Penetration into low-density media: In situ observation of penetration process of various projectiles
Highlights
► We carry out hypervelocity impact experiments using low-density targets. ► We observed the track formation process using a high-speed camera. ► At early stages, the evolution of tracks is similar in any type of projectiles. ► The evolution is different depending on the projectile conditions at later stages. ► We briefly discuss the track formation process based on the experimental results and some previous models.
Introduction
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., 1993, Hörz et al., 1998, Hörz et al., 2009, Burchell et al., 1999, Burchell et al., 2001, Burchell et al., 2008, Burchell et al., 2009, Kitazawa et al., 1999, Niimi et al., 2011, Niimi 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). For more quantitative description, the depth L and the maximum diameter Dm of final tracks are often measured as a function of the parameters varied in the experiments such as, projectile density ρp, projectile diameter Dp (radius Rp), projectile and target strengths, target density ρt, and impact velocity v0. The features about L/Dp and Dm suggested by previous various experiments with low-density targets are roughly summarized as follows: L/Dp increases with v0, and takes a peak, then decreases (e.g., Fechtig et al., 1980, Werle et al., 1981, Ishibashi et al., 1990, Tsou, 1990, Barrett et al., 1992, Kitazawa et al., 1999, Burchell et al., 2001, Hörz et al., 2009), and is scaled by ρp/ρt (e.g., Barrett et al., 1992, Hörz et al., 1993, Love et al., 1993, Burchell et al., 1999, Burchell et al., 2009, Niimi et al., 2011, Niimi et al., 2012), and Dm is proportional to Dp regardless of ρp/ρt and proportional to v0 (e.g., Ishibashi et al., 1990, Kitazawa et al., 1999, Burchell et al., 2008, Niimi et al., 2012).
Several models have been proposed on the penetration depth (e.g., Anderson and Ahrens, 1994, Trucano and Grady, 1995, Westphal et al., 1998, Kadono, 1999, Domínguez et al., 2004, Kadono and Fujiwara, 2005, Trigo-Rodríguez et al., 2008, Domínguez, 2009, Coulson, 2009a, Coulson, 2009b, Niimi et al., 2011). Most models suggest that projectiles receive the hydrodynamic force (Cd/2)ρtv2S, where Cd, v, and S are the drag coefficient, instantaneous velocity, and the cross-sectional area, respectively, for v higher than the sound velocity of target materials. Thus, the feature about the depth L is relatively understood (e.g., the feature that L/Dp increases with v0, takes a peak, and then decreases, is explained such that, at low v0, projectiles are intact and L/Dp indicates a logarithmic increase with v0, then at a critical v0, the breakup of projectiles occurs, and, as v0 increases, the largest fragment becomes smaller and L/Dp decreases). On the other hand, there are a relatively small number of researches on the growth of track diameter (Westphal et al., 1998, Kadono, 1999, Domínguez et al., 2004, Trigo-Rodríguez et al., 2008, Coulson, 2009b, Domínguez, 2009). As the mechanisms of the expansion of the track diameter, shock waves (Domínguez et al., 2004), thermal effects (Kadono, 1999, Coulson, 2009b, Domínguez, 2009), and fragmentation of projectiles (Kadono, 1999, Trigo-Rodríguez et al., 2008) have been considered. However, among the common features for any track about track diameter, in particular, the track profile near the entrance is still not clear.
To understand the track diameter evolution, we focus on the similarities and differences on the formation process of various tracks, based on the in situ observation in laboratory experiments. There have been a few attempts so far for in situ observation of penetration process into low-density materials, such as breakwire (Tsou, 1990), magnetic pickup (Tsou, 1990), X-ray shadow graphs (Tsou, 1990, Trucano and Grady, 1995) and an optical high-speed camera with aerogel targets (Niimi et al., 2011). However, these results mainly consider the penetration depth of hard projectiles. In this paper, we used three types of projectiles and observed the track formation process (the track length, the growth rate of bulbous tracks, and the position of the maximum diameter), using a high-speed camera as a function of time. Based on the results and the previous models, we briefly discuss the track formation process.
Section snippets
Experiments
Impact experiments were carried out using a two-stage hydrogen gun at Institute of Space and Astronautical Science, JAXA. We used three types of projectiles: (A) a spherical soda–lime–glass (SLG) with a diameter of 491 μm and a density of 2.5 g cm−3 (Fig. 1a), (B) a sintered mixture of small silica (the size of the constituting silica particles less than 20 μm) and a large SLG particle (∼300 μm) with a bulk diameter of ∼500 μm and a bulk density of ∼1.4 g cm−3 (Fig. 1b), and (C) sintered small silica
Track morphology
Fig. 2 shows the consecutive images of penetration process. (a) Projectile (A), SLG. The time interval between each frame is 8 μs from the first to the 11-th frame and 80 μs from the 11th to the 15th one. (b) Projectile (B), the mixture of SLG and small silica. The time interval between each frame is 8 μs from the first to the 8th frame and 80 μs from the 8th to the 10th frame. (c) Projectile (C), small silica aggregates. The time interval is 4 μs for all frames. The projectiles come from left. In
Discussion
In this section, we discuss a track formation mechanism, particularly near the entrance. Various excavation mechanisms have been considered by several authors such as shock waves (Domínguez et al., 2004, Coulson, 2009b, Iida et al., 2010), fragmentation of projectiles (Kadono, 1999, Kadono and Fujiwara, 2005, Trigo-Rodríguez et al., 2008), and thermal effects (Kadono, 1999, Trigo-Rodríguez et al., 2008, Domínguez, 2009, Coulson, 2009b). In this paper, we focus on the roles of shock waves to
Conclusion
We carried out hypervelocity impact experiments using low-density (60 mg cm−3) aerogel targets and various types of projectiles, and observed the track formation process in the targets using a high-speed camera. A carrot shaped track, a bulbous one, and a hybrid one consisting of bulbous and thin parts were formed. Based on the results of the high-speed camera observation, we consider the similarity and differences on the temporal penetration depth and maximum diameter of these tracks, comparing
Acknowledgments
We thank N. Machii and A.M. Nakamura for cooperating the production of sintered projectiles, N. Onose for cooperating the experiments, and M. Arakawa, M. Yasui and K. Kurosawa for useful comments. The authors are also grateful to H.J. Melosh and G. Dominguez for helpful comments. This work was supported by the Space Plasma Laboratory, ISAS, JAXA. A. Tsuchiyama was supported by a grant-in-aid of the Japan Ministry of Education, Culture, Sports, Science, and Technology (19104012).
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