The fatigue limits and fracture characteristics for three Pd–Ag dental casting alloys (Super Star, Heraeus Kulzer; Rx 91,
Pentron; W-1, Ivoclar Vivadent) were studied. Specimens meeting the dimensions for ADA Specifications No. 5 and 38, and having
the as-cast surface condition, were subjected to heat treatment simulating dental porcelain firing cycles and fatigued in
air at room temperature under uniaxial tension-compression at 10 Hz. A ratio of compressive stress amplitude to tensile stress
amplitude (R-ratio) of −1 was used. Alloy microstructures and fracture surfaces were examined with a scanning electron microscope
and a transmission electron microscope. Fatigue limits for the three alloys had low values of approximately 15% of the yield
strength for 0.2% permanent tensile strain. Complex fracture surfaces with characteristic striations were observed for all
three fatigued alloys. Planar slip of dislocations occurred in the Pd solid solution matrix, along with dislocation-precipitate
interactions and dislocation networks in the interfaces between the precipitates and surrounding matrix. Twinning occurred
in the Pd solid solution matrix of Rx 91, and within discontinuous precipitates in Super Star and Rx 91. The low fatigue limits
for these alloys are attributed to their complex microstructures and perhaps to casting defects. 相似文献
This paper presents a numerical study on the enhancement of mixing within a micro cavity by use of transient induced-charge
electroosmotic flows created by a periodic on-off-switching of the electric field applied across a pair of electrodes. For
the first time, we demonstrate that the electro-osmotic flows observed during the charging and decharging stages are not symmetric
with respect to each other and can lead to a chaotic mixing. Lyapunov exponents and particle scattering patterns are obtained
for the symptom of the chaotic advection at various external fields. 相似文献
In this work, a hybrid method based on the lattice Boltzmann method (LBM) combined with smoothed profile method (SPM) is proposed for simulating the fluid flow and heat transfer in particulate systems. LBM is used to evaluate the flow field, while the temperature distribution is computed by a finite difference method. The no-slip and constant temperature boundary conditions on particle surfaces are treated with SPM. The following benchmarking studies are considered to validate the accuracy and robustness of the proposed method: natural convection in a square cavity with a heated circular cylinder, natural convection in a concentric annulus, and the sedimentation of single and two circular particles in a long narrow container. In all the studies, the results obtained from the present technique based on hybrid method show good agreement with the previously published results and those given by LBM combined with SPM based on the double-population model. After evaluating the central processing unit time taken by the two methods, it was found that the proposed method is about 50% more computationally efficient than the method based on double-population model in all the simulation cases considered in this work. 相似文献
The goals of this study were to investigate the fatigue limits of two Pd–Ag alloys (Ivoclar Vivadent) with differing mechanical
properties and varying proportions of secondary alloying elements, examine the effect of casting porosity on fatigue behavior,
and determine the effect of casting size on microstructures and Vickers hardness. The alloys selected were: IPS d.SIGN 59
(59.2Pd–27.9Ag–8.2Sn–2.7In–1.3Zn); and IS 64 (59.9Pd–26.0Ag–7.0Sn–2.8Au–1.8 Ga–1.5In–1.0Pt). Tension test bars, heat-treated
to simulate dental porcelain application, were subjected to cyclic loading at 10 Hz, with R-ratio of −1 for amplitudes of
compressive and tensile stress. Two replicate specimens were tested at each stress amplitude. Fracture surfaces were examined
with a scanning electron microscope (SEM). Sectioned fatigue specimens and additional cast specimens simulating copings for
a maxillary central incisor restoration were also examined with the SEM, and Vickers hardness was measured using 1 kg load.
Casting porosity was evaluated in sectioned fatigue fracture specimens, using an image analysis program. The fatigue limit
(2 × 106 loading cycles) of IS 64 was approximately 0.20 of its 0.2% yield strength, while the fatigue limit of d.SIGN 59 was approximately
0.25 of its 0.2% yield strength. These relatively low ratios of fatigue limit to 0.2% yield strength are similar to those
found previously for high-palladium dental alloys, and are attributed to their complex microstructures and casting porosity.
Complex fatigue fracture surfaces with striations were observed for both alloys. Substantial further decrease in the number
of cycles for fatigue failure only occurred when the pore size and volume percentage became excessive. While the heat-treated
alloys had equiaxed grains with precipitates, the microstructural homogenization resulting from simulated porcelain firing
differed considerably for the coping and fatigue test specimens; the latter specimens had significantly higher values of Vickers
hardness. 相似文献
In this paper, low profile cavity-backed cross shaped slot antenna using substrate integrated waveguide (SIW) technology is proposed for obtaining quad-band response. In order to generate four distinct frequencies, a crossed slot is etched on the back of the dielectric substrate and three balanced shorting vias (metallic vias) are included across the longitudinal slot. Furthermore, the electric field distribution has been utilized for understanding the working principle of the proposed design. Balanced shorting vias modify the current distributions of the cavity, which consequently produces two resonant frequencies in X-band and two more resonant frequencies in Ku-band. Finally, the proposed quad-band antenna is fabricated using printed circuit board (PCB) technology and the results are experimentally verified. With the height of 0.026λ0, the measured results show that the proposed design resonates at 8.6 , 10.2 , 13.2 , and 14.8 GHz and exhibits a –10 dB impedance bandwidth more than 150 MHz at corresponding frequencies. The proposed antenna attains a peak gain of more than 6 dBi at the corresponding resonant frequencies, respectively. Those amicable characteristics, e.g., low profile, uni-directional radiation pattern, ease of fabrication, quad-band response and moderate gain, make the proposed antenna suitable for both radar and satellite systems.
We conducted a numerical study on the onset of electro-convection as well as the complex flow phenomena of dielectric liquid subjected to unipolar autonomous charge injection in the annular gap between two concentric circular cylindrical electrodes. The Nernst-Planck equations governing the charge density transport, the Poisson equation for the electric potential and the Navier-Stokes equations for the fluid flow are solved numerically using the finite volume method. The developed code is validated by comparing the critical stability parameter values for the onset of electro-convection with those obtained from the linear stability analysis. We identify in a parameter space the stable hydrostatic state and the electro-convection state. The electro-convection is again divided into three regimes: stationary, oscillatory and chaotic. For inner cylinder radius ri ≥ 1.0, we observed an increase in the number of charged plumes and vortex pairs with stability parameter T before the electro-convection becomes chaotic. For outer injection, although the onset of electroconvection starts at T higher than the inner injection, the onset of chaotic motion occurs at lower T. 相似文献
We present a numerical simulation of two-phase flow in a three-dimensional cross-junction microchannel by using the lattice
Boltzmann method (LBM). At first, we validated our LBM code with the velocity profile in a 3-dimensional rectangular channel.
Then, we developed a lattice Boltzmann code based on the free energy model to simulate the immiscible binary fluid flow. The
parallelization of the developed code is implemented on a PC cluster using the MPI program. The numerical results of two-phase
flow in the microchannel reveal droplet formation process, which compares well with corresponding experimental results. The
size of droplet decreases with increase of the flow-rate ratio and the capillary number. The movement of a droplet through
the microchannel induces three-dimensional circulating flow inside the droplet. This complex flow is thought to enhance the
mixing and reaction of reagents. 相似文献