Mechanical and damage evolution properties of sandstone under triaxial compression

To study the mechanical and damage evolution properties of sandstone under triaxial compression, we analyzed the stress strain curve characteristics, deformation and strength properties, and failure process and characteristics of sandstone samples under different stress states. The experimental results reveal that peak strength, residual strength, elasticity modulus and deformation modulus increase linearly with confining pressure, and failure models transform from fragile failure under low confining pressure to ductility failure under high confining pressure. Macroscopic failure forms of samples under uniaxial compression were split failure parallel to the axis of samples, while macroscopic failure forms under uniaxial compression were shear failure, the shear failure angle of which decreased linearly with confin- ing pressure. There were significant volume dilatation properties in the loading process of sandstone under different confining pressures, and we analyzed the damage evolution properties of samples based on acoustic emission damage and volumetric dilatation damage, and established damage constitutive model, realizing the real-time Quantitative evaluation of samnles damage state in loading process.

Evolution Properties of Modules in Complex Networks

In complex networks, network modules play a center role, which carry out a key function. In this paper, we introduce the spatial correIation function to describe the relationships among the network modules. Our focus is to investigate how the network modules evolve, and what the evolution properties of the modules are. In order to test the proposed method, as the examples, we use our method to analyze and discuss the ER random network and scale-free network. Rigorous analysis of the existing data shows that the introduced correlation function is suitable for describing the evolution properties of network modules. Remarkably, the numerical simulations indicate that the ER random network and scale-free network have different evolution properties.

Structural Evolution and Phase Change Properties of C-Doped Ge_2Sb_2Te_5 Films During Heating in Air

We elucidate the importance of a capping layer on the structural evolution and phase change properties of carbondoped Ge2 Sb2 Te5（C-GST） films during heating in air. Both the C-GST films without and with a thin SiO2 capping layer（C-GST and C-GST/SiO2） are deposited for comparison. Large differences are observed between C-GST and C-GST/SiO2 films in resistance-temperature, x-ray diffraction, x-ray photoelectron spectroscopy,Raman spectra, data retention capability and optical band gap measurements. In the C-GST film, resistancetemperature measurement reveals an unusual smooth decrease in resistance above 110℃ during heating. Xray diffraction result has excluded the possibility of phase change in the C-GST film below 170℃. The x-ray photoelectron spectroscopy experimental result reveals the evolution of Te chemical valence because of the carbon oxidation during heating. Raman spectra further demonstrate that phase changes from an amorphous state to the hexagonal state occur directly during heating in the C-GST film. The quite smooth decrease in resistance is believed to be related with the formation of Te-rich GeTe4-n Gen（n = 0, 1） units above 110℃ in the C-GST film. The oxidation of carbon is harmful to the C-GST phase change properties.

Fractional description of mechanical property evolution of soft soils during creep

The motion of pore water directly influences mechanical properties of soils, which are variable during creep. Accurate description of the evolution of mechanical properties of soils can help to reveal the internal behavior of pore water. Based on the idea of using the fractional order to reflect mechanical properties of soils, a fractional creep model is proposed by introducing a variable-order fractional operator, and realized on a series of creep responses in soft soils. A comparative analysis illustrates that the evolution of mechanical properties, shown through the simulated results, exactly corresponds to the motion of pore water and the solid skeleton. This demonstrates that the proposed variable-order fractional model can be employed to characterize the evolution of mechanical properties of and the pore water motion in soft soils during creep. It is observed that the fractional order from the proposed model is related to the dissipation rate of pore water pressure.

Evolution of Microstructure and Mechanical Property during Long-Term Aging in Udimet 720Li

Thermal stabilities of microstructure and mechanical property have been investigated on superalloy U720Li, which is of great interest of application for jet engine and land-based turbine disc. The results showed that, the primary and secondary gamma' particles maintain good thermal stability at 650 and 700 degreesC with aging time up to 3000 h, while the tertiary gamma' is apparently dependent on aging temperature and time. The tertiary gamma' particles undergo a procedure of coarsening, dissolution and eventually complete disappearance with the increasing of aging time and temperature. They exhibit unusual high sensibility upon aging temperature, which is attributed to the lattice misfit between the gamma' precipitates and the matrix in the alloy. The grain boundary phase M23C6 remains stable without forming of sigma phase even with aging time up to 3000 h at 700 degreesC. Microhardness decreases apparently with increasing aging time and aging temperature. Theoretical analysis based on dislocation mechanism indicates that the change of microhardness should be attributed to the evolution of the tertiary gamma' during aging.

Effects of Heating and Hot Extrusion Process on Microstructure and Properties of Inconel 625 Alloy

The effects of the heating process and hot extrusion on the microstructure and properties of inconel 625 alloy were studied. The experimental results showed that the properties of Inconel 625 alloy could be improved through the heating process and hot extrusion concomitant with a reduced corrosion rate. The M23C6 carbide, generated in the heating process, was retained and distributed at the grain boundary during the process of hot extrusion, which had an important influence on both elongation and corrosion resistance. The improvement of the comprehensive properties of the material, as measured by a tensile test at room temperature, was correlated with the dissolution of segregation Nb. A typical ductile fracture changed to a cleavage fracture where secondary cracks could be clearly seen. With the increase of the extrusion ratio, the real extrusion temperature was higher, which led to more dissolution of the M23C6 carbide, decreased the number of secondary cracks, enhanced the effect of solid solution strengthening, and reduced the intergranular corrosion rate. Under the condition of a high extrusion ratio and a high extrusion speed, the less extrusion time made it possible to obtain organization with a smaller average grain size. Moreover, in this case, the M23C6 carbide and segregated Nb did not have enough time to diffuse. Thus all samples exhibited medium strengths and corrosion rates after extrusion.

Monitoring of velocity changes based on seismic ambient noise: A brief review and perspective

Over the past two decades,the development of the ambient noise cross-correlation technology has spawned the exploration of underground structures.In addition,ambient noise-based monitoring has emerged because of the feasibility of reconstructing the continuous Green’s functions.Investigating the physical properties of a subsurface medium by tracking changes in seismic wave velocity that do not depend on the occurrence of earthquakes or the continuity of artificial sources dramatically increases the possibility of researching the evolution of crustal deformation.In this article,we outline some state-of-the-art techniques for noise-based monitoring,including moving-window cross-spectral analysis,the stretching method,dynamic time wrapping,wavelet cross-spectrum analysis,and a combination of these measurement methods,with either a Bayesian least-squares inversion or the Bayesian Markov chain Monte Carlo method.We briefly state the principles underlying the different methods and their pros and cons.By elaborating on some typical noisebased monitoring applications,we show how this technique can be widely applied in different scenarios and adapted to multiples scales.We list classical applications,such as following earthquake-related co-and postseismic velocity changes,forecasting volcanic eruptions,and tracking external environmental forcing-generated transient changes.By monitoring cases having different targets at different scales,we point out the applicability of this technology for disaster prediction and early warning of small-scale reservoirs,landslides,and so forth.Finally,we conclude with some possible developments of noise-based monitoring at present and summarize some prospective research directions.To improve the temporal and spatial resolution of passive-source noise monitoring,we propose integrating different methods and seismic sources.Further interdisciplinary collaboration is indispensable for comprehensively interpreting the observed changes.

Microstructure Evolution and Mechanical Properties of HR3C Steel during Long-term Aging at High Temperature

Microstructure evolution and the changes in mechanical properties of HR3 Csteel during long-term aging at650,700 and 750℃ were investigated.The precipitated phases of the aging steel included M23C6 carbides,Z-phase and a trace amount of Nb（C,N）.The M23C6 carbides were distributed mainly at the grain boundary,while Z-phase was mainly inside the grains.Amounts of both M23C6 carbides and Z-phase during the aging process increased with increasing aging period and temperature.Coarsening of M23C6 carbides was influenced significantly by aging time and temperature,while the size of the Z-phase was relatively less affected by the aging time and temperature,which had a steady strengthening effect.Coarsening of the M23C6 carbides was the main reason for the decline in high temperature yield strength during long-term aging at 750℃.The M23C6 carbides were linked into a continuous chain along the grain boundary which accounted for the decrease of toughness during aging.

Anodic Oxidation on Structural Evolution and Tensile Properties of Polyacrylonitrile Based Carbon Fibers with Different Surface Morphology

Polyacrylonitrile （PAN） based carbon fibers with different surface morphology were electrochemically treated in 3 wt% NH4HCO3 aqueous solution with current density up to 3.47 A/m 2 at room temperature, and surface structures, surface morphology and residual mechanical properties were characterized. The crystallite size （La） of carbon fibers would be interrupted due to excessive electrochemical etching, while the crystallite spacing （d（002）） increased as increasing current density. The disordered structures on the surface of carbon fiber with rough surface increased at the initial oxidation stage and then removed by further electrochemical etching, which resulting in continuous increase of the extent of graphitization on the fiber surface. However, the electrochemical etching was beneficial to getting ordered morphology on the surface for carbon fiber with smooth surface, especially when the current density was lower than 1.77 A/m 2 . The tensile strength and tensile modulus could be improved by 17.27% and 5.75%, respectively, and was dependent of surface morphology. The decreasing density of carbon fibers probably resulted from the volume expansion of carbon fibers caused by the abundant oxygen functional groups intercalated between the adjacent graphite layers.

Effects of Strain Rate,Temperature and Grain Size on the Mechanical Properties and Microstructure Evolutions of Polycrystalline Nickel Nanowires:A Molecular Dynamics Simulation

Through molecular dynamics（MD） simulation, the dependencies of temperature, grain size and strain rate on the mechanical properties were studied. The simulation results demonstrated that the strain rate from 0.05 to 2 ns–1 affected the Young＇s modulus of nickel nanowires slightly, whereas the yield stress increased. The Young＇s modulus decreased approximately linearly; however, the yield stress firstly increased and subsequently dropped as the temperature increased. The Young＇s modulus and yield stress increased as the mean grain size increased from 2.66 to 6.72 nm. Moreover, certain efforts have been made in the microstructure evolution with mechanical properties association under uniaxial tension. Certain phenomena such as the formation of twin structures, which were found in nanowires with larger grain size at higher strain rate and lower temperature, as well as the movement of grain boundaries and dislocation, were detected and discussed in detail. The results demonstrated that the plastic deformation was mainly accommodated by the motion of grain boundaries for smaller grain size. However, for larger grain size, the formations of stacking faults and twins were the main mechanisms of plastic deformation in the polycrystalline nickel nanowire.

Distinct microstructure and property evolution of 0.76(Bi_(0.5)Na_(0.5))TiO_(3)-0.24SrTiO_(3) ferroelectric ceramics synthesized with different TiO_(2) reactants

Single phase 0.76(Bi_(0.5)Na_(0.5))TiO_(3)–0.24SrTiO_(3) ferroelectric ceramics have been synthesized with homogenous anatase and hierarchical rutile TiO_(2) raw reactants(BNST-A and BNST-R).Either calcined powder persists the microstructure characteristics of raw reactants.As the result,when the sintering temperature increases from 1000 to 1200℃,the average grain size and density of BNST-A increase from 0.49 to 1.48μm and 5.02 to 5.61 g/cm^(3),while those of BNST-R from 0.86 to 1.44μm and 5.37 to 5.61 g/cm^(3).BNST-A illustrates a predominant ergodic relaxor state,and BNST-R prefers a non-ergodic relaxor state,as evidenced by the distinct polarization-electric field loops and current-electric field curves.Especially,such a distinct ferroelectric state is independent of sintering temperature.It is believed that the special hierarchical microstructure of rutile TiO_(2) reactant is beneficial to form denser ceramics with larger grains,and thus suppresses the contributions of polar nanoregions and defect-induced built-in field to ferroelectric property,leading to non-ergodic relaxor state.This work clearly demonstrates the nonnegligible effects of TiO_(2) reactants on the microstructure and properties of BNST ferroelectric ceramics.

Propagation properties of foreshock cavitons: Cluster observations

Foreshock cavitons are transient phenomena observed in the terrestrial foreshock region.They are characterized by a simultaneous depression of magnetic field magnitude and plasma density,which are bounded with enhancements of these two parameters and surrounded by ultralow frequency(ULF)waves.Previous studies focused on the interplanetary magnetic field(IMF)conditions,solar wind(SW)conditions,and the growth of the foreshock waves related to the generation of foreshock cavitons.Previously,a multipoint spacecraft analysis method using Cluster data was applied to analyze only two foreshock cavitons,and this method did not consider uncertainties.In this study,multipoint spacecraft analysis methods,including the timing method,the minimum directional derivative(MDD)method,and the spatiotemporal difference(STD)method are applied to determine the velocity in both spacecraft and solar wind frames.The propagation properties show good agreement with previous results from simulations and observations that most cavitons move sunward in the solar wind frame,with the velocities larger than the Alfvén speed.The propagation properties of foreshock cavitons support the formation mechanism of cavitons in previous simulations,which suggested that cavitons are formed due to the nonlinear evolution of compressive ULF waves.We find that there is clear decreasing trend between the size of cavitons and their velocity in the solar wind frame.In addition,the timing method considering errors has been applied to study the evolution properties by comparing the velocities with errors of the leading and trailing edges,and we identify three stable cavitons and one contracting caviton,which has not been studied before.Most cavitons should remain stable when they travel toward the Earth’s bow shock.The relationship between the size of foreshock cavitons and their distance from the bow shock is also discussed.

Distribution of modified Circumpolar Deep Water and its threat in Vincennes Bay,East Antarctica

The Antarctic Bottom Water formation site Vincennes Bay,East Antarctica is experiencing a substantial intrusion of modified Circumpolar Deep Water(mCDW),which may inhibit the formation of Dense Shelf Water(DSW)and drive basal melting of the ice shelves.Based on hydrographic data obtained from March to November in 2012,we evaluated the spatial spread of mCDW over the continental shelf region of Vincennes Bay and the associated temporal evolution of water properties,as well as the sea ice formation effect on water column in the coastal polynya.Results show that two branches of mCDW occupied the deep layers of the continental shelf,distinguished by the potential density(smaller than 27.8 kg/m^(3) or not)when potential temperatureθ=0.5°C in theθ-salinity space.The warmer and less dense branch observed on the east plateau,accessed the eastern ice shelves in the coastal polynya to drive basal melting of ice shelves.In contrast,the other colder and denser branch in the mid-depression reached the western Underwood Ice Shelf.DSW formation was detectable in the coastal polynya during September-November,proving the occurrence of deep convection.Surface heat loss and brine rejection during the intensive sea ice formation contributed to the destratification of the water column in the coastal polynya.It was estimated that at least 1.11±0.79 TW heat carried by mCDW into the inner part of the polynya.