2.2 iSALE simulation set-up
Davison et al (2011) show that simulations at this scale can be approximated well with resolutions of 12-20 cells per projectile radius (CPPR), with little increase in accuracy beyond 20 CPPR for crater depth. As such, our simulations employ an Eulerian grid with square 1.5785x10-4 m cells, corresponding to resolutions of 20 CPPR. The high-resolution zone (HRZ) of this mesh consists of 220 x 360 cells. Beyond the HRZ, cell-size increases by a constant fraction of 1.05. As these are 2D simulations, we use cylindrical symmetry to approximate vertical impacts along the axis of symmetry (the y-axis). The right and left sides of the mesh have free-slip boundary conditions. The bottom and top have no-slip and outflow boundary conditions, respectively. Projectiles were simulated as 3.175 mm (aluminum and quartz) and 6.35 mm (quartz only) spheres with impact velocities that correspond to those reported from the experiments by Marchi et al. (2020).
For small-scale simulations such as these, the time when the transient crater is formed (when there is the maximum volume) is generally the same time as when the crater has finished forming (change in depth and diameter is minimized). Further, observations from the in situ experiments from Marchi et al (2020) show that the modification stage for impacts into these metallic materials is virtually non-existent, resulting in crater rims that appear frozen in place. For the materials in this work, the timeframe in which the transient crater is formed (and therefore, where we are approximating the end of the crater formation) seems to occur after 30-80 microseconds. For continuity, we take the longer end-member of 80 microseconds as the final crater stage to compare the results of all simulated craters with the experimental results. We ran simulations through 150 microseconds.
We simulated impacts into four different target materials with two different impactor materials based on laboratory measurements of the targets used in Marchi et al (2020). Table 3 lists the full details of the properties for each material. The quartz projectiles were approximated by using the ANEOS equation of state for granite (Ivanov, 2000 and Pierazzo et al., 1997) and a thermal softening model after Ohnaka et al. (1995). Strength in the quartz projectiles is approximated by the “ROCK” strength model (Collins, 2004)—a pressure-dependent strength model which reduces shear strength with the accumulation of damage. The aluminum projectiles were approximated using the Tillotson equation of state for aluminum (Tillotson, 1962) and the JC strength and thermal softening (Johnson and Cook, 1983) models.
The experiments performed by Marchi et al. (2020) simulated in this work were on three distinct manufactured types, henceforth referred to as I-90 (a), I-90 (b), and I-94 (b) where each refers to Ingot (I), the compositional make-up (90 or 94 % Fe, and 10 or 6 % Ni, respectively), and which batch of experiments (a - hypervelocity experiments which took place in 2016 versus b- hypervelocity experiments that took place in 2019). We also simulated the five vertical impacts into the Gibeon meteorite targets. All target materials were simulated using the ANEOS equation of state for iron (Ivanov, 2000) and the JC strength and thermal softening (Johnson and Cook, 1983) models. For each distinct target material, we determined the necessary JC strength model parameters (see Table 2) from mechanical characterization tests performed in the laboratory. Finally, we define the surface temperature of the target in the simulation to match that of the temperature of the target from the in situ impact experiments (as temperature was noted to have a distinct effect on the material strength, discussed further in the next section).