Results

During the passage of the shock wave, the dunite part of the sample remains rather cold and reaches only a temperature of about 400 K, the quartzite part is heated up considerably more to about 700 K due to its higher compressibility compared to dunite. The quartzite section is strongly compressed, and pressure and temperature in this section show considerable peaks near the interfaces to the denser and less compressible

Fig. 4. Zone plots of materials showing the geometry of the sample at impact and selected later times. Width of container: 4.5 cm. Height of container: 5 cm. Thickness of flyer plate: 0.4 cm. Width of rock sample: 1.5 cm. Height of rock sample: 1 cm. Velocity of flyer plate: 2.54 km/s. Time in seconds. Spatial resoluti materials during compression and fall earlier than in the center of the quartzite during decompression. Figure 5 shows density, pressure, and temperature changes in a horizontal cross-section cutting through the center of the rock sample 18 mm below the top of the iron container, a phase transition is observed in quartzite beginning at 3.5 microseconds, and the reverse transition occurs at 5.6 microseconds after impact of the flyer plate. Hence, quartzite stays in the high-pressure phase for about 2 microseconds. The onset of the phase transitions occurs at the material interface and subsequently affects the entire quartzite section with advancing time. From first principles of thermodynamics one can see that the adiabatic (standard) release above the lpp-hpp Hugoniot curve will result in a considerable amount of heat and correspondingly high residual temperatures. The occurrence of these phase transitions underlines the need for an equation of state, which can take them into account properly. Near the edges of the quartzite sample, temperature and pressure peaks can be observed which are due to a focusing of the shock wave by the surrounding denser and less compressible materials, but at the resolution of our simulations and within the restrictions of the employed material model they do not reach values necessary for the formation of melt. The temperatures in the interior of the rock samples during shock passage remain well below the melting point of any of the materials. At the interface between rock sample and iron container a reflected wave can be observed.

According to data obtained from vertical sections through the centers of both rock samples, the phase transitions on top of the quartzite block begin about 3 microseconds (low pressure phase - high pressure phase) after the impact of the flyer plate. Pressure release and reverse transformation (high pressure phase - low pressure phase) start at 4 microseconds after impact. In the density profile the phase changes are visible by a step-like increase or decrease in density. Due to the thin flyer plate the shock extends only over a short vertical distance, and the quartzite section is never completely in its high-pressure phase. The phase changes are reversible under unloading, but affect strongly the propagation of the shock wave.

Fig. 5. (The following 3 pages) Horizontal sections across the middle of the rock sample, 18 mm below top of the iron container and 5 mm below top of the sample in uncompressed state (22 mm below top of the impacting flyer plate), showing variations of density, pressure and temperature across the probe at different times and illustrating phase transitions in quartzite (note the density changes in the quartzite section). Next to the interface to the denser and less compressible materials, pressure and temperature in quartzite show notable peaks in compression at t=3.5 microseconds.

t = 3,5 microseconds

iron

quartzite

dunito

iron r

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 honzonlal distance |m]

iron

quarlzile

dunile

iron

"""-^

- ,

0 0005 0.01 0015 0 02 O.025 003 0.035 0 04 0.045 horizontal distance [m]

0 0005 0.01 0015 0 02 O.025 003 0.035 0 04 0.045 horizontal distance [m]

t ■ 7.0 microaeoonda

iron

qiiartzite

dunile

iron

j

0 0 005 0.01 0015 0.02 0.025 0 03 0.035 0.04 0.045 horizontal distance [m]

0 0 005 0.01 0015 0.02 0.025 0 03 0.035 0.04 0.045 horizontal distance [m]

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