Low-velocity impact cratering experiments in a wet sand target

摘要

Low-velocity impact cratering experiments were conducted in a wet sand target.With the addition of interstitialnwater, the sand stiffens and the yield stress σy increases by a factor of 10 and we observe a significant changenin the resulting crater shape. A small water saturation (S ∼ 0.02) is sufficient to inhibit the crater wall collapse,nwhich causes the crater diameter d to decrease and the crater depth to increase, and results in the steepening of thencrater wall.With a further addition of water (S ∼ 0.04), the collapse is completely inhibited such that cylindricalncraters form and the impactor penetration depth δ and ejecta dispersal are suppressed. However, forS > 0.7, thenwet sand becomes fluidized such that both d and δ increase thereafter. Comparing the relevant stresses, we findnthat cylindrical craters form when the yield stress is more than about three times larger than the gravitationalnstress such that it can withstand collapse. Experiments with different impactor sizes D and velocities indicatenthat for S u0002 0.02, gravity-regime scaling applies for d. However, the scaling gradually fails as S increases. Inncontrast, we find that δ/D can be scaled by the inertial stress normalized by the yield stress, for a wide range ofnS. This difference in the scaling is interpreted as arising from d being affected by whether or not the crater wallncollapses, whereas δ is determined by the penetration process that occurs prior to collapse. The experimentalnparameter space in terms of dimensionless numbers indicates that our experiments may correspond to impactncratering in small asteroids.