Hypersonic impact flash characteristics of a long-rod projectile collision with a thin plate target

Impact flash occurs when objects collide at supersonic speeds and can be used for real-time damage assessment when weapons rely on kinetic energy to destroy targets.However,the mechanism of impact flash remains unclear.A series of impact flash experiments of flat-head long-rod projectiles impacting thin target plates were performed with a two-stage light gas gun.The impact flash spectra for 6061 aluminum at 1.3-3.2 km/s collision speeds were recorded with a high-speed camera,a photoelectric sensor,and a time-resolved spectrometer.The intensity of the impact flash exhibited a pulse charac-teristic with time.The intensity(I)increased with impact velocity(V0)according to I∝Vn0,where n = 4.41 for V0 ＞ 2 km/s.However,for V0 ＜ 2 km/s,n = 2.21,and the intense flash duration is an order of magnitude less than that of higher V0.When V0 ＞ 2 km/s,a continuous spectrum(thermal radiation background)was observed and increased in intensity with V0.However,for V0 ＜ 2 km/s,only atomic line spectra were detected.There was no aluminum spectral lines for V0 ＜ 2 km/s,which indicated that it had not been vaporized.The initial intense flash was emission from excited and ionized ambient gases near the impact surface,and had little relationship with shock temperature rise,indicating a new mechanism of impact flash.     

Shock-induced reaction behaviors of functionally graded reactive material

In this paper, the ballistic impact experiments, including impact test chamber and impact double-spaced plates, were conducted to study the reaction behaviors of a novel functionally graded reactive material (FGRM), which was composed of polytetrafluoroethylene/aluminum (PTFE/Al) and PTFE/Al/bismuth trioxide (Bi₂O₃). The experiments showed that the impact direction of the FGRM had a significant effect on the reaction. With the same impact velocity, when the first impact material was PTFE/Al/Bi₂O₃, compared with first impact material PTFE/Al, the FGRM induced higher overpressure in the test chamber and larger damaged area of double-spaced plates. The theoretical model, which considered the shock wave generation and propagation, the effect of the shock wave on reaction efficiency, and penetration behaviors, was developed to analyze the reaction behaviors of the FGRM. The model predicted first impact material of the FGRM with a higher shock impedance was conducive to the reaction of reactive materials. The conclusion of this study provides significant information about the design and application of reactive materials.     

Effect of change of sand properties on travel distance of ricocheted debris

The debris from exploded buildings can ricochet after colliding with the ground, thus increasing the debris travel distance and danger from any associated impacts or collisions. To reduce this danger, the travel distance of ricocheted debris must be accurately predicted. This study analyzed the change in the travel distance of ricocheted concrete debris relative to changes in the properties of a sand medium. Direct shear tests were conducted to measure the change in internal friction angle as a function of temperature and water content of the sand. Finite element analysis (FEA) was then applied to these variables to predict the speed and angle of the debris after ricochet. The FEA results were compared with results of low-speed ricochet experiments, which employed variable temperature and water content. The travel distance of the debris was calculated using MATLAB, via trajectory equations considering the drag coefficient. As the internal friction angle decreased, the shear stress decreased, leading to deeper penetration of the debris into the sand. As the loss of kinetic energy increased, the velocity and travel distance of the ricocheted debris decreased. Changes in the ricochet velocity and travel distance of the debris, according to changes in the internal friction angle, indicated that the debris was affected by the environment.     

Investigation on the process parameters of TIG-welded aluminum alloy through mechanical and microstructural characterization

Multi-pass TIG welding was conducted on plates (15×300×180 mm3) of aluminum alloy Al-5083 that usually serves as the component material in structural applications such as cryogenics and chemical processing industries. Porosity formation and solidification cracking are the most common defects when TIG welding Al-5083 alloy, which is sensitive to the welding heat input. In the experiment, the heat input was varied from 0.89 kJ/mm to 5 kJ/mm designed by the combination of welding torch travel speed and welding current. Tensile, micro-Vicker hardness and Charpy impact tests were executed to witness the impetus response of heat input on the mechanical properties of the joints. Radiographic inspection was performed to assess the joint's quality and welding defects. The results show that all the specimens displayed inferior mechanical properties as compared to the base alloy. It was established that porosity was progressively abridged by the increase of heat input. The results also clinched that the use of me-dium heat input (1-2 kJ/mm) offered the best mechanical properties by eradicating welding defects, in which only about 18.26% of strength was lost. The yield strength of all the welded specimens remained unaffected indicated no influence of heat input. Partially melted zone (PMZ) width also affected by heat input, which became widened with the increase of heat input. The grain size of PMZ was found to be coarser than the respective grain size in the fusion zone. Charpy impact testing revealed that the absorbed energy by low heat input specimen (welded at high speed) was greater than that of high heat input (welded at low speed) because of low porosity and the formation of equiaxed grains which induce better impact toughness. Cryogenic (-196 C) impact testing was also performed and the results corroborate that impact properties under the cryogenic environment revealed no appreciable change after welding at designated heat input. Finally, Macro and micro fractured surfaces of tensile and impact specimens were analyzed using Stereo and Scanning Electron Microscopy (SEM), which have supported the experimental findings.     

Survivability assessment of spacecraft impacted by orbit debris

To help optimize the spacecraft design and reduce the risk of spacecraft mission failure, a new approach to assess the survivability of spacecraft in orbit is presented here, including the following three steps:1) Sensitivity Analysis of spacecraft. A new sensitivity analysis method, a ray method based on virtual outer wall, is presented here. Using rays to simulate the debris cloud can effectively address the component shadowing issues. 2) Component Vulnerability analysis of spacecraft. A function"Component functional reduction degree — Component physical damage degree"is provided here to clearly describe the component functional reduction. 3) System-level Survivability Assessment of spacecraft. A new method based on expert knowledge reasoning, instead of traditional artificial failure tree method, is presented here to greatly improve the efficiency and accuracy of calculation.     

Research and development on hypervelocity impact protection using Whipple shield: An overview

Whipple shield, a dual-wall system, as well as its improved structures, is widely applied to defend the hypervelocity impact of space debris (projectile). This paper reviews the studies about the mechanism and process of protection against hypervelocity impacts using Whipple shield. Ground-based experiment and numerical simulation for hypervelocity impact and protection are introduced briefly. Three steps of the Whipple shield protection are discussed in order, including the interaction between the projectile and bumper, the movement and diffusion of the debris cloud, and the interaction between the debris cloud and rear plate. Potential improvements of the protection performance focusing on these three steps are presented. Representative works in the last decade are mentioned specifically. Some prospects and suggestions for future studies are put forward.     

Numerical investigation on free air blast loads generated from center-initiated cylindrical charges with varied aspect ratio in arbitrary orientation

In current guidelines, the free air blast loads (overpressure and impulse) are determined by spherical charges, although most of ordnance devices are more nearly cylindrical than spherical in geometry. This may result in a great underestimation of blast loads in the near field and lead to an unsafe design. However, there is still a lack of systematic quantitative analysis of the blast loads generated from cylindrical charges. In this study, a numerical model is developed by using the hydrocode AUTODYN to investigate the influences of aspect ratio and orientation on the free air blast loads generated from center-initiated cylindrical charges. This is done by examining the pressure contours, the peak overpressures and impulses for various aspect ratios ranged from 1 to 8 and arbitrary orientation monitored along every azimuth angle with an interval of 5°. To characterize the distribution patterns of blast loads, three regions, i.e., the axial region, the vertex region and the radial region are identified, and the propagation of blast waves in each region is analyzed in detail. The complexity of blast loads of cylindrical charges is found to result from the bridge wave and its interaction with primary waves. Several empirical formulas are presented based on curve-fitting the numerical data, including the orientation where the maximum peak overpressure emerges, the critical scaled distance beyond which the charge shape effect could be neglected and blast loads with varied aspect ratio in arbitrary orientation, all of which are useful for blast-resistant design.     

区间正交小波变换域FLP算法及其在捷联寻北中的应用

针对机抖激光陀螺角随机游走和发动机干扰等影响静态捷联寻北精度的问题,提出了一种新的基于区间正交小波变换的前向线性预测算法。该算法利用前向线性预测技术以及区间正交小波变换抑制边界效应的能力,有效地减小了角随机游走和发动机干扰。车载实验证明该算法的有效性。     

低入射余角下雷达地杂波反射率模型

在常用雷达地杂波反射率模型的基础上,综合考虑雷达频率、地形种类和入射余角等对地杂波反射率的影响,建立了一个低入射余角下雷达地杂波反射率模型,并在不同的情况下,该模型与其他常用地杂波反射率模型相比较。结果表明:该雷达地杂波反射率模型更符合实际,进而证明了该模型的有效性和实用性。     

低入射余角下雷达海杂波的建模与仿真

研究了低入射余角下雷达海杂波的建模、仿真算法及实现问题。根据实际背景 ,对实测数据进行深入讨论 ,建立起一个较为准确的海杂波幅度分布模型与功率谱模型 ;针对非相参和相参两种雷达体制 ,分别采用修正的零记忆非线性变换法和球不变随机过程法来实现具有给定相关性的K分布随机数序列的产生 ,并以VisualC + +为平台实现了仿真软件。仿真结果证明了模型和算法的有效性。