This paper reviews major findings of the Multidisciplinary Experimental and Modeling Impact Crater Research Network ( MEMIN ). MEMIN is a consortium, funded from 2009 till 2017 by the German Research Foundation, and is aimed at investigating impact cratering processes by experimental and modeling approaches. The vision of this network has been to comprehensively quantify impact processes by conducting a strictly controlled experimental campaign at the laboratory scale, together with a multidisciplinary analytical approach. Central to MEMIN has been the use of powerful two‐stage light‐gas accelerators capable of producing impact craters in the decimeter size range in solid rocks that allowed detailed spatial analyses of petrophysical, structural, and geochemical changes in target rocks and ejecta. In addition, explosive setups, membrane‐driven diamond anvil cells, as well as laser irradiation and split Hopkinson pressure bar technologies have been used to study the response of minerals and rocks to shock and dynamic loading as well as high‐temperature conditions. We used Seeberger sandstone, Taunus quartzite, Carrara marble, and Weibern tuff as major target rock types. In concert with the experiments we conducted mesoscale numerical simulations of shock wave propagation in heterogeneous rocks resolving the complex response of grains and pores to compressive, shear, and tensile loading and macroscale modeling of crater formation and fracturing. Major results comprise (1) projectile–target interaction, (2) various aspects of shock metamorphism with special focus on low shock pressures and effects of target porosity and water saturation, (3) crater morphologies and cratering efficiencies in various nonporous and porous lithologies, (4) in situ target damage, (5) ejecta dynamics, and (6) geophysical survey of experimental craters.

Planar-plate impact tests were carried out for the measurement of Hugoniot data of dry Seeberger sandstone in the pressure range of up to 7 GPa. A special inverse testing method suitable for rock targets was applied together with a laser interferometer for the measurement of the free-surface velocity of the target plate required for the calculation of the particle velocity (up) and the shock wave velocity (Us) in the sandstone specimen. In our measurement range (impact velocity between about 50 m/s and about 1300 m/s), the Us up data show significant scattering. The travel times of the impact-induced shock waves indicate that pore crushing occurs. A strong dependance of the Us up data on the test batch selected for the impact experiments and, thus, on the mining location was observed. This dependance is greater than the scattering of the individual data sets. (C) 2016 Elsevier Ltd. All rights reserved.

In multiple engineering fields such as rock drilling or building constructions or extreme events like earthquakes or impacts, the dynamic properties of rock play an important role. A way to model these events and define measures to minimize the damage derived from these events is created by means of numerical analysis. Hence, the knowledge of the dynamic material behavior is essential for studying the effects of such a loading scenario. Solid geological materials, from the family of the sedimentary rocks, have been analyzed under quasi-static loads. However, there is a lack of knowledge when high strain rate loadings are involved. Within this context, the paper focuses on the experimental characterization of two sedimentary rocks, sandstone and limestone, under impact loading using the Hopkinson-Bar spallation and compression tests. The analysis encompasses the determination of the tensile and compressive properties as well as the comparison between the quasi-static and dynamic behavior (dynamic increase factors). The paper fills the gap of information existing about dynamic behavior of sedimentary rocks under strain rates between 100 and 5.2 × 102 s−1. Furthermore, the fragmentation under different strain rates is investigated and conclusions with respect to energy absorption capacity are drawn.

Introduction: During hypervelocity impact into rock targets a large amount of material (“ejecta”) is ejected backwards. For this reason, the momentum transferred to the target is greater than the impactor momentum. This effect, often called “momentum multiplication”, is generally expressed by a dimensionless quantity: The momentum multiplication factor β. This factor is defined as the ratio of the momentum transferred to the target, ∆pt, and the momentum of the projectile, pp. Thus, β = ∆pt/pp = (pp+pe)/pp = 1+(pe/pp), where pe denotes the ejecta momentum. In the hypervelocity regime, where a large amount of ejecta is generated, β can be significantly greater than 1. In a recent study it was shown that target porosity leads to reduced β-values [1]. This behavior can be explained by the reduced cratering efficiency in porous targets compared with non-porous targets and, thus, by the smaller amount of ejected mass [1]. Furthermore, a slower and shallower ejection was observed in porous targets [1]. Walker et al. [2] investigated scale size effects for impacts of different projectile materials into aluminum and rock targets. These authors showed that β increases with increasing impactor size. Hypervelocity impact experiments into rock targets using different projectile sizes have shown that increased projectile size leads to a cratering efficiency (πV = Vρt/mp, where V denotes the crater volume, ρt and mp denote the target density and the projectile mass, respectively) higher than predicted by strength scaling laws [3]. This effect was attributed to an increased spallation volume if larger projectiles are used. Based on this observation, the goal of the present study was to investigate possible projectile size scale effects and, hence, a potential influence of increased spallation on the momentum multiplication factor β. Methods: The impact experiments were conducted using a two-stage light-gas gun at Fraunhofer EMI in Freiburg, Germany. In addition to 5 mm aluminum projectiles (see [1]), 2 mm and 7 mm aluminum spheres were used as projectiles. Seeberger sandstone (see [4] for detailed material description) was used as target material. The 20 cm cubic target blocks were attached to a ballistic pendulum. A laser vibrometer was used to measure the pendulum displacement after the impact. Impact craters were digitized using a light scanner and crater volumes were calculated. A special method for the measurement of the transient crater volume [5, 6] was applied using parabola fits to the transient crater. The spallation volume was calculated using the difference between final crater volume and transient crater volume. Results: In Figure 1 the measured β-values are shown as a function of projectile velocity in scaled form [7]. The results are given in Table 1. Density and uniaxial compressive strength of the target material are ρt = 2.04 g/cm and Yt = 42.3 ± 2.4 MPa, respectively (see [1]). The scaling parameter ν was set to 0.4 [8]. The trend for the 5 mm projectiles taken from [1] is given as a dashed line. The results show that the βvalues for the 7 mm projectiles lie above the 5 mm trend line. The β-values for the 2 mm projectiles are below this line but, however, one impact experiment (Exp.# 5649) yielded a β-value which exceeds the 5 mm trend line.

A special phenomenon observed in hypervelocity impacts on rock targets is the so-called momentum multiplication, i.e. the momentum transferred to the target is greater than the original momentum of the projectile. This effect is caused by ejection of debris in the direction opposite to the flight direction of the projectile. In the present study momentum multiplication was investigated as a function of target material properties and projectile velocity. Hypervelocity impact experiments on target materials with different porosities were conducted and the momentum transfer was measured using a ballistic pendulum.

Hypervelocity impact experiments on porous tuff targets were carried out to determine the effect of porosity on deformation mechanisms in the crater's subsurface. Blocks of Weibern Tuff with about 43% porosity were impacted by 2.5 mm and 12.0 mm diameter steel spheres with velocities between 4.8 km s −1 and 5.6 km s −1 . The postimpact subsurface damage was quantified with computer tomography as well as with meso‐ and microscale analyses of the bisected crater subsurface. The intensity and style of deformation in mineral clasts and the tuff matrix were mapped and their decay with subsurface depth was determined. Subsurface deformation styles include pore space compaction, clast rotation, as well as microfracture formation. Evaluation of the deformation indicates near‐surface energy coupling at a calculated depth of burial of ~2 projectile diameters ( d p ), which is in conflict with the crater shape, which displays a deep, central penetration tube. Subsurface damage extends to ~2 d p beneath the crater floor in the experiments with 2.5 mm projectiles and increases to ~3 d p for 12 mm projectiles. Based on overprinting relationships and the geometrical orientation of deformation features, a sequence of subsurface deformation events was derived (1) matrix compaction, (2) intragranular crack formation in clasts, (3) deformation band formation in the compacted matrix, (4) tensile fracturing.

NEOShield, a project funded by the European Commission, brings together an international team of 13 partner organizations to address the global issue of near-Earth objects (NEO) impact prevention. The project’s goals are to investigate the feasibility of techniques to prevent a potentially catastrophic impact on Earth by an asteroid or a comet and to develop detailed designs of appropriate missions to test deflection techniques. This chapter highlights some of the NEOShield research results obtained to date. The focus will be on mitigation-related science with a brief discussion of ongoing technology development and test-mission designs. Following a brief introduction to the NEOShield project, the three main NEO deflection techniques investigated are described (the kinetic impactor, blast deflection, and the gravity tractor), and the required or desirable payload instrumentation for each technique is discussed. A necessary prerequisite for the design of a successful deflection mission is accurate knowledge of the relevant physical properties of the threatening object; therefore, some of the key physical properties are addressed. A crucial component of NEOShield is laboratory and numerical modeling work to complement investigations of NEO physical properties based on observational data. Experiments to measure the momentum transfer during hypervelocity impacts into different asteroid analog materials are described, as well as initial results of numerical simulations of kinetic impacts at various velocities into small asteroids with different porosities.

In the present study we introduce an innovative method for the measurement of impact-induced pressure waves within geological materials. Impact experiments on dry and water-saturated sandstone targets were conducted at a velocity of 4600m/s using 12mm steel projectiles to investigate amplitudes, decay behavior, and speed of the waves propagating through the target material. For this purpose a special kind of piezoresistive sensor capable of recording transient stress pulses within solid brittle materials was developed and calibrated using a Split-Hopkinson pressure bar. Experimental impact parameters (projectile size and speed) were kept constant and yielded reproducible signal curves in terms of rise time and peak amplitudes. Pressure amplitudes decreased by 3 orders of magnitude within the first 250mm (i.e., 42 projectile radii). The attenuation for water-saturated sandstone is higher compared to dry sandstone which is attributed to dissipation effects caused by relative motion between bulk material and interstitial water. The proportion of the impact energy radiated as seismic energy (seismic efficiency) is in the order of 10(-3). The present study shows the feasibility of real-time measurements of waves caused by hypervelocity impacts on geological materials. Experiments of this kind lead to a better understanding of the processes in the crater subsurface during a hypervelocity impact.

COMPARISON TO EXPERIMENTAL OBSERVATIONS N. Güldemeister1, D. Moser2, K. Wünnemann1, T. Hoerth3 and F. Schäfer3, 1Museum für Naturkunde, Leibniz-Institut für Evolutionsund Biodiversitätsforschung, Berlin, Germany. (nicole.gueldemeister@mfn-berlin.de) 2Technische Universität München, Non-destructive Testing Laboratory, München, Germany. 3Fraunhofer-Institute for High-Speed Dynamics, Freiburg, Germany.

Within the frame of the MEMIN research unit (Multidisciplinary Experimental and Numerical Impact Research Network), impact experiments on sandstone targets were carried out to systematically study the influence of projectile mass, velocity, and target water saturation on the cratering and ejection processes. The projectiles were accelerated with two-stage light-gas guns (Ernst-Mach-Institute) onto fine-grained targets (Seeberger sandstone) with about 23% porosity. Collection of the ejecta on custom-designed catchers allowed determination of particle shape, size distribution, ejection angle, and microstructures. Mapping of the ejecta imprints on the catcher surface enabled linking of the different patterns to ejection stages observed on high-speed videos. The increase in projectile mass from 0.067 to 7.1 g correlates with an increase in the total ejected mass; ejecta angles, however, are similar in range for all experiments. The increase in projectile velocity from 2.5 to 5.1 km s-1 correlates with a total ejecta mass increase as well as in an increase in comminution efficiency, and a widening of the ejecta cone. A higher degree of water saturation of the target yields an increase in total ejecta mass up to 400% with respect to dry targets, higher ejecta velocity, and a steeper cone. These data, in turn, suggest that the reduced impedance contrast between the quartz grains of the target and the pores plays a primary role in the ejecta mass increase, while vaporization of water determines the ejecta behavior concerning ejecta velocity and particle distribution.

Hypervelocity (2.57.8 km s-1) impact experiments into sandstone were carried out to investigate the influence of projectile velocity and mass, target pore space saturation, target-projectile density contrast, and target layer orientation on crater size and shape. Crater size increases with increasing projectile velocity and mass as well as with increasing target pore space saturation. Craters in water-saturated porous targets are generally shallower and larger in volume and in diameter than craters from equivalent impacts into dry porous sandstone. Morphometric analyses of the resultant craters, 540 cm in diameter, reveal features that are characteristic of all of our experimental craters regardless of impact conditions (I) a large central depression within a fragile, light-colored central part, and (II) an outer spallation zone with areas of incipient spallation. Two different mechanical processes, grain fragmentation and intergranular tensile fracturing, are recorded within these crater morphologies. Zone (I) approximates the shape of the transient crater formed by material compression, displacement, comminution, and excavation flow, whereas (II) is the result of intergranular tensile fracturing and spallation. The transient crater dimensions are reconstructed by fitting quadric parabolas to crater profiles from digital elevation models. The dimensions of this transient and of the final crater show the same trends: both increase in volume with increasing impact energy, and with increasing water saturation of the target pore space. The relative size of the transient crater (in percent of the final crater volume) decreases with increasing projectile mass and velocity, signifying a greater contribution of spallation on the final crater size when projectile mass and velocity are increased.

Impact cratering experiments were performed on quartzite, tuff, and dry and water-saturated sandstones in the framework of the MEMIN research unit. 2.5-12 mm diameter projectiles were accelerated to similar to 5 km/s. Evaluation of the resulting craters shows that crater volumes and crater efficiencies of large-scale experiments are greater than predicted by strength scaling laws. A method to approximate the transient crater volume shows that this effect is largely due to an increase in spallation. Strength scaling laws are used to determine the reduction of tensile strength in large-scale experiments and show a decrease by a factor of 1.8-3.6. This strength reduction can be correlated with a decrease in strain rate for larger projectiles, and with the Weibull theory of strength reduction for larger rock sample sizes. Further variations in spallation are observed between different target materials; a decrease in spall is suggested to be controlled by increased porosity. (C) 2014 Elsevier Inc. All rights reserved.

The MEMIN research unit (Multidisciplinary Experimental and Modeling Impact research Network) is focused on analyzing experimental impact craters and experimental cratering processes in geological materials. MEMIN is interested in understanding how porosity and pore space saturation influence the cratering process. Here, we present results of a series of impact experiments into porous wet and dry sandstone targets. Steel, iron meteorite, and aluminum projectiles ranging in size from 2.5 to 12 mm were accelerated to velocities of 2.57.8 km s-1, yielding craters with diameters between 3.9 and 40 cm. Results show that the targets porosity reduces crater volumes and cratering efficiency relative to nonporous rocks. Saturation of pore space with water to 50% and 90% increasingly counteracts the effects of porosity, leading to larger but flatter craters. Spallation becomes more dominant in larger-scale experiments and leads to an increase in cratering efficiency with increasing projectile size for constant impact velocities. The volume of spalled material is estimated using parabolic fits to the crater morphology, yielding approximations of the transient crater volume. For impacts at the same velocity these transient craters show a constant cratering efficiency that is not affected by projectile size.

This study deals with the investigation of highly dynamic processes associated with hypervelocity impacts on porous sandstone. For the impact experiments, two light-gas accelerators with different calibers were used, capable of accelerating steel projectiles with diameters ranging from 2.5 to 12 mm to several kilometers per second. The projectiles impacted on dry and water-saturated Seeberger Sandstone targets. The study includes investigations of the influence of pore water on the shape of the ejecta cloud as well as transient crater growth. The results show a significant influence of pore water on ejecta behavior. Steeper ejecta cone angles are observed if the impacts are conducted on wet sandstones. The transient crater grows at a faster rate and reaches a larger diameter if the target is water saturated. In our experiments, target porosity leads to smaller crater sizes compared with nonporous targets. Water within the pore space reduces porosity and counteracts this process. Power law fits were applied to the crater growth curves. The results show an increase in the scaling exponent mu with increasing pore space saturation.