75 research outputs found

    Apparent negative motion of vortex matter due to inhomogeneous pinning

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    We investigate the transport of vortices in superconductors with inhomgeneous pinning under a driving force. The inhomogeneity of pinning is simplified as strong-weak pinning regions. It is demonstrated that the interactions between the vortices captured by strong pinning potentials and the vortices in the weak pinning region cause absolute negative motion (ANM) of vortices: The vortices which are climbing toward the high barriers induced by the strong pinning with the help of driving force move toward the opposite direction of the force and back to their equilibrium positions in the weak pinning region as the force decreases or is withdrawn. Our simulations reveal that the hysteresis of ANM is determined by the competition between the speed of the negative motion which depends on the piining inhomogeneity in superconductors and the speed of the driving force. Under the conditions of either larger force scanning rate or higher pinning inhomogeneity, a marked ANM and a larger hysteretic speed-force loop could be observed. This indicates that the time window to observe the ANM should be chosen properly. Moreover, the V-1 characteristics of Ag-sheathed Bi=2223 tapes are measured, and experimental observations are qualitatively in agreement with the simulation

    Isothermal Fatigue and Creep-Fatigue Interaction Behavior of Nickel-Base Directionally Solidified Superalloy

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    The creep-fatigue interaction in directionally solidified nickel-base superalloy was analyzed with the modified Chaboche-based unified viscoplasticity constitutive model. The model features the anisotropic material behavior, hardening/ softening, and stress relaxation. Simple low-cyclic fatigue and specified hold time experiments were conducted on a directionally solidified superalloy (DZ125) at temperatures over 760Β°C. The material parameters were optimized considering its tensile, cyclic and creep behavior with the Levenberg–Marquardt optimization procedure. The model was constructed in FORTRAN and integrated in FEA software UMAT/ABAQUS. The results show that experimental and simulated hysteresis loop size/shape, peak stresses, stress relaxation, and related areas are closely matched. The modified constitutive model was found to be instrumental for revealing the fatigue and creep-fatigue interaction behavior of such materials and can be used for practical applications.ВзаимодСйствиС ползучСсти ΠΈ усталости Π² супСрсплавС Π½Π° основС никСля, ΠΏΠΎΠ»ΡƒΡ‡Π΅Π½Π½ΠΎΠ³ΠΎ Π½Π°ΠΏΡ€Π°Π²Π»Π΅Π½Π½ΠΎΠΉ кристаллизациСй, ΠΏΡ€ΠΎΠ°Π½Π°Π»ΠΈΠ·ΠΈΡ€ΠΎΠ²Π°Π½ΠΎ с ΠΏΠΎΠΌΠΎΡ‰ΡŒΡŽ ΠΌΠΎΠ΄ΠΈΡ„ΠΈΡ†ΠΈΡ€ΠΎΠ²Π°Π½Π½ΠΎΠΉ ΡƒΠ½ΠΈΡ„ΠΈΡ†ΠΈΡ€ΠΎΠ²Π°Π½Π½ΠΎΠΉ ΠΎΠΏΡ€Π΅Π΄Π΅Π»ΡΡŽΡ‰Π΅ΠΉ ΠΌΠΎΠ΄Π΅Π»ΠΈ вязкопластичности Шабоша. МодСль Ρ…Π°Ρ€Π°ΠΊΡ‚Π΅Ρ€ΠΈΠ·ΡƒΠ΅Ρ‚ Π°Π½ΠΈΠ·ΠΎΡ‚Ρ€ΠΎΠΏΠ½ΠΎΠ΅ ΠΏΠΎΠ²Π΅Π΄Π΅Π½ΠΈΠ΅ ΠΌΠ°Ρ‚Π΅Ρ€ΠΈΠ°Π»Π°, ΡƒΠΏΡ€ΠΎΡ‡Π½Π΅Π½ΠΈΠ΅/Ρ€Π°Π·ΡƒΠΏΡ€ΠΎΡ‡Π½Π΅Π½ΠΈΠ΅ ΠΈ Ρ€Π΅Π»Π°ΠΊΡΠ°Ρ†ΠΈΡŽ напряТСний. ЭкспСримСнты ΠΏΠΎ ΠΎΠΏΡ€Π΅Π΄Π΅Π»Π΅Π½ΠΈΡŽ ΠΌΠ°Π»ΠΎΡ†ΠΈΠΊΠ»ΠΎΠ²ΠΎΠΉ усталости ΠΈ Π²Ρ€Π΅ΠΌΠ΅Π½ΠΈ прСбывания Π²Ρ‹ΠΏΠΎΠ»Π½Π΅Π½Ρ‹ Π½Π° супСрсплавС (DZ125) ΠΏΡ€ΠΈ Ρ‚Π΅ΠΌΠΏΠ΅Ρ€Π°Ρ‚ΡƒΡ€Π°Ρ… Π²Ρ‹ΡˆΠ΅ 760Β°Π‘. ΠŸΠ°Ρ€Π°ΠΌΠ΅Ρ‚Ρ€Ρ‹ ΠΌΠ°Ρ‚Π΅Ρ€ΠΈΠ°Π»Π° ΠΎΠΏΡ‚ΠΈΠΌΠΈΠ·ΠΈΡ€ΠΎΠ²Π°Π½Ρ‹ с ΡƒΡ‡Π΅Ρ‚ΠΎΠΌ Π΅Π³ΠΎ повСдСния ΠΏΡ€ΠΈ растяТСнии, цикличСской Π½Π°Π³Ρ€ΡƒΠ·ΠΊΠ΅ ΠΈ ползучСсти ΠΏΡ€ΠΈ ΠΏΠΎΠΌΠΎΡ‰ΠΈ ΠΌΠ΅Ρ‚ΠΎΠ΄Π° ΠΎΠΏΡ‚ΠΈΠΌΠΈΠ·Π°Ρ†ΠΈΠΈ Π›Π΅Π²Π΅Π½Π±Π΅Ρ€Π³Π°β€“ΠœΠ°Ρ€ΠΊΠ²Π°Ρ€Π΄Ρ‚Π°. МодСль построСна Π½Π° языкС ЀОРВРАН ΠΈ ΠΈΠ½Ρ‚Π΅Π³Ρ€ΠΈΡ€ΠΎΠ²Π°Π½Π° Π² ΠΏΡ€ΠΎΠ³Ρ€Π°ΠΌΠΌΡƒ FEА UMAT/ABAQUS. Показано, Ρ‡Ρ‚ΠΎ ΡΠΊΡΠΏΠ΅Ρ€ΠΈΠΌΠ΅Π½Ρ‚Π°Π»ΡŒΠ½Ρ‹Π΅ ΠΈ ΠΌΠΎΠ΄Π΅Π»ΡŒΠ½Ρ‹Π΅ Ρ€Π°Π·ΠΌΠ΅Ρ€Ρ‹/ Ρ„ΠΎΡ€ΠΌΠ° ΠΏΠ΅Ρ‚Π»ΠΈ гистСрСзиса, ΠΏΠΈΠΊΠΎΠ²Ρ‹Π΅ напряТСния, рСлаксация напряТСний ΠΈ ΠΈΡ… ΠΎΠ±Π»Π°ΡΡ‚ΡŒ находятся Π² соотвСтствии. УстановлСно, Ρ‡Ρ‚ΠΎ ΠΌΠΎΠ΄ΠΈΡ„ΠΈΡ†ΠΈΡ€ΠΎΠ²Π°Π½Π½ΡƒΡŽ ΠΎΠΏΡ€Π΅Π΄Π΅Π»ΡΡŽΡ‰ΡƒΡŽ модСль цСлСсообразно ΠΏΡ€ΠΈΠΌΠ΅Π½ΡΡ‚ΡŒ ΠΏΡ€ΠΈ Π°Π½Π°Π»ΠΈΠ·Π΅ усталости ΠΈ взаимодСйствия ползучСсти ΠΈ усталости ΠΏΠΎΠ΄ΠΎΠ±Π½Ρ‹Ρ… ΠΌΠ°Ρ‚Π΅Ρ€ΠΈΠ°Π»ΠΎΠ², ΠΎΠ½Π° ΠΌΠΎΠΆΠ΅Ρ‚ Ρ‚Π°ΠΊΠΆΠ΅ Π½Π°ΠΉΡ‚ΠΈ практичСскоС ΠΏΡ€ΠΈΠΌΠ΅Π½Π΅Π½ΠΈΠ΅

    Nonequilibrium dynamics in type-II superconductors with inhomogeneous vortex pinning

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    We study numerically the dynamics relating to negative vortex motion in inhomogeneous pinning systems. We show that this dynamical phenomenon results from the internal field effect produced by the growing local barriers with decreasing temperature. We find that the negative motion is characterized by a peak of negative voltage or resistance in resistance–temperature transport measurements. We also demonstrate that the time window to observe the negative motion is determined by the magnitude of driving force in addition to the temperature scanning rat

    Nucleon-nucleon momentum correlation function for light nuclei

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    Nucleon-nucleon momentum correlation function have been presented for nuclear reactions with neutron-rich or proton-rich projectiles using a nuclear transport theory, namely Isospin-Dependent Quantum Molecular Dynamics model. The relationship between the binding energy of projectiles and the strength of proton-neutron correlation function at small relative momentum has been explored, while proton-proton correlation function shows its sensitivity to the proton density distribution. Those results show that nucleon-nucleon correlation function is useful to reflect some features of the neutron- or proton-halo nuclei and therefore provide a potential tool for the studies of radioactive beam physics.Comment: Talk given at the 18th International IUPAP Conference on Few-Body Problems in Physics (FB18), Santos, Brasil, August 21-26, 2006. To appear in Nucl. Phys.

    Scaling of anisotropy flows in intermediate energy heavy ion collisions

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    Anisotropic flows (v1v_1, v2v_2 and v4v_4) of light nuclear clusters are studied by a nucleonic transport model in intermediate energy heavy ion collisions. The number-of-nucleon scalings of the directed flow (v1v_1) and elliptic flow (v2v_2) are demonstrated for light nuclear clusters. Moreover, the ratios of v4/v22v_4/v_2^2 of nuclear clusters show a constant value of 1/2 regardless of the transverse momentum. The above phenomena can be understood by the coalescence mechanism in nucleonic level and are worthy to be explored in experiments.Comment: Invited talk at "IX International Conference on Nucleus-Nucleus Collisions", Rio de Janeiro, Aug 28- Sept 1, 2006; to appear on the proceeding issue in Nuclear Physics

    Stress State Analysis of Iosipescu Shear Specimens for Aerogel Composite with Different Properties in Tension and Compression

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    AbstractThe objective of present research is to establish guidelines for preparing shear tests of ceramic-fiber-reinforced aerogel. The aerogel composite prepared by Sol-gel methods exhibits bi-modulus properties under tension and compression loadings. Bi- modulus constitutive model of the aerogel composite was built, and then verified by analysis of bending test. Finite element analysis (FEA) was presented on Iosipescu specimens with different V-notch and Round-notch configurations. Several trends have become evident: stress state of bi-modulus aerogel is different from that of isotropic material; stress concentration is a strong function of V-notch tip radius, and increasing V-notch tip radius can effectively reduce normal stress concentration at notch tips; shear stress distribution in the test region can be modified by varying notch angle when V-notch tip radius≀1.3mm; increasing the Round-notch radius can also effectively reduce the normal stress concentration at notch tips. Based on these observations, Round-notch specimen seems to be a favorable choice for reducing the normal stress concentration at notch tips, especially the longitudinal tensile stress concentration. Round-notch specimen with r=4mm is favorable for the uniformly distributed shear stress in the test region. Digital image correlation (DIC) was suggested for shear strain measuring
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