Penetration scaling in atomistic simulations of hypervelocity impact

We present atomistic molecular dynamics simulations of the impact of copper nano particles at 5 km s⁻¹ on copper films ranging in thickness from from 0.5 to 4 times the projectile diameter. We access both penetration and cratering regimes with final cratering morphologies showing considerable similarity to experimental impacts on both micron and millimetre scales. Both craters and holes are formed from a molten region, with relatively low defect densities remaining after cooling and recrystallisation. Crater diameter and penetration limits are compared to analytical scaling models: in agreement with some models we find the onset of penetration occurs for 1.0 < f/d_p < 1.5, where f is the film thickness and d_p is the projectile diameter. However, our results for the hole size agree well with scaling laws based on macroscopic experiments providing enhanced strength of a nano-film that melts completely at the impact region is taken into account.     

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.     

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.     

Residual temperature measurements of light flash under hypervelocity impact

Experimental and theoretical results for light flash temperatures are presented for impacts on soda-lime glass by iron projectiles. The experiments were performed with a 2 MV Van de Graaff, where iron dust particles (0.14–0.63 μm in diameter) impacted soda-lime glass at a velocity range of 5–20 km s⁻¹. Theoretical calculations were based on the assumption of the Mie–Gruneisen equation of state (EoS) with different values for the Gruneisen coefficient and hydrodynamic behaviour (no strength effects were considered). Within the scatter of experimental data, results suggest a constant value for the average light flash temperature of approximately 2600 K independent of iron dust impact velocity. Although theoretical calculations are limited by the use of the Mie–Gruneisen EoS up to the point of incipient vaporisation of the target material, relatively good agreement with experiments is observed. This agreement suggests that the observed constant temperature may be due to material phase change from incipient to complete vaporisation over the range of velocities considered.     

Response of composite materials to hypervelocity impact

Investigation of composite materials response to hypervelocity impact by space debris has been carried out. In order to simulate hypervelocity impact, a unique laser driven flyer plate (LDFP) system was used, generating hypervelocity debris with velocities of up to 3 km/s. The materials studied in this research were Kevlar 29/epoxy and Spectra1000/epoxy thin film micro-composites (thickness of about 100 μm). Both Spectra and Kevlar fibers are used in long-duration spacecraft outer wall shielding to reduce the perforation threat. The micro-mechanical response of different composites was studied and correlated to the fiber, the matrix and the fiber/matrix interface properties. Visual and microscopic examinations of the damaged area identified fiber debonding as the prevailing failure mechanism. On the basis of a simple energy balance model it can be stated that for Spectra/epoxy composite the dominant mechanism is new surface creation, whereas for Spectra surface-treated fibers/epoxy the fiber pull out is the dominant mechanism. For Kevlar/epoxy fiber, pull out mechanism plays an important role.     

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.     

Comparison study of MPM and SPH in modeling hypervelocity impact problems

Due to the high nonlinearities and extreme large deformation, the hypervelocity impact simulation is a challenging task for numerical methods. Meshfree particle methods, such as the smoothed particle hydrodynamics (SPH) and material point method (MPM), are promising for the simulation of hypervelocity impact problems. In this paper, the material point method is applied to the simulation of hypervelocity impact problems, and a three-dimensional MPM computer code, MPM3D, is developed. The Johnson–Cook material model and Mie–Grüneisen equation of state are implemented. Furthermore, the basic formulations of MPM are compared with SPH, and their performances are compared numerically by using MPM3D and LS-DYNA SPH module.     

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.     

Further validation of a general approximation for impact penetration depth considering hypervelocity gouging data

A first-order approximation of penetration depth is developed for use in engineering design. A survey of available penetration data is used to construct a one-dimensional approach for estimating the geometry of a crater resulting from high-energy impact. The results are generalized to allow approximations to be made using existing experimental data without the requirement for laboratory testing. This approach for penetration depth approximation is validated using the hypervelocity gouging data from the Holloman High Speed Test Track (HHSTT) and hypervelocity gouging impact tests conducted by the authors.     

On hypervelocity penetration of the mesh-bumper strings into a projectile

An elastic–plastic model describing hypervelocity penetration of a mesh-bumper string by an arriving projectile is presented. An analytical solution for the case of rigid penetration is derived whereby the dependence between penetration depth and impact velocity is established. A comparative analysis between rigid penetration and penetration with a spreading string is performed. The mesh strings interact between each other with the aid of flow arising during penetration. The model which is developed allows us to estimate destruction depth of the projectile during its interaction with the mesh strings.     

Fracture behavior of silicon nitride ceramics subjected to hypervelocity impact

A new advanced ceramic thruster made of monolithic silicon nitride ceramics was developed for the planetary exploration spacecraft AKATSUKI (PLANET-C) at Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA). To ensure its operation onboard the spacecraft, the reliability of the ceramic thruster against micrometeoroid hypervelocity impact has been investigated. Silicon nitride plates were impacted by spheres of stainless-steel and other materials with 0.2-0.8-mm diameters in the velocity range up to 8.0 km/s using a two-stage light-gas gun. Using crater depth data under various impact conditions, the penetration equation of silicon nitride was determined. The impacted samples showed fracture patterns of three types: cratering, cratering with spallation, and perforation. These fracture patterns were well categorized by the multiple forms of the penetration equation.     

Characterizing the transient response of CFRP/Al HC spacecraft structures induced by space debris impact at hypervelocity

To quantify the disturbance induced by the impact of micrometeoroid and space debris particles at hypervelocity on vibration-sensitive CFRP/Al HC SP satellite platforms a method is presented which uses experimentally validated hydrocode models to characterize the impact-induced transient wave in the local structure. Key features of the transient waveform are simplified by a mathematical function which is expressed in terms of impactor momentum. Evolution of the transient waveform is characterized using multiple measurement gauges located on the sandwich panel facesheets outside the area of mechanical damage. The characterization is then used to extrapolate the elastic waveform back to the impact location. The elastic-equivalent excitation of a CFRP/Al HC SP is defined in terms of force with respect to time for application in finite element structural codes for propagation of the local disturbance to vibration-sensitive locations (i.e. measurement devices).     

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.     

Preliminary study on sustained arc due to plasma excited by hypervelocity impact of space debris on the solar array coupon

Recently, solar array has become higher in potential and larger in capacitance. Therefore, possibility of collision between space debris and enlarged solar array has been pointed out. Many debris and dust impacts were confirmed on fuselage of the retrieved satellite space flyer unit (SFU) and solar array paddle of satellite Eureca. If space debris collides with the solar array of an orbiting satellite, it may cause generation of high-density plasma by debris impact-induced dielectric breakdown of satellite component and the phenomenon called discharge. This discharge short circuits the solar array and current will not flow into pay load of the satellite. And the very worst event by this discharge is the operational end of the satellite. However, any events of discharge phenomenon by debris impact cannot be yet confirmed. But we cannot ignore such possibility of discharge by debris impact. The purpose of the present paper is to investigate discharge condition due to debris impact which yields us reduction of electric power of solar array, and to reduce influence of the impact on satellite missions. In this study, a solar array coupon was tested under hypervelocity impact in which a projectile was launched by a two-stage light gas gun installed in Kyushu Institute of Technology (KIT). As a result, we verified discharge event in the hypervelocity impact ground test.     

Experimental evidence of triboluminescence induced by hypervelocity impact

The emission of light due to crystal fracture, or triboluminescence (TL), is a phenomenon that has been known for centuries. One of the most common examples of TL is the flash created from chewing wintergreen Lifesavers^®. For the last couple of years, the authors have been measuring fluorescence properties of phosphors like zinc sulfide doped with manganese (ZnS:Mn). Preliminary results indicate that impact energies greater than 16 mJ produced measurable TL from ZnS:Mn. Light was generated from the interaction of a dropped mass and a small number of luminescence centers in the ZnS:Mn powder. To extend this research, a two-stage hypervelocity light gas gun located at NASA's Marshall Space Flight Center (MSFC) was used to evaluate equipment and settings that show promise for hypervelocity TL detection. In these experiments, a projectile was accelerated to approximately 5–6 km/s before striking a ZnS:Mn phosphor-coated aluminum plate. This paper will provide an overview into the first experimental evidence of TL emission from ZnS:Mn due to hypervelocity impact. It is hoped that these results will generate interest in future hypervelocity research.     

Multi-layer insulation material models suitable for hypervelocity impact simulations

MLI (multi-layer insulation) is present in many spacecraft missions and typically consists of multiple layers of aluminized Kapton separated by fine gauze. It has been observed, depending on the type and position within the structure, that MLI can influence the ballistic performance of panels under hypervelocity impact (HVI) despite being extremely lightweight. Due to the very thin nature of the foils, <10 μm, it is often considered too computationally expensive to explicitly include such materials in HVI simulations of typical structures used in space. Accurate resolution of the foils would require a prohibitive number of elements. This paper reports on the development of a discrete modelling approach that efficiently facilitates the inclusion of such materials and allows for each layer of the MLI to be explicitly represented in the numerical model. Mesomechanical simulations of planar plate impact experiments (PPI) and an HVI event on MLI are presented where each layer of the MLI is explicitly represented with a number of elements through the thickness. The results of these models are then compared with the developed discrete approach suitable for including in larger scale simulations of impacts on real space structures. The current study applies previously developed material models to structural materials such as Carbon Fiber Reinforced Plastics (CFRP). This paper further describes simulation of an HVI event on a CFRP-Aluminum/honeycomb structure at oblique incidence, thereby illustrating that the developed approach for modelling MLI provides a practical method for the inclusion of such materials in full-scale simulations.     

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.     

Analysis of formation of the Odessa crater

The impact of the meteorite that formed the Odessa crater is examined in this numerical study. Extensive information collected from excavation of the site has made it possible to use the Odessa crater as a code validation test, to the extent that a calculated impact can produce equivalent cavity dimensions and stratification. In addition, the calculations can be used to provide a better estimate of velocity and trajectory of the meteorite at impact. The initial set of simulations performed in this study suggests that original estimates of the impact conditions may not have been sufficient to produce a crater of the size and shape known from the Odessa site. Supplemental analysis was performed and suggested that the meteor impacted at a much larger obliquity angle and may have been much larger than originally speculated. Details of the analysis leading to these observations are presented.