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.     

An engineering fragmentation model for the impact of spherical projectiles on thin metallic plates

In this paper, an engineering fragmentation model is presented for the case of hypervelocity impact of a spherical projectile on a thin bumper plate at normal incidence. The range of impact velocities covered is the solid fragmentation regime up to the limits of complete melting of projectile and target material. The model was developed for an axisymmetric fragment cloud by consideration of the conservation laws for mass, momentum, and energy, as well as making a few assumptions on the morphology of the cloud. The fragment cloud is modeled discretely, i.e. each particle of the fragment cloud is considered separately in the analytical calculation. The model consists of mainly analytical relationships and a few empirical fit functions, where no analytical formulation was available. The model distinguishes between fragments originating from the projectile and fragments originating from the bumper plate. The projectile fragments are split into the central fragment and spall fragments. An exponential distribution function is assumed for the mass distribution of the projectile's spall fragments. The fragments from the bumper are assumed to have a uniform mass. All fragments are assumed to be of spherical shape. The fragmentation model was applied and calibrated during experiments, in which Al spheres impact on thin Al plates. The calibration experiments, performed using a two-stage light gas gun, were in the range of impact velocities between 4.8 and 6.7 km/s. In this velocity range, the model was calibrated against residual velocities measured and fragment mass distribution, which was indirectly determined by measuring the crater depth distributions in rear walls.     

Fragmentation of a sphere initiated by hypervelocity impact with a thin sheet

Selected results of tests in which 9.53-mm-diameter, 2017-T4 aluminum spheres impacted 0.25-mm- to 4.80-mm-thick, 6061-T6 aluminum sheets are presented. Impact velocities for these tests ranged from 1.98 km/s to 7.38 km/s. Flash x-rays were used to view the debris clouds produced by the impacts. As impact velocity was increased, failure of the aluminum sphere progressed through the following stages of fracture and fragmentation: (1) formation of a spall failure at its rear surface, (2) development of a detached shell of spall fragments, and (3) complete disintegration of the sphere. The threshold impact velocity for development of the spall failure in the sphere was observed to be a function of the bumper-thickness-to-projectile-diameter ratio (t/D), and to increase as the t/D ratio decreased. When the debris cloud was fully developed, the disintegrated projectile formed its dominant feature--an internal structure, composed of a front, center, and rear element, located at the front of the debris cloud. The front element was small and consisted of finely-divided projectile and bumper material. The bulk of the fragmented projectile was contained in the center element, a disc-like structure made up of a large central fragment surrounded by numerous smaller fragments. A shell of fragments, spalled from the rear of the sphere, formed the rear element. Radiographs of the debris clouds were analyzed to determine the size and size distribution of certain fragments within the cloud. The size of the large fragment was shown to be dependent on impact velocity and t/D ratio. The smaller fragments in the center element were several times larger than the fragments in the shell of spall fragments forming the rear element. Detailed analyses of fragments in the shell of spall fragments were made. The analyses indicated their median Martin's statistical diameter exhibited an orderly dependence on impact velocity and t/D ratio.     

Flow-field center migration during vertical and oblique impacts

The downrange-directed momentum from an oblique impact affects crater excavation. Ejecta dynamics were measured within growing ejecta curtains for experimental impacts with incidence angles of 90°, 45°, and 30° above horizontal, all impacting at velocities near 1.0 km/s. These ejecta dynamics constrain the horizontal migration of the flow-field center between the impact point and the crater center for three curtain segments (uprange, lateral, and downrange) during oblique impacts and are compared with vertical impacts. At angles as high as 45°, the flow-field center migration occurs throughout a substantial portion of crater growth, thereby demonstrating that impact angle affects crater excavation even at relatively high angles. Stationary point sources are found unable to account for this detailed excavation flow during oblique impacts.     

Effects of scale on debris cloud properties

Results of tests using various thicknesses of 6061-T6 aluminum sheet and 6.35-, 9.53-, 12.70-, and 15.88-mm-diameter, 2017-T4 aluminum spheres are described. Impact velocities for these tests ranged from 3.77 to 7.38 km/s. Multiple-exposure, orthogonal-pair, flash radiographs of the debris clouds produced by the impacts were analyzed to provide quantitative data which described the size and velocity of a number of characteristic morphologic features in the debris clouds and the sizes and size distributions of fragments in the structural elements of the debris cloud.The axial and diametral velocities of these morphologic features were shown to be the same, regardless of sphere diameter, when debris clouds produced by impacts with similar bumper-thickness-to-projectile-diameter ratios and impact velocities were compared. As a result, the dimensions of these debris clouds differed only by the differences in the diameters of the spheres that produced them.An analyses of fragment sizes showed that the equivalent diameter of the large projectile fragment along the center line of the debris cloud scaled with projectile diameter; the dimensions of fragments forming the shell of spall fragments at the rear of the debris cloud did not scale with projectile diameter. The large central fragment appeared to originate from near the center of the sphere and was a part of the sphere which remained intact after all processes that worked to reduce the size of the sphere were complete. Formation of spall-shell fragments was a shock-related process which was sensitive to rate effects and other material properties that did not scale.     

Measuring hypervelocity impact velocity from micrometeoroid crater geometry

Guided by half-space computer simulations showing hypervelocity impact crater formation for an iron particle impacting an aluminum target and characteristic crater geometry changes with impact velocity over the range 8–40 km s⁻¹, we examined normal surface crater views and cross-sectional views through craters (>0.5 mm diameter) from samples retrieved from the NASA LDEF satellite and examined in the scanning electron microscope (SEM). While geometrical features suggested in the computer simulations were indeed observed for micrometeoroid craters in 6061-T6 aluminum targets and 303 stainless steel targets, there was no consistent estimate for impact velocities in any of the experimental samples, and velocity estimates based on measuring ratios of ejecta width/crater diameter and ejecta height/crater depth as well as ejecta height/crater diameter varied from 8 to 42 km s⁻¹; over the same range simulated. These results point to the need to create reference data from actual hypervelocity impact experiments in the laboratory, and systematic observation of residual crater geometries in the SEM. These experiments also demonstrate the uncertainty in assuming a fixed impact velocity for all impact craters in space materials as well as an apparent futility in attempting to correlate impacting particle velocity with post-mortem characteristics of a given crater.     

Propagation of hypervelocity impact fragment clouds in pressure gas

This study investigated the propagation of hypervelocity impact fragment clouds in pressure gas. Fragment clouds were generated through perforation of thin aluminium bumper plates by spherical aluminium projectiles. A thick aluminium backwall plate, placed inside a pressure container at a given distance from the bumper plate, caught the fragments to act as a witness plate for the residual damage potential of the fragments. Crater depth statistics are presented as a function of container pressure. The fragment cloud was photographed by means of an image converter camera. The images showed a strong deformation of the fragment cloud for increased container pressures and were used to extract residual velocities until up to 50 μs after impact. The deceleration of the velocity as a function of time after impact suggested an exponential decay function as the best fit to the curve. Thus, maximum fragment impact velocities on the backwall plate could be extrapolated from the axial cloud velocities. The extrapolated curves were compared with experimental time-of-flight measurements, and proved a good match. Fragment impact velocities and maximum crater depths were used to calculate maximum fragment particle sizes as a function of the container gas pressure.     

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.     

Crater ejecta environment from near-surface high-explosive detonations

Near-surface high-explosive detonations produce many large high-velocity ejecta fragments which impact the surface far from the detonation point at high velocity. The fragment size distribution, the ejection and impact velocities, and the impact range depend on the explosive energy, the geometry and position of the explosive charge relative to the surface, and the strength and layered nature of the cratering medium. A model that includes all of these effects accurately reproduces the observed envelopes and fractional size distributions of the airborne ejectra fragments for the cases presented. The calculated radial distribution of accumulated crater debris tends to be thinner than the measured distribution.     

The size distributions of fragments ejected at a given velocity from impact craters

The mass distribution of fragments that are ejected at a given velocity for impact craters is modeled to allow extrapolation of laboratory, field, and numerical results to large-scale planetary events. The model is semi-empirical in nature and is derived from (1) numerical calculations of cratering and the resultant mass versus ejection velocity, (2) observed ejecta blanket particle size distributions, (3) an empirical relationship between maximum ejecta fragment size and crater diameter, (4) measurements and theory of maximum ejecta size versus ejecta velocity, and (5) an assumption on the functional form for the distribution of fragments ejected at a given velocity. This model implies that for planetary impacts into competent rock, the distribution of fragments ejected at a given velocity is broad, e.g. 68% of the mass of the ejecta at a given velocity contains fragments having a mass less than 0.1 times a mass of the largest fragment moving at that velocity. The broad distribution suggests that in the impact process, additional comminution occurs after the initial shock has passed. This additional comminution produces the broader size distribution in impact ejecta as compared to that obtained in simple brittle failure experiments.     

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.     

Response of simulated propellant and explosives to projectile impact—II. Fragmentation

This paper is the second of a series concerned with the penetration and perforation phenomena in two types of propellant and explosive simulant, named Propergol, due to the impact at normal incidence of both blunt and conically-tipped steel strikers. The collision results in fragmentation, plug formation and generation of a cloud of debris that includes particles of measurable dimensions traveling with significant velocities. Both the fragment size and area as well as the ejecta mass are determined experimentally as a function of Propergol specimen thickness and impact velocity or energy. The cumulative number of fragments as a function of size for the Propergol is uniformly found to be a bi-linear semi-logarithmic relationship with the bifurcation occurring at the mean crystal radius. Individual crystals and the crater generated are examined by means of a scanning electron microscope.

A phenomenological model of the fragmentation process is constructed, based on an assumed spherical shape of the fragments and the bi-linear fragment distribution, using energy methods. This is combined with a perforation analysis that considers the process to be sequentially composed of initial indentation, fragmentation, and sliding and deflection of the Propergol disks. An evaluation of this model providing fragment volumes as a function of impact velocity is compared with experimental results and found to be in good agreement.     

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.     

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.     

Analysis of debris clouds produced by impact of aluminum spheres with aluminum sheets

Results of two-stage light gas gun testing of two diameters of aluminum spheres impacting 0.5 mm and 1.0 mm thickness aluminum plates were described in this paper. Impact velocities for these tests were between 3.16 km/s and 5.17 km/s. The components of debris cloud and damage patterns in the witness plate were described. The morphologic features of debris clouds such as shape, axial velocity, and diametral velocity were discussed. The size and number of fragments in the internal structure of debris cloud were not evaluated quantitatively, but described qualitatively. As a result, the shape of the leading face of the internal structure of debris cloud appeared to be sensitive to impact velocity, but not t/D ratio (bumper-thickness-to-projectile diameter ratio). The point at which the maximum diameter of the external bubble of debris cloud occurred had a same half spray angle of 30 degree and the last fragments ejected from bumper had a same half spray angle of 42 degree for each test. Fragments after the point mentioned above in the external bubble of debris cloud were ejected as several chains, the number of which is sensitive to impact velocity, but not t/D ratio. The changes in normalized velocity of the measurement points at debris cloud appeared the same trend as conclusions presented by Piekutowski except for the normalized internal structure expanding velocity. A certain value of t/D ratio, at two sides of which, the normalized internal structure expanding velocity appeared different variety trend existed.     

Debris clouds behind double-layer targets

It is demonstrated that the impedance mismatch and the order of the layers in two-layer sandwiches strongly influences the crater hole size formed in the target, the down-range debris cloud peak velocity, the fragment number and size, and the angles of downrange and uprange debris. Full and half scale test series with aluminum spheres of 10 mm and 5 mm diameter are performed with two-stage light gas guns against glued sandwiches of two layers at about equal areal density and different as well as equal shock impedances in the velocity range of 3–8 km/s. In the case of the titanium/tungsten plate sequence the transmitted shock wave is much stronger than for the tungsten/titanium target. This leads to a higher degree of fragmentation of the participated materials. For titanium/tungsten the hole diameter formed in the titanium layer is distinctly larger than in the tungsten layer for tungsten/titanium. For the titanium/tungsten target the larger crater diameter on the impact side is in agreement with the lower maximum debris cloud velocity.     

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.     

Application of a 100-kV electric gun for hypervelocity impact studies

The Lawrence Livermore National Laboratory 100-kV electric gun has been used to launch flat-plate projectiles for use in studies of spall and hypervelocity impact penetration of thin plates. Impactors were 0.3-mm thick Kapton with dimensions and velocities ranging from 100 mm² at 4 km/s to 10 mm² at 18 km/s. A Fabry-Perot laser velocimeter, an electronic streak camera, and a flash x ray were used as diagnostics of the flyer-plate impact on the selected specimen. Experiments generally included the recovery of the remnant specimen and fragments for detailed examination, permitting a study of incipient spall, onset of melting, and fraction fragmented. Experiments to be described include spall measurements on simple and composite target walls at normal and oblique incidence and “reverse ballistics” impacts of the thin-plate impactor on a stationary penetrator (e.g., Kapton impactors at 15 km/s incident on rods of steel, aluminum, and lead) for calibration of hypervelocity impact codes.     

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.     

Micrometeorite impact simulation at EMI — a review

Activities at EMI in the field of hypervelocity impact techniques are reported. Optimization experiments have been carried out with a light gas gun in order to achieve projectile velocities up to 10 km/s. Different methods for measuring the projectile velocities have been developed and adapted according to respective velocity and mass ranges of projectiles. Experimental efforts have been undertaken to accelerate also microgram particles in light gas guns. Masses as small as 37 μg can be accelerated as individual particles. As examples, several contributions to recent space projects are described.