多胞牺牲层的抗爆炸分析

运用一维冲击波模型和三维细观有限元模型分析了多胞牺牲层的抗爆炸行为. 基于刚性-塑性硬化(R-PH)的多胞材料模型,建立了一维冲击波模型,得到了多胞牺牲层中冲击波传播的控制方程. 揭示了冲击波在多胞牺牲层中的传播特性,并阐述了附加质量和爆炸载荷强度两个参数对牺牲层设计的重要影响. 比较了基于刚性-理想塑性-锁定(R-PP-L) 模型和基于刚性-塑性硬化(R-PH) 模型的多胞牺牲层的结构设计,指出了两种模型的适用范围. 通过基于三维Voronoi 技术的细观有限元方法验证了基于R-PH 模型的多胞牺牲层结构的设计准则.      

梯度多胞牺牲层的抗爆炸分析

运用一维非线性塑性冲击波模型和细观有限元模型对密度梯度多胞牺牲层的抗爆炸性能进行了分析。基于率无关的刚性-塑性硬化模型,建立了描述冲击波在多胞牺牲层中传播的控制方程,分别给出了正、负密度梯度多胞材料在指数型爆炸载荷作用下的响应特性。研究了可正好吸收爆炸能量的梯度多胞牺牲层的临界厚度与载荷强度、覆盖层质量、多胞材料的密度梯度等参数之间的关系,给出了以临界厚度和支撑端应力峰值为指标的密度梯度设计图。运用二维细观有限元模型验证了基于非线性塑性冲击波模型的抗爆炸分析的有效性。     

多孔泡沫牺牲层的动态压溃及缓冲吸能机理研究

本文对强动载荷下多孔泡沫牺牲层的动态压溃行为及缓冲吸能机理进行了研究. 基于刚性-理想塑性-锁定(R-PP-L)及刚性-塑性硬化(R-PH)两类多孔泡沫材料本构, 建立了强动载荷下多孔泡沫牺牲层动态响应的理论分析模型, 分析了一维冲击波在多孔泡沫牺牲层中的传播规律; 利用Voronoi方法建立了多孔泡沫牺牲层的二维细观有限元模型, 获得了冲击载荷下多孔泡沫牺牲层的变形模式和动态响应曲线, 讨论了多孔泡沫材料的层间界面效应对多孔泡沫牺牲层缓冲吸能的影响. 研究结果表明, 考虑多孔泡沫材料塑性硬化影响的理论分析模型能够预测入射波在远端的反射及对多孔泡沫牺牲层的二次压缩过程和端部应力增强现象; 相比较存在界面的多孔泡沫牺牲层, 连续设计的多孔泡沫牺牲层可增强其缓冲吸能能力, 但在界面处增加设计刚性面板则能够降低界面胞元不完整对缓冲吸能的影响; 相同冲量载荷下, 端部应力峰值随冲击能量增大而增大, 而端部冲击波的反射可能是端部应力增强的主要诱因.      

Crashworthiness design of density-graded cellular metals

Crashworthiness of cellular metals with a linear density gradient was analyzed by using cell-based finite element models and shock models. Mechanisms of energy absorption and deformation of graded cellular metals were explored by shock wave propagation analysis. Results show that a positive density gradient is a good choice for protecting the impacting object because it can meet the crashworthiness requirements of high energy absorption, stable impact resistance and low peak stress.     

On the local nature of the strain field calculation method for measuring heterogeneous deformation of cellular materials

A strain field calculation method based on the optimal local deformation gradient technique has been developed to calculate the ‘local’ strain tensor of cellular materials using cell-based finite element models. The local nature and accuracy of this method may be strongly dependent on the cut-off radius, which is introduced to collect the effective nodes for determining the optimal local deformation gradient of a node. Two different schemes are first analyzed to determine the suitable cut-off radius by characterizing the heterogeneous deformation of Voronoi honeycombs under uniaxial compression and we suggest that in Scheme 1, the cut-off radius defined based on the reference configuration is about 1.5 times the average cell radius; in Scheme 2, the cut-off radius defined based on the current configuration is about 0.5 times the average cell radius. Then, Scheme 3, a combined scheme of the two former schemes, is further suggested. It is demonstrated that the optimal cut-off radius in Scheme 3 characterizes the local strain reasonable well whether the compression rate is low or high. Finally, the strain field calculation method with the optimal cut-off radius is applied to reveal the evolution of the heterogeneous deformation of two different configurations of double-layer cellular cladding under a linear decaying blast load. The 2D fields and the 1D distributions of local engineering strain are calculated. These results interpret the shock wave propagation mechanisms in both claddings and provide useful understanding in the design of a double-layer cellular cladding.     

多胞材料的动态应力应变状态及其一致近似关系

多胞材料在高速冲击下呈现出逐层压溃的变形模式,塑性冲击波模型可以用来表征这种集中变形带的传播行为。本文中采用截面应力计算方法得到了随机蜂窝在恒速冲击下的一维应力分布,进而对冲击波的传播规律进行了分析。比较了高速冲击下由不同方法得到的冲击波速度与冲击速度的关系,结果表明R-PP-L（率无关,刚性-理想塑性-锁定）模型高估了冲击波速度,但R-PH（率无关,刚性-塑性硬化）模型以及一维冲击波理论得到的冲击波速度与有限元结果比较接近。冲击波速度与冲击速度在高速情形下趋于线性关系,但随着冲击速度的减小,冲击波速度不断减少并趋于常数。根据这一特征和塑性冲击波模型,发展了可以表征冲击波速度与冲击速度的关系、动态应力应变关系的一致近似模型。     

具有恒定冲击载荷的梯度泡沫金属材料设计

多胞材料可通过大变形大量地吸收冲击能量,引入密度梯度可进一步提高其耐撞性。梯度多胞材料的宏观力学响应对材料密度分布极为敏感,不同类型的细观构型的影响也极为不同。已有的研究工作主要局限在对给定的密度梯度分析其动态响应,较少对耐撞性设计方法进行研究。本文针对梯度闭孔泡沫金属材料,基于非线性塑性冲击波模型发展了耐撞性反向设计方法,以维持冲击物受载恒定为目标,运用级数法获得了简化模型和渐近解。利用变胞元尺寸法构建了连续梯度变化的三维Voronoi细观有限元模型,并利用ABAQUS/Explicit有限元软件对理论设计进行数值验证。结果表明,反向设计理论简化模型的渐近解对于梯度闭孔泡沫金属材料的耐撞性设计是有效的,所提出的耐撞性设计方法在控制冲击吸能过程和冲击物受载方面具有指导意义。     

高聚物牺牲包层对钢筋混凝土板的爆炸毁伤缓解效应

为研究高聚物牺牲包层对钢筋混凝土结构的爆炸毁伤缓解效应,开展了带高聚物牺牲包层钢筋混凝土板的接触爆炸试验,同时设置了普通钢筋混凝土板作为对照组,对比分析了高聚物牺牲包层对钢筋混凝土板毁伤特征的影响。此外,运用AUTODYN软件建立了现场爆炸试验的SPH-FEM耦合模型,通过与试验结果的对比,验证了所建耦合模型的可靠性。在此基础上,通过参数敏感性分析,探究了炸药量和高聚物牺牲包层密度、厚度对带高聚物牺牲包层钢筋混凝土板毁伤特性以及吸能特性的影响。结果表明：接触爆炸下,高聚物牺牲包层能够有效地分散爆炸荷载,缓解爆炸荷载对钢筋混凝土板的冲击作用,具有良好的防护性能;药量在一定范围内增大时,高聚物牺牲包层依然能维持较高的吸能水平,增大包层密度和厚度有利于增强高聚物牺牲包层的吸能特性,包层厚度的变化会造成被保护钢筋混凝土板毁伤模式的改变。     

弹性半空间二维出平面自由波场的一维化时域算法

提出了一种计算出平面SH波斜入射时弹性半空间自由波场时域计算的一维化有限元方法。首先利用Snell定律确定平面波沿水平方向的传播规律,在用有限元法对弹性半空间进行离散化时,竖向单元尺寸根据波动有限元模拟精度要求确定,而水平向有限元网格尺寸根据水平向波的传播规律和采用的离散时间步长确定,使得有限元离散模型中任意节点的运动可以用水平向相邻节点的运动表示,从而将二维有限元节点运动方程组化为一维的形式。求解此一维方程组,可得到弹性半空间中一列节点的运动,再根据行波的传播规律,可确定全空间自由波场。理论分析和数值算例表明,该方法具有较高的精度和良好的稳定性。     

爆轰波强间断问题的伪弧长算法及其人为解验证

针对该问题开展了伪弧长数值算法研究,通过引入弧长参数,使网格按照一定的形式自适应移动,达到在强间断区域自动加密的效果,从而提高网格分辨率。基于伪弧长算法编写了二维程序,并对程序进行人为解方法验证。将伪弧长算法和直接有限体积法的数值结果进行对比,通过误差分析,显示出伪弧长算法能有效提高计算精度。最后将伪弧长算法应用于气相爆轰波在二维管道中的传播问题,研究了波阵面的捕捉效果和爆轰波胞格结构的形成过程。     

炸药水中爆炸冲击波超高速同时分幅/扫描摄影技术

研究了超高速同时分幅/扫描摄影技术在炸药水中爆炸冲击波传播中的应用效果,并采用自研的同时分幅/扫描超高速光电摄影系统对炸药水中爆炸过程进行拍摄。结果表明,该系统能够拍摄同一时基、同一空基且具有超高时空分辨的一维和二维图像,进而成功观察到?60 mm×66 mm TNT炸药在水中爆炸冲击波传播过程,获得该过程冲击波阵面平均传播速度及压力。这可为水中兵器战斗部的设计及毁伤效能评估提供参考。     

EFFECTS OF 2-TUNNEL AND 3-TUNNEL FIXATION ON BIOMECHANICS OF TIBIOFEMORAL JOINT

基于正常膝关节4个屈曲角度(0°, 25°, 60°, 80°)下的磁共振(MR)图像数据, 建立正常/两隧道固定/三隧道固定内侧半月板移植术后的胫股关节模型. 对各屈曲角度下的4种模型分别施加单一轴向压载和压扭组合载荷进行有限元仿真, 得到各模型在2种载荷作用下的应力及位移分布. 提取关节软骨、半月板表面的等效应力以及半月板的最大位移进行对比分析, 得到内侧半月板移植术后两/三隧道固定对胫股关节中软骨及半月板的影响. 总体上讲内侧半月板移植中的三隧道固定要优于双隧道固定, 更利于胫股关节恢复正常的应力特性.     

Wave propagation in double helical rods

The propagation of waves in helical rods has been studied extensively. However, studying the wave propagation in double helical rods have received less attention although this can be useful in multiple fields of science and engineering. Obtaining an analytical model for a double helical rod is challenging since the curvature and tortuosity are not constant. Thus, resolving the wave behaviour analytically is nearly impossible. In this paper, wave propagation in a double helical rod will be studied using the wave and finite element method which is a technique that can be used to model homogeneous and periodic one and two dimensional structures based on the periodic structure theory. For modelling a double helical rod, the finite element model of a single turn is processed using Bloch waves. The dispersion curves and wavemodes are obtained and the similarities and differences of waves in helical and double helical rods are highlighted.     

Symplectic analysis for elastic wave propagation in two-dimensional cellular structures

On the basis of the finite element analysis, the elastic wave propagation in cellular structures is investigated using the symplectic algorithm. The variation principle is first applied to obtain the dual variables and the wave propagation problem is then transformed into two-dimensional （2D） symplectic eigenvalue problems, where the extended Wittrick-Williams algorithm is used to ensure that no phase propagation eigenvalues are missed during computation. Three typical cellular structures, square, triangle and hexagon, are introduced to illustrate the unique feature of the symplectic algorithm in higher-frequency calculation, which is due to the conserved properties of the structure-preserving symplectic algorithm. On the basis of the dispersion relations and phase constant surface analysis, the band structure is shown to be insensitive to the material type at lower frequencies, however, much more related at higher frequencies. This paper also demonstrates how the boundary conditions adopted in the finite element modeling process and the structures＇ configurations affect the band structures. The hexagonal cells are demonstrated to be more efficient for sound insulation at higher frequencies, while the triangular cells are preferred at lower frequencies. No complete band gaps are observed for the square cells with fixed-end boundary conditions. The analysis of phase constant surfaces guides the design of 2D cellular structures where waves at certain frequencies do not propagate in specified directions. The findings from the present study will provide invaluable guidelines for the future application of cellular structures in sound insulation.     

有限元畸变单元离散模型中的SH波动

本文采用晶格动力学分析方法,研究了有限元畸变单元离散模型中的SH波的传播规律。结果表明单元畸变对波动规律影响显著,当单元畸变较小时,单元畸变对波动传播并不产生明显的不利影响,当单元畸变较大时,这一影响不可忽略。通过详细分析和对比集中质量、混合质量和一致质量模型中的波动规律,评价了不同有限元模型的抗畸变能力。文中还讨论了时域离散化与单元畸变联合作用对波动传播的影响。     

闭孔泡沫铝的多轴唯象受压本构参数

基于显微计算机断层扫描影像信息, 逆向重建闭孔泡沫铝试件的三维细观有限元模型, 定量研究闭孔泡沫铝在多轴压缩载荷作用下的大变形力学行为. 讨论了泡沫金属唯象弹塑性本构参数的确定方法, 根据计算结果确定了3 个有代表性的泡沫材料本构模型的本构参数, 并验证了这些本构模型在描述多轴压缩应力状态下的精度. 研究表明, 对于单轴压缩, 3 个本构模型的屈服面均有很好的精度;对于静水压缩, 有限元软件"ABAQUS"的可压缩泡沫本构模型屈服面会发生严重偏离, 陈-卢本构模型"屈服面" 略微低估静水压缩的屈服应力, 而体积强化本构模型的屈服面有很好的精度.      

接触爆炸荷载作用下带孔防护结构内冲击波的传播规律

采用有限元分析软件ATUODYN,对接触爆炸荷载作用下带孔防护结构内冲击波的传播进行了数值模拟,得到了防护结构孔口和内部中心处冲击波超压－时间曲线;分析了炸药量和爆心距孔口距离对防护结构内部超压、正压冲量的影响;以数值计算结果为基础,结合量纲分析,拟合得到了结构内部中心处爆炸冲击波特征参数的预估公式。     

Shock wave propagation and interactions

激波的传播特性既取决于激波的产生条件,也与所处的传播环境密切相关.驱动条件、几何边界、介质的物理化学属性等发生变化时,都会引起激波传播特性的改变,而激波的变化反过来又会对其波及的流场产生影响.尽管激波传播及其干扰现象广泛存在于自然界和人类科技活动之中,其复杂机理的认识、规律的描述乃至应用潜力的挖掘仍有漫长的路要走.本文根据气体中激波传播和干扰现象以及与之相关的理论描述特征,在对激波传播以及反射、折射等基本现象进行简要阐述的基础上,重点围绕目前的热点问题,包括激波/激波干扰、激波/边界层干扰、激波与湍流作用、激波的聚焦与点火以及激波作用下气体界面不稳定性等研究进行了介绍和讨论,旨在对近年来该领域的进展及获得的成果做一个概述和归纳,期望对将来的深入研究有一个鉴借意义.     

BAND STRUCTURE AND TRANSMISSION CHARACTERISTICS CALCULATION OF PHONONIC CRYSTALS BASED ON MULTI-LEVEL SUBSTRUCTURE TECHNIQUE

基于多重多级子结构方法提出一种快速的声子晶体能带与传输特性的计算策略. 主要思想是将声子晶体划分成多层级子结构有限元模型,在能带计算中采用静凝聚和子结构周游树技术将子结构的内部刚度阵凝聚到Bloch 边界上. 由于内部刚度阵并不随着简约波矢变化,所以这种计算策略可以大大降低求解规模并提高计算效率,并不对整体有限元模型引入近似. 在传输特性计算中同样采用该策略,由于声子晶体单胞具有周期性,所以各个单胞的系数矩阵是相同的,从而减少计算量,并且可以灵活地选择是否回代求解单胞内部自由度. 数值算例以三维局域共振型声子晶体和二维Bragg 散射型声子晶体为例,计算结果验证了这种求解策略的正确性和高效性,并适用于复杂声子晶体分析.     

Numerical modeling of PZT-induced Lamb wave-based crack detection in plate-like structures

This paper presents the numerical modeling and simulations of PZT-induced Lamb wave propagation in plate-like structures by using the spectral finite element method. A novel spectral plate finite element, which can efficiently model the three-dimensional (3D) behavior of Lamb waves, is proposed. In the formulation, linear displacement distributions in the thickness direction are assumed for both the PZT layer and the base plate. A way to avoid the thickness locking is proposed and used in the formulations. Two examples, one for the validation of the proposed two-dimensional (2D) spectral finite element and the other for the demonstration of crack detection in plates, are presented and discussed. The contact between the two faces of crack is considered. Numerical results show that (1) only the anti-symmetric mode is prone to thickness locking thus remedy should be made only on this part, (2) the proposed 2D spectral finite element can adequately model the Lamb wave propagation in plate-like structures and the complex scattering for the crack, and (3) crack location can be well determined by a PZT-induced Lamb wave-based diagnosis algorithm.