横向划痕对光场调制的近场模拟

熔石英亚表面划痕对入射激光的近场调制是导致光学元件低阈值损伤的主要因素之一. 用三维时域有限差分方法研究了连续横向划痕的近场分布, 对比了尖锐截面与光滑截面场调制的差异, 着重探讨了光场调制与划痕宽深比R的关系. 研究表明: 酸蚀后的光滑截面有助于减弱近场调制, 这类划痕的R>10.0时调制较弱且相互接近, R<5.0时调制显著增强. 当R取1---3时, 亚表面的调制达最大值, 最大电场幅值为入射波幅值的4.3倍. 当R取1.0---3.5时, 缺陷附近有80%以上取样点的最大电场幅值超过入射波幅值的2倍. 随着深度的增大, 强场区具有明显的"趋肤效应": 位于划痕正下方的强场区首先往左右两侧移动, 然后移向抛物口界面以及水平界面, 同时衍生出的多条增强线诱导整个亚表面层的光场增强.     

黏滞等离子体中双撕裂模不稳定性的数值模拟研究

本文采用磁流体力学模型,数值研究了平板位形下双撕裂模线性增长率关于等离子体电阻η和黏滞v的定标关系.结果表明,对于有理面间距较大的情况,线性增长率关于电阻和黏滞的指数定标率随着黏滞的增加逐渐由γ∝ η3/5v0的定标变化到γ∝η5/6v-1/6的定标;而对于有理面间距较小的情况,其指数定标率随着黏性的增加从γ∝η 1/3 v0的定标逐渐变化到γ∝η2/3v-1/3的定标.本文还给出了初始阶段对称的双撕裂模的非线性演化,发现在非线性阶段对称的双撕裂模将转化为反对称的双撕裂模,并解释了相应的物理机理.     

FeCoCrCuNi高熵合金裂纹及孔洞结构力学与微观构象演化机理的分子动力学模拟研究

本文采用分子动力学方法研究了FeCoCrCuNi高熵合金裂纹及孔洞模型结构在不同轴向拉伸应变速率下的力学与微观结构演化机理. 结果表明：应变速率越高FeCoCrCuNi裂纹结构对应更高的过冲应变和过冲应力,其主要原因是高拉伸速率会导致高强度的BCC结构及孪晶结构的生成,而BCC结构及孪晶结构的产生进而会抑制应力的下降,通过应力-应变曲线,可知FeCoCrCuNi裂纹模型在轴向应力作用下表现为塑性形变. 对于不同尺寸的孔洞FeCoCrCuNi裂纹模型的应力模拟与结构分析,可以得出：孔洞尺寸越大, FeCoCrCuNi裂纹结构对应的过冲应变和过冲应力越小,其主要原因是大尺寸的孔洞造成孔洞之间产生裂纹的,进而会影响这个材料的屈服应变和屈服强度.     

Use of early acoustic emission to evaluate the structural condition and self-healing performance of textile reinforced cements

Textile reinforce cements (TRC) are innovative materials that are used for repair of existing structures or recently as stand-alone lightweight structural members. Fracture and thermal behavior of these materials are very complicated due to multiple failure modes. The undergoing processes cannot be described by the simple constitutive equation. In this perspective the contribution of monitoring techniques is crucial. Acoustic emission (AE) is used to check the behavior of TRC beams and plates in different states of structural health: intact, thermally pre-cracked and self-healed by polymer powder. This is the first time that the AE behaviors of intact, cracked and self-healed TRC laminates are compared in literature.     

利用高压热处理技术提高Bi-2223/Ag高温超导带材临界电流

提高临界电流密度(Jc)对促进Bi-2223/Ag高温超导带材的应用有着重要意义.在常压热处理条件下所制备的带材中存在大量的裂纹、孔洞和第二相颗粒等,这些因素都会对带材的超导连接性能产生破坏作用.我们成功建立了一套可用氧气氛的热等静压设备,工作气压和温度可同时达到15MPa和850°C.通过引入裂纹愈合热处理过程,我们成功对带材进行了高压热处理,所得样品中第二相颗粒和裂纹数量大幅度降低,77K、自场下的临界电流相比常压热处理样品提高了20%左右.     

An energy-based microstructure model to account for fatigue scatter in polycrystals

Scatter observed in the fatigue response of a nickel-based superalloy, U720, is linked to the variability in the microstructure. Our approach is to model the energy of a persistent slip band (PSB) structure and use its stability with respect to dislocation motion as our failure criterion for fatigue crack initiation. The components that contribute to the energy of the PSB are identified, namely, the stress field resulting from the applied external forces, dislocation pile-ups, and work-hardening of the material is calculated at the continuum scale. Further, energies for dislocations creating slip in the matrix/precipitates, interacting with the GBs, and nucleating/agglomerating within the PSB are computed via molecular dynamics simulations. Through this methodology, fatigue life is predicted based on the energy of the PSB, which inherently accounts for the microstructure of the material. The present approach circumvents the introduction of uncertainty principles in material properties. It builds a framework based on mechanics of microstructure, and from this framework, we construct simulated microstructures based on the measured distributions of grain size, orientation, neighbor information, and grain boundary character, which allows us to calculate fatigue scatter using a deterministic approach. The uniqueness of the approach is that it avoids the large number of parameters prevalent in previous fatigue models. The predicted lives are in excellent agreement with the experimental data validating the model capabilities.     

Generic structure of externally driven tearing modes instabilities

A major limit to steady state and advanced high operation of tokamaks of reactor class is due to the onset of tearing modes that develop magnetic and may cause loss of energy confinement or a major disruption. Here the structure of a classical problem about the effects of external control helical fields is analysed and it is shown to offer a general paradigm of response of low order classical and neoclassical tearing modes to a wide class of external perturbations. New results of principle on the structural stability of the response model are obtained, leading to a clear interpretation of the role of “seed islands" in the onset of neo-classical tearing modes and the role of finite ion larmor radius corrections to Ohm's law. Received 12 November 2001 and Received in final form 4 January 2002     

Boundary integral equation method in micropolar elasticity

This paper presents the integral representations of the displacement and rotation fields of the micropolar continuum, and the regularized integral representations of the traction and couple vector. Thus the boundary integral equations and the boundary integro-differential equations are derived. In addition, the formulation for crack problems is given by both the boundary integral equations and the boundary integro-differential equations.     

Turbulence investigations during ergodic divertor operation with induced 2/1 tearing mode

Within the last year of TEXTOR operation a major part of the experiments were dedicated to the interaction of the ergodized plasma edge, induced by the dynamic ergodic divertor (DED), with the core plasma. Systematic experiments are performed in the 3/1 configuration of the DED. Depending on , the DED generates a locked 2/1 mode in the plasma at a critical current in the DED coils. Different diagnostics were used to measure the plasma response in the core and edge. The investigations in this paper are focused on the measurement of density fluctuation and the analysis of the coherent-mode structure outside the q = 2 surface using O-mode poloidal correlation reflectometry. A reduction of the quasi-coherent mode amplitude and frequency is observed, caused by the 2/1 mode together with a decrease of the density scale length. Moreover, the deduced poloidal rotation velocity at the q = 3 surface changes from the electron to the ion diamagnetic drift direction with the onset of the 2/1 island. Also a reduction of the density fluctuations is observed as well as a decrease in the poloidal correlation length. From Ar-injection a delayed inward propagation of the emission maximum is observed for different Ar ionization stages when the 2/1 mode is generated.Similar experiments with a slowly rotating DED show that the steepened density gradients are spatially localized and confirm the hypothesis that the 2/1 mode is responsible for the observations.Presented at the Workshop Electric Fields Structures and Relaxation in Edge Plasmas, Nice, France, October 26–27, 2004.     

On the Path of a Quasi-static Crack in Mode III

A method for finding the path of a quasi-static crack growing in a brittle body is presented. The propagation process is modelled by a sequence of discrete steps optimizing the elastic energy released. An explicit relationship between the optimal growing direction and the parameters defining the local elastic field around the tip is obtained for an anti-plane field. This allows to describe a simple algorithm to compute the crack path. Mathematics Subject Classifications (2000) 74R05, 74B05, 74G70.Gerardo E. Oleaga: Supported by EU-Project Front Singularities University of Leipzig and the Max Planck Institute MIS. Partial support was also provided by the Spanish DGES project BFM2000-0605.