Physics – Geophysics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p31a1695t&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P31A-1695
Physics
Geophysics
[4475] Nonlinear Geophysics / Scaling: Spatial And Temporal, [5420] Planetary Sciences: Solid Surface Planets / Impact Phenomena, Cratering, [8160] Tectonophysics / Rheology: General
Scientific paper
Rheology of the target material is one of the important factors which controls the crater morphology. There have been several experiments using dry granular materials, one of which showed the importance of particle size which in turn affects the rheology (Walsh et al., PRL, 2003). Wet granular material which is partially saturated with liquid, has a yield stress and therefore its rheology differs from a dry granular material. Here we report the results of impact cratering experiments using a wet sand target with volumetric liquid saturation (S) as the changeable parameter. Experiments were performed by releasing a steel ball from a holder so that it fell vertically into a cylindrical container filled with beach sand (grain size 0.2mm). We use a high-speed camera to record the crater formation process and laser displacement meter to measure crater profiles, diameter and depth. We change water saturation (S=0 - 80%), projectile size (diameter 10 - 22.2 mm) and release height (h = 200-1700mm) and study how the crater size and morphology change with these parameters. For S=0-3%, a cone-shaped crater forms. For S=4.1-5.5% it changes to a cylindrical crater with an outer ring, which forms as the ejecta settles. For S > 5.8% the outer ring disappears because of less ejecta volume. For S > 57%, it becomes accompanied by a depression surrounding the crater. When cylindrical craters form, ejection occurs only from the surface and the ejecta volume decreases with S. However its volume increases again for S > 72% when the target becomes a slurry. Similarly, the crater diameter and depth becomes smaller with S, and then it increases again from about S = 60%. We studied how the depth and the diameter scales with the impactor energy. For dry sand, we confirmed that the power-law exponent becomes close to 1/4, consistent with previous works. However for wet sand (S = 5-20%), we find that it cannot be scaled with energy. We find that the crater diameter is approximately equal to projectile diameter, and the depth/diameter ratio changes with release height. When the projectile is released from a fixed height, the ratio is the same regardless of the projectile size. We also measured the yield stress σ y of the wet sand and found that it is larger than the dry sand by an order of magnitude. Comparing σ y with the inertial stress σ I ˜ (m v2 / R)/ π R2 (m: ball mass, v: impact velocity, R: ball radius) we find that for dry sand, σ y / σ I ˜ 10-2, whereas for wet sand, σ y / σ I ˜ 10-1. This suggests that morphological change can be associated with this ratio exceeding of the order of 10-1.
Sumita Ikuro
Takita H.
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