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Time scale in concrete fracture: a model based on partitions of unity
Intense research efforts have been put in the formulation of theories for crack initiation and propagation in concrete. Yet, little attention has been paid to the time-dependence of fracture, despite evidence of its significance. This paper reports a finite element model which captures the time scale in concrete fracture. Visco-elasticity is employed to capture bulk creep. In the fracture process zone a different time scale acts. Therefore, a rate-dependent cracking resistance is modelled. A recently developed finite element method for modelling cohesive cracks is employed. It is based on partitions of unity, by which means displacement jumps are introduced independently of the mesh structure. This avoids the requirement of dense meshes by regularised continuum approaches to model localisation, and a priori knowledge of where cracks occur for standard discrete cracking approaches via interfaces
Pressure induced Raman and fluorescence singularities in
The pressure effect on the fluoride scheelite laser host is studied
at room temperature up to 26 GPa by Raman scattering and up to 40 GPa by
fluorescence of doped sample. The Raman spectra exhibit three
singularities at the vicinity of 6 GPa, 10-12 GPa and 16-17 GPa. The samples
pressurized to 21 GPa or higher do not recover the original phase after being
released, giving more Raman lines than original samples. The luminescence
spectra of are collected in the energy range corresponding to
following transitions , and
. Singularities are observed in the vicinity of 6 GPa, 10 GPa, 16
GPa, 23 GPa in agreement with the Raman study. Moreover, an irreversible
transition occurs at 23 GPa. The samples pressurized to above 26 GPa become
amorphous when released and all the sharp lines disappear. Above 31 GPa, the
spectra at high pressures show only some broad bands corresponding to
transitions between two multiplets of the configuration of .
These singularities suggest possible phase transformations leading to lowering
of the lattice symmetry.Comment: 12 pages, 13 figures, 2 table, LaTe
Studies on the Weak Itinerant Ferromagnet SrRuO3 under High Pressure to 34 GPa
The dependence of the Curie temperature Tc on nearly hydrostatic pressure has
been determined to 17.2 GPa for the weak itinerant ferromagnetic SrRuO3 in both
polycrystalline and single-crystalline form. Tc is found to decrease under
pressure from 162 K to 42.7 K at 17.2 GPa in nearly linear fashion at the rate
dTc/dP = -6.8 K/GPa. No superconductivity was found above 4 K in the pressure
range 17 to 34 GPa. Room-temperature X-ray diffraction studies to 25.3 GPa
reveal no structural phase transition but indicate that the average Ru-O-Ru
bond angle passes through a minimum near 15 GPa. The bulk modulus and its
pressure derivative were determined to be B =192(3) GPa and B' = 5.0(3),
respectively. Parallel ac susceptibility studies on polycrystalline CaRuO3 at 6
and 8 GPa pressure found no evidence for either ferromagnetism or
superconductivity above 4 K
Tin-selenium compounds at ambient and high pressures
SnxSey crystalline compounds consisting of Sn and Se atoms of varying
composition are systematically investigated at pressures from 0 to 100 GPa
using the first-principles evolutionary crystal structure search method based
on density functional theory (DFT). All known experimental phases of SnSe and
SnSe2 are found without any prior input. A second order polymorphic phase
transition from SnSe-Pnma phase to SnSe-Cmcm phase is predicted at 2.5 GPa.
Initially being semiconducting, this phase becomes metallic at 7.3 GPa. Upon
further increase of pressure up to 36.6 GPa, SnSe-Cmcm phase is transformed to
CsCl-type SnSe-Pm3m phase, which remains stable at even higher pressures. A
metallic compound with different stoichiometry, Sn3Se4-I43d, is found to be
thermodynamically stable from 18 GPa to 70 GPa. Known semiconductor tin
diselenide SnSe2-P3m1 phase is found to be thermodynamically stable from
ambient pressure up to 18 GPa. Initially being semiconducting, it experiences
metalization at pressures above 8 GPa
Studies of superconductivity and structure for CaC6 to pressures above 15 GPa
The dependence of the superconducting transition temperature Tc of CaC6 has
been determined as a function of hydrostatic pressure in both helium-loaded gas
and diamond-anvil cells to 0.6 and 32 GPa, respectively. Following an initial
increase at the rate +0.39(1) K/GPa, Tc drops abruptly from 15 K to 4 K at 10
GPa. Synchrotron x-ray measurements to 15 GPa point to a structural transition
near 10 GPa from a rhombohedral to a higher symmetry phase
Post-spinel transformations and equation of state in ZnGa2O4: Determination at high-pressure by in situ x-ray diffraction
Room temperature angle-dispersive x-ray diffraction measurements on spinel
ZnGa2O4 up to 56 GPa show evidence of two structural phase transformations. At
31.2 GPa, ZnGa2O4 undergoes a transition from the cubic spinel structure to a
tetragonal spinel structure similar to that of ZnMn2O4. At 55 GPa, a second
transition to the orthorhombic marokite structure (CaMn2O4-type) takes place.
The equation of state of cubic spinel ZnGa2O4 is determined: V0 = 580.1(9) A3,
B0 = 233(8) GPa, B0'= 8.3(4), and B0''= -0.1145 GPa-1 (implied value); showing
that ZnGa2O4 is one of the less compressible spinels studied to date. For the
tetragonal structure an equation of state is also determined: V0 = 257.8(9) A3,
B0 = 257(11) GPa, B0'= 7.5(6), and B0''= -0.0764 GPa-1 (implied value). The
reported structural sequence coincides with that found in NiMn2O4 and MgMn2O4.Comment: 20 pages, 4 figures, 2 Table
Dependence of the Superconducting Transition Temperature of MgB2 on Pressure to 20 GPa
The dependence of Tc on nearly hydrostatic pressure has been measured for an
isotopically pure (11B) MgB2 sample in a helium-loaded diamond-anvil-cell to
nearly 20 GPa. Tc decreases monotonically with pressure from 39.1 K at ambient
pressure to 20.9 K at 19.2 GPa. The initial dependence is the same as that
obtained earlier (dTc/dP = -1.11(2) K/GPa) on the same sample in a He-gas
apparatus to 0.7 GPa. The observed pressure dependence Tc(P) to 20 GPa can be
readily described in terms of simple lattice stiffening within standard
phonon-mediated BCS superconductivity.Comment: 9 pages, 3 figure
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