dust Debye length lD. This is an improvement over experimental data as current
experiments can only verify the theory up to k = 1.5.Comment: 8 pages, Presented at COSPAR '0
Determination of material properties in the Chaboche unified viscoplasticity model
An experimental programme of cyclic mechanical testing of a 316 stainless steel, at temperatures up to 600°C, under isothermal conditions, for the identification of material constitutive constants, has been carried out using a thermo-mechanical fatigue (TMF) test machine with induction coil heating. The constitutive model adopted is a modified Chaboche unified viscoplasticity model, which can deal with both cyclic effects, such as combined isotropic and kinematic hardening, and rate-dependent effects, associated with viscoplasticity. The characterization of 316 stainless steel is presented and compared with results from cyclic isothermal tests. A least-squares optimization algorithm has been developed and implemented for determining the material constants in order to further improve the general fit of the model to experimental data, using the initially obtained material constants as the starting point in this optimization process. The model predictions using both the initial and optimized material constants are compared to experimental data
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