A New Deformation Theory with Strain Gradient Effects

A new phenomenological deformation theory with strain gradient effects is proposed. This theory, which belongs to nonlinear elasticity, fits within the framework of general couple stress theory and involves a single material length scale l. In the present theory three rotational degrees of freedom omega(i) are introduced in addition to the conventional three translational degrees of freedom u(i). omega(i) has no direct dependence upon ui and is called the micro-rotation, i.e. the material rotation theta(i) plus the particle relative rotation. The strain energy density is assumed to only be a function of the strain tensor and the overall curvature tensor, which results in symmetric Cauchy stresses. Minimum potential principle is developed for the strain gradient deformation theory version. In the limit of vanishing 1, it reduces to the conventional counterparts: J(2) deformation theory. Equilibrium equations, constitutive relations and boundary conditions are given in details. Comparisons between the present theory and the theory proposed by Shizawa and Zbib (Shizawa, K., Zbib, H.M., 1999. A thermodynamical theory gradient elastoplasticity with dislocation density Censor: fundamentals. Int. J. Plast. 15, 899) are given. With the same hardening law as Fleck et al. (Fleck, N.A., Muller, G.H., Ashby, M.F., Hutchinson, JW., 1994 Strain gradient plasticity: theory and experiment. Acta Metall. Mater 42, 475), the new strain gradient deformation theory is used to investigate two typical examples, i.e. thin metallic wire torsion and ultra-thin metallic beam bend. The results are compared with those given by Fleck et al, 1994 and Stolken and Evans (Stolken, J.S., Evans, A.G., 1998. A microbend test method for measuring the plasticity length scale. Acta Mater. 46, 5109). In addition, it is explained for a unit cell that the overall curvature tensor produced by the overall rotation vector is the work conjugate of the overall couple stress tensor. (C) 2002 Elsevier Science Ltd. All rights reserved.

Formation, microstructure and development of the localized shear deformation in low-carbon steels

An investigation has been made into the effect of microstructural parameters on the propensity for forming shear localization produced during high speed torsional testing by split Hopkinson bar with different average rates of 610, 650 and 1500 s(-1) in low carbon steels. These steels received the quenched, quenched and tempered as well as normalized treatments that provide wide microstructural parameters and mechanical properties. The results indicate that the occurrence of the shear localization is susceptible to the strength of the steels. In other words, the tendency of the quenched steel to form a shear band is higher than that of the other two steels. It is also found that there is a critical strain at which the shear localization occurs in the steels. The critical strain value is strongly dependent on the strength of the steels. Before arriving at this point, the material undergoes a slow work-hardening. After this point, the material suffers work-softening, corresponding to a process during which the deformation is gradually localized and eventually becomes spatially correlated to form a macroscopic shear band. Examinations by SEM reveal that the shear localization within the band involves a series of sequential crystallographic and non-crystallographic events including the change in crystal orientation, misorientation, generation and even perhaps damage in microstructures such as the initiation, growth and coalescence of the microcracks. It is expected that the sharp drop in the load-carrying capacity is associated with the growth and coalescence of the microcracks rather than the occurrence of the shear localization, but the shear localization is seen to accelerate the growth and coalescence of the microcracks. The thin foil observations by TEM reveal that the density of dislocations in the band is extremely high and the tangled arrangement and cell structure of dislocations tends to align along the shear direction. The multiplication and interaction of dislocations seems to be responsible for work-hardening of the steels. The avalanche of the dislocation cells corresponds to the sharp drop in shear stress at which the deformed specimen is broken. Double shear bands and kink bands are also observed in the present study. The principal band develops first and its width is narrower than that of the secondary band.

障碍物扰动对半开口管道中火焰传播特性的影响

自从1926年Chanman和Wheeler率先开创有障碍物管道中的火焰传播研究工作以来，管道中障碍物扰动引起的火焰加速现象引起了广泛的关注。由于这类现象在燃烧科学上的学术意义及其在生产中诸如安全问题等方面的实际意义，人们相继进行了一系列的研究工作。归纳起来，可以分为两大类：1）封闭管道的火焰传播。人们在不同形状的管道或容器中研究了障碍物对火焰加速的影响，在理论分析和数值计算方面也作了一些有益的工作；2）开口管道的火焰传播。相对于闭口管道，开口管道中的火焰传播研究则逊色得多。这方面尚缺乏系统的实验数据，理论方面的分析也欠缺得多。但当前在实际中，并不乏开口体系的应用，如，现已广泛应用于电力系统的一种燃气除灰装置，是一燃烧气体燃料的半开口系统。因此，研究半开口管道中非稳态燃烧的加速机制具有重要意义。在本论文工作中，通过大量的实验研究，比较系统地研究了障碍物的扰动对预混火焰传播特性的影响。实验在一长L=5m、内径D＝80mm的一端封闭、一端开口的火焰传播管内进行，管内均匀布置障碍物，通过改变障碍物的形状、间距、阻塞比大小，同时选用五种不同的可燃气体，探索了障碍物结构对预混湍流火焰加速和管内压力上升的影响。实验表明，对于敏感气体如氢气和乙炔，由于障碍物扰动产生的影响，火焰不断加速，并最终达到一准稳定状态；在适当的条件下，火焰传播状态可由爆燃向爆轰转变，此时火焰速度发生跃变；而对干不敏感气体如甲烷，则爆燃转爆轰现象不容易发生。在不同的火焰传播状态，障碍物结构特性对火焰速度和压力产生的影响各不相同。在缓燃态，随着阻塞比的变化，最大火焰速度先上升后下降，在BR=0.3～0.4之间存在一最大值；在銮塞态，最大火焰速度受阻塞比变化的影响不明显，略低于燃烧产物的声速；在阻塞比BR＝0.5附近，压力达到最大值。而在爆轰态，随着阻塞比的增加，最大火焰速度和压力逐渐降低，爆燃转爆轰的浓度范围变小。由于在有障碍物的管道中，火焰速度很容易达到声速（变塞态）或超声速（爆轰态），必须考虑流体的马赫数效应。本文在前人研究成果的基础上，给出了湍流马赫数修正的可压缩性两方程湍流模型，模拟了半开口狭长管道中重复布置的障碍物引起的湍流火焰加速现象。最大火焰速度和管内压力的计算结果与实验测量值吻合良好，这表明用修正后的湍流模型能够比较真实地模拟障碍物管内预混火焰的发展过程。通过对管道内障碍物扰动引起的燃烧波加速的机理和技术研究，对流场扰动对燃烧波产生和发展的影响规律有了比较全面的了解，研究结果对气脉冲除灰技术的完善具有直接的指导意义。

Single attosecond pulse generation in He+ by controlling the instant ionization rate using attosecond pulse trains combined with an intense laser pulse

High-order harmonics and single attosecond pulse generation by using an infrared laser pulse combined with attosecond pulse trains (APT) interacting with He+ have been investigated. We show that the ionization for different instant time intervals can be controlled by altering the time delay between the APT and the infrared pulse. Consequently, APT can be used as a tool to control the efficiency of high-order harmonics emitted at different times. By choosing appropriate APT and time delay, the driving pulse width for single attosecond pulse generation can be extended up to six optical cycles. (c) 2007 Optical Society of America.