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751.
The acceleration of aluminum particles with a 5μm diameter in the flow field behind an incident shock wave was investigated
experimentally in a 10-m long and 70 mm inner diameter shock tube. By means of instantaneous Laser Doppler Velocimetry (LDV)
the velocity of the particles was observed directly. The light scattered by the moving particles is Doppler shifted and sent
to the laser Doppler velocimeter. The velocimeter essentially consists of a phase-stabilized Michelson interferometer used
as a sensitive spectrometer. An electro-optical circuit ensures the phase stabilization that results in a voltage signal independent
of the scattered light intensity and proportional to the mean velocity of the particles at the measurement point. Because
of the very short response time (1μs) of the LDV system used here, the latter gives a continuous real-time signal of the particle
acceleration. To avoid particle oxidation the particles were accelerated by a high-speed nitrogen gas flow. From the measured
velocity the dimensionless drag coefficient was calculated. The drag coefficient is related to the fluid dynamic force exerted
by the gas on the particles. The experimental data were compared to theoretical models from the literature. A significant
deviation between the model and the experimental data was observed. This deviation is supposed to be induced by the shock
wave, which hits the particles and breaks them into pieces of a smaller diameter. Further experiments will be carried out
in the future to check the size distribution of the particles after the shock has gone past them.
相似文献
752.
The time and depth of vertical one-dimensional projectile penetration into sandy media in the near shore region are derived.
A precise definition for the physical properties and for the behavior of the sandy medium following the projectile impact
are evaluated. Three separate time intervals following projectile impact are identified. During the first 3 ms of penetration,
the deviatoric friction stress is shown to be negligible and the integrated Mie–Grüneisen equation of state (or, equivalently,
the Hugoniot-adiabat) may be applied to compute the normal penetration resistance force from the sand pressure. In order to
compute sand pressure as a function of the sand density D by the integrated Mie–Grüneisen equation of state, the Mie–Grüneisen
dimensionless constants γ0 and s and the dimensional speed of sound C
0 in the sandy medium are required. In order to illustrate the one-dimensional shock wave propagation in both wet and dry sands,
Hugoniot data for wet and dry silica sands are evaluated by a three degrees of freedom algorithm to compute these required
constants. The numerical results demonstrate that the amplitude of the shock wave pressure in the wet silica sand (41% porosity)
is approximately one-third of the shock wave pressure amplitudes in the dry silica sands (22% and 41% porosity). In addition,
the shock wave pressure dampens quicker in the wet sand than in the dry sands. 相似文献
753.
Search for conditions of compressive fracture of hard brittle ceramics at impact loading 总被引:1,自引:0,他引:1
G.I. Kanel E.B. Zaretsky A.M. Rajendran S.V. Razorenov A.S. Savinykh V. Paris 《International Journal of Plasticity》2009
In this paper we discuss three different experimental configurations to diagnosing the modes of inelastic deformation and to evaluating the failure thresholds at shock compression of hard brittle solids. One of the manifestations of brittle material response is the failure wave phenomenon, which has been previously observed in shock-compressed glasses. However, based on the measurements from our “theory critical” experiments, both alumina and boron carbide did not exhibit this phenomenon. In experiments with free and pre-stressed ceramics, while the Hugoniot elastic limit (HEL) in high-density B4C ceramic was found to be very sensitive to the transverse stress, it was found relatively less sensitive in Al2O3, implying brittle response of the boron carbide and ductile behavior of alumina. To further investigate the effects of stress states on the shock response of brittle materials, a “divergent flow or spherical shock wave” based plate impact experimental technique was employed to vary the ratio of longitudinal and transversal stresses and to probe conditions for compressive fracture thresholds. Two different experimental approaches were considered to generate both longitudinal and shear waves in the target through the impact of convex flyer plates. In the ceramic target plates, the shear wave separates a region of highly divergent flow behind the decaying spherical longitudinal shock wave and a region of low-divergent flow. Experiments with divergent shock loading of alumina and boron carbide ceramic plates coupled with computer simulations demonstrated the validity of these experimental approaches to develop a better understanding of fracture phenomena. 相似文献
754.
D.B. Hayes G.T. Gray III R.S. HixsonC.A. Hall M.E. ByersJ.E. Vorthman 《International Journal of Plasticity》2009
When uranium alloyed with 6-wt% niobium (U–6Nb) is rapidly compressed in uniaxial strain experiments, shear stress is observed to relax with a characteristic time of 30 ± 7 ns. In shock wave experiments, this relaxation inhibits the development of an elastic precursor commonly seen in other materials. When U–6Nb is cold-rolled to pre-twin and significantly increase the density of dislocations in the material, stress relaxation effects are diminished suggesting that twinning causes relaxation in the un-worked material. Separate ramp wave compression experiments produce effects that agree with those observed in shock-loading experiments. A phenomenological model is introduced that allows accurate simulation of all experiments. Estimates of residual shear stress after relaxation are obtained. 相似文献
755.
The recent diagnostic capability of the Omega laser to study solid-solid phase transitions at pressures greater than 10 GPa and at strain rates exceeding 107 s−1 has also provided valuable information on the dynamic elastic-plastic behavior of materials. We have found, for example, that plasticity kinetics modifies the effective loading and thermodynamic paths of the material. In this paper we derive a kinetics equation for the time-dependent plastic response of the material to dynamic loading, and describe the model’s implementation in a radiation-hydrodynamics computer code. This model for plasticity kinetics incorporates the Gilman model for dislocation multiplication and saturation. We discuss the application of this model to the simulation of experimental velocity interferometry data for experiments on Omega in which Fe was shock compressed to pressures beyond the α-to-ε phase transition pressure. The kinetics model is shown to fit the data reasonably well in this high strain rate regime and further allows quantification of the relative contributions of dislocation multiplication and drag. The sensitivity of the observed signatures to the kinetics model parameters is presented. 相似文献
756.
We report on the characterisation of the upstream medium ahead of a radiative cylindrical blast wave launched in an argon cluster gas with a 1 J, 1 ps, 1054 nm Nd:Glass laser system. By launching two perpendicular blast waves and introducing a time delay between the heating beams it is possible to determine the extent of the cluster medium by observing the high energy absorption region associated with clusters, as apposed to the low energy deposition in monatomic gas. It was found that argon ions launched from the initial laser driven cluster ionisation created a ballistic ion wave which sweeps out ahead of the hydrodynamic blast wave at an initial velocity of 1000 kms−1. This ballistic wave disassembles the clusters ahead of the blast wave into a neutral gas medium before the arrival of a radiative precursor. This observation gives us confidence that the dynamics of a radiative blast wave in cluster based experiments is determined primarily by the properties of an upstream atomic gas, and is not significantly influenced by cluster affects on energy transport or other material properties. 相似文献
757.
758.
Chen Shuxing 《数学年刊B辑(英文版)》1996,17(4):445-456
Inthispaperwecontinueourdiscussionin[5].Mainlywewillestablishaprioriestimatesforthelinearizedproblem(G),andthenprovetheexistenceofthesolutiontononlinearproblem.In§5wegivesometechnicalpreparations.Thenthewhole§6isdevotedtotheestablishmentofenergyestimates.I… 相似文献
759.
760.
Jiˇí Fürst 《Flow, Turbulence and Combustion》2006,76(4):331-342
The article describes the development of a high order finite volume method for the solution of transonic flow problems. The method is based on a reconstruction procedure similar to the weighted essentially non-oscillatory scheme (WENO). The analysis of accuracy and stability of the method is carried out for the case of smooth data and for simple discontinuity. The computational results demonstrate the performance of the WLSQR method for the solution of several flow problems in 2D and 3D using both structured and unstructured meshes. 相似文献