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The high conductivity of the Earth's core discovered through first-principles and experimental studies requires that the core must start very hot and cool down slowly to generate the Earth's magnetic field by thermal buoyancy. The requirement is difficult to satisfy due to the fast cooling of the overlying magma ocean and consequently of the underlying core. This is in direct conflict with the early appearance of the Earth's paleomagnetic field. Recently, it was proposed that significant amount of magnesium (Mg) can be partitioned into the core through the high temperature created by the Moon-forming Giant Impact. Due to its intrinsic low solubility, subsequent cooling would cause Mg precipitation to generate compositional buoyancy to power the geodynamo in the early history of the Earth. Here we show using first-principles molecular dynamics simulations that the equilibrium constant of magnesium dissolution in molten iron depends on temperature, entirely consistent with recent experimental data. We further show that Mg partitioned into the core during giant impacts and reaching a concentration of about 2 wt% can precipitate out at around 3.5 Ga, much earlier than the onset of inner core nucleation. During the subsequent evolution of the Earth, silicon (Si) concentration of the Earth's core will remain constant while Mg and oxygen (O) concentrations decrease significantly. Consequently, the current Si concentration in the core reflects the accretion processes of the Earth while O and Mg concentrations in the core is the combined result of both accretion and the subsequent evolution of the Earth core. Forward modeling shows that for MgO precipitation to provide enough power to generate the magnetic field in the early history of the Earth, initially high silicon content of the core is preferred, which is accommodated readily in the Grand Tack accretion scenario. The geodynamo driven by MgO precipitation explains the secular decline of palaeomagnetic field intensity in the early history of the Earth.

In this paper, a software–hardware mixed design of synchronization and triggering system for multi-functional LXI instruments is proposed. A solution based on open-source software PTPd and DP83640 is introduced and the synchronous deviation can be limited to ±20 ns. And the trigger state machine is implemented in a FPGA while trigger service requests and trigger resources are processed or managed by Linux application programs. Equipped with IVI-COM programming interface and Lua scripting platform, the design can be used to build an efficient and interchangeable test system. In addition, an event log mechanism is also provided to assist fault analysis. Test results indicate that the design strictly conforms to the IviLxiSync Specification, and the precision of synchronization and triggering system satisfies the recommendations of LXI Device Specification.

Quantity determination of the Aircraft spares is affected by many factors, the indeterminate effect is not quantitative factors on the quantity of spare parts, analytic hierarchy process is applied to the quantity calculation of the aircraft spares. General principles of administrative levels to analyze law dealing with the aircraft spares calculation of multi factor problem is introduced, 5 influence factors is economy, importance, difficulty degree, affected, string etc. The use of analytic hierarchy process used in random spare parts calculation is illustrated.

In the field of coming war, integrated logistics support for military equipment is a asked high-reliability system engineering. Based on the theory of Man-Machine-Environment reliability, the paper set forth the significance and conception of the question discussed in the paper. A discussion was given to get over the present challenge of reliability engineering and To ensure the reliability and the trend of the research, the basic direction of improving reliability are introduced in detail are introduced.

With the continuous development of national economy and the construction technology, a lot of large-span spatial structures with unique form and complex construction are emerging. The health monitoring system that mainly used in bridge structure is applied in large-span spatial structures gradually. Parameter identification and damage alarming are the core technology of health monitoring. In this paper some structural modal parameter identification methods based on ambient excitation are introduced. Finally, for large span structures with characteristics of low-intensive modes, the response data based on large span structures for modal parameter identification method was summed up, and the critical issues for further research in this field are also presented.

The discrete finite element (FE) model often cannot reflect structure characteristics accurately due to imply more idealistic assumptions and simplifications. Therefore, it is necessary to update FE model for structural damage identification, response calculation, safety evaluation, optimization design, and so on. This article will illustrate respectively three key steps of updating parameters selection, target function selection and optimization method in process of dynamic FE model updating of footbridge structures based on ambient excitation, and put forward a feasible updating method: combine empirical method with sensitivity analysis method to select updating parameters; joint natural frequencies, MAC and modal flexibility as target function; adopt optimization algorithm based on the optimization theory.

Construction control is the key technique for cable dome. During the actual engineering, however, it is difficult to ensure no difference between the real prestress distribution and the designed value due to existed construction errors. The practical problem for cable dome construction is to determine how much is allowable errors limit. To solve the problem, sensitivity analysis of manufacture errors of bearing and cable length are carried out in this paper. The random errors are simulated by Monte Carlo method. In a numerical model of cable dome with the diameter of 62m, the sensitivity of effect of bearing and cable length manufacture errors on structural performance is analyzed. Numerical results show that the manufacture errors of radial bearing, hoop cable 2, ridge cable 3, diagonal cable 3 and hoop cable 1 should be strictly controlled. Besides, the maximum allowable errors are proposed referring to the domestic existing specifications.

The static stability capacity is the main index to measure the whole mechanical performance of single-layer latticed shells. Three single-layer cylindrical latticed shells with different height to span ratio were modeled, and their seismic damage were assessed by the degradation of static stability capacity incurred by earthquakes. Two different static load patterns were considered: dead load and the combination of dead load and half span live load. The results show that the damage assessment method is applicable to single-layer cylindrical latticed shells, and the static load patterns have no big influence on the damage assessment of the single-layer cylindrical latticed shells.

In order to study the friction contact effect between cable and membrane in cable-membrane structures, a tensioned cable-membrane FE model which has a stable cable (valley cable) is created with FEA software ANSYS. The friction contact between the valley cable and membrane surface is simulated by contact elements in ANSYS, and the effect of friction contact surface is considered by the introduction of friction coefficients. Three different design valley cable forces (15kN, 20kN and 25kN) are considered in form-finding and static analysis, and valley cable force losses are analyzed. Results show that valley cable force loss due to friction contact should be considered obviously when design valley cable force is relatively small.

To analysis progressive collapse behavior of single-layer reticulated shell under severe earthquake, based on spatial fiber beam element failure criteria, a simulation method on progressive collapse of shell is presented, which account of mechanical properties of components after buckling and damage accumulation effect of combine hardening material. And a computation program is developed correspondingly. At last the method and program proves to be applicable by an example.

Elastic constants of akimotoite at high-temperatures and pressures are calculated by using molecular dynamics simulations to study the anisotropic properties of akimotoite. We find that akimotoite becomes more anisotropic with increasing temperature but less with increasing pressure. On the (0 0 1) face, S-wave splitting and azimuthal anisotropy will occur simultaneously while the P-wave velocity remains almost constant. Polarization directions of the two S-waves are neither strictly perpendicular nor parallel to the crystallographic face. The polarization direction of the fast S-wave is more parallel with the plane than that of the slow S-wave for all wave propagation directions. On the (1 0 0) face, no S-wave splitting will occur for waves passing along the crystallographic c-axis, and the P-wave velocity along the axis is smaller than that in other directions. Polarization anisotropy changes its sign with the variation of wave propagation directions, giving rise to the dependence of the relative magnitude of VSH and VSV on the seismic wave propagation directions. We further calculate the anisotropic properties of a mineral assemblage containing ringwoodite, akimotoite, and Ca perovskite to infer the effect that akimotoite may have on the anisotropy of the lower portion of the mantle transition zone. We find that the mineral assemblage is much less anisotropic than akimotoite, but possesses many anisotropy features of akimotoite. Comparison of the anisotropy features with seismological observations indicates that a vertical flow direction is required in areas close to the subduction zones, which may be caused by the downward penetration of slabs to the lower mantle as well as upwelling of fluids and light materials in the mantle wedge associated with deep dehydration reactions of the slabs.

Bulk viscosity is closely related to bulk dissipation of seismic waves. To understand the potential bulk dissipation in the Earth’s outer core, molecular dynamics simulations were conducted in the present study to calculate the bulk viscosity of liquid iron–nickel (Fe–Ni) alloy at the temperatures and pressures of the Earth’s outer core. Results from these simulations showed that bulk viscosities of liquid Fe–Ni alloy are in the order of 10 −3 Pa s. These values are in the same order of magnitude as those of pure liquid iron [Phys. Chem. Minerals 27 (2000) 164], indicating that nickel has a negligible effect on the bulk viscosity of liquid iron. Consequently, the large bulk viscosity of the outer core inferred from seismological studies should be accounted for through concentration fluctuations or other mechanisms with light elements such as sulfur or oxygen involved in [Nature 285 (1980) 204; Science 277 (1997) 219], or using the particle suspension model [J. geophys. Res. 88 (1983) 2445]. In addition, using the activation energy and volume calculated from these bulk viscosity data, it was found that the seismically induced phase change model for a partial melt system [J. Geophys. Res. 73 (1968) 7675] might not be capable to account for the bulk attenuation of the outer core.

The compositions of aqueous fluids in equilibrium with enstatite + forsterite were investigated at temperatures from 900 to 1200 °C and pressures from 1.0 to 2.0 GPa. Experiments, performed in a piston-cylinder apparatus, involved the location of phase boundaries between the stability fields of enstatite and enstatite + forsterite, and enstatite + forsterite and forsterite. The intersection of these two phase-boundaries near the H2O apex was used to define the fluid composition. The results indicated a systematic increase in the concentration of silica in the fluid phase with increasing temperature. The experiments indicated that the concentration of dissolved MgO was below 0.3 mol% and not resolvable using our techniques. This finding was corroborated by microprobe analyses of quench precipitates from the fluid phase, which gave on average 0.2 mol% MgO. Because of the low MgO concentrations, the mean values of the intercepts with SiO2-H2O binary of the fitted lines representing, respectively, the phase boundaries between the enstatite and enstatite + forsterite and between the enstatite + forsterite and forsterite stability fields were taken to represent the concentrations of dissolved silica at the various temperatures and pressures of the present study. The concentrations increased from 0.6 mol% at 900 °C to 3.9 mol% at 1200 °C at 1.0 GPa. The pressure effect from 1.0 to 2.0 GPa at 1000 °C appeared to be minor and not resolvable using our techniques. At 1.0 GPa, the base 10 logarithm of the molal concentration of dissolved silica in equilibrium with enstatite + forsterite was obtained combining data from the present study with those from Nakamura and Kushiro (1978) and Manning and Boettcher (1994): ![Formula][1] Comparison with studies of the solubility of quartz (Manning 1994) indicated that thermodynamic properties of aqueous silica derived from silica-saturated systems may not be applicable to calculations in silica-deficient systems at high pressure. [1]: /embed/tex-math-1.gif

[1] The magma ocean process in the early history of the Earth has a great influence on the light element identities and contents of the core which subsequently affect the energy of the geodynamo provided by the compositional convection and the inner core growth through their effect on the phase diagram of iron alloy. In the present work, a twophase ab-initio molecular dynamics method is established to study the solubility of silicon and oxygen in liquid iron in equilibrium with silicate melt. The ab-initio results are found to be in close agreement with experimental data. At the base of a deep magma ocean (39 GPa and 3116 K), liquid iron contains 2.7 wt% silicon and 0.5 wt% oxygen at the current bulk Earth composition. The oxygen content is low compared with its current estimate in the core, indicating a deeper magma ocean may need to be invoked. Citation: Zhang, Y., and G. Guo (2009), Partitioning of Si and O between liquid iron and silicate melt: A two-phase ab-initio molecular dynamics study, Geophys. Res. Lett., 36, L18305