Implications Outlook and Some Precautionary Notes

At the conditions used in the simulations, the materials do not melt by shock heating alone. However, experiments and Eulerian simulations suggest considerable shear at the interface of different materials and the possibility of massive thermal damage of material near interfaces under shock loading. If organic material were present in such regions (as it is usually expected to be found in cracks and near interfaces of rocks), it would very probably suffer destruction when its host rock is subjected to impacts with high shock pressures. Different compressibility of materials in contact can lead to a concentration of plastic work connected with a significant rise in temperature which makes localised melting possible. However, the material model, which was employed, was not fully adequate to describe this effect. Nevertheless we conclude that the presence of material interfaces can lead to increased thermal effects in shocked geomaterials, and should be considered if impact conditions are to be derived from material changes.

Fig. 10. Pressure dependence of melting temperatures for SiO2, enstatite, forsterite, and forsterite/enstatite eutectic mixture after Chen and Presnall (1975) and Presnall et al. (1998).

The results of simulations like those presented in this work depend not only on the material models used, but also on details of the equation of state. Since ANEOS is mainly based on equilibrium thermodynamics, its usage for materials exhibiting complex unloading behavior and irreversible phase transitions is complicated. Their occurrence requires the separate treatment of low-pressure and high-pressure phases (Ivanov 2003) and the inclusion of transition kinetics. However, the differences in release paths were not of significant influence on the outcome of the simulations described above because the principal changes occurred in the compressed state of the material.

Future simulations must take into consideration effects of yield strength and brittle fragmentation of the material, and for a proper description of the melting process which is not possible using purely deviatoric stresses it is necessary to include the viscoelastic behavior of the material (in particular near the melting point) as a further sink of dissipated energy. The temperature dependence of the maximum shear stress, the kinetics of solid-solid phase transitions and melt production, and the viscous relaxation times of the materials need also to be taken into consideration.

Three-dimensional simulations are necessary for a full description of experiments. It is planned to investigate the effects of the cylindrical shape of the samples. The Eulerian version of the program needs to be extended further in order to be able to simulate systems where three materials (e. g. iron and two kinds of rock) may be in mutual contact. Alternatively, the input parameters may be adjusted such that the velocity of the shock wave and the pressure on top of the inclusion reach the values they would have for an iron container and an iron flyer plate so that the experimental situation is approximated. Studies of this problem and systematic simulations of the effect of the inclination of the material boundary on the temperature rise (Hertzsch 2003) have to be subject of future work.

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