Elasticity of polycrystalline stishovite

Ultrasonic data for the velocities of SiO₂-stishovite have been determined as a function of pressure to 10 kbar at room temperature for polycrystalline specimens hot-pressed at pressures P = 120kbar and temperatures T = 900°C. These cylindrical specimens are 2 mm in diameter and 0.9–1.4 mm long and have a grain size less than 10 μm. Compressional and shear wave velocities were measured both parallel and perpendicular to the axis of pressing and were found to be isotropic at 10 kbar with ν_p = 11.0 ± 0.2km/sec andν_s = 6.9 ± 0.3km/sec; this shear velocit is substantially higher than that of Mizutani et al. (1972) perhaps due to the presence of crack orientations in their specimen which affected ν_s but not ν_p. The Murnaghan P-V trajectories calculated from the ultrasonic data [bulk modulus K_s = 2.5 ± 0.3Mbar and assuming (?K_s/?P)_T = 6 ± 2] are consistent with recent hydrostatic compression data and with the shock wave compression data above 600 kbar. The combined evidence from the data of the ultrasonic and hydrostatic compression techniques suggests that the most probable value of the bulk modulus of stishovite at zero pressure is close to the upper limit of the uncertainty of our ultrasonically determined value, K₀ = 2.7?2.8Mbar. Elasticity data for rutile-type oxides are not compatible with normal K_s-V₀ systematics perhaps due to the neglect of non-central forces in the lattice model. These new stishovite data would make it impossible to satisfy the elasticity-density data of the lower mantle using an oxide mixture with either olivine or pyroxene stoichiometry.     

Thermal diffusivity of stishovite

The thermal diffusivity of synthetic polycrystalline stishovite was determined by the Ångstrom method in the temperature range 300–550 K at 1 atm. The calculated thermal conductivity of stishovite at 300 K is 0.086 W cm^?1 K^?1 which is comparable to that for TiO₂-rutile but much lower than for GeO₂ and SnO₂. The observed thermal conductivities of rutile-structure oxides increase systematically with increasing density, in contrast with the expected behavior for isostructural compounds.     

Thermal expansion of stishovite

The thermal expansion of stishovite has been determined by an X-ray camera technique in a temperature range of 18 – 600°C at an atmospheric pressure. The thermal-expansion coefficients along the crystallographic a- and c-axes at 300 K are α_a = (6.0 ± 0.6) · 10^?6K^?1 and α_c = (1.4 ± 0.5) · 10^?6K^?1, respectively. The volume coefficient at 300 K is α_ν = (13.5 ± 0.6) · 10^?6K^?1.     

Molecular dynamics modeling of stishovite

A Morse-stretch potential charge equilibrium force field for silica system has been employed to simulate the thermodynamics of stishovite with the molecular dynamics (MD) method. The equation of state, thermal expansivity and melting curve of stishovite have been obtained. This simple force field yielded results in accordance with the static and dynamic experiments. The stishovite melting simulation appears to validate the interpretation of superheating of the solid along the Hugoniot in the shock melting experiments. MD simulations show that the thermal expansivity of stishovite at lowermost mantle conditions is a weak function of temperature. The phase diagram of silica up to the mega bar regime is proposed based on the experimental and theoretical studies. The related physical and geophysical implications are addressed.     

Magnetic susceptibility under hydrostatic pressure of synthetic magnetite samples

Artificial specimens containing magnetite of various grain sizes and in different concentrations have been used to study the behaviour of 15 directional susceptibilities under hydrostatic pressure of up to 20 MPa (0.2 kbar). With increasing pressure a moderate but systematic decrease of all directional susceptibilities has been measured. These changes are reversible with respect to the pressure applied. The maximum decrease in susceptibility has not exceeded 3% of the original value, and the differences in pressure sensitivity of magnetite with varying grain size are minimal. In the case of relatively homogeneous transfer of stress across an isotropic matrix, the cause of the susceptibility decrease is probably the decreased mobility of the domain walls under hydrostatic pressure.     

静水条件下振动对测压水位影响实验

将自行设计的井-承压含水层渗流系统置于地震模拟振动台之上,探究静水条件下测压水位的同震响应特征.该实验含12组工况,加载频率f_in分别为0.5,1,2,5,10,15 Hz和加速度a_in分别为1.5,2.5 m/s2的正弦波.研究结果表明:测压水位呈现阶升、阶升—振荡、振荡和阶降—振荡等4种响应变化形态,与天然地震同震响应井水位变化形态具有一定的相似性;12组工况中, 测压水位和孔隙水压力在加载低频(f_in=1,2 Hz)工况时均呈下降变化,中高频(f_in=5,10 Hz)时为上升变化,高频(f_in=15 Hz)时为振荡变化,且加速度峰值也会影响二者的变化形态;测压水位和孔隙水压力的变幅均随输入频率的增大呈先增大后减小的变化规律,其最大变幅分别出现在a_in=1.5 m/s2, f_in=10 Hz和a_in=2.5 m/s2, f_in=5 Hz两种工况中.      

再生混凝土受压循环试验研究

着眼于再生混凝土的应用,为了解再生混凝土与普通混凝土在循环荷载下损伤性能的差别,进行了本次试验研究。采用0%,50%和100%3种取代率,70mm×70mm×148mm棱柱体试件,电-液伺服试验机,等位移速率加载。试验结果表明,相同残余应变时再生混凝土的损伤系数随再生骨料取代率的增加而增加。     

Disproportionation of kyanite to corundum plus stishovite at high pressure and temperature

Natural kyanite (Al₂SiO₅) has been found to disproportionate into a mixture of its component oxides, corundum and stishovite, at a loading pressure of about 160 kbar and temperature between 1000–1400°C in a diamond-anvil press. The exact transition pressure is not certain due to transient increases in pressure during the local and rapid heating by a continuous YAG laser. The phase boundary, however, has been estimated to be P(kbar) = (138 ～ 174) + 0.011 T (°C) on the basis of the available thermodynamic data. The shock-wave Hugoniot data above 650 kbar for andalusite (Al₂SiO₅) and sillimanite (Al₂SiO₅) as starting materials are consistent with the present results.     

砖砌体单轴压缩与拉伸应力应变关系

砖砌体是被广泛应用的建筑材料,大量学者对砖砌体的压缩和拉伸行为特性以及应力-应变关系进行了试验研究。研究发现:砖砌体表现出与混凝土相近的应力-应变关系,为基于混凝土应力-应变关系模型发展砖砌体的应力-应变关系模型提供了基础。对已经提出的几种不同的砖砌体应力-应变关系模型进行了比较,通过比较分析,基于中国混凝土应力-应变关系模型,推荐了可用于估计砖砌体单轴压缩和拉伸载荷下应力-应变曲线的简单分析模型,模型中使用的参数不仅可以很容易地通过实验获得,而且常在规范中被规定使用。     

干砂土高频振动下压缩变形特性试验研究

在自行改装高频振动系统基础上,通过对干砂土进行高频振动压缩室内试验,研究了干砂土变形特性。试验研究表明:在18~27 Hz的频率范围内,塑性压缩变形随着频率的增加而减少速度很快;在27~37 Hz的频率范围内,塑性压缩变形随着频率的增加而减少速度减缓。重点分析了相对密实度、动应力幅值等因素对高频振动下频率与塑性压缩变形之间关系的影响。分析结果表明,虽然在频率较低时,这2个因素对塑性压缩变形影响很大,在高频时,塑性压缩变形对这2个因素不敏感;但是它们并不会改变频率与塑性压缩变形关系曲线变化的趋势。     

A study of the magnetic susceptibility of rocks under uniaxial compression

Summary Preliminary experiments conducted on rocks under uniaxial compression seem to indicate that the magnetic susceptibility parallel to the domain wallsK does not play an effective role in stress-induced anisotropy.     

Thermal conductivity of NaCl,MgO, coesite and stishovite up to 40 kbar

The thermal conductivity of NaCl, MgO, coesite and stishovite have been measured as a function of pressures up to 40 kbar (4 Gpa) at room temperature. Polycrystalline coesite and stishovite were synthesized under high pressures and temperatures at our laboratory. An improved version of the comparative method suitable for the thermal conductivity measurement of small samples under high pressures was designed. The zero-pressure values are 0.0189 and 0.0412 cal. cm^?1 s^?1 °C^?1 for coesite and stishovite, respectively. The thermal conductivities were found to increase linearly with pressure, and the increase rates relative to their zero-pressure values were 3.1, 0.68, 0.39 and 0.90% per kbar for NaCl, MgO, coesite and stishovite, respectively. On the thermal conductivity of coesite, the pressure dependence is small and the zero-pressure value is almost the same as that of polycrystalline quartz. On the other hand, the zero-pressure value of stishovite is 2.2 times as large as that of coesite.     

Preliminary location of microcracks in several rock specimens under true triaxial compression

ＰｒｅｌｉｍｉｎａｒｙｌｏｃａｔｉｏｎｏｆｍｉｃｒｏｃｒａｃｋｓｉｎｓｅｖｅｒａｌｒｏｃｋｓｐｅｃｉｍｅｎｓｕｎｄｅｒｔｒｕｅｔｒｉａｘｉａｌｃｏｍｐｒｅｓｓｉｏｎＺｈａｏ－ＹｏｎｇＸＵ（许昭永），Ｓｈｉ－ＲｏｎｇＭＥＩ（梅世蓉），Ｃａｎ－ＴａｏＺＨ...     

Frequency-dependence of velocity and attenuation of elastic waves in granite under uniaxial compression

Velocity as well as attenuation factorQ ^–1 ofP-wave in a dry granitic rock sample under uniaxial compressions were measured in the range of frequency between 100 kHz and 710 kHz by using the pulse transmission technique. Above the stress of 0.5 _f , where _f is the fracture stress, theP-wave velocity decreases with increasing axial stress, whereasQ ^–1 increases. Particularly, the change ofQ ^–1 is greater for high frequency than for low frequency. At a given stress level, the higher the frequency, the higher theP-wave velocity and the largerQ ^–1. This result means that the velocity decrease with increasing stress is smaller for higher frequency. Because of this frequency-dependence of velocity decrease, theP-wave in the rock under dilatant state shows dispersion. The body wave dispersion is more remarkable at higher stress, and is not found in a homogeneous material with no cracks. Thus the disperison is attributed to the generation of cracks. When the frequency-dependence ofQ ^–1 is approximated asf ⁿ in the present frequency range, the exponentn takes a value from 0.63 to 0.77.     

Tidal evolution and hydrostatic equilibrium of phobos

Summary A dynamical model of Phobos has been set up: its boundary surface is defined by the 2nd degree Stokes zonal and sectorial parameters, the mass and angular velocity of rotation of the body (assumed homogeneous), plus the constant part of the tidal potential due to Mars which is relatively very large. The tidal evolution has been outlined, based on the condition that, at a certain epoch, the sum of the tidal and centrifugal forces on the surface of Phobos should be equal to the gravitation of opposite sign. The problem of hydrostatic equilibrium of Phobos is discussed.

---

ma ¶rt;uaua ¶rt; a: aa nm n¶rt;a m a u mua mu naamau, a u m au ma uma ¶rt;¶rt;, n nm am nuu nmuaa m aa, m aum aum. aa nuua u, aa nu u, m m n a nuu u m u a nmu a aum u numu. ¶rt;am nau¶rt;mamu au a.

     

3D-FEM modeling of the microscopic stress field of forsterite aggregate under hydrostatic pressure: Significance of the crystal orientation

Three Dimensional Finite Element Method(3D-FEM)has been used to model the deviatoric stress field in a forsterite aggregate with a sandwich geometry:two forsterite cubes aligned in the same crystallographic orientation(the"breads")sandwich a third forsterite cube(the"filling"),which might have an identical or different crystallographic orientation.The results show that there is no von Mises stress in the forsterite sandwich if the sandwiching and sandwiched forsterite cubes are aligned in the same crystallographic orientation.If the crystallographic orientations are different,however,von Mises stress and heterogeneous stress distribution occur both along the boundary and in the forsterite cubes.For the investigated P-T conditions(up to 6.4GPa and 500°C),the resulted deviatoric stress is much lower than the yield strength of forsterite,so that higher P,higher T,or other means to create higher deviatoric stress is necessary,in order to constrain the yielding behavior of forsterite.     

圆不锈钢管混凝土短柱轴压承载力性能研究

为了研究圆不锈钢管混凝土柱轴压承载力性能,在试验的基础上,利用有限元软件ABAQUS建立合理的有限元模型,并进行不同混凝土强度和约束效应系数参数化分析及不锈钢管和核心混凝土的轴力分配研究.结果表明:壁厚相同时,CFSST短柱的极限荷载增大幅度由大到小为:30MPa增加到40 MPa,50 MPa增加到60 MPa,40...     

Single-crystal elasticity of stishovite: New experimental data obtained using high-frequency resonant ultrasound spectroscopy and a Gingham check structure model

Single-crystal elasticity of stishovite was examined using a new experimental technique and an empirical macroscopic model. Employing high-frequency resonant ultrasound spectroscopy, single-crystal elastic constants of stishovite were determined: C₁₁ = 443(3), C₃₃ = 781(4), C₁₂ = 193(2), C₂₃ = 199(2), C₄₄ = 256(2), and C₃₃ = 316(2) GPa. The frequency range of the resonant ultrasound spectroscopy techniques was 6–20 MHz, which is much lower than the ～10 GHz range of the Brillouin scattering technique. Of the elastic constants, the shear elastic constants C₄₄ and C₆₆ are consistent with the average value of the previously mentioned Brillouin scattering. Conversely, the four elastic constants, C₁₁, C₃₃, C₁₂, and C₂₃, slightly deviate outside the range of previous Brillouin scattering results. The present results, except those for C₁₂, are consistent with recent lattice dynamic analysis of inelastic X-ray scattering data. The adiabatic bulk modulus was calculated as 298 GPa, which is smaller and more consistent with the result of compression experiments than any other Brillouin scattering results (301–312 GPa). The present result shows greater P-wave velocity anisotropy (24.7%) than any preceding work. To understand the unique elastic properties of stishovite, the Gingham check model was proposed and examined. The result shows that the octahedron of 6-coordinated Si in stishovite crystal has stiffness comparable to that of diamond.     

Disproportionation of Ni2SiO4 to stishovite plus bunsenite at high pressures and temperatures

Samples of Ni₂SiO₄ in both olivine and spinel phases have been compressed to pressures above 140 kbar in a diamond-anvil cell and heated to temperatures of 1400–1800°C using a continuous YAG laser. After quenching and releasing pressure, X-ray diffraction examination indicates that the samples disproportionate to a mixture of stishovite (SiO₂) and bunsenite (NiO) at pressures between 140 and 190 kbar. The exact disproportionation pressure is not certain due to transient increases in pressure during the local and rapid heating. However, thermodynamic calculations suggest that the transition pressure is about 192 ± 4 kbar at 1545°C and that the equation of the spinel-mixed oxides phase boundary isP(kbar) = 121 + (0.046 ± 0.020) T (°C).