A simple model for the optimazation of stand-off hypervelocity particle shields

Impacts at extreme velocities of many tens or even hundreds of kilometers per second are essentially inaccessible to experiment; hence, theory and the resulting scaling laws are the only means to describe the relevant phenomena. One area amenable to this approach involves analyzing the efficacy of stand-off shields for protection against hypervelocity particles. A simple model that allows for optimal design of these shields is developed. Examples and scaling laws that provide estimates for the minimum requirements for such shields are derived from the model.     

A simple model for debris clouds produced by hypervelocity particle impact

A simple debris cloud model is developed by considering the one dimensional shock wave motion in the material together with the catastrophic fragmentation theory by Grady. The model provides a simple method for calculating the velocities at the outer perimeter of the cloud and the average particle size in the cloud.     

A hypervelocity projectile launcher for well perforation

Current oil well perforation techniques use low- to medium-velocity gun launchers for completing wells in soft rock. Shaped-charge jets are normally used in harder, more competent rock. A launcher for a hypervelocity projectile to be used in well perforation applications has been designed. This launcher will provide an alternative technique to be used when the conventional devices do not yield the maximum well perforation. It is an adaptation of the axial cavity in a high explosive (HE) annulus design, with the axial cavity being filled with a low density foam material. Two configurations were tested; both had an HE annulus filled with organic foam, one had a projectile. Comparison of the two shots was made. A time sequence of Image Intensifier Camera photographs and sequential, orthogonal flash x-ray radiographs provided information on the propagation of the foam fragments, the first shock wave disturbance, the projectile motion and deformation, and the direct shock wave transmission from the main HE charge. Perforation tests of both device configurations (with and without the pellet) into steel-jacketed sandstone cylinders were made. Static radiographs of the cavities in the sandstone showed similar cavities, however, the perforation of the steel cap was larger in response to the pellet. DYNA2D calculations were made to assist in the interpretation of the experimental records. The preliminary results show promise that a useful perforating tool can be developed. Plans for an extended experimental program are outlined.     

Measurement of stress wave asymmetries in hypervelocity projectile impact experiments

Asymmetries in both structure and ejecta are observed around a number of craters on planetary surfaces. Similar asymmetries have been documented for hypervelocity impact experiments. Such asymmetries arise from the stress front developed around oblique impacts. The onset angle for asymmetric stress distributions indicates that geologic asymmetries should be present in a significant fraction of impact structures.     

弹丸超高速撞击半无限厚铝板数值模拟

微流星体及空间碎片的超高速撞击威胁着长寿命、大尺寸航天器的安全运行,导致其严重的损伤和灾难性的失效.撞击损伤特性研究是航天器防护设计的一个重要问题.本文采用AUTODYN软件的Lagrange法对半无限铝板的超高速斜撞击和与其具有相同法向速度的正撞击进行了模拟,给出了不同撞击角和不同法向速度下半无限厚铝板弹坑深度、宽度、长度的变化规律及多弹坑的形成过程,并与经验方程进行了比较分析.结果发现:随撞击角的增加,弹坑的深度和宽度减小,而弹坑的长度增加;随撞击速度的增加弹坑的直径和深度增加;在撞击角大于70度时出现多弹坑.     

Empirical hole size and crack length models for dual-wall systems under hypervelocity projectile impact

This paper presents the results of a study whose objective was to develop empirical equations to characterize the hole formation and cracking phenomena associated with the penetration of some of the multi-wall systems being considered for the International Space Station Alpha. The significance of the work performed is that these equations can be used in a survivability analysis to determine whether or not module unzipping would occur under a specific set of impact conditions and the time available for module evacuation prior to the onset of incapacitation due to air loss. The correlation coefficients for the equations obtained exceeded 0.8 in most cases; hence, it is believed that the equations developed have a rather good fit to the experimental data.     

A new design criteria for explosively-formed hypervelocity projectile (EFHP)

In this work, we present a simple analysis, concerning the shaped charge parameters that might have a significant effect on the shape and velocity of the explosively-formed hypervelocity projectile. The study is based on theoretical considerations which result in a simple and practical method of a combined experimental and computational steps to obtain stable projectiles in the range of velocities above 10 km/sec. The method has been tested and verified experimentally. Stable finite mass projectiles made of copper were formed moving at a speed close to 12 km/sec.     

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.     

高速粒子撞击作用下LF6合金多层靶结构防护能力研究

针对总厚度为４ｍｍ的ＬＦ６合金双层靶和总厚度为２ｍｍ的三层靶进行了直径为２ｍｍ,速度分别为５．８和７．２ｋｍ／ｓ的ＧＣｒ１５粒子 撞击试验,并对双层靶进行了不同前靶厚度和靶间距的撞击试验,试验结果表明：与同样碰撞条件下半无限体靶上产生的破坏情况相比,多层靶被击穿的总厚度远淖于半无限体靶上形成的弹坑深度,采用多层靶结构可显著提高材料的抗高速粒子撞击能力,并大大降低航天器抗高速粒子撞击的防护结构的重量     

A review of explosive accelerators for hypervelocity impact

A technical overview of experimental methods using high explosive techniques for conducting hypervelocity impact studies is presented. The explosive techniques use the explosive detonation fronts as means of accelerating the projectile, or as means of compressing a light gas which is then used to launch the projectile.

The explosive launchers are in six subdivisions: high explosive pellet accelerators, flyer plate accelerators, shaped charges, explosive-formed projectiles, fragment and microparticle accelerators, and explosive gas guns. Each one of the subdivisions presents the various techniques, their advantages and disadvantages, the range of mass and velocity capable of being accelerated, and whether the technique can be scaled for larger or smaller masses.     

An investigation of metal matrix composites as shields for hypervelocity orbital debris impacts

The improvement of space vehicle shield designs to resist penetration by hypervelocity impacts of meteoroids or man-made orbital debris can lengthen mission life and increase mission efficiency. One option to improve shields is to create new bumper materials which can be tailored to meet the requirements for effective shielding. Metal matrix composites are one such material. Fiber content, type, and orientation could be varied to tailor the material to the specific properties needed for weight efficient shielding. In this study, two varieties of aluminum matrix composites were investigated, one with continuous graphite fibers and one with silicon carbide particulates. The objectives of the study were: to compare the penetration resistance of the composite with the known resistance of aluminum; to study the penetration mechanics by comparing the condition of the composite after the impact test with the pre-test condition; to study the effects of fiber content and fiber orientation on penetration resistance; and to recommend a material “design” for metal matrix composites which would best protect a space vehicle from orbital debris. The composite bumpers did not perform significantly better than aluminum bumpers. The particulate composites are more effective bumpers than the continuous fiber composites for the conditions tested. The differences in the measured hole diameters resulting from the impact tests as compared to predicted hole diameters for the particulate composite bumpers, are within the expected differences for metallics. However, the continuous fiber composites had much larger holes than predicted.     

A simple finite element model for vibration analyses induced by moving vehicles

This study developed a simple finite element method combining the moving wheel element, spring–damper element, lumped mass and rigid link effect to simulate complicated vehicles. The advantages of this vehicle model are (1) the dynamic matrix equation is symmetric, (2) the theory and formulations are very simple and can be added to a standard dynamic finite element codes easily and (3) very complicated vehicle models can be assembled using the proposed elements as simple as the traditional finite element method. The Fryba's solution of a simply supported beam subjected to a moving two‐axle system was analysed to validate this finite element model. For a number of numerical simulations, the two solutions are almost identical, which means that the proposed finite element model of moving vehicles is considerably accurate. Field measurements were also used to validate this vehicle model through a very complicated finite element analysis, which indicates that the current moving vehicle model can be used to simulate complex problem with acceptable accuracy. Copyright © 2006 John Wiley & Sons, Ltd.     

A study of hypervelocity impact on cryogenic materials

This paper reports a result of hypervelocity impact experiments on cryogenically cooled aluminum alloys and a composite material. Experiments are carried out on a target palate at 122 K. Aluminum spheres at 1.95 km/s in 50 kPa air were impinged against the target plate at cryogenic temperature and the result was compared with room temperature target plates. Hypervelocity impact (HVI) processes were visualized with shadowgraph arrangement and recorded with high-speed video camera and to ensure the temperature dependence we compared HVI tests with metal target plates with AUTODYN 2D and SPH numerical simulations. We found that cryogenic impacts created slight differences of impact damage from room temperature ones, i.e., the shape and averaged diameters of HVI crater holes were less at cryogenic impacts.     

The application of the integral theory of impact to model penetration of hypervelocity impact

The Integral Theory of Impact (ITI) (Swanson and Donaldson, 1978) is a unique formulation of the equations of motions of projectiles to describe the penetration process. The model requires only basic material properties, no empirical data is needed. The original model was modified to divide the penetration process into three consecutive phases. The new model has successfully modeled impacts over a wide range of velocities, at normal or oblique impact, for infinite or finite targets. In order to better match the experimental observations of impacts in the hypervelocity range, it was necessary to include a thermal softening effect on flow stress. For finite targets, the back-face effect is proposed to be a function of both penetration velocity and the speed of sound, extending the model's applicability to hypervelocity penetration.     

Advanced algorithms for computer simulation of hypervelocity impact

Presented are several algorithms that allow a Lagrangian hydrodynamic computer code to simulate hypervelocity impact. Such impacts typically involve very large material deformations and ordinarily can be handled only by Eulerian codes, in which the materials flow through a stationary grid. In Lagrangian codes, the mesh is attached to the material and deforms with it; in problems involving severe distortions, the mesh can become tangled, resulting in loss of accuracy or even a breakdown of the computational scheme. However, Lagrangian codes present several advantages in economy, ease of programming and use, and interpretation of results.

This paper describes several modifications of the DEFEL code (Flis, Miller, and Clark, 1984). This code, a descendant of EPIC-2 (Johnson, 1976), was originally developed to simulate low- to medium-velocity impact and explosive-metal interaction. Modifications include automatic rezoning, a surface-erosion model, and treatment of fracture (cracking) by automatic introduction of sliding lines between elements. These fractures allow DEFEL to handle a variety of problems in the hypervelocity regime. Included are discussions of several example problems.     

A lumped mass numerical model for cellular materials deformed by impact

When impacted by a relatively rigid body, cellular materials undergo severe deformation and extensive material failure. However, such behaviour may not be well described using traditional numerical approaches such as the finite element method. This paper presents a lumped mass numerical model which can accommodate high degrees of deformation and material failure. The essence of this model is to discretize a block of material into contiguous element volumes, each represented by a mass point. Interactions between a node and its neighbours are accounted for by defining ‘connections’ that represent their interfaces which transmit stresses. Strains at a node are calculated from the co‐ordinates of the surrounding nodes; these also determine the stresses on the interfaces. The governing equations for the entire solution domain are then converted into a system of equations of motion with nodal positions as unknowns. Failure criteria and possible combinations of ‘connection’ breakage are incorporated to model the occurrence of damage. A practical contact algorithm is also developed to describe the contact interactions between cellular materials and rigid bodies. Simulations for normal and oblique impacts of rigid rectangular, cylindrical and wedge‐tipped impactors on crushable foam blocks are presented to substantiate the validity of the model. The generally good correlation between the numerical and experimental results demonstrates that the proposed numerical approach is able to model the impact response of the crushable foam. However, some limitations in modelling crack propagation in oblique impacts by a rigid impactor on foam blocks are observed. Copyright © 2001 John Wiley & Sons, Ltd.