In situ observation of penetration process in silica aerogel: Deceleration mechanism of hard spherical projectiles

In situ flash X-ray observation of projectile penetration processes and crater cavity growth in porous gypsum target analogous to low-density asteroids

“…In the case of the penetration hole shown in Fig. 4a, the C d was determined to be about 0.9, a value almost consistent with those obtained for other porous materials such as aerogel and polyurethane foam, and the C d is 1.1 for aerogel, as obtained by Niimi et al (2011), and 0.9 for polyurethane foam, as obtained by Trucano and Grady (1995). In the case of a hemispherical cavity, we fit the data between 0 ls and 20 ls corresponding to t s of the cavity growth by using Eq.…”

“…(2) The drag coefficient, C d , could be estimated from the results of d by using an equation of projectile motion by Niimi et al (2011), and we found that the C d was 0.9 for a penetration hole, which was almost consistent with other porous materials such as aerogel, while the C d for a hemispherical cavity was 2.3-3.9, larger than that for a penetration hole because of the deformation and destruction of projectile. Furthermore, we could estimate the compressional strength of the target, Y ct , by using a stopping time, t s , assuming the shock pressure P = q t C t v i , and we found Y ct $ 400 MPa, that is, the projectile cannot penetrate the target after the shock pressure was attenuated to less than 400 MPa.…”

“…4, we can derive the drag coefficient, C d . We used the equation of motion proposed by Niimi et al (2011) for a hard spherical projectile decelerating in a porous target at a wide range of impact velocity. In this study, the depth of the crater cavity was equal to the penetration depth of a projectile.…”

“…Furthermore, it is difficult to directly observe the interior of rocky targets at high speed with visible light. To observe the target interior during the impact directly, Niimi et al (2011) conducted impact experiments of aerogel (density of 60 kg/m 3 ) at impact velocities of $200 m/s and $4 km/s to study in situ observations of the track trajectory formation and the deceleration mechanism of projectiles. However, the density of aerogel is much lower than those of asteroids (1000-3000 kg/m 3 ), so the crater shapes in aerogel were quite different from those in asteroids.…”

Penetration into low-density media: In situ observation of penetration process of various projectiles

“…Hypervelocity impact experiments on very low-density materials have been carried out to extend the cratering experiments (e.g., Cannon and Turner, 1967;Fechtig et al, 1980;Werle et al, 1981;Love et al, 1993;Trucano and Grady, 1995) and to develop and calibrate the instruments for intact capture of interplanetary dust samples using foams (e.g., Ishibashi et al, 1990;Tsou, 1990) and for aerogels (e.g., Barrett et al, 1992;Hörz et al, 1993Hörz et al, , 1998Hörz et al, , 2009Burchell et al, 1999Burchell et al, , 2001Burchell et al, , 2008Burchell et al, , 2009Kitazawa et al, 1999;Niimi et al, 2011Niimi et al, , 2012. These previous studies indicate that the impacts between high-density projectiles and low-density targets generate ''penetration tracks'': track diameter is small at the entrance (= impact point), then increases with depth, takes a peak, and decreases (this qualitative feature is common for any track).…”