超高速撞击厚靶过程的能量分配研究

超高速撞击过程的能量分配研究,对于解决动能撞击、发展导弹拦截技术、判定空间飞行器被撞事件及评估碰撞破坏程度具有重要的理论意义。本文在总结前人关于超高速撞击过程能量分配的基础上,将超高速撞击厚靶过程中弹丸的动能分配归纳为靶板的变形能、弹丸与靶板作用过程应力波传播使靶板内能的增加、撞击产生碎片的崩溅能和产生电磁辐射的辐射能,并结合理论推导、实验和数值模拟对撞击速度为2.61km/s且正碰撞2A12铝靶的能量分配进行了定量计算。研究结果表明：无论在弹坑的形貌、尺寸还是辐射温度等方面,实验测量结果、理论推导结果与数值模拟的结果均基本吻合。该研究成果在解决行驶中的车辆碰撞问题以及航空飞行器遭遇鸟撞等领域亦有重要的参考价值。     

基于拟流体模型的SPH新方法及其在弹丸超高速碰撞薄板中的应用

引入颗粒动力学理论(拟流体模型)建立了适用于超高速碰撞的SPH新方法。将超高速碰撞中处于损伤状态的碎片等效为拟流体,在描述其运动过程中引入了碎片间相互作用和气体相对碎片的作用。采用该方法对球形弹丸超高速碰撞薄板形成碎片云的过程进行了数值模拟,得到了弹坑直径、外泡碎片云和内核碎片云的形状、分布,并与使用传统SPH方法、自适应光滑粒子流体动力学(ASPH)方法的模拟结果进行对比,结果显示:新方法在内核碎片云形状和分布上计算结果更加准确。同时对Whipple屏超高速碰撞问题进行了研究,分析了不同撞击速度下防护屏弹坑尺寸及舱壁损伤特性等特性,计算结果与实验吻合较好且符合Whipple防护结构的典型撞击极限曲线。     

基于能量原理确定系泊碰撞载荷的有限元法

准确地确定出船舶系泊时的撞击载荷, 对船舶及海洋平台的强度研究都是极其重要的. 提出了通过能量原理和有限元分析确定系泊撞击载荷的方法. 碰撞时, 动能和变形能满足能量守恒, 动能是包括附连水质量在内的船体总动能, 此动能在撞击时转化为变形能, 根据变形能与载荷的关系, 求出碰撞载荷. 通过具有碰撞载荷理论解的梁的验证, 表明该方法是完全可行的. 为实际工程提供了确定护舷撞击力的理论依据和计算方法.     

Oldroyd-B黏弹性液滴弹跳行为的改进SPH模拟

基于光滑粒子流体动力学SPH(Smoothed Particle Hydrodynamics)方法对Oldroyd-B黏弹性液滴撞击固壁面产生的弹跳行为进行了模拟与分析。首先,为了解决SPH模拟黏弹性自由表面流出现的张力不稳定性问题,联合粒子迁移技术提出了一种改进SPH方法。然后,对Oldroyd-B黏弹性液滴撞击固壁面产生的铺展行为进行了改进SPH模拟,与文献结果的比较验证了方法的有效性。最后,通过降低Reynolds数捕捉到了液滴的弹跳行为;并在此基础上,分析了液滴黏度比、Weissenberg数和Reynolds数对液滴弹跳行为的影响。结果表明,改进SPH方法可有效地模拟黏弹性自由表面流问题;液滴黏度比、Weissenberg数和Reynolds数对液滴最大回弹高度均有显著的影响。     

光滑粒子模拟方法在超高速碰撞现象中的应用

简要介绍了基于黎曼解的光滑粒子法,并将改进的SPH方法应用于超高速碰撞,对二维轴对称条件下的弹丸超高速碰撞薄板问题进行了数值模拟,研究了靶板厚度、弹丸速度、弹丸形状等因素对形成碎片云的影响。通过与实验数据比较,该算法模拟的碎片云的形状及特征与实验相吻合,验证了光滑粒子法对冲击动力学问题数值模拟的有效性。     

Q235钢板对半球形头弹抗侵彻特性

利用轻气炮进行了半球形头杆弹正撞击单层板和等厚接触式三层板的实验, 得到了这两种结构靶体的初始-剩余速度曲线以及弹道极限。采用ABAQUS/EXPLICIT数值模拟软件对杆弹撞击金属板的过程进行了数值模拟研究, 通过对比数值模拟和实验结果, 验证了数值模拟材料模型和参数的有效性。研究了靶体结构对抗侵彻特性的影响, 并分析了弹体对靶体的撞击过程。研究结果表明:多层板的弹道极限高于等厚单层板。单层板主要失效模式为剪切, 而多层板的主要失效模式为整体的蝶形变形和局部的盘式隆起。对于多层板, 靶板具体的失效模式与其在靶中位置相关。     

柔性头部模型与大挠度板接触撞击力的预报方法

针对被撞击板是大挠度的特点,提出了模拟函数预报撞击力的方法,根据撞击系统的动量守恒原理推导出了撞击力的预报方程,用恢复系数考虑头皮对撞击缓冲的影响．得到了头部与发动机罩板大变形、非线性、瞬态撞击过程的响应函数．分析结果与鸿巢敦宏、石川博敏的试验数据进行了对比,结果显示有较好的一致性,表明该方法能够较好地描述柔性球体模型与大挠度板接触撞击的响应过程．同时还利用该预报方程分析了各种因素对接触撞击力的影响．     

不同壁面条件下液滴撞击铺展特性的模拟研究

液滴撞击不同润湿性壁面的传热流动问题在自然界和工业生产中广泛存在。研究采用CLSVOF方法,引入描述壁面润湿特性的动态接触角,并考虑液滴物性参数随温度的变化,建立液滴撞壁模型,模拟研究液滴撞击流动行为,通过与实验对比验证,确定模型有效性。在此基础上,对传热作用下考虑壁面润湿性的液滴撞击问题展开研究,探讨壁面传热作用对液滴撞击铺展特性的影响。研究表明,在撞击过程中,液滴先铺展后逐渐收缩,与静态接触角模型相比,采用动态接触角模型所得的液滴流动特性与实验结果更加吻合;随着接触角增大,液滴在撞壁初期不易铺展,随后则易于收缩;虽然固液传热作用会影响液滴铺展直径,但不改变液滴的运动趋势。     

薄膜断裂和穿透的向量式有限元分析及应用

向量式有限元基于牛顿运动定律,通过质点描述和向量分析来求解整体结构的动力响应。首先,给出了向量式有限元三角形薄膜单元的基本理论,进而针对薄膜结构的断裂和穿透破坏过程,提出相应解决方案。对于薄膜断裂问题,采用Mises应力状态变量达到失效应力限值作为断裂判据,通过质点分裂方式将相连单元的对应节点断开,并对分裂后的新质点进行状态更新来模拟其断裂过程;对于薄膜穿透问题,则同时结合碰撞和断裂过程模拟来实现。在此基础上,编制了薄膜结构的断裂和穿透求解计算程序,算例分析表明,程序可很好地完成薄膜结构的大变形大转动、断裂和穿透等不连续行为的模拟,验证了理论及程序的可靠性和有效性,体现了本文方法进行薄膜结构复杂不连续行为分析的优势。     

薄膜断裂和穿透的向量式有限元分析及应用

向量式有限元基于牛顿运动定律,通过质点描述和向量分析来求解整体结构的动力响应。首先,给出了向量式有限元三角形薄膜单元的基本理论,进而针对薄膜结构的断裂和穿透破坏过程,提出相应解决方案。对于薄膜断裂问题,采用Mises应力状态变量达到失效应力限值作为断裂判据,通过质点分裂方式将相连单元的对应节点断开,并对分裂后的新质点进行状态更新来模拟其断裂过程;对于薄膜穿透问题,则同时结合碰撞和断裂过程模拟来实现。在此基础上,编制了薄膜结构的断裂和穿透求解计算程序,算例分析表明,程序可很好地完成薄膜结构的大变形大转动、断裂和穿透等不连续行为的模拟,验证了理论及程序的可靠性和有效性,体现了本文方法进行薄膜结构复杂不连续行为分析的优势。     

Experimental Observations on Dynamic Response of Selected Transparent Armor Materials

Structural transparent material systems are critical for many military and civilian applications. Transparent armor systems can consist of a wide variety of glass laminate assemblies with polymeric bonding interfaces and backing as well as the inclusion of polycrystalline ceramic (AlON, spinel) and single crystals (sapphire) as front facing materials. Over the last 20 years as the threats have escalated and become more varied, the challenges for rapidly developing optimized threat specific transparent armor packages have become extremely complex. Ultimate failure of structural ceramics in impact events is a function of the temporal and spatial interaction of the macro-stresses at the macro-, micro- and nano-structural scale, including elastic and inelastic (plastic) deformation, crack nucleation, damage evolution and resulting failure from the macro-scale (top down) and/or from the nano-scale (bottom up). In order to accelerate the development of validated design and predictive performance models, a systematic series of experimental investigations have been carried out on various non-crystalline ceramics (glass), single crystal (sapphire) and polycrystalline ceramics (AlON). The Edge-on Impact (EOI) test coupled with a high-speed Cranz-Schardin film camera has been extensively used on a variety of monolithic and laminated glasses, AlON and crystallographically controlled sapphire single crystals to visualize and quantify stress wave, crack and damage propagation. A modified Kolsky bar technique instrumented with a high speed digital camera has been utilized in an unconfined and confined test sample mode to examine the dynamic deformation and failure of AlON undergoing uniaxial, high strain rate compression. Real time photography has clearly demonstrated the critical influence of defects and post mortem characterization of fragments resulting from these tests have revealed the influence of micro-deformational twining and cleavage down to the nano-scale. Finally, a brief summary of work using ultra-high-speed photography of the impact of conventional projectiles on glass and AlON will be presented. These experimental results will be absolutely critical to help evolve and validate existing models used in computer codes to simulate the impact performance of brittle materials.     

Stochastic bifurcation analysis of a friction-damped system with impact and fractional derivative damping

Nonlinear Dynamics - Impact together with friction can be widely found in the mechanical engineering. Although some scholars have investigated the stochastic systems with impact and friction, they...     

Modelling inelastic behaviour of orthotropic metals in a unique alignment of deviatoric plane within the stress space

A finite strain constitutive model to predict the deformation behaviour of orthotropic metals is developed in this paper. The important features of this constitutive model are the multiplicative decomposition of the deformation gradient and a new Mandel stress tensor combined with the new stress tensor decomposition generalized into deviatoric and spherical parts. The elastic free energy function and the yield function are defined within an invariant theory by means of the structural tensors. The Hill’s yield criterion is adopted to characterize plastic orthotropy, and the thermally micromechanical-based model, Mechanical Threshold Model (MTS) is used as a referential curve to control the yield surface expansion using an isotropic plastic hardening assumption. The model complexity is further extended by coupling the formulation with the shock equation of state (EOS). The proposed formulation is integrated in the isoclinic configuration and allows for a unique treatment for elastic and plastic anisotropy. The effects of elastic anisotropy are taken into account through the stress tensor decomposition and plastic anisotropy through yield surface defined in the generalized deviatoric plane perpendicular to the generalized pressure. The proposed formulation of this work is implemented into the Lawrence Livermore National Laboratory-DYNA3D code by the modification of several subroutines in the code. The capability of the new constitutive model to capture strain rate and temperature sensitivity is then validated. The final part of this process is a comparison of the results generated by the proposed constitutive model against the available experimental data from both the Plate Impact test and Taylor Cylinder Impact test. A good agreement between experimental and simulation is obtained in each test.     

The role of epistemic uncertainty of contact models in the design and optimization of mechanical systems with aleatoric uncertainty

Epistemic uncertainty, the uncertainty in the physical model used to represent a phenomenon, has a significant effect on the predictions of simulations of mechanical systems, particularly in systems with impact events. Impact dynamics can have a significant effect on a system’s functionality, stability, wear, and failure. Because high-fidelity models of systems with impacts often are too computationally intensive to be useful as design tools, rigid body dynamics and reduced order model simulations are used often, with the impact events modeled by ad hoc methods such as a constant coefficient of restitution or penalty stiffness. The choice of impact model, though, can have significant ramifications on design predictions. The effects of both epistemic and aleatoric (parametric) uncertainty in the choice of contact model are investigated in this paper for a representative multiple-degree of freedom mechanical system. Six contact models are considered in the analysis: two different constant coefficient of restitution models, a piecewise-linear stiffness and damping (i.e., Kelvin–Voight) model, two similar elastic-plastic constitutive models, and one dissimilar elastic-plastic constitutive model. Results show that the optimal mechanism design for each contact model appears extremely different. Further, the effects due to epistemic uncertainty are differentiated clearly in the response from the effects due to aleatoric uncertainty. Lastly, when the mechanisms are optimized to be robust against aleatoric uncertainty, the resulting designs show some robustness against epistemic uncertainty.     

Numerical simulation: The dynamic behavior of reinforced concrete plates under normal impact

This investigation deals with the use of the finite element method on the reinforced concrete structural dynamic response and failure behavior when subjected to the projectile impacts of different velocities, using the test conducted in [S.J. Hanchak, M.J. Forrestal, E.R. Young, J.Q. Ehrgott, Perforation of concrete slabs with 48 MPa (7 ksi) and 140 MPa (20 ksi) unconfined compressive strengths, Int. J. Impact Eng. 12 (1992) 1–7]. The Johnson–Holmquist concrete material constitutive law model is employed to simulate the large strains, high strain states and high pressures to which the concrete is subjected. The projectile impact velocity ranges from 381 m/s to 1058 m/s. Numerical simulations demonstrate that the Johnson–Holmquist concrete material constitutive model can describe the different failure modes without any predefined defects in the element mesh, and normally obtain good agreement between the numerical simulations and test results.     

Green water impact pressure on a three-dimensional model structure

Green water impact pressure due to plunging breaking waves impinging on a simplified, three-dimensional model structure was investigated in the laboratory. Two breaking wave conditions were tested: one with waves impinging on the vertical wall of the model at the still water level and the other with waves impinging on the horizontal deck surface. Pressure measurements were taken at locations in two vertical planes on the deck surface with one at centerline of deck and the other between the centerline and an edge. Impact pressure was found to be quite different between the two wave conditions even though the incoming waves are essentially identical. Two types of pressure variations were observed??impulsive type and non-impulsive type. Much higher pressure was observed for the deck impingement wave condition, even though the flow velocities were quite close. Void fraction was also measured at selected points. Impact pressure was correlated with the mean kinetic energy calculated based on the measured mean velocities and void fraction. Impact coefficient, defined as the ratio between the maximum pressure at a given point and the corresponding mean kinetic energy, was obtained. For the wall impingement wave condition, the relationship between impact pressure and mean kinetic energy is linear with the impact coefficient close to 1.3. For the deck impingement wave condition, the above relationship does not show good correlation; the impact coefficient was between 0.6 and 7. The impact coefficient was found to be a function of the rate of pressure rise.     

Numerical simulation and experimental study of Parallel Seismic test for piles

The theoretical capabilities of the Parallel Seismic (PS) test for determining the length of piles and characterizing possible defects are investigated in this paper. In deriving the theory, a correction factor is proposed in this paper to enhance the accuracy of the prediction. An axisymmetric finite element (FE) model was developed to carry out a series of parametric studies which included the effects of the pile length, the pile-to-borehole distance and the stiffness of surrounding soil. A miniature experiment using an aluminum bar embedded in epoxy prism was also designed and tested in the lab to verify the theory and to demonstrate the use of the correction factor. The results from the experiment and the numerical FE model were also compared to expose the potential of this nondestructive method to in situ application. The model was further modified to simulate the PS test on defective piles with axisymmetric necks and bulges. It can be concluded that the geometric configuration of a pile and the possible significant defect can be characterized with this nondestructive test.     

一种用于材料高应变率剪切性能测试的新型加载技术

高应变率下的冲击剪切实验技术是材料动态力学行为及其微观机理研究的重要基础.采用分离式霍普金森压杆(split Hopkinson pressure bar)装置一般可以获得材料在104s-1以内应变率的动态力学性能.在超过104s-1的应变率下对材料进行冲击剪切测试时,通常需要采用高速压剪飞片技术或由气炮发射子弹对试样进行直接加载.本文提出一种可用于传统霍普金森压杆技术的新型双剪切试样,可以在103~105s-1剪应变率范围实现对材料剪切性能的精确测量;同时,可以对材料的变形及失效过程进行直接观测.试样与压杆之间避免了复杂的界面或连接装置,通过转接头可以保证试样与压杆直接接触,提高测试精度,同时可以防止因试样的横向位移而导致的非均匀变形.获得了紫铜在1400~75000s-1应变率下的剪应力-剪应变曲线,并采用计算软件"ABAQUS/Explicit"对双剪切试样的动态加载过程进行了数值模拟和结果验证.分析表明,剪切区的主要区域内剪切成分占主导地位,其应力应变场沿厚度及宽度方向基本呈均匀分布.实验得到的剪应力-剪应变曲线与模拟结果吻合较好,说明所提出的基于分离式霍普金森压杆系统的双剪切试样可以为材料的高应变率力学性能测试提供一种方便有效的加载技术.      

高强度钢的超高周疲劳裂纹扩展模型研究

当疲劳寿命在106或107周时,Wöhler S-N 曲线被看作渐近于水平轴,107的疲劳强度被看成是疲劳极限。现代应用要求延长零件的工作寿命,实际齿轮部件应用超过107循环的疲劳失效。本文应用压电超声疲劳试验机对经过热处理和渗碳处理后的低铬合金钢材料进行研究,采用红外摄像仪观测试件表面的温度场随疲劳裂纹萌生和扩展的过程。试验条件是室温,应力比为0.1（R=0.1）,频率为20KHz。通过对表面渗碳处理后试件的断口分析,探讨表面渗碳处理、微观结构和与杂质有关的断裂机理,根据Paris公式建立超高周疲劳裂纹扩展模型。对裂纹扩展过程中裂纹尖端的塑性区的分析结果,结合传热学原理,建立热耗散模型,有限元方法的数值解结果较好地符合红外摄像仪的观测的试验结果。     

单向KFRP的应变率相关的本构方程

在旋转盘式杆杆型冲击拉伸试验装置上 ,对单向Kevlar 49纤维增强酚醛树脂复合材料(KFRP)进行了冲击拉伸试验 ,得到了应变率为 15 0 ,40 0和 15 0 0s-1下的单向KFRP的完整拉伸应力应变曲线 ;结果表明 ,单向KFRP的力学性能是应变率相关的。通过改进复合丝束模型 ,建立了计及应变率效应的单向KFRP的一维损伤宏观本构方程。