Enhanced momentum transfer from hypervelocity particle impacts

When hypervelocity particles impact thick targets, the momentum generated in the target can be substantially greater than that associated with the incident projectiles. Adapting an existing model for vaporization-induced impulse generation, a simple model and various scaling laws are derived for this type of momentum enhancement. The model is compared to limited experimental data and is contrasted with other theoretical predictions. The sensitivity to uncertainties in the various input parameters is determined, and the variation of the momentum coupling efficiency is examined. To emphasize the importance of the enhancement phenomenon, illustrative system-level requirements for delivering specific target loads are calculated.     

Momentum enhancement in hypervelocity impact

Momentum enhancement is the increase in momentum transferred to a target due to ejecta material being thrown backwards during crater formation. Quantitative estimates of momentum enhancement are of interest in determining the effectiveness of hypervelocity impactors in deflecting potentially hazardous cosmic objects. This paper explores the influence of impactor density and shape on momentum enhancement when striking aluminum, rock, and ice for impacts up to 10 km/s. Computations are performed and compared to the relatively sparse available data for validation. The computations show that momentum enhancement is most sensitive to the tensile fracture stress of the target material. Further, it is shown that for consolidated (low porosity) targets, momentum enhancement is maximized when the density of the impactor is similar to that of the target and the shape of the impactor is close to a sphere.     

Numerical modeling of hypervelocity impacts

This research presents a new “finite-size particle in cell” method developed for numerical modeling of processes at high energy density. It uses the Lagrangian–Eulerian representation of media which allows one to match contact and free surfaces and to calculate flows with strong deformations. Efficient models of thermodynamic properties, elastic–plastic deformation and fragmentation have been employed in the gas dynamic code adapted for parallel computations. 3D and 2D numerical modeling of plate penetration by impactors of different geometry has been done in a wide range of velocities. The influence of used materials properties models on numerical results has been investigated.     

Oblique hypervelocity impacts into graphite

Investigations have been conducted into the morphology of craters formed by impacts of aluminium and HDPE projectiles at oblique angles to graphite target plates. The experiments were conducted with a two-stage gas gun capable of launching projectiles of differing density and strength to speeds of about 6 kms⁻¹ at right angles into target plates. It was found that, as the impact angle is decreased from the normal, the crater dimensions scaled as the normal component of the impact velocity as predicted by the ‘2/3 power law’ until a critical normal velocity was reached below which the conditions for a hypervelocity impact no longer apply. In this regime, new scaling laws were derived for the crater dimensions. It was also possible to identify a fragmentation angle below which the projectile remains intact as it ricochets across the target surface.     

Hole growth characterisation for hypervelocity impacts in thin targets

Interpretation of the flux data of hypervelocity impact craters and perforations from recovered metallic satellite surfaces (such as from NASA's LDEF and ESA's Eureca) is primarily directed towards the derivation of particle diameters. This requires careful calibration, especially in the transition region near to marginal perforation for targets of finite thickness. This calibration is usually in the form of a “hole growth equation”. Although several thin foil formulae exist and many thick target formulae, the only established equation which attempts to interpret near marginal impacts does not seek to interpret impacts where the perforation is smaller than the foil thickness. In this work intuitive trends are used to select an appropriate mathematical form for a new equation, after which hypervelocity impact data on thin foils are used for defining the parameters.     

Debris fragment characterization in oblique hypervelocity impacts

A case history in debris characterization is presented for oblique impacts of chunky tungsten projectiles against thin plates. The integrated approach of scaled experiments and hydrocode simulations led to a semi-analytic model of behind the plate debris fragment distributions. This debris distribution model agreed quite well with the experimental fragment distributions derived from witness plate measurements. The 1/4 scale test program included three projectile masses, two target geometries (single and dual plates), a velocity range of 4–7 km/s and a strike angle range of 15–55 degrees. Close correlation of measured and predicted fragment distributions encouraged the extension of the model to higher velocities not currently obtainable in the laboratory.

The paper also includes discussions of critical features of debris in oblique hypervelocity impact, the scalability of fragment data, and the utilization of the derived fragment models in semi-analytic damage assessment codes.     

Scaling numerical models for hypervelocity test sled slipper-rail impacts

Hypervelocity test sled slipper-rail impacts have been simulated numerically using the finite volume hydrocode, ChartD to the Three-Halves (CTH). This study addresses the difficulties of applying CTH model solutions to real test sled runs. Past CTH models using dimensions different than actual test sleds have been used to study phenomenological aspects of the problem. However, quantitative results from the CTH model solution do not apply directly to actual test sled runs due to strain rate effects and time scale differences. The Buckingham Pi Theorem is applied to two potential hypervelocity gouging models. Validity of the invariant products is tested using sample CTH hypervelocity gouging models that are scaled up to simulate dimensions of a real test sled. Real test sled dimensions are desired in order to more closely simulate actual test sled runs. Invariant products developed from application of the Buckingham Pi Theorem can be used as guidelines for determining whether a CTH model is applicable to a test sled with specific dimensions. Strain rate effects are investigated to study whether deviations between scaled CTH models may be reduced by modifying the constitutive model.     

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.     

Vulnerability of spacecraft harnesses to hypervelocity impacts

During a study performed in framework of a European Space Agency contract, the vulnerability of spacecraft harnesses has been assessed. The harnesses consisted of three different space-grade cable types: power cables, screened twisted pair data cables and radio frequency (RF) cables. They were alternately shielded by two different types of representative spacecraft structure walls. Ten hypervelocity impact (HVI) tests at 0° incidence have been performed with impact velocities ranging from 6.4 km/s to 7.7 km/s. Projectiles have been aluminium spheres with diameters ranging from 1.5 mm to 4.0 mm. During the tests, all cables were operated at their representative conditions and the disturbances, induced by the impacts, were measured. The malfunction observed could be related to two physical failure mechanisms: (1) short circuits caused by a conducting cloud of molten and evaporated aluminium and, for the data cables, (2) strands being bent by impacting fragments creating a short circuit between screen and signal cable. The influence of a more complex structure wall to the failure mechanism in (1) is shown. Malfunction was dependent on mechanical damage, but no clear correlation between severity of malfunction and mechanical damage could be established. The data cables were the most vulnerable cable, while the RF cables were the most robust. The disturbances recorded could pose a significant threat to connected electronic equipment. Examples of electrical performance are given.     

The shape effect of non-spherical projectiles in hypervelocity impacts

This paper provides qualitative and quantitative analyses of regular non-spherical projectile hypervelocity impacts on basic Whipple shields using test data obtained by light-gas guns, flat plate accelerators and shaped charge launchers. Surrogate cadmium and zinc test results are used to extend light-gas gun data beyond 8 km/s. Advanced Whipple shield derivatives are shown to be necessary to protect against non-spherical projectiles.     

Response of woven ceramic bumpers to hypervelocity impacts

The multi-shock shield concept devised by Crews and Cour-Palais,¹ composed of multiple ceramic cloth bumper layers and an aluminum back sheet, was used to investigate the response of woven ceramic bumpers to a hypervelocity impact. Observations made on past hypervelocity impact test data show that areal density is the most important bumper characteristic for initially breaking up solid particles. Our research has shown that once the solid particle has been shocked into a cloud of liquid and vapor, the weave pattern of the cloth bumper can influence the ability of the shield to absorb and contain the energy of the debris cloud.

To design a weave that will absorb particle energy more efficiently, we need to understand the micromechanics of the interaction between the debris cloud and the cloth bumper. In this paper we discuss our observations on the response of a ceramic cloth bumper to a hypervelocity impact and the failure mode occurring at the individual strand level.     

Acceleration fields induced by hypervelocity impacts on spacecraft structures

This paper presents an overview of the hypervelocity impact test campaign ongoing in the frame of the ESA contract “spacecraft disturbances from hypervelocity impact”. The project aims at analyzing the propagation of shocks due to hypervelocity impacts from the external shell of a spacecraft to its internal components. The object of the study is the GOCE satellite, which has been recognized to be very sensitive to small disturbances because of its payload that has been designed to measure even very low acceleration levels. In the first step presented hereafter, the test campaign has been focused on the qualification of the background environment inside the impact chamber and on the determination of the vibration levels induced by perforating and non-perforating hypervelocity projectiles on simple aluminum plates. The results currently obtained and a preliminary data analysis will be presented in the following.     

基于小波分析技术的高速撞击声发射源定位

针对在轨航天器遭受空间碎片撞击损伤的定位问题,利用二级轻气炮发射铝弹丸撞击铝板来模拟空间碎片对航天器结构的损伤;通过声发射传感器实时记录结构中的时域波形信号,结合板波的频散特性,分析了高速撞击穿孔损伤时波动在铝板中的传播模式.基于小波时频分析技术研究了高速撞击损伤的定位问题,分析了波速对定位精度的影响.结果表明,基于小波分析技术的高速撞击声发射源定位能够满足撞击损伤的定位要求,为解决航天器在轨遭受空间碎片撞击的损伤定位提供了一种新的方法.     

Analysis and simulation of hypervelocity gouging impacts for a high speed sled test

An analysis and simulation of the gouging impact phenomenon which occurs at the Holloman Air Force Base High Speed Test Track (HHSTT) during hypervelocity impact testing is presented. Simulations of the sled/rail interactions were conducted using the hydrocode, CTH. These simulations utilize the most accurate and validated material models for the sled shoes (VascoMax 300) and rail (1080 steel) – which were recently developed. Sled shoe impacts with the rail were evaluated using various geometries possible in the field. The conditions leading to hypervelocity gouging were identified, as well as the condition which resulted in rail wear. The CTH simulations match results observed in the field extremely well. Recommendations are made, based on the latest material models and simulations, which should significantly reduce the occurrences of hypervelocity gouging at the HHSTT.     

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.     

Acoustic response of aluminium and Duroid plates to hypervelocity impacts

The growing need for real-time impact sensors for deployment on both space vehicles and space habitats (in orbit or on the surface of atmosphere-less bodies such as the Moon) has stimulated sensor development programmes. The sensors should be low mass, low power, easily read-out electronically, cover large areas and be sensitive to impacts which can cause damage up to and including penetration. We propose that piezo-strain acoustic sensors can play an important role in this work. Accordingly we report on a series of hypervelocity impact tests of acoustic sensors mounted on thin plates (aluminium and Duroid plates). The acoustic sensors gave strong signals for impacts of sub mm-mm scale projectiles. We investigated dependences on impactor speed and size, angle of incidence and tested the difference between cratering and penetrating impacts.     

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.     

Numerical simulations of target hole diameters for hypervelocity impacts into elevated and room temperature bumpers

Normal impacts between 3.175 mm diameter 1100-O Al spherical projectiles and 1.6 mm thick 6061-T6 Al targets are considered for both room temperature and heated targets. The elevated target temperatures under examination are 110 and 210 °C, and the simulated impact velocities range from approximately 2 to7 km/s. The AUTODYN v.5 hydrocode is used to generate data that are compared to empirical data obtained from the literature, and to that obtained with the University of Denver Research Institute's (DRI) two-stage light gas gun. The effect of using applicable strength and fracture models currently available in AUTODYN to replicate both room and elevated temperature target hole diameters is also examined. It was found that the numerical simulations for both room and elevated temperature targets predict hole diameters that differ approximately 0.1–13% from the DRI empirical data.     

Generation of transient vibrations on aluminum honeycomb sandwich panels subjected to hypervelocity impacts

This paper presents the results of a set of experiments aimed at discovering the main features of impact-induced vibrations on all-aluminum honeycomb sandwich panels, representative of the GOCE satellite's top floor, which is exposed to the orbital debris environment. The activity focused on the characterization of the vibrations induced in the vicinity of internal payloads by hypervelocity impacts occurring on the vehicle's external shell. More than 30 tests were realized by launching 0.8–2.3 mm aluminum projectiles in the velocity range 4–5.5 km/s on targets with tri-axial accelerometer assemblies mounted on both the front and rear face of the panel, at a nominal distance of 150 mm from the impact point. It was found that a hypervelocity impact produces in both the front and rear side of the sandwich panel a vibration environment which can be described through the shock response spectrum (SRS) of three different types of waves that can be distinguished on the basis of the acceleration direction: out-of-plane, in-plane longitudinal and in-plane shear. The influence of projectile mass and velocity on SRS appeared to vary with frequency, with the most significant difference in the range between ∼10³ and ∼10⁴ Hz. The results of whole experimental set were used to derive an interpolation law through standard techniques of nonlinear fit. The empirical equation obtained makes it possible to predict the near-field vibration environment produced by hypervelocity impacts with debris having given size and velocity, reproducing all the test data with an average uncertainty of ±6 dB.