Investigation of Hypervelocity Impact Phenomena Using Real-time Concurrent Diagnostics

In the fields of space engineering and planetary science, hypervelocity impact phenomena have been studied as they relate to the space debris problem and planetary impact. With regard to hypervelocity-impact-induced damage, many studies focus on the evaluation of impact-damage geometry and morphology, for example, to construct the ballistic limit equations and/or penetrating equations for space structures, and to predict the size and shape of crater and fragments generated by planetary impact [1-4]. While the final state or late stage of an impact event are of primal interest, damage accumulation at early stages affect the overall outcome of the impact event. The understanding of hypervelocity-impact-damage processes lead to improvement of material-response models for hypervelocity impact and higher fidelity simulations of hypervelocity impact events. Under such a background, we have performed real-time imaging of hypervelocity-impact events on transparent materials to investigate the impact-damage formation and evolution processes [5-7]. In our previous work, the stress-wave-propagation behavior and damage evolution were observed by means of a transmitted light shadowgraph. In these measurements, the shape of the longitudinal-stress-wave front, crater and spall fracture were successfully visualized. On the other hand, these shadowgraph images provide little information about damage microstructure. The shadowgraph has difficulty in visualizing ramped waves, such as the release wave, and also for the shear wave which is not accompanied by the change of volumetric strain. Those play important role in initiating damage. This occurs because the intensity of the shadowgraph image depends on the second spatial derivative of the refractive index. In this study, we try two types of real-time imaging of impact events. One is imaging by using scattered light on the impacted target to visualize the microstructure of the impact-induced damage, the other is a shadowgraph using polarized light to visualize propagation of the impact-induced stress field.

Developing a fundamental understanding of how polymers respond during hypervelocity impact (HVI), a high strain rate process, is crucial for the potential use of polymers in HVI protection systems. Here, we present the high-strain-rate impact behavior of commercially available polycarbonates with two different molecular weights, 42 and 21 kg/mol. A powder gun and a two-stage light-gas gun were used to achieve the velocities (v0) of a 4 mm projectile impacting the polycarbonate targets over the range of 400–6,500 m/s. The deformation behavior of the polymer during the impact event was captured using high-speed cameras. The impact caused a variety of responses, ranging from deflection of the projectile to complete perforation for higher v0, leading to major debris cloud formation. These debris clouds consist of both fluid- and dust-like ejecta. The fluid-like debris cloud indicates the melting of polymer caused by the conversion of kinetic energy to thermal energy and subsequent adiabatic heating. The dust-like response can likely be attributed to the brittle failure behavior of polymers, as the polymer became brittle at a high strain rate. Dynamic mechanical analysis (DMA) and dielectric thermal analysis (DETA) on the polycarbonate samples used here indicate an approximately 40 °C increase of Tg with increasing frequency from 1 Hz to 106 Hz, and such an increase has likely led to brittle behavior. While the leading-edge velocity of the debris clouds and perforation diameters scale linearly with v0, we found negligible differences in the HVI response for the two molecular weights of polycarbonate tested here. This study displays the importance of v0 and thickness contributing to responses of polycarbonates undergoing HVI.

In this paper, we introduce a computational model for the simulation of hypervelocity impact (HVI) phenomena which is based on the Discrete Element Method (DEM). Our paper constitutes the first application of DEM to the modeling and simulating of impact events for velocities beyond 5 . We present here the results of a systematic numerical study on HVI of solids. For modeling the solids, we use discrete spherical particles that interact with each other via potentials. In our numerical investigations we are particularly interested in the dynamics of material fragmentation upon impact. We model a typical HVI experiment configuration where a sphere strikes a thin plate and investigate the properties of the resulting debris cloud. We provide a quantitative computational analysis of the resulting debris cloud caused by impact and a comprehensive parameter study by varying key parameters of our model. We compare our findings from the simulations with recent HVI experiments performed at our institute. Our findings are that the DEM method leads to very stable, energy–conserving simulations of HVI scenarios that map the experimental setup where a sphere strikes a thin plate at hypervelocity speed. Our chosen interaction model works particularly well in the velocity range where the local stresses caused by impact shock waves markedly exceed the ultimate material strength.

3D characterisation of debris clouds under hypervelocity impact with large-field pulsed digital in-line holography

Validation of the Preston–Tonks–Wallace strength model at strain rates approaching ∼1011 s−1 for Al-1100, tantalum and copper using hypervelocity impact crater morphologies

1. The Shock Hugoniot of Solid Ice: We present a complete description of the solid ice Hugoniot based on new shock wave experiments conducted at an initial temperature of 100 K and previously published data obtained at 263 K. We identify five regions on the solid ice Hugoniot: (1) elastic shock waves, (2) ice Ih deformation shocks, transformation shocks to (3) ice VI, (4) ice VII, and (5) liquid water. In each region, data obtained at different initial temperatures are described by a single Us - Δup shock equation of state. The dynamic strength of ice Ih is strongly dependent on temperature. The Hugoniot Elastic Limit varies from 0.05 to 0.62 GPa, as a function of temperature and peak shock stress. We estimate the entropy and temperature along the 100 and 263 K Hugoniots and derive the critical pressures for shock-induced incipient (IM) and complete (CM) melting upon release. On the 100 K Hugoniot, the critical pressures are about 4.5 and between 5-6 GPa for IM and CM, respectively. On the 263 K Hugoniot, the critical pressures are 0.6 and 3.7 GPa for IM and CM, lower than previously suggested. Shock-induced melting of ice will be widespread in impact events. 2. Rampart Crater Formation on Mars: We present a model for the fluidization of Martian rampart crater ejecta blankets with liquid water based on the shock physics of cratering onto an ice-rich regolith. We conducted simulations of crater formation on Mars, explicitly accounting for the equations of state and shock-induced melting criteria for both the silicate and ice components and using strength models constrained by the observed transition diameter DTr from simple to complex craters on Mars, where DTr = 8 km corresponds to an effective yield strength of 107 Pa. For the observed size range of rampart craters (diameters D ≾ 30 km) and typical asteroidal impact conditions (silicate impactors, D ≾ 1 km, at 10 km s-1), we find that the hemispherical volume where subsurface ice is partially melted by the impact shock has a radius of about 15 projectile radii (rp), much larger than previous predictions of about 6 rp. The radius of the final crater is comparable to the radius of partial melting and more than half the ice within the excavated material is melted. Thus, the amount of shock-melted water incorporated into the continuous ejecta blanket is within a factor of two of the near-surface ground ice content. We find that fluidized ejecta blankets may form in the current climate with mean surface temperatures of 200 K. Decreasing the effective yield strength of the modeled materials, e.g., by increasing the ice content or porosity, modifies the impact-induced flow in the excavated cavity, resulting in deeper projectile penetration, steeper ejection angles, higher crater rim uplift, and reduced final crater diameter. The volume fraction of shock-melted water in the ejecta blanket increases with distance from the crater rim. The horizontal flow velocities during emplacement of fluidized ejecta (~ 10 - 1000 m s-1) is nearly constant in the continuous ejecta blanket and within the range of large terrestrial landslides. Therefore, ground-hugging debris flow conditions are achieved. The ejecta blanket properties from impacts into a Martian regolith containing 20-40%vol near-surface ice are consistent with the fraction of liquid water inferred from models of ejecta flow rheologies which produce rampart morphologies, about 10-30% liquid water by volume [Ivanov, B. A., Solar System Research, 30, 43-58, 1996]. We present a model for the formation of different rampart ejecta morphologies which may be used in conjunction with an ejecta blanket debris flow model to map the distribution of ground ice. In addition, we find that formation of single or multiple-rampart ejecta blankets does not require pre-existing liquid water in the Martian crust. We estimate the minimum water content in observed rampart ejecta blankets to be equivalent to a global layer of water 0.6 m thick. Based on the crater sampling efficiency, the implied global Martian ice content, within the upper 2 km of the crust, is equivalent to a global layer of water 100 m deep. This result is comparable to other estimates of HM2O content in the Martian crust.

Hypervelocity impact events are generally defined as collisions that occur at velocities greater than a few kilometers per second, greater than the speed of sound in the material. Such impact events are relatively common in space, where even the smallest of particles or debris can cause significant damage when colliding at these speeds with satellites or manned spacecraft. However, the physics behind the formation of plasma, resulting from the hypervelocity impact, and the dynamics of its expansion that can cause electrical damage remain largely unknown. Experimental studies and numerical simulation are being performed in order to understand the underlying physical processes that occur upon formation and expansion of the impact plasma. This understanding will help to develop suitable impact measuring and analysis tools as well as new materials and designs for protecting man and machines in space. A hypervelocity impact (HVI) event also occurs when a kinetic warhead missile hits an enemy missile or target missile, such as the intercept missiles developed by countries for defense. The objective in this case is to maximize lethality, i.e., the ability to completely destroy an incoming missile threat. As in the spacecraft application, experimental studies and numerical simulation are being performed for missile impact events in order to understand the extent of lethality (i.e., the destruction of the enemy missile) and predict its effects (e.g., debris patterns, etc.). Stanford University is working to improve the understanding of the physical mechanisms driving the plasma expansion process and associated RF emission and their role in spacecraft failure. Invocon, Inc. has been pursuing the same understanding, but primarily for the purpose of assessing lethality and damage progression as it relates to missile intercepts. This paper provides an overview of the advancements made in HVI research by a major university and a research and technology company that have pursued the same objectives from different perspectives, and explains why HVI matters. doi: 10.12783/SHM2015/359

Simulation method of debris cloud from fiber-reinforced composite shield under hypervelocity impact

The material point method (MPM) is an extension of particle-in-cell method to solid mechanics. A parallel MPM code is developed using FORTRAN 95 and OpenMP in this study, which is designed primarily for solving impact dynamic problems. Two parallel methods, the array expansion method and the domain decomposition method, are presented to avoid data races in the nodal update stage. In the array expansion method, two-dimensional auxiliary arrays are created for nodal variables. After updating grid nodes in all threads, the auxiliary arrays are assembled to establish the global nodal array. In the domain decomposition method, the background grid is decomposed into some uniform patches, and each thread deals with a patch. The information of neighbor patches is exchanged through shared variables. After updating nodes in all patches, their nodal variables are assembled to establish the global nodal variables. The numerical tests show that the domain decomposition method has much better parallel scalability and higher parallel efficiency than the array expansion method. Therefore, a parallel computer code, MPM3DMP, is developed based on the domain decomposition method. Finally, MPM3DMP is applied to a large-scale simulation with 13,542,030 particles for obtaining the high-resolution results of debris cloud in hypervelocity impact.

The Hypervelocity Impact Society is devoted to the advancement of the science and technology of hypervelocity impact and related technical areas required to facilitate and understand hypervelocity impact phenomena. Topics of interest include experimental methods, theoretical techniques, analytical studies, phenomenological studies, dynamic material response as related to material properties (e.g., equation of state), penetration mechanics, and dynamic failure of materials, planetary physics and other related phenomena. The objectives of the Society are to foster the development and exchange of technical information in the discipline of hypervelocity impact phenomena, promote technical excellence, encourage peer review publications, and hold technical symposia on a regular basis. It was sometime in 1985, partly in response to the Strategic Defense Initiative (SDI), that a small group of visionaries decided that a conference or symposium on hypervelocity science would be useful and began the necessary planning. A major objective of the first Symposium was to bring the scientists and researchers up to date by reviewing the essential developments of hypervelocity science and technology between 1955 and 1985. This Symposia--HVIS 2007 is the tenth Symposium since that beginning. The papers presented at all the HVIS are peer reviewed and published as a special volume of the archival journal International Journal of Impact Engineering. HVIS 2007 followed the same high standards and its proceedings will add to this body of work.

Hypervelocity impact in metals, glass and composites

A debris cloud model for hypervelocity impact of the spherical projectile on reactive material bumper composed of polytetrafluoroethylene and aluminum

View Video Presentation: https://doi.org/10.2514/6.2021-0725.vid The study of hypervelocity impacts (HVIs) is of increasing interest in numerous engineering applications involving micrometeoroids, orbital debris, hypersonic vehicles, and hypervelocity weapons systems. The associated relative velocities are typically in excess of several kilometers per second. The experimental and modeling studies of HVIs are instrumental in damage estimation and designing protective armor for personnel and hardware. In typical HVIs, debris clouds are formed at the front and rear of the target and propagate outwards. By unraveling the physics of these debris clouds, it is possible to gain a fundamental understanding of the energy of the impact, energy absorption and dissipation, and material failure mechanisms. Such knowledge also helps to determine the feasibility of materials for protective applications. Innovative optical diagnostic techniques can be applied to study the temporal and spatial evolution of these debris clouds. In this work, we use digital particle tracking and schlieren imaging to analyze HVIs produced by a two-stage light gas gun that can launch metal or polymer projectiles of 2–10 mm in diameter at speeds of 2–8 km/s. The target material in these experiments is ultra-high molecular weight polyethylene (UHMWPE). A dynamic delay generator system was developed to obtain synchronized ultra-high-speed videos at frame rates greater than 1M fps of the formation and expansion of the debris cloud. A particle tracking code enables the estimation of particle size and velocity from these videos. Additionally, a lens-type schlieren imaging system is used to track the density gradients produced by the shock waves formed by the fragments. These results are vital to better characterize, develop, and validate materials failure models during HVI events.

In situ diagnostics for a small-bore hypervelocity impact facility

Projectile fragmentation and debris cloud formation behaviour of wavy plates in hypervelocity impact