Observations and simulations of the low velocity-to-hypervelocity impact crater transition for a range of penetrator densities into thick aluminum targets

Projectile/target impact crater systems involving soda-lime glass/1100 aluminum, ferritic stainless steel/1100 aluminum, and tungsten carbide/1100 aluminum (corresponding to projectile densities of 2.2, 7.89, and 17 Mg (m³) at impact velocities ranging from 0.56 to 3.99 km/s were examined by light metallography, SEM, and TEM. Plots of crater depth/crater diameter ratio (p/D _c) versus impact velocity exhibited anomalous humps with p/D _c ranging from 0.8 to 5.5 between 1 and 2 km/s, with hypervelocity threshold or steady-state values of p/D _c (>5 km/s) ranging from 0.4 to 1.0; with the p/D _c values increasing with increasing projectile density in each case. This hump-shaped regime, with exaggerated target penetration depths, appears to occur because projectiles remain relatively intact and unfragmented. The crater geometry begins to change when the projectile fragmentation onset velocity (>2 km/s) is exceeded and fragmentation increases with increasing impact velocity. Computer simulations and validation of these simulations were developed which fairly accurately represented residual crater shapes/geometries and correlated experimentally measured microhardness maps with simulated residual yield stress contour maps. Validated computer simulations allowed representative extrapolations of impact craters well beyond the laboratory where melting and solidification occurred at the crater wall, especially for hypervelocity impact (>5 km/s).     

Experimental observations and computer simulations for metallic projectile fragmentation and impact crater development in thick metal targets

Stainless steel (3.18 mm diameter) spherical projectiles impacting 2.5 cm thick targets of nickel, copper, 304 stainless steel, and 70/30 brass at velocities ranging from 0.52 to 5.12 km/s were observed by SEM to form decreasing average fragment sizes with increasing impact velocity, beyond a fragmentation onset velocity of 0.7 km/s. Crater observations by optical microscopy and SEM were qualitatively simulated using an AUTODYN numerical analysis code, which also illustrated a decrease in fragmentation density within the target craters with increasing impact velocity. However, extrapolated simulations corresponding to impact velocities as high as 10 km/s showed residual fragmentation within these craters in contrast to extrapolations of the experimental fragment size versus impact velocity data indicative of zero fragment size at 6 km/s.     

Projectile density, impact angle and energy effects on hypervelocity impact damage to carbon fibre/peek composites

This paper explores the effects of projectile density, impact angle and energy on the damage produced by hypervelocity impacts on carbon fibre/PEEK composites. Tests were performed using the light gas gun facilities at the University of Kent at Canterbury, UK, and the NASA Johnson Space Center two-stage light gas gun facilities at Rice University in Houston, Texas. Various density spherical projectiles impacted AS4/PEEK composite laminates at velocities ranging from 2.71 to 7.14 km/s. In addition, a series of tests with constant size aluminum projectiles (1.5 mm in diameter) impacting composite targets at velocities of 3, 4, 5 and 6 km/s was undertaken at incident angles of 0, 30 and 45 degrees. Similar tests were also performed with 2 mm aluminum projectiles impacting at a velocity of approximately 6 km/s. The damage to the composite was shown to be independent of projectile density; however, debris cloud damage patterns varied with particle density. It was also found that the entry crater diameters were more dependent upon the impact velocity and the projectile diameter than the impact angle. The extent of the primary damage on the witness plates for the normal incidence impacts was shown to increase with impact velocity, hence energy. A series of tests exploring the shielding effect on the witness plate showed that a stand-off layer of Nextel fabric was very effective at breaking up the impacting debris cloud, with the level of protection increasing with a non-zero stand-off distance.     

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.     

Crater morphology at impact velocities between 8 and 17 km/s

The crater morphology of impacts in the velocity range between 8 and 17 km/s has been investigated Glass projectiles with diameters between 20 ωm and 200 ωm were impacted on targets of gold, tungsten, iron and aluminum. Data have been compiled for the dependence of the crater diameter D_c and the crater depth T_c on the projectile velocity.     

铝球弹丸超高速正撞击薄铝板穿孔尺寸研究

利用2017-T4铝球弹丸高速正撞击不同厚度的2A12铝合金板,模拟空间碎片对航天器防护屏的高速撞击作用,分析铝合金板撞击穿孔尺寸特征。铝球弹丸直径为3.18mm~6.35mm,弹丸直径与铝板厚度之比dp/t为1.00~9.96,撞击速度为1.50km/s~6.98km/s,得到了铝球弹丸高速正撞击铝板的穿孔经验公式。实验结果表明:薄铝板高速撞击穿孔直径扩张率与弹丸直径、铝合金板厚度及撞击速度有关。当弹丸直径与铝合金板厚度之比dp/t一定时,薄铝板撞击穿孔直径扩张率随着撞击速度的增大而增大;当撞击速度一定时,薄铝板撞击穿孔直径扩张率与dp/t呈非线性关系,且随着dp/t的增加,对薄铝板撞击穿孔直径扩张率的影响减弱。     

Hypervelocity impact damage to composites

This paper presents the results of hypervelocity impact tests conducted on graphite/PEEK laminates. Both flat plate and circular cylinders were tested using aluminum spheres of varying size, travelling at velocities from 2–7 km/s. The experiments were conducted at several facilities using light gas guns. Normal and oblique angle impacts were investigated to determine the effect of impact angle, particle energy and laminate configuration on the material damage and ejecta plumes. Correlations were established between an energy parameter and the impact crater size, spallation damage and debris cone angle. Secondary damage resulting from the debris plume on adjacent composite structures was studied using high-speed photography and witness plates. It was observed that for hypervelocity impacts, the debris plume particles have sufficient energy to penetrate adjacent structures and cause major structural damage as well.     

Searching for momentum enhancement in hypervelocity impacts

In conjunction with the Los Alamos National Laboratory hypervelocity microparticle impact (HMI) team effort to produce higher impact velocities and to understand the physics of crater formation and momentum transfer, we have implemented a low noise microphone as a momentum detector on both the 6 MV Van de Graaff and 85 KV “test stand” particle accelerators. Calculations are presented showing that the impulse response of a circular membrane. When used as a momentum impulse detector, the microphone theoretically may detect impulses as small as 8.8 × 10⁻¹⁵ N s. Sensitivity of the microphone in this application is limited by the noise threshold of the electronic amplifiers and the ambient microphonic vibration of the system. Calculations lead us to anticipate detection of particles over the full range of the Van de Graaff acceleration capability and up to 7 km/s on the test stand. We present momentum enhancement data in the velocity range between 10 km/s and 20 km/s. Preliminary work is presented on momentum impulse calibration of the microphone using laser-pulse photon momentum as an impulse source.     

Pressure-shear impact and the dynamic viscoplastic response of metals

Pressure-shear plate impact experiments are used to investigate the viscoplastic response of metals at shear strain rates ranging from 10⁵ s⁻¹ to 10⁷ s⁻¹. Flat specimens with thicknesses between 300 μm and 3 μm are sandwiched between two hard, parallel plates that are inclined relative to their direction of approach. Nominal stresses and strains in the specimens are determined from elastic wave profiles monitored at the rear surface of one of the hard plates. Results are reviewed for two fcc metals: commercially pure aluminum and an aluminum alloy. New results are presented for bcc high purity iron, a high strength steel alloy and vapor deposited aluminum. For commercially pure aluminum the flow stress increases strongly with strain rate as strain rate increases from 10⁴ s⁻¹ to 10⁵ s⁻¹. At strain rates above 10⁵ s⁻¹ the flow stress, based on results for thin vapor-deposited aluminum specimens, increases strongly, but less than linearly, with increasing strain rate until it saturates at strain rates between 10⁶ s⁻¹ and 10⁷ s⁻¹. Preliminary results for high purity alpha-iron indicate that the flow stress increases smoothly over eleven decades of strain rate, and faster than logarithmically for strain rates from 10² s⁻¹ to greater than 10⁶ s⁻¹. In contrast, for a high strength steel alloy the flow stress depends only weakly on the strain rate, even at strain rates at high as 10⁵ s⁻¹. Such contrasting behavior is attributed to differences in the relative importance of viscous glide and thermal activation as rate controlling mechanisms for dislocation motion in the various metals. Numerical studies indicate that experiments performed at the highest strain rates on the thinnest specimens are not adiabatic, thus requiring a full thermal-mechanical analysis in order to interpret the data.     

Computer simulations of large asteroid impacts into oceanic and continental sites--preliminary results on atmospheric, cratering and ejecta dynamics

Computer simulations have been completed that describe passage of a 10-km-diameter asteroid through the Earth's atmosphere and the subsequent cratering and ejecta dynamics caused by impact of the asteroid into both oceanic and continental sites. The asteroid was modeled as a spherical body moving vertically at 20 km/s with a kinetic energy of 2.6 × 10³⁰ ergs (6.2 × 10⁷ Mt ). Detailed material modeling of the asteroid, ocean, crustal units, sedimentary unit, and mantle included effects of strength and fracturing, generic asteroid and rock properties, porosity, saturation, lithostatic stresses, and geothermal contributions, each selected to simulate impact and geologic conditions that were as realistic as possible. Calculation of the passage of the asteroid through a U.S. Standard Atmosphere showed development of a strong bow shock wave followed by a highly shock compressed and heated air mass. Rapid expansion of this shocked air created a large low-density region that also expanded away from the impact area. Shock temperatures in air reached 20, 000K near the surface of the uplifting crater rim and were as high as 2000K at more than 30 km range and 10 km altitude. Calculations to 30 s showed that the shock fronts in the air and in most of the expanding shocked air mass preceded the formation of the crater, ejecta, and rim uplift and did not interact with them. As cratering developed, uplifted rim and target material were ejected into the very low density, shock-heated air immediately above the forming crater, and complex interactions could be expected. Calculations of the impact events showed equally dramatic effects on the oceanic and continental targets through an interval of 120 s. Despite geologic differences in the targets, both cratering events developed comparable dynamic flow fields and by 29s had formed similar-sized transient craters 39km deep and 62km across. Transient-rim uplift of ocean and crust reached a maximum altitude of nearly 40 km at 30s and began to decay at velocities of 500 m/s to develop large-tsunami conditions. After 30s, strong gravitational rebound drove both craters toward broad flat-floored shapes. At 120 s, transient crater diameters were 80km (continental) and 105km (oceanic) and transient depths were 27km; crater floors consisting of melted and fragmented hot rock were rebounding rapidly upward. By 60 s, the continental crater had ejected 2 × 10¹⁴t, about twice the mass ejected from the oceanic crater. By 120 s, 70, 000km³ (continental) and 90, 000km³ (oceanic) target material were excavated (no mantle) and massive ejecta blankets were formed around the craters. We estimate that in excess of 70% of the ejecta would finally lie within 3 crater diameters of the impact, and the remaining ejecta (10¹³t), including the vaporized asteroid, would be ejected into the atmosphere to altitudes as high as the ionosphere. Effects of secondary volcanism and return of the ocean over hot oceanic crater floor could also be expected to contribute substantial material to the atmosphere.     

Hypervelocity crater formation in aluminum alloys at low temperatures

The penetration and perforation of three kinds of aluminum alloys at room temperatures and low temperatures in the velocity range from about 0.5 to about 3.7 km/s were investigated experimentally. Main interests were focused on the depth and diameter of craters and their relations to the impact velocity. As a result, very distinctive features of the temperature effect on the shape and size of craters were found. Also, the effect of impact-induced phase transition of projectiles on the crater formation was examined about carbon steel, aluminum alloy and NaCl projectiles.     

Penetration mechanics and performance of segmented rods against metal targets

Terminal ballistic experiments confirm theoretical predictions that a segmented rod will penetrate a semi-infinite metal target deeper than a continuous rod of the same material and having equal mass, diameter and velocity. For copper segmented rods impacting aluminum targets and tantalum segmented rods impacting 4340 (BHN 300) steel, penetration depths of at least 50 percent greater than that for a corresponding continuous rod are measured at impact velocities of 4 to 5 km/s. Spacing between segments of only about 2.5 segment diameters or more are required to achieve these results. Reducing the Li/D of the segments to less than 1 improves the penetration efficiency of a segmented rod. For segmented rods with segment L_i/D < 1, experiments suggest that penetration may increase with impact velocity rate greater than V^2/3.     

Ballistic limit for shielded plates subjected to hypervelocity impact

A new method of determining ballistic limits for hypervelocity impact is proposed. This method applies to cases of impulsive impact on a plate protected by a multi-shock shield, which corresponds to projectile velocities in the range above 6 km s⁻¹. It is shown, by using experimental and analytical results, that the plate will not fail, provided the impact parameters are bounded by certain critical values. A simple equation that relates the critical values is established. Curves are presented for the critical projectile radius versus the projectile velocity, and for the critical plate thickness versus the velocity. These curves are in good agreement with curves that have been generated empirically.     

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 on isotropic composites with metal or ceramic inclusions

Experimental results of studying the hypervelocity impact on isotropic heterogeneous composites consisting of an epoxy or aluminum matrix containing fine-grained metal (Al, Pb) or ceramic (SiO₂) inclusions are given. The aim of the study is to develop composite materials offering higher penetration resistance to a high-velocity projectile than the component material. This resistance is characterized by the magnitude of the ratio of the crater depth in a thick target to the diameter of spherical projectile. In the case of two particulate composites studied it is shown that the crater depth from impact of steel projectiles is lower about by one projectile diameter than for homogeneous lead or aluminum over the impact velocity ranged from 3 up to 11 km/s.     

Hypervelocity impact testing of micrometeorite capture cells in conjunction with a PVDF thin-film velocity/trajectory sensor and a simple plasma velocity detector

Five different small particle capture cell designs were evaluated for their ability to capture fragments and residue from 10–200 μm diameter glass projectiles and oblong olivine crystals impacting at 1–15 km/s in sufficient quantity for chemical and isotopic analyses. Aluminum multi-foils (0.1–100 μm thick with ≈10, 000 and 1800 μm spacing), foil covered germanium crystals, and 0.50 and 0.120 g/cm³ Aerogels, were positioned behind either multi-film (1.4–6.0 μm thick) polyvinylidene fluoride (PVDF) velocity/trajectory sensor devices of a simple wire-grid plasma velocity detector. All capture cells collected significant amounts of impactor debris behind the PVDF sensors from nominal 100 μm diameter glass projectiles and olivine crystals which struck the sensor at velocities up to 6.4 km/s. At velocities >8 km/s little or no debris penetrated the second PVDF film. Results were incolsive for velocities between 6.5 and 8 km/s. Plasma detector results showed identifiable impactor residue on Al foils for velocities up to 8.7 km/s and impact tracks with apparent debris imbedded in the Aerogels for velocities up to 12.7 km/s. Maximum foil penetration of glass spheres and olivine crystals were the same, but more particulate debris was associated with olivine crystal ipacts versus glass impacts. Foil spacing beyond one particle diameter had no effect on total penetration. Aerogels are identified as a capture cell media that warrants further investigation. The Al multi-foil capture cell with 100 μm net spacers is identified as the most effective of the other designs and offers the advantages of compact structure, low secondary ejecta from impacts, and easy recovery of impactor debris for analysis.     

Microstructural phenomena associated with micrometeoroid impact craters in aluminum and stainless steel

We explore the metallurgical and materials implications for hypervelocity impact crater formation in some representative materials exposed in space in low-Earth orbit. Radial cracks associated with small size (<0.2 mm) craters in anodized aluminum alloy illustrate the importance of impacting particle flux and size distributions. Novel sectioning and etching of selected craters in stainless steel bolt heads has illustrated the potential for detailed characterization of cracking, phase changes, and extreme deformation proximate to the crater wall while thin sections through the crater and selectively ion-milled to electron transparency have illustrated shock pressure effects on microstructures below the crater for the first time. The use of optical, acoustic, and electron microscopy is illustrated in the characterization of hypervelocity impact crater-related microstructures and these observations point to the essential role to be played by imaging techniques in understanding the environmental effects of space in low-Earth orbit on the behavior of materials and space structures. Reprinted from Microstructural Science, vol. 20, Metallographic Characterization of Metals after Welding, Processing and Service, Proc. of the Twenty Fifth Annual Tech. Meeting of the International Metallographic Society, W.R. Kanne Jr., G.W.E. Johnson, J.D. Braun, and M.R. Louthan Jr., ed., The International Metallographic Society, Columbus, Ohio, and ASM International, 1993, pp. 261–80.     

High-velocity solid ejecta fragments from hypervelocity impacts

The recent discovery of meteorites from the moon and the strong probability that the 8 SNC (Shergottite, Nakhlite and Chassignite) meteorites originated on Mars indicate that large hypervelocity impacts eject some solid debris at very high speed (more than 2.5 and 5 km/sec in the above cases). The standard Hugoniot relation between particle velocity and shock pressure predicts that lunar ejecta should be very heavily shocked (40–50 GPa) and Martian ejecta should be vaporized (100–200 GPa). However, the lunar meteorite ALHA 81005 was in fact subjected to less than 15 GPa, while the most highly shocked SNC meteorite was exposed to ca. 50 GPa, while others showing no detectable shock damage at all.

Theoretical work shows that the normal Hugoniot relation doesn't apply in the vicinity of a free surface. The free surface is, by definition, a pressure-free boundary, so shock pressures on it must be identically zero. On the other hand, the acceleration of debris is proportional to the pressure gradient, so that near-surface material may be accelerated to high speed and still escape compression to correspondingly high pressure. This process occurs only in a restricted zone near the free surface. The thickness of this zone is proportional to the rise time of the stress-wave pulse generated by the impact.

The rise time of the stress wave generated by a large impact is typically a/v_i, where a is the projectile radius and v_i its impact velocity. The near-surface zone in this case is comparable in thickness to a fraction of the projectile radius. Since the cratering event itself displaces many thousands of times the projectile mass, the quantity of lightly-shocked, high speed ejecta is small, amounting to only a few percent of the projectile's mass (for ejecta speed>few km/sec). The fastest solid ejecta leave at about 1/2 the impact velocity.

Although the total quantity of high speed solid ejecta is thus small in comparison to the total crater ejecta, it is significant because no other process yields such high velocity fragments. Many meteorites appear to be near-surface samples of their parent bodies (many are regolith samples and one is a vesicular lava) and so may have been ejected by this process.     

Highy oblique impacts into thick and thin targets

Hypervelocity impact (HVI) tests have been conducted at the JSC Hypervelocity Impact Test Facility (HIT-F) with aluminum projectiles impacting semi-infinite (thick) and thin aluminum plates (with plate thickness to projectile diameter ratios of 6.4 and 0.14, respectively) at impact angles ranging from normal to the plate (0°) to highly oblique (88°). The targets were impacted by solid homogeneous aluminum spheres from 1 mm to 3.6 mm diameter. Results of the HVI tests were not unusual up to 65°, where impact damage is characterized as smooth craters and holes that become progressively elliptical and distended along the projectile flight path. Above 65° angles, however, a transition occurs to an irregularly shaped hole in thin materials and rough bottomed crater in thick targets. Above 80°, multiple damage sites in the targets were formed with the damage areas separated by variable distances of undamaged target surface. Analytical and numerical simulations of the impact process at oblique angles above 65° demonstrates that shock compression and release of the projectile into multiple fragments occurs before the projectile fully engages the target. The resulting projectile fragments are then responsible for the multiple impact sites observed on the targets.     

Oblique hypervelocity impacts: Implication for the analysis of material retrieved after exposure to space

In the past few years, a wide variety of surfaces have been brought back to Earth after being exposed to the space particulate environment. The impact features found on this material can give clues to the characteristics of the impacting man-made debris and meteoroids. Many investigations have been carried out to deduce projectile parameters (size, shape, and velocity vector) from the morphology of impact features and their origin from the analysis of projectile remnants inside craters formed. However, there are still ambiguities in the interpretation of these results. Recently, the post-flight analysis of solar arrays retrieved from the hubble space telescope (HST) showed the lack of data concerning the interpretation of many impact features. In the present study, we have examined especially the distinctive features of craters caused by particles at oblique incidence. These craters represent more than one third of impacts with a size between 5 μm and 1 mm observed on the European Retrievable Carrier (EuReCa) and HST solar arrays. Interpretation difficulties of this kind of impacts on solar cells led to hypervelocity impacts test onto pure silica targets performed with iron projectiles at different incidence angles and different velocity ranges. They were made in order to find a possible link between the incidence angle of a projectile, the impact velocity, and the parameters, which could be deduced from the analysis of the crater and projectile remnants. A detailed survey of impacts features formed was done for each couple angle velocity. High-resolution observations show an evolution of the crater morphology and circularity with the increasing angles of incidence and velocity, and some changes in the projectile remnants amount, appearance, and position are also noted.