79 research outputs found
Mechanical properties of three-dimensional interconnected alumina/steel metal matrix composites
Three-dimensional interconnected alumina/steel metal matrix composites (MMCs) were produced by pressureless Ti-activated melt infiltration method using three types of Al2O3 powder with different sizes and shapes. By partial sintering during infiltration an interpenetrating ceramic network was realised. The effect of the ceramic particle size and shape on the resulting ceramic network, volume % fraction and the MMC properties is presented. The MMCs were characterised for mechanical properties at room temperature and elevated temperature. An increase in flexural strength and Young's modulus with decreasing particle size has been observed. In addition, the effect of the volume of ceramic content and the surface finish of the MMCs on the wear behaviour is show
Buckling and crush resistance of high-density TRIP-steel and TRIP-matrix composite honeycombs to out-of-plane compressive load
AbstractThe mechanical and structural responses of high-density TRIP steel and TRIP-steel/zirconia composite honeycomb structures were studied under uniaxial compression in the out-of-plane loading direction over a wide range of strain rates. Their mechanical response, buckling, and failure mechanisms differ considerably from those of conventional thin-walled, low-density cellular structures. Following the linear-elastic regime and the yield limit of the bulk material, the high-density square honeycombs exhibited a uniform increase in compression stress over an extended range of (stable) plastic deformation. This plastic pre-buckling stage with axial crushing of cell walls correlates with the uniaxial compressive response of the bulk specimens tested. The dominating material effects were the pronounced strain hardening of the austenitic steel matrix accompanied by a strain-induced α’-martensite nucleation (TRIP effect) and the strengthening effect due to the zirconia particle reinforcement. The onset of critical plastic bifurcation was initiated at high compressive loads governed by local or global cell wall deflections. After exceeding the compressive peak stress (maximum loading limit), the honeycombs underwent either a continuous post-buckling mode with a folding collapse (lower relative density) or a symmetric extensional collapse mode of the entire frame (high relative density). The densification strain and the post-buckling or plateau stress were determined by the energy efficiency method. Apart from relative density, the crush resistance and deformability of the honeycombs were highly influenced by the microstructure and damage evolution in the cell walls as well as the bulk material’s strain-rate sensitivity. A significant increase in strain rate against quasi-static loading resulted in a measured enhancement of deformation temperature associated with material softening. As a consequence, the compressive peak stress and the plastic failure strain at the beginning of post-buckling showed an anomaly with respect to strain rate indicated by minimum values under medium loading-rate conditions. The development of the temperature gradient in the stable pre-buckling stage could be predicted well by a known constitutive model for quasi-adiabatic heating
Neutrinos with Mixing in Twisting Magnetic Fields
Transitions in a system of neutrinos with vacuum mixing and magnetic moments,
propagating in matter and transverse magnetic field, are considered. It is
shown that in the realistic case of magnetic field direction varying along the
neutrino path qualitatively new phenomena become possible: permutation of
neutrino conversion resonances, appearance of resonances in the
neutrino-antineutrino () transition
channels, neutrino-antineutrino resonant conversion, large amplitude
oscillations, merging of different
resonances (triple resonances). Possible phenomenological implications of these
effects are briefly discussed.Comment: LaTeX, 35 pages, 4 figures (not included but available upon request).
In memoriam of Ya.A. Smorodinsky. SISSA-170/92/E
The Sum over Topologies in Three-Dimensional Euclidean Quantum Gravity
In Hawking's Euclidean path integral approach to quantum gravity, the
partition function is computed by summing contributions from all possible
topologies. The behavior such a sum can be estimated in three spacetime
dimensions in the limit of small cosmological constant. The sum over topologies
diverges for either sign of , but for dramatically different reasons:
for , the divergent behavior comes from the contributions of very
low volume, topologically complex manifolds, while for it is a
consequence of the existence of infinite sequences of relatively high volume
manifolds with converging geometries. Possible implications for
four-dimensional quantum gravity are discussed.Comment: 12 pages (LaTeX), UCD-92-1
New Jacobi-Like Identities for Z_k Parafermion Characters
We state and prove various new identities involving the Z_K parafermion
characters (or level-K string functions) for the cases K=4, K=8, and K=16.
These identities fall into three classes: identities in the first class are
generalizations of the famous Jacobi theta-function identity (which is the K=2
special case), identities in another class relate the level K>2 characters to
the Dedekind eta-function, and identities in a third class relate the K>2
characters to the Jacobi theta-functions. These identities play a crucial role
in the interpretation of fractional superstring spectra by indicating spacetime
supersymmetry and aiding in the identification of the spacetime spin and
statistics of fractional superstring states.Comment: 72 pages (or 78/2 = 39 pages in reduced format
Properties of 3-manifolds for relativists
In canonical quantum gravity certain topological properties of 3-manifolds
are of interest. This article gives an account of those properties which have
so far received sufficient attention, especially those concerning the
diffeomorphism groups of 3-manifolds. We give a summary of these properties and
list some old and new results concerning them. The appendix contains a
discussion of the group of large diffeomorphisms of the -handle 3-manifold.Comment: 20 pages. Plain-TeX, no figures, 1 Table (A4 format
Cyclic Statistics In Three Dimensions
While 2-dimensional quantum systems are known to exhibit non-permutation,
braid group statistics, it is widely expected that quantum statistics in
3-dimensions is solely determined by representations of the permutation group.
This expectation is false for certain 3-dimensional systems, as was shown by
the authors of ref. [1,2,3]. In this work we demonstrate the existence of
``cyclic'', or , {\it non-permutation group} statistics for a system of n
> 2 identical, unknotted rings embedded in . We make crucial use of a
theorem due to Goldsmith in conjunction with the so called Fuchs-Rabinovitch
relations for the automorphisms of the free product group on n elements.Comment: 13 pages, 1 figure, LaTex, minor page reformattin
Synthesis of niobium-alumina composite aggregates and their application in coarse-grained refractory ceramic-metal castables
Niobium-alumina aggregate fractions with particle sizes up to 3150 µm were produced by crushing pre-synthesised fine-grained composites. Phase separation with niobium enrichment in the aggregate class 45–500 µm was revealed by XRD/Rietveld analysis. To reduce the amount of carbon-based impurities, no organic additives were used for the castable mixtures, which resulted in water demands of approximately 27 vol.% for the fine- and coarse-grained castables. As a consequence, open porosities of 18% and 30% were determined for the fine- and coarse-grained composites, respectively. Due to increased porosity, the modulus of rupture at room temperature decreased from 52 MPa for the fine-grained composite to 11 MPa for the coarse-grained one. However, even the compressive yield strength decreased from 49 MPa to 18 MPa at 1300 °C for the fine-grained to the coarse-grained composite, the latter showed still plasticity with a strain up to 5%. The electrical conductivity of fine-grained composite samples was in the range between 40 and 60 S/cm, which is fifteen magnitudes above the values of pure corundum
Steel ceramic composite anodes based on recycled MgO–C lining bricks for applications in cryolite/aluminum melts
Novel manufacturing route for composite inert anodes containing 60:40 of 316 L stainless steel and MgO powder obtained from recycled MgO–C brick material has been developed and evaluated. After burnout of residual carbon from the recycled MgO–C powder, MgO and steel were granulated and pre-sintered in order to generate agglomerates of composite material acting as coarse grains within the composite material, and thus lowering the sintering-related shrinkage. The pre-sintered granules were mixed with raw steel and MgO powder in order to achieve a high particle packing and subsequently cold isostatically pressed in the form of electrodes. All manufactured anode samples were subjected to sintering at 1350 °C and pre-oxidation at different temperatures – 800 °C, 900 °C, and 1000 °C. Afterwards, mechanical and electrical properties of the manufactured electrodes were characterized. The results show that upcycling of the MgO–C material enables manufacturing of sophisticated electrode products, which can be applied in the aluminum industry
Low-density, one-dimensional quantum gases in a split trap
We investigate degenerate quantum gases in one dimension trapped in a
harmonic potential that is split in the centre by a pointlike potential. Since
the single particle eigenfunctions of such a system are known for all strengths
of the central potential, the dynamics for non-interacting fermionic gases and
low-density, strongly interacting bosonic gases can be investigated exactly
using the Fermi-Bose mapping theorem. We calculate the exact many-particle
ground-state wave-functions for both particle species, investigate soliton-like
solutions, and compare the bosonic system to the well-known physics of Bose
gases described by the Gross-Pitaevskii equation. We also address the
experimentally important questions of creation and detection of such states.Comment: 7 pages, 5 figure
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