含能活性材料防护结构超高速撞击特性数值模拟研究

含能活性材料超高速撞击数值模拟研究对于新型空间碎片防护机理探索研究具有重要意义。采用AUTODYN动力学仿真程序,基于改进的Lee-Tarver点火增长模型,在验证计算模型有效性的基础上,开展球形弹丸超高速撞击含能活性材料防护结构数值模拟研究,对弹丸临界破碎速度、碎片云形貌特征及后墙损伤等进行分析。给出了铝合金弹丸超高速撞击PTFE/Al含能活性材料防护屏的临界破碎速度公式,得到弹丸直径、撞击速度和含能活性材料防护屏板厚对碎片云特征参数的影响规律,进一步揭示了含能活性材料防护结构超高速撞击条件下的新型防护机理。     

椭球弹丸超高速撞击防护屏碎片云数值模拟

低地球轨道的各类航天器易受到微流星体及空间碎片的超高速撞击.本文采用AUTODYN软件进行了椭球弹丸超高速正撞击及斜撞击防护屏碎片云的数值模拟.给出了三维模拟的结果.研究了在相同质量的条件下,不同长径比椭球弹丸以不同速度和入射角撞击防护屏所产生碎片云的特性,并与球形弹丸撞击所应产生的碎片云特性进行了比较.结果表明:在相同的速度下,不同长径比椭球弹丸撞击的碎片云形状、质量分布和破碎程度是不同的,随撞击入射角的增加弹丸的破碎程度增大,滑弹碎片云的数量增加;随撞击速度的增加,弹丸的破碎程度也增加.     

基于帧间相关性的高动态视频色调映射研究

目前高动态范围视频的色调映射基本采用逐帧处理的方式,效率不高。本文利用视频的帧间相关性,摈弃每帧做色调映射处理,研究讨论了一种新的处理方法。基于颜色直方图提取视频序列的关键帧,将视频序列分组并设置每组参考帧,对参考帧做双边滤波的色调映射处理,将组内每帧图像分块,与参考帧进行块匹配的运动估计,块匹配运动估计采用自适应十字模式搜索法和平均绝对误差匹配准则,利用参考帧中匹配块的信息得到对应的低动态图像,由此避免了对视频序列进行逐帧色调映射处理。通过结构相似性和处理速度两个维度对处理方法进行评估,仿真实验结果表明,与逐帧进行双边滤波压缩处理结果有约89%的结构相似度时,处理速度约有2. 7倍的提高。     

一种基于HVS特性的视频质量评测方法

本文针对视频质量的评测应用,对传统峰值信噪比(PSNR)算法加以改进.通过在视频帧内图像和帧间图像的处理过程中引入人眼视觉系统(HVS)的主要特性,克服传统PSNR算法在序列质量检测应用方面的缺陷.方法在帧内图像处理上利用人眼对边缘轮廓失真具有较强敏感性的特点,设计了基于图像边缘的检测方案以提高对典型空域失真的检测性能;在帧间图像处理上,通过测量帧间时域能量的变化,获得序列在时域轴上的典型特征,并据此对空域检测结果进行修正.通过上述改进,算法能在保持传统PSNR算法简易性的同时,提升其检测结果与主观感受的相关性;同时算法的计算量并不复杂,易于在检测设备中实现系统集成     

机载光电跟踪系统连续帧图像快速识别定位

针对机载光电跟踪系统连续帧图像快速识别、准确定位的要求,本文在深入分析连续图像序列特性和机载光电跟踪系统特点的基础上,结合精密转台的运动变化信息,提出了基于转角信息的连续帧图像处理算法.该方法根据精密转台的转角变化信息,采用连续的图像序列进行多帧间差值运算以获得目标残差图,残差图之间相乘.除去大部分随机噪声点,结合中值滤波和自适应渡门跟踪算法,大大的减小了运算量,实现了运动目标的快速跟踪定位.通过对目标实测图像序列的试验,结果表明,该算法具有快速、稳定等优点,能满足机载光电跟踪系统实时图像跟踪的要求.     

序列图像中运动目标检测

提出动态背景下序列图像中的运动目标检测算法。利用像素邻域的各向同性对图像进行归一化,消除亮度变化等因素的影响;利用光流信息并结合小波变换由粗及精计算速度场来配准图像;用当前帧作参考图像,通过时域积分校正背景图像。当前帧与校正后背景图像作差得到差分图像。假设该差分图像中噪声分布为高斯分布,由高斯分布的3σ特性滤除差分图像中的噪声,则粗定位出目标;最后以聚类方法确定运动目标区域。分别对200帧可见光和200帧红外图像序列进行实验,检测率分别为95%和94%。     

基于自适应前景分割及粒子滤波的人体运动跟踪

提出了在图像序列中用自适应前景分割及粒子滤波对人体的3-D运动轨迹进行跟踪的方法．首先建立了像素点的高斯模型,并结合图像帧间的差分信息以及灰度分布的先验概率等因素完成了图像中人体的自适应分割．根据所得到的分割结果建立了透视投影下的运动平面跟踪模型．根据投影过程的非线性以及图像中噪声分布的未知性,提出了粒子滤波的跟踪方法,并最终得到了人体运动平面的3-D轨迹．实际人体运动图像序列的实验证明,本文方法能有效地跟踪人体运动的3-D轨迹,并反映出在此跟踪问题上粒子滤波比传统的扩展卡尔曼滤波更具优势．     

运动背景下多目标跟踪的小波方法

为了能从运动背景中检测其中的运动目标,并进行跟踪,提出一种基于小波变换的分层匹配跟踪算法。利用小波分解的多层子图进行分层匹配,估计整个背景的运动矢量;利用差分算法从运动背景中检测出多个运动目标,计算出多个动目标的形心坐标,绘出各动目标的运动轨迹。该算法与传统的块匹配算法相比,滤除了原图像的高频噪声,防止了在含噪原图像上进行块匹配不准确的缺点;另外,在低频分量图像上N×N范围进行块匹配,相当于在原图像上2nN×2nN的范围进行匹配搜索,搜索速度快。当相邻两帧背景运动向量小于10个像素,运动目标相对背景的运动向量小于5个像素时,实验结果证明了此算法的有效性和可行性。     

局部特征点的鲁棒性数字稳像

针对视频序列受载体的影响,本文提出了基于Harris局部特征点的数字稳像方法。局部特征点在图像处理、模式识别、计算机视觉以及目标检测与跟踪领域得到了广泛的研究。首先,利用Harris算法提取动载体摄像系统中每帧图像的局部特征点,采用基于归一化互相关匹配算法进行局部特征点的匹配,提出了一种双向搜索的匹配策略来提取具有较强鲁棒性的特征点;然后,利用随机抽样一致算法进一步筛选所选定的特征点,并将保留下来的精确匹配特征点带入仿射运动模型求出全局运动矢量;最后,提取全局运动矢量中的抖动参数,并对原始图像进行补偿。实验结果表明,该方法可以快速有效地处理图像序列的仿射运动,水平和垂直方向的精确度小于1 pixel,并且具有较好的鲁棒性,可用于动载体摄像系统。     

一种特征点跟踪的运动目标检测

提出一种基于特征点跟踪的运动目标分割算法。在角点跟踪过程中,8等分匹配点邻域,构造方向子邻域提高匹配精度;提出一种新的聚类准则抽取最优特征子集估计运动参数;结合统计方法消除残差噪声;采用时域滤波滤除孤立区域检测出运动目标。对200帧可见光视频序列和100帧红外图像序列中运动汽车进行检测分割实验,检出率分别达到96%和94%。     

Crater distribution on the rear wall of AL-Whipple shield by hypervelocity impacts of AL-spheres

All spacecraft in low orbit are subject to hypervelocity impact by meteoroids and space debris, which can in turn lead to significant damage and catastrophic failure. In order to simulate and study the hypervelocity impact of space debris on spacecraft through hypervelocity impact on AL-Whipple shield, a two-stage light gas gun was used to launch 2017-T4 aluminum alloy sphere projectiles. The projectile diameters ranged from 2.51 mm to 5.97 mm and impact velocities ranged from 0.69 km/s to 6.98 km/s. The modes of crater distribution on the rear wall of AL-Whipple shield by hypervelocity impact of AL-spheres in different impact velocity ranges were obtained. The characteristics of the crater distribution on the rear wall were analyzed. The forecast equations for crater distribution on the rear wall of AL-Whipple shield by normal hypervelocity impact were derived. The results show that the crater distribution on the rear wall is a circular area. As projectile diameter, impact velocity and shielding spacing increased, the area of crater distribution increased. The critical fragmentation velocity of impact projectile is an important factor affecting the characteristics of the crater distributions on the rear wall.     

A single energy-based parameter to assess protection capability of debris shields

Protecting spacecraft structures against hypervelocity impacts (HVIs) of space debris, which may cause fatal damage to the spacecraft structures, has received wide attention. In this paper, the numerical simulation of hypervelocity solid–solid impacts is conducted and an energy-based parameter to assess protection capability of debris shields is proposed. To numerically simulate the hypervelocity impact phenomena, a two-dimensional improved smoothed particle hydrodynamics (SPH) method with new particle generation and particle merger techniques is used. The spatial and temporal distributions of the kinetic energy flux density of a debris cloud at the position of the upper surface of a pressure wall are calculated, and the correlation between the kinetic energy of the debris cloud and the deformation and fracture behavior of the pressure wall is discussed. Finally, based on the maximum value of the total kinetic energy of debris cloud per unit area at the position of the upper surface of a pressure wall, an energy-based parameter to assess protection capability of debris shields is proposed.     

Debris clouds generated by hypervelocity impact of cylindrical projectiles with thin aluminum plates

Orthogonal, flash x rays were used to observe the debris clouds produced by the hypervelocity impact of cylindrical aluminum projectiles with thin aluminum sheets or bumpers. Three major structural features were observed in the debris clouds--a front cone, a bulbous main debris cloud, and an inner cone. Inclination of the projectile at impact changed the orientation of these features and the severity of damage to the rear wall of a double-sheet structure; projectiles with the greatest inclination produced the most damage. Two experiments, using aluminum and copper as projectile and target or target and projectile, respectively, were performed to determine the source of material in each of the three structural features of the debris clouds. The front cone and main cloud were shown to consist of bumper debris while the inner cone was composed of projectile fragments.     

铝双层板结构撞击损伤的板间距效应实验研究

为了研究空间碎片对航天器防护结构的超高速撞击损伤特性,采用二级轻气炮发射球形弹丸,对铝双层板结构进行了超高速撞击实验研究.弹丸直径为3.97 mm,撞击速度分别为(2.58±0.08)km/s、(3.54±0.25)km/s和(4.35±0.11)km/s,板间距为10~100 mm.实验得到了铝双层板结构在不同撞击速度区间的后板损伤模式.结果表明,弹丸撞击速度一定时,后板弹坑分布随前后板间距的不同而不同.前板背面返溅影响区和后板弹坑分布区随板间距的增大而增大,各弹坑分布区扩散角随板间距的增大而减小.     

Hypervelocity impact computations with finite elements and meshfree particles

The ability of computations to model characteristics of hypervelocity impact is demonstrated using an algorithm for the automatic conversion of distorted finite elements to meshfree particles. The Lagrangian formulation tracks material boundaries and properties without the errors typical in an Eulerian formulation as the material traverses large distances. A computation of a sphere impacting a bumper is shown to reproduce three regions in the debris cloud that are observed in tests: a front region composed of droplets of melted projectile and target, a middle region of fragmented projectile, and a back region of spalled projectile. Additional computations reproduce the observed traits that result from variations in the projectile shape and obliquity. The computation of a projectile impacting spaced plates demonstrates the ability of the method to model the damage to the rear plate of a Whipple shield for spacecraft protection.     

碎片云撞击声发射信号能量特征小波包分析

利用小波包分析技术对碎片云撞击载荷作用下铝合金板声发射信号的能量分布特征进行了研究。首先,介绍了碎片云撞击信号的获取方案及撞击引起的损伤情况;其次,对获取的声发射信号进行小波包分析,得到了信号在频率带上能量分布特征图。最后,讨论了碎片云撞击损伤特征与声发射信号能量分布的关系。分析结果发现,对于相同质量的弹丸,随着其破碎程度的提高,形成的碎片云对后板的损伤程度减少;弹丸具有的初始速度越大,弹丸破碎越完全,碎片云撞击声发射信号中的能量越小;当弹丸破碎程度低时,碎片云撞击引起的声发射信号能量集中在约488kHz以下;弹丸破碎程度越高,信号中488kHz以上的能量所占总能量的比例越大。     

圆柱形长杆超高速正碰撞薄板结构破碎效应

超高速碰撞多层板结构破碎效应研究对空间碎片防护及动能武器毁伤效应研究有着重要意义。采用ANSYS/AUTODYN程序的SPH方法,对超高速碰撞碎片云的形成过程进行了数值模拟,某典型时刻一次及二次碎片云形貌的数值模拟结果与实验结果吻合较好,验证了计算方法和模型参数的正确性。在此基础上采用数值模拟方法,对钨合金、轧制均质装甲(Rolled Homogeneous Armor,RHA)及LY12铝三种材料的圆柱形弹体超高速碰撞薄板的破碎规律进行了研究,基于量纲分析方法得出了弹体破碎长度随弹靶材料特性、弹靶尺寸及初始撞击速度变化的关系式。并研究了钨合金及RHA两种材料的长杆弹对八层RHA板结构的超高速碰撞效应。     

纤维织物FEM-SPH耦合单胞模型及超高速碰撞特性

目前,对纤维织物超高速碰撞过程中的变形、断裂、破碎等力学行为已有较广泛的研究,但对碰撞过程中纱线间接触问题的分析尚未见公开文献报道。考虑纱线间的相互作用,建立了纤维织物的FEM-SPH耦合单胞模型,该模型不仅能够进行纤维织物超高速碰撞过程中的穿孔断裂、破碎、碎片云扩展等损伤行为分析,还能够进行纱线间的接触作用过程分析。结果表明,该模型分析结果与试验结果具有较好的一致性。      

Debris cloud expansion studies

Properties of fragment clouds produced by hypervelocity perforation of metal plates have been experimentally investigated. Replica model techniques have been applied. Targets consisted of steel dual-plate systems. Projectiles were hard metal spheres of tungsten carbide (3 mm, 7 mm and 10 mm diameter, HRc 79) and steel spheres (6 mm and 12 mm diameter, HRc 63) at velocities ranging between 2.3 km/s and 4.5 km/s. Cloud expansion velocities have been measured by means of in-flight flash X-ray photograph series. Maximum and minimum fragment velocities at front and rear side of clouds have been determined. From impact crater patterns on witness plates, and X-ray photographs of debris clouds, projectile and shield fragments have been identified. It has been found that plate perforation holes and debris cloud parameters scale geometrically for 6 mm and 12 mm diameter steel and 7 mm and 10 mm diameter hard metal spheres. For the 3 mm diameter hard metal spheres only the maximum debris cloud velocity v_rmax scales; all other parameters show deviations, indicating non-uniformity of the plate perforation process at different plate thicknesses. The shape of the inner part of debris clouds of steel spheres is different from that of hard metal spheres, caused by the density difference. For steel spheres the debris cloud shape is a convex lense, the shape of the hard metal fragments becomes in the rear nearly hemispherical. Increasing of the impact velocity causes an increasing of the expansion velocity and a flattening of the debris clouds.