K. Tsembelis

On the survivability and detectability of terrestrial meteorites on the moon.

Materials blasted into space from the surface of early Earth may preserve a unique record of our planets early surface environment. Armstrong et al. (2002) pointed out that such materials, in the form of terrestrial meteorites, may exist on the Moon and be of considerable astrobiological interest if biomarkers from early Earth are preserved within them. Here, we report results obtained via the AUTODYN hydrocode to calculate the peak pressures within terrestrial meteorites on the lunar surface to assess their likelihood of surviving the impact. Our results confirm the order-of-magnitude estimates of Armstrong et al. (2002) that substantial survivability is to be expected, especially in the case of relatively low velocity (ca. 2.5 km/s) or oblique (<or=45 degrees) impacts, or both. We outline possible mechanisms for locating such materials on the Moon and conclude that searching for them would be a scientifically valuable activity for future lunar exploration.

Hydrocode modelling of hypervelocity impact on brittle materials: depth of penetration and conchoidal diameter

Summary The Johnson-Holmquist brittle material model has been implemented into the AUTODYN hydrocode and used for Lagrangian simulations of hypervelocity impact of spherical projectiles onto soda-lime glass targets. A second glass model (based on a shock equation of state and the Mohr-Coulomb strength model) has also been used. Hydrocode simulations using these two models were compared with experimental results. At 5 km s−1, the Mohr-Coulomb model under-predicted the depth of penetration, whilst adjustment of the Johnson-Holmquist model bulking parameter was required to match the experimental data to the simulation results. Neither model reproduced the conchoidal diameter; a key measured parameter in the analysis of retrieved solar arrays, so two failure models were used to investigate the tensile failure regime. A principal tensile failure stress model, with crack softening, when used with failure stresses between 100 and 150 MPa and varying bulking parameters, reproduced the conchoidal diameter morphology. Empirically-determined, power-law damage equation predictions for the range 5–15 km s−1 were compared with simulations using both models since no experimental data was available. The powerlaw velocity dependence of the depth of penetration simulations was found to be significantly lower than the 0.67 predicted by the empirically-determined damage equations.

Velocity thresholds for impact plasma production

Abstract Experiments have been performed on the dust accelerator facilities at the University of Kent at Canterbury (UK) and the Max-Planck-Institut fur Kernphysik (Germany) in which the production of plasma from impacts of micron and sub-micron particles at velocities from 1 to 90 km s −1 has been measured. Various projectile and target materials have been investigated. Time-of-flight mass spectrometry of the positive ions in the plasma allows their atomic species to be identified. By accumulating large amounts of data over a range of impact velocities it has been possible to identify the threshold velocities required to produce ions of different species, whether present in the system as the nominal projectile and target materials or as contaminants. The results obtained have been compared with theoretical predictions based on the principles of molecular dynamics and with the results of hydrocode simulations.

Dynamic compaction of porous silica powder

The dynamic compaction characteristics of a porous silicon dioxide (SiO2) powder are reported. The initial specific volumes of the samples were either V00=1.30, 4.0, or 10.0cm3∕g whereas the silicon dioxide has a matrix specific volume of V0=0.455cm3∕g. The impact velocity ranges from 0.25to1.0km∕s and the shock incident pressure on the silica ranges from 0.77to2.25GPa. The shock velocity–particle velocity exhibited a linear relationship within this range. Although these tests represent the low end of dynamic compaction, the dynamic tests compare favorably to extrapolated data available in the open literature. Theoretical pressure–particle velocity and shock velocity–particle velocity curves were generated using a P-α compaction curve. The P-α compaction curve accurately represented the pressure–particle velocity and shock velocity–particle velocity Hugoniot curves for the low specific volume powder, specifically V00=1.30cm3∕g. However, the P-α compaction curve did not accurately represent the pressure–pa...

THE DYNAMIC COMPACTION OF SAND AND RELATED POROUS SYSTEMS

Porous and granular materials are widely found in a number of environments. One of the most important groups both geographically and in the construction industry are the sands. A review of the response of sand (42% porous) over a wide range of strain rates is presented. Factors such as water content and density variation are addressed. In addition a very low‐density silica dust (95% porous) is also discussed in relation to its contrasting behaviour.

The Principal Hugoniot and Dynamic Strength of Dolerite under Shock Compression

A series of plate impact experiments was performed on Dolerite (diabase) igneous rock. Longitudinal stresses were measured using embedded manganin stress gauges up to ca. 11 GPa. In addition, lateral stresses were also measured up to ca. 7 GPa. In combination with the longitudinal stresses, these results have been used to obtain the material shear stress under shock compression. Results indicate that the longitudinal behaviour is elastic for the stress range involved although shear stresses indicate deviation from elastic loading for longitudinal stresses higher than ca. 4.3 GPa. The results are then compared and contrasted to data for other geologic materials.

Longitudinal and lateral stress measurements in shock-loaded gabbro and granite

Plate impact experiments have been performed on two igneous rocks of different grain size. Shock stresses have been measured using embedded manganin stress gauges, up to ∼12 GPa. In the coarser grained material data was obtained by impacting rock flyer plates onto previously characterized targets. Results are compared and contrasted to the existing data for other geological materials. In the finer-grained material lateral stress was also measured. In combination with the longitudinal stresses, these results have been used to obtain the materials shear strength under shock loading conditions. Results suggest that the material is deforming in an inelastic manner.

A method for determining the main mechanical properties of soft soils at high strain rates (103–105 s−1) and load amplitudes up to several gigapascals

A new method has been developed for determining the main laws of the deformation of soft soils under conditions of dynamic loading with amplitudes of up to several gigapascals. The method is based on the results of high-strain-rate tests under uniaxial deformation conditions, which were obtained using a modified Kolsky method and the plane shock wave technique. The possibilities of the proposed method are illustrated by determining the dynamic properties of sand.

Measurement of Equation of State of Silicone Elastomer

Silicone Elastomer, (“Sylgard 184 ®”), samples were mounted between copper plates. Manganin stress gauges were placed within the front copper plate, halfway through the Sylgard and at the interface between the Sylgard and the rear copper plate. A series of experiments was performed in which the front plate was impacted by copper plates projected at a range of velocities. It was assumed that a Gruneisen Gamma form with a constant Γ could fit the Equation of State of the sample. A trial set of EoS parameters, including Gamma, was entered into a spreadsheet, then the state variables for the different stress jumps were calculated with the aid of a “Goalseek” function. This enabled the stresses and times for each jump to be calculated. Comparing these predictions with the experimentally determined parameters enabled optimum values of the EoS parameters to be identified.

The Dynamic Strength of Cement Paste under Shock Compression

A series of plate impact experiments on cement paste (grout) has been performed to assess the dynamic strength of this material. Lateral stresses have been directly measured by means of embedded manganin stress gauges. In combination with longitudinal stresses, measured previously [1], these results have been used to obtain shear strength under shock loading. Results indicate that the material is behaving in an inelastic manner with the shear strength increasing with increasing pressure.