A finite deformation analysis of polycrystal by considering the influence of grain boundary

By combining grain boundary (GB) and its influence zone, a micromechanic model for polycrystal is established for considering the influence of GB. By using the crystal plasticity theory and the finite element method for finite deformation, numerical simulation is carried out by the model. Calculated results display the microscopic characteristic of deformation fields of grains and are in qualitative agreement with experimental results.

A stochastic jump and deterministic dynamics model of impact failure evolution with rate effect

Motivated by the observation of the rate effect on material failure, a model of nonlinear and nonlocal evolution is developed, that includes both stochastic and dynamic effects. In phase space a transitional region prevails, which distinguishes the failure behavior from a globally stable one to that of catastrophic. Several probability functions are found to characterize the distinctive features of evolution due to different degrees of nucleation, growth and coalescence rates. The results may provide a better understanding of material failure.

Crack growth by grain boundary cavitation in the transient and extensive creep regimes

Crack growth due to cavity growth and coalescence along grain boundaries is analyzed under transient and extensive creep conditions in a compact tension specimen. Account is taken of the finite geometry changes accompanying crack tip blunting. The material is characterized as an elastic-power law creeping solid with an additional contribution to the creep rate arising from a given density of cavitating grain boundary facets. All voids are assumed present from the outset and distributed on a given density of cavitating grain boundary facets. The evolution of the stress fields with crack growth under three load histories is described in some detail for a relatively ductile material. The full-field plane strain finite element calculations show the competing effects of stress relaxation due to constrained creep, diffusion and crack tip blunting. and of stress increase due to the instantaneous elastic response to crack growth. At very high crack growth rates the Hui-Riedel fields dominate the crack tip region. However. the high growth rates are not sustained for any length of time in the compact tension geometry analyzed. The region of dominance of the Hui-Riedel field shrinks rapidly so that the near-tip fields are controlled by the HRR-type field shortly after the onset of crack growth. Crack growth rates under various conditions of loading and spanning the range of times from small scale creep to extensive creep are obtained. We show that there is a strong similarity between crack growth history and the behaviour of the C(t) and C(t) parameters. so that crack growth rates correlate rather well with C(t) and C(t). A relatively brittle material is also considered that has a very different near-tip stress field and crack growth history.

The influence of momentum addition on the acceleration of the solar and stellar winds

The influence of the momentum addition, which may be associated with the average or fluctuation transverse component of the magnetic field or others, on the acceleration the solar wind or stellar wind is studied in a local streamtube. The results show that the larger the momentum addition the stronger the acceleration of the wind. For example, if the typical transverse magnetic field is about 0.1 of the longitudinal field, the velocity of the solar wind at 1 AU may be increased by 40%. The coronal hole may be considered as a streamtube, the presence of a high stream from the coronal hole may be explained by the existence of an average or fluctuation transverse magnetic field in the streamtube. A similar conclusion may be applied to the polar region, where the velocity of the solar wind will be larger than elsewhere as if there is a transverse component of magnetic field, as well as to the stellar wind. The influence of other parameters on the acceleration of the solar wind is also discussed. From the viewpoint of the solar wind mechanism, the present paper shows that the momentum addition in the subsonic flow region can increase the velocity of the solar wind at 1 AU.

燃烧物理学基础

Surface-enhanced Raman scattering substrate fabricated by femtosecond laser direct writing

We report the fabrication of a novel surface-enhanced Raman scattering (SERS) substrate with a controllable enhancement factor (EF) using femtosecond laser direct writing on Ag+-doped phosphate glass followed by chemical plating at similar to 40 degrees C. Silver seeds were first photoreduced using a femtosecond laser in a laser-irradiated area and then transformed into silver nanoparticles of suitable size for SERS application in the subsequent chemical plating. Rhodamine 6G was used as a probing molecule to investigate the enhancement effect of a Raman signal on the substrate. Nearly homogenous enhancement of the Raman signal over the Substrate was achieved, and the EF of the substrate was controlled to some extent by adjusting fabrication parameters. Moreover, the ability of forming a SERS platform in an embedded microfluidic chamber would be of great use for establishing a compact lab-on-a-chip device based on Raman analysis.

Upconversion luminescence and optical power limiting effect based on two- and three-photon absorption processes of ZnO crystal

Near-infrared to UV and visible upconversion luminescence was observed in single-crystalline ZnO under an 800 nm infrared femtosecond laser irradiation. The optical properties of the crystal reveal that the UV and VIS emission band are due to the exciton transition (D0X) bound to neutral donors and the deep luminescent centers in ZnO, respectively. The relationship between the upconversion luminescence intensity and the pump power of the femtosecond laser reveals that the UV emission belongs to three-photon sequential band-to-band excitation and the VIS emission belongs to two-photon simultaneous defect-absorption induced luminescence. A saturation phenomenon and polarization-dependent effect are also observed in the upconversion process of ZnO. A very good optical power limiting performance at 800 nm has been demonstrated. The two- and three-photon absorption coefficients of ZnO crystal were measured to be 0.2018 cm GW(-1) and 7.102 x 10(-3) cm(3) GW(-2), respectively. The two- and three-photon cross sections were calculated to be 1.189 x 10(-51) cm(4) s and 1.040 x 10(-80) cm(6) s(2), respectively. (c) 2007 Elsevier B.V. All rights reserved.

LiNbO3：Fe晶体光栅擦除特性

在光折变过程中，光波对掺杂中心的激发系数S是一个重要的物理量。为了求出光激发系数S对波长A的依赖，研究了在不同的擦除波长条件下，记录在光折变晶体LiNbO3：Fe中的光栅衰减特性。实验结果表明lnP（P为光擦除灵敏度）与擦除波长λ呈线性关系，即波长越短，光栅擦除越快，光擦除灵敏度越高。在单中心电子带传输模型的基础上，理论计算也证实了实验结果。利用带传输模型，给出了理论上光激发系数S对波长的指数依赖关系。同时微观量φμγ（φ为量子效率）也可由光栅擦除实验求出。