The 600 keV electron injections in the Earth's outer radiation belt:A statistical study

Relativistic electron injections are one of the mechanisms of relativistic(≥0.5 MeV) electron enhancements in the Earth’s outer radiation belt. In this study, we present a statistical observation of 600 keV electron injections in the outer radiation belt by using data from the Van Allen Probes. On the basis of the characteristics of different injections, 600 keV electron injections in the outer radiation belt were divided into pulsed electron injections and nonpulsed electron injections. The 600 keV electron injections were observed at 4.5 < L <6.4 under the geomagnetic conditions of 450 nT < AE < 1,450 nT. An L of ~4.5 is an inward limit for 600 keV electron injections. Before the electron injections, a flux negative L shell gradient for ≤0.6 MeV electrons or low electron fluxes in the injected region were observed. For600 keV electron injections at different L shells, the source populations from the Earth’s plasma sheet were different. For 600 keV electron injections at higher L shells, the source populations were higher energy electrons(~200 keV at X ~–9 R_(E)), whereas the source populations for 600 keV electron injections at lower L shells were lower energy electrons(~80 keV at X ~–9 R_(E)). These results are important to further our understanding of electron injections and rapid enhancements of 600 keV electrons in the Earth’s outer radiation belt.

Longitudinal connection effect on initial support steel frames in tunnels--Take the traffic tunnels as examples

Most tunnel projects are designed with cross-sectional loads,and the inhomogeneity of the longitudinal forces is ignored.In theory,such a support structure can resist large loads,but in practice,large deformation,concrete cracking,steel frame distortion,and other phenomena often occur in tunnels under poor surrounding rock conditions.Hence,the longitudinal stability of the tunnel must be considered.In this study,the mechanism of longitudinal connecting ribs(LCRs)of tunnels was investigated through element tests,theoretical analyses,and numerical simulations,and the effect of the LCRs was evaluated experimentally.The applicability of the constitutive relations and boundary conditions of the numerical model was verified.The instability mode of the steel frame reflecting the longitudinal stress gradient of the tunnel was analyzed,and the longitudinal surrounding rock pressure and the verified numerical model were applied to analyze the LCR using the load structure method.The results indicate the following:(1)LCRs can effectively improve the ultimate bearing capacity and stability of a structure and reduce the area and degree of damage;(2)Two types of instability modes occur in tunnel steel frames,and the main factor is bending failure caused by the axial force;(3)The distance sensitivity of the LCR in the tunnel is higher than the stiffness sensitivity.For large deformations of tunnels,double rows of rebars with a spacing of less than 1.5 m should be used as longitudinal connections.

On the superplastic deformation mechanisms of near-αTNW700 titanium alloy

The new near-αTNW700 titanium alloy is a potential candidate material for high performance ultrasonic/hypersonic aircrafts,which is designed for short-term service at 700℃.This study systematically investigated the superplastic deformation microstructure evolution and mechanism of TNW700 alloy at different strain rates and true strains at 925℃.Results show that TNW700 alloy exhibits excellent superplastic behavior in a constant strain rate range of 0.0005-0.005 s^(-1) with elongation above 400%.The peak stress decreases with decreasing strain rate,which is related to the increase ofβ-phase volume fraction caused by the increase of thermal exposure time.In addition,significant strain hardening is observed in early-middle stage of superplastic deformation,and flow softening is followed in middle-late stage.To rationalize these complex flow behaviors,electron backscatter diffraction(EBSD)and high resolution transmission electron microscopy(HRTEM)were used to characterize the microstructure.Strain hardening is correlated to the synergistic effect ofβgrain growth,dislocation accumulation,silicide precipitate,and solid solution strengthening ofαphase.Continuous dynamic recrystallization(CDRX)induced the fragmentation of primaryαgrains in middle-late stage of superplastic deformation,and the refinement ofαgrains,the increase ofβphase volume fraction and dynamic dislocation recovery are main causes of high strain softening.In addition,EBSD and TEM observations confirmed texture randomization,fine equiaxed primaryαgrains and intragranular dislocation movement,indicating that grain boundary sliding(GBS)accommodated by dislocation sliding/climb is the dominant superplastic deformation mechanism of TNW700 alloy.