Historical overview of hypervelocity impact diagnostic technology

The ability to understand hypervelocity impact phenomena and to validate predictive models of material behavior is largely dependent on the diagnostic tools available to the experimenter. These tools range from simple, post-impact examination of targets to extremely sophisticated and complex techniques, which simultaneously measure a myriad of impact parameters on a time scale of nanoseconds to milliseconds. This wide range of available techniques represents the challenge and opportunity if hypervelocity impact experimentation.

There has been a continual challenge to develop diagnostic techniques with ever-increasing resolution, as higher velocities and pressures are achieved. Today's techniques provide the experimenter the means of measuring most hypervelocity impact parameters, including velocity, displacement, temperature, radiation, volumetric change, impluse, stress, and strain of the materials involved. However, the prospective quantum leap in impact velocities to be produced by electromagnetic and electrothermal launchers will require corresponding advances in diagnostic systems.

This paper examines the capabilities and limitations of the major diagnostic techniques for hypervelocity impact experimentation and traces their evolvement as useful laboratory tools.     

High-speed image-forming instrumentation for hypervelocity impact studies

Fast image-forming instrumentation is, by far, the most widely used means for studying hypervelocity impacts when time-resolved information is sought. Single frame (snapshot) cameras are available whose exposure times are short enough to effectively “freeze” material movements in even the fastest impact events. Flash X-radiography equipment is available for taking “snapshot” silhouettes through smoke/dust that defeat visible-light cameras. Similar X-ray equipment operating at higher voltage allows taking flash radiographs through massive materials so that internal features of impacts can be observed. A second series of imaging techniques take cine' sequences of photographs of dynamic events analogous to conventional motion-picture photography. Currently, available equipment operating in the visible is fast enough to time-resolve almost any conceivable macro-impact. X-ray cine' equipment is available, but it is in a considerably earlier stage of development. Streak photography has found wide use in the shock wave analysis community and has also been applied successfully to diagnosing hypervelocity impacts. Streak cameras produce position-vs.-time plots of visualized events that occur along a pre-selected line across the image of an impact process. To the author's knowledge, streak photography has been applied to ballistic research applications exclusively in the visual spectral region … although appropriate energy sources and electronic streak cameras for operating in the X-ray spectral regime have been available for at least a decade.     

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.     

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.     

Scale modeling of hypervelocity impact

The significance of different Pi terms which result from a dimensional analysis of the parameters involved in hypervelocity impact is discussed. The consequences of distorting some physical phenomena in models are analyzed, and experimental verification is presented for the use of replica and dissimilar material models.     

Characteristics of hypervelocity impact debris clouds

This paper describes an experimental investigation of hypervelocity impact debris clouds produced by impacting metal rods with Kapton flyers in an electric gun facility. Soft copper witness plates placed in the path of the debris were cratered and coated with rod material. From the sizes of the craters on the witness plates we could obtain values for a cratering parameter containing the ,asses and velocities of the debris fragments that formed the craters. By combining the cratering parameter with rough estimates of the fragment masses, we then estimated the fragment velocities. By measuring the thickness and extent of the coating on the witness plates, we obtained a bound on the amount of material vaporized by the impact.     

Review of modern hypervelocity impact facilities

The paper describes three hypervelocity impact facilities, their instrumentation, the laucher systems, the launch techniques for a large variety of projectile geometries, and the launcher performance. A very brief review is given of the research in the areas of military applications, meteorite impact simulation, basic impact research, and efforts to improve the performance of the light-gas guns and measurement techniques. An outline at future research programs is given.     

Spectral measurements of hypervelocity impact flash

We have revisited the well-known phenomena of impact flash by examining the optical spectra generated by hypervelocity impacts at velocities up to several tens of kilometers per second. This particular effort, sponsored by an LDRD from our laboratories, has looked at the flash from impacts over a range of velocities from 6 to 25 km/s, using two different experimental environments. Both two- and three-stage light-gas guns and magnetically driven flyers on the Z accelerator were used. Here, we describe the latter set of experiments. Using standard tabulations of atomic spectral data, we were able to identify strong spectral lines from the principal materials used in the various shots. We demonstrated that the impact-flash spectra were qualitatively reproducible from shot to shot, and have confirmed the feasibility and credibility of using impact-flash spectroscopy for remote sensing applications such as meteoroid-impact or missile-defense engagement analyses.     

Cavity and crater depth in hypervelocity impact

We discuss the depth of cavities and craters caused by hypervelocity impacts as a function of impact parameters such as impact velocity, projectile and target densities, and projectile diameter, in two extreme cases: the penetration of intact projectiles at low impact pressure and the hemispherical excavation at very high impact pressure. The relations between the depth and the impact parameters are obtained. Then, previous experimental results are compiled; crater depth normalized by projectile diameter and the ratio of projectile and target densities is plotted for glass, plastic, and metal projectiles and metal, rock, ice, foam, sheet-stack, and aerogel targets. The trends of the data are consistent with the relations in the extreme cases.     

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.     

Recent improvements in SPH modeling of hypervelocity impact

Four improvements to Smoothed Particle Hydrodynamics which enhance its ability to simulate hypervelocity impact are discussed and applied to the impact fracture of a steel cube on an aluminum plate at 2.2 km/s.     

History and application of hydrocodes in hypervelocity impact

Hydrocode enhancements have evolved as a result of the desire of researchers to be predictive, generally for some particular application. A review of the development of hydrocodes is given from a historical viewpoint. The paper then ends with a discussion of possible future enhancements in the codes.     

ANEOS extensions for modeling hypervelocity impact

Several extensions have been implemented into the ANEOS analytical equation of state package [1]. The extensions permit more precise control of the properties of the liquid-vapor dome region of the equation of state. Different algebraic forms for the cold component were examined, including a modified form of the Morse-Coulomb potential currently used in ANEOS, the Lennard-Jones potential, and an analytic EXP-N formulation [2]. Extensions to the interpolation method for the nuclear component between the solid and vapor regions were also performed. In general, these extensions allow for more positive control over the position of the critical point and the shape of the liquid-vapor dome. The critical point and liquid-vapor dome shape can have a significant influence on the results from hypervelocity impact events, since release isentropes often pass near the critical point and directly through the high-pressure regions of the dome. To demonstrate this, several impact calculations from projectiles striking thin shields are performed using CTH [3]. The results show that the form of the equation of state can alter the structure and properties of the resulting debris cloud.     

Pulsed holography for hypervelocity impact diagnostics

The development of pulsed holography has two principal objectives. The first objective is to quantify the three dimensional characteristics of hypervelocity impact events, and the second is to provide a diagnostic with the ability to capture high fidelity information for the validation of sophisticated three-dimensional hydrocodes. The holographic image-capturing subsystem uses a Q-switched, seeded, frequency-doubled Nd-YAG laser which produces 5 ns, 750 mJ, coherent pulses at 532 nm. Holographic images have been captured of the back-surface debris bubble from 4 km/s perforating impacts and crater ejecta from 2 km/s non-perforating impacts. A prototype holographic reconstruction and image analysis subsystem has been assembled that provides the ability to measure both the spatial distribution of particles and the morphology of individual particles produced in a hypervelocity impact event. The demonstrated image resolution of this system is 20 μm; however, higher resolutions are possible with magnification optics.     

Gouge development during hypervelocity sliding impact

The sliding interface of a hypersonic flyer/target system is subject to immense forces due to dynamic loads and the highly oblique impact of the flyer with the target. Under these severe loading conditions, the material in both the flyer and target will experience large non-linear deformation known as gouging. Using the hydrocode CTH, a numerical model of the gouging process has been created and studied to examine the underlying physics of the phenomenon. Results of simulated gouges are presented along with a discussion of the mechanics involved. Under the combination of a normal load sufficient to cause penetration of the flyer into the target, and hypervelocity tangential motion, stresses may exceed the plastic flow stress of the materials. In this case, material jets may develop at the material interface and impinge into the adjacent material. This jet formation, and the material deformation it causes, are the underlying causes of gouging.     

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.     

Fracture modes in alumina at hypervelocity impact conditions

Experiments were performed to find the fracture patterns of alumina at hypervelocity impact conditions using short pulsed laser induced shock waves. For planar shock waves spalling was obtained, while using spherical shock waves, the samples developed Herzian (conical) fracture mode.     

Meso-scale modeling of hypervelocity impact damage in composite laminates

ObjectivesThis paper presents an approach to numerical modeling of hypervelocity impact on carbon fiber reinforced plastics (CFRP).MethodsThe approach is based on the detailed meso-scale representation of a composite laminate. Material models suitable for explicit modeling of laminate structure, including fiber-reinforced layers and resin-rich regions, are described. Two numerical impact tests with significantly different impact energies were conducted on thermoplastic AS4/PEEK materials with quasi-isotropic layups. Simulations employed both SPH and Finite element methods.ResultsResults of simulations are verified against experimental data available from the literature and demonstrate good correlation with the experiments.ConclusionsDeveloped modeling approach can be used in simulations where post-impact damage progression in composite material is of the main focus.