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Experimental hypervelocity impacts: Implication for the analysis of material retrieved after exposure to space environment
Acta Astronautica 167 (2020) 429–439
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Acta Astronautica journal homepage: www.elsevier.com/locate/actaastro
Research paper
Experimental hypervelocity impacts: Implication for the analysis of material retrieved after exposure to space environment
T
Jean-Claude Mandevillea,∗, Jean-Marie Perrinc, Loïc Vidalb a
MANDESPACE, 8 Rue des Régans, 31000, Toulouse, France Institut de Science des Matériaux de Mulhouse (IS2M), CNRS UMR 7361, 15 Rue Jean Starcky, BP2488 68057, Mulhouse, France c SPACENVIR, Observatoire de Haute Provence, 04870, Saint Michel l'Observatoire, France b
ARTICLE INFO
ABSTRACT
Keywords: Meteoroids Orbital debris Hypervelocity impact SEM EDX Brittle material
During the last three decades a wide variety of surfaces have been brought back to Earth after being exposed to the space particulate environment. The impact features found on these surfaces can give clues to the characteristics of the orbital debris and meteoroids that created them. Many investigations have been carried out to derive projectile parameters (size, shape, impact angle, origin) from the morphology of impact features and the analysis of projectile their remnants inside the crater. However, there are still some ambiguities in the interpretation of these results. In this study we have investigated carefully the distinguishing features for craters caused by the impact of high velocity iron particles on thick brittle targets, using a micro-particle accelerator. Up to date instrumentation, SEM and EDX, and in depth results analysis provides an extensive data-set that can be used to improve the scientific return of the investigation of material retrieved after exposure to space.
1. Introduction
materials. However fragmentation and the formation of spalls during impacts on brittle materials increase the difficulties of analytical studies or numerical modelling (see for instance Refs. [10–12]). In this paper we used only phenomenological calculations, the analytical and numerical studies of our impacts on brittle targets will be the subject of a forthcoming publication. The aim of this study is to refine criteria allowing an assessment of size, velocity and origin of a projectile from parameters could be measured on the impact craters. Indeed, samples exposed on MIR (see Refs. [13,14]) and HST solar cells retrieved after more than 8 years of exposure to space [15,16]) showed a large number of elliptical features, mainly for impacts smaller than 100 μm because larger ones have seen their material ejected losing the evidence of ellipticity. These craters are not uncommon, indeed they represent more than one third of impacts smaller than 100 μm observed. Interpretation of this kind of impacts rely on hypervelocity impact tests in the laboratory. In addition, a larger amount of projectile residue is found in oblique craters rather than in circular ones. In section 2 the experimental approach used is described. Section 3 presents the analysis of impact craters to obtain their main geometric parameters in function of a number of projectile masses and size and several impact velocities. Section 4 is devoted to the study of spalls ejecta observed for some oblique impacts while Section 5 presents the analysis of projectile remnants. We conclude in Section 6.
Space environment is hostile to spacecraft [1–3]. Threats come from its components and among them meteoroids and space debris. It is therefore vital to understand the damage they can cause and to take adequate measures to design spacecraft that resist their degrading effects. Accurate debris and meteoroid flux models are crucial for the design of manned and unmanned space missions. For particles sizes smaller than a few millimetres, data on the environment can only gained by in-situ detectors, hardware retrieved after exposure to space and active sensors that provide real-time impact information. Data not only give information on the meteoroid and debris environment but also provide a better knowledge of the hypervelocity impact phenomena on materials exposed to space. However it is still difficult to derive with precision, from the analysis of impact features, the parameters of the projectiles and their possible origin [4,5].In this study we consider the impacts of small size iron particles (< 10 μm) onto brittle glass targets. A study dealing of similar impacts but on ductile material has been published earlier in this journal [6]. In the past many laboratory impact simulations have been done using hypervelocity launchers for large particles and dust particles accelerators for small particles on ductile and on brittle targets [7]. A large number of publications present analytical (see for instance Refs. [6,8]) and numerical (see Ref. [9]) solutions concerning hypervelocity impacts on ductile ∗
Corresponding author. E-mail address: [email protected] (J.-C. Mandeville).
https://doi.org/10.1016/j.actaastro.2019.11.020 Received 20 September 2019; Received in revised form 8 November 2019; Accepted 13 November 2019 Available online 18 November 2019 0094-5765/ © 2019 IAA. Published by Elsevier Ltd. All rights reserved.
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2–12 km/s. In order to study the influence of angle of incidence in the formation of the craters, 4 angles of incidence have been used: 0°, 30°, 45° and 60°, (measured from the normal to the surface). Five velocity ranges were used per incidence angle. 2.4. Observation equipment Impact craters are studied with a scanning electronic microscope (SEM), a IS2M Quanta 400 instrument, with a resolution of about 3.5 nm. The observations were performed at an acceleration voltage of 30 kV. As the samples are conductive (original gold coating), no preparation was needed for the observations. The micrographs were acquired at magnifications ranging from 100× to 100000 X. For depth computation stereoscopic views were done using pictures acquired by tilting the sample at 0°, 7° and 14°, see for details ref. 6. In addition 30°, 37° and 44° tilt angles were used to study the bottom of the craters when the incidence angle is 30° while, 37°, 45° and 52° tilt angles were used when the incidence angle is 45° and 46°, 53°, 60° when the incident angle is 60°. The chemical analysis were performed using an Energy Dispersive X-ray (EDX) spectrometer (Oxford Instruments Inca X-Act) attached to the microscope. Six to eleven representative impacts features per impacted area were analysed (each area correspond to a velocity range). Each impact was observed with SEM, in a secondary electron mode in order to obtain a high resolution picture. Then, analysis of the remnants gives the elemental composition of melted residue. As targets were pure glass and projectiles were carbonyl-iron particles this analysis is rather easy to interpret, the only elements studied being Au, Si, O and Fe. Nevertheless the thin layer of vacuum deposited gold and the depth of the craters could be the cause of artefacts.
Fig. 1. Size distribution of particles.
2. Experimental approach 2.1. Acceleration of microparticles One of the best, if not the only one, method to accelerate small particles to hypervelocities consists in the use of an electrostatic accelerator. The Max Planck Institute for Nuclear Physics in Heidelberg (Germany) has been operating such a facility for many years [7]. Details concerning the experimental process have given in a previous paper [6]. Velocity, charge and mass of the particles are measured by dedicated measuring devices. Due to operational constraints, it was only possible to select several velocity ranges and to allow a large number of particles of different sizes within this velocity interval. The largest particles are accelerated at the lowest velocity and the smallest at the highest velocity. This constraint make the analysis of the impact features a difficult one, as discussed later. Fig. 1 shows the size distribution of particles according to their velocity, as delivered by the accelerator. This dataset correspond to a small number of particles. Upon experimental tests a much larger number of particles are used in each velocity range as shown for instance on Fig. 5.
3. Analysis of impact features 3.1. Energy dependence of craters morphology The constraint above mentioned, i.e. the low resolution of projectiles size in a selected velocity interval, needs comparison with experiments performed with individual projectile whose size and mass are known These experiments permit to link the kinetic energy (KE) of the impact to the morphology of the crater and are useful to understand the results obtained when a number of projectiles simultaneously impact a target. Such simulation experiments were carried out in the past at the NASA Ames Research Centre using a unique electrostatic accelerator [18]. Then it is possible to distinguish the following changes in the morphology of the craters:
2.2. Target material The targets are 1 mm thick, 25 mm in diameter disks of fused quartz. The disks were vacuum-coated with gold before experiments, in order to identify clearly the impact craters. Target can thus be considered as thick as compared to the size of the projectiles. In these conditions the side and rear walls play no significant part in the cratering process.
1. Craters with only a central smooth cavity diameter of the pit Dc for circular craters due to normal incidence, or elliptical axis 2a and 2b for craters due to oblique incidence), surrounded by a circular lip. 2. Central cavity with a small number of radial cracks (2–3) and an inner ring of shattered glass. 3. Central cavity, with an irregular lip (due a beginning of fragmentation), with several radial cracks (5–8), an inner ring of shattered glass and incipient conchoidal fractures; 4. Central cavity partly broken and surrounded by a spallation zone (size Ds), with several (4–6) large spalls either ejected or not. 5. The central cavity disappears and the impact crater has a conical shape, surrounded by conchoidal and radial fractures.
2.3. Impact tests rationale The purpose of experiments was primarily to investigate: - variation in crater morphology with mass (m), velocity (v), kinetic energy (KE) and angle of incidence θ of the projectile. - size of central part of the craters (pit) and size of the spallation zone - depth (P) of the craters as compared to projectile diameter and velocity - assessment of material fragmented and ejected (amount and energy used for the process) - amount of projectile material present in the crater after the impact and its variation with impact parameters (kinetic energy)
For normal impacts, the threshold for the occurrence of the different sequences observed in the analysis of the craters depends mainly on the kinetic energy of the particles. Table 1 summarizes the occurrence the various features. For the experiments described in this paper the kinetic energy is lower than 2.5 erg and it is not possible to see all the features described above. However impacts experiments performed at NASA Ames, with
Projectiles used during the present tests were 0.15–2.3 μm diameter (d) carbonyl-iron spheres accelerated to velocities in the range of 430
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more developed on the far side of the incoming projectile. Fig. 3 shows that the measurement of circularity is a good parameter to assess the angle of incidence of the projectile when the velocity is lower than 10–12 km/s. This is in agreement with crater pictures shown on Fig. 2. In particular the circularity and thus angle of impact are useful in determining the source of secondary impacts observed at many instances on material retrieved after exposure to space.
Table 1 Impact energy and occurrence of spallation. Dc, μm
Ds, μm
E, ergs
Comments
<2 2–3 >3 5–10 10–20 20–50
0 0 10 15–30 30–60 60–150
<1 1–2 >3 3,5–25 30–150 200–500
Central pit only Lip partly broken, inner spall ring Lip broken, inner spall ring, large spalls Spalls not ejected Spalls partially ejected, Halo Spalls ejected
3.2.2. Determination of Dc/d ratio for normal impacts and 2 b/d ratio for oblique impacts The determination of the variation of Dc/d ratio (or 2 b/d ratio) with impact velocity is necessary to derive from the measurement of Dc (or 2b the minor axis of the crater for oblique impacts) the value of the size of the projectile responsible of the impact crater. This determination is primeval for the computation of micrometeoroid or space debris fluxes. In this section we study the Dc/d ratio for normal impacts and the 2 b/d ratio for oblique impacts obtained from experimental simulations in laboratory. More precisely we investigate the variations of these ratios with the velocity of the projectiles and their angle of incidence on the targets. For this determination we use a number of crater sizes Dc (or 2b) measured with an accuracy better than ± 0.05 μm/Dc (or ± 0.05 μm/2b). For each velocity range and for each angle of incidence, we have:
kinetic energies between 1 and 4 ergs display many of the features. This can be seen on the figures of the corresponding publication [15–17]. As it will be shown later the impact craters present on material exposed to space display the various morphologies. 3.2. Impacts geometry One of the objectives of these impact simulations is the interpretation of micro-craters found on material retrieved after exposure to space. In particular to estimate the size of the meteoroids or debris and to assess the possibility to find any residues from the impactor. In this section we consider only the small impacts, such as impacts with a crater diameter Dc (the diameter of the pit), smaller than 5 μm. We were confronted with a difficulty concerning the validity of the choice of impact craters, in particular a possible bias towards selection of large craters (considering that in each velocity interval particles with different size were allowed to strike the target). As mentioned above, a careful analysis of impact diameter measurements and a comparison with previous experiments performed in similar conditions, but using particles with known diameter and velocity [17,18] was necessary to solve this problem. Fig. 2 (a) to 2 (p) show examples of impact craters obtained on the gold-coated fused quartz targets. In general, when the projectile incidence is normal to the target, the impact process produces a central cavity, circular and nearly hemispherical (depth depending on the relative density of projectile and target), surrounded by a zone characterized by radial and conchoidal fractures. When the projectile incidence is oblique, the cavity is hemispheroidal while the radial and conchoidal fractures are concentrated downstream. The conchoidal fractures produce spalls which are ejected or not ejected. For high kinetic energy, the central cavity is fractured and ejected and the shape of the crater is nearly conical (this is not observed with our simulation experiments, but this can be seen on material retrieved from space such on HST solar cells). Thus it is no longer possible to use the Dc/d ratio to characterize the impact, because there is no crater pit. Fig. 2 shows typical impact craters.
- the low magnification pictures (250× and 500x) of the target showing the impact craters due to these projectiles and among them the selected craters whose Dc (or 2b) values are determined (Fig. 4) - the distribution of the sizes of the projectiles (Fig. 5). As an example, let us consider the 4–5 km/s velocity range: Fig. 5 shows the size distribution of the projectiles when the incidence is normal to the target. On this figure, 5 zones are delimited by 4 lines and these zones do not depend on the angle of incidence of the projectiles on the targets. As the size of the craters is an increasing function of the size of the projectiles, using the low magnification pictures ( Fig. 4 for normal incidence), for each incidence angle, we can observe that the largest selected craters are due to projectiles whose size is include within zone 4 while the other selected craters are impacts of projectiles whose size is include within zone 3. If v (km/s) is the velocity of the projectiles, the zone 3 is bounded by:
the bottom line d(µm) = 0.986
the top line
d(µm) = 1.685
0.060( v
0.502( v
4)
(1)
4)
(2)
4)
(3)
While the zone 4 is bounded by: The bottom line equation (2)
3.2.1. Circularity Oblique impacts create elliptical craters and the lip is seen mainly downstream. Furthermore the spallation occurs extensively downstream and sometimes laterally. The spallation occurs for a kinetic energy lower than for a normal impact. It is caused by the horizontal component of the impact velocity vector. Most of the kinetic energy is apparently dissipated in a material volume smaller than for a normal impact when the kinetic energy is dissipated in a larger material hemispherical volume around the point of impact (see Fig. 2e–p). A circularity index, Ci is defined from the semi minor b and semi major an axis ratio, such as:
the top line
d(µm) = 1.819
0.461( v
These equations are derived from Fig. 5. From the largest size value of the selected craters (2.71 μm), we calculate the bounds of the domain of validity of Dc/d ratio using equations (2) and (3). In this same way from the smallest value (2.01 μm), we obtain the bounds of the domain of validity of the ratio using equations (1) and (2). The Dc/d ratio for the overall selected craters are elements of the common part of these domains (Fig. 6). We use the same approach for the other velocity ranges and for oblique incidence (using 2b instead of Dc). The variation of the ratio within the velocity range 3–11 km/s is shown on Fig. 7). On Fig. 7a the craters obtained at Ames are outside the domain of validity. This means that the size of the projectiles are larger than those obtained at Heidelberg. In summary, for the set of selected craters in each velocity range, it is possible to conclude that they are located with the area included into the 2 lines.
Ci = b/a For an impact angle of 0° craters are circular or quasi circular and a lip of deformed and melted material surrounds the central pit. For impact angles of 30°, 45°and 60°, the crater pit is more and more elliptical and the lip has a shape related to the impact direction: the major axis of the crater is aligned with this direction. The edge of the lip is 431
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Fig. 2. a-d. Impacts incidence 0°, Iron particles on gold-coated fused quartz; a: impact at 2–4 km/s, b: impact at 2–4 km/s, c: impact at 4–5 km/s and d: impact at 4–5 km/s e-h. Impacts incidence 30°, Iron particles on gold-coated fused quartz; e: impact at 4–5 km/s, f: impact at 4–5 km/s, g: impact at 5–6 km/s and h: impact at 5–6 km/s i-l. Impacts incidence 45°, Iron particles on gold-coated fused quartz; i: impact at 2–4 km/s, j: impact at 4–5 km/s, k: impact at 5–6 km/s and l: impact at 6–8 km/s m-p. Impacts incidence 60°, Iron particles on gold-coated fused quartz; m: impact at 6–8 km/s, n: impact at 4–6 km/s, o: impact at 10–12 km/s and p: impact at 8–10 km/s.
The ratio Dc/d can be compared with results from Paul and Berthoud model [15], see Fig. 8. In our mathematical expression of Dc, rt (resp. rp) refers to the density of the target (resp. of the projectile) Dc = 1.12 10−4 ρt− 0.5 ρp 0.743 d1.076 v0.65 cos θ0.15 (cgs).
- and, in the case of brittle target, the spalls. The effect on the jetting is indeed visible on Fig. 2i and j, where it is possible to see perforations on the thin gold film torn after impact. The mass of the jetting is lower than 1% of the total ejected mass. The respective mass of cone and spalls depends on the size of the crater: the mass of the ejected spalls increases with the crater size. For craters larger than 100 μm, the mass of the spalls is more than 50% of the total ejected mass and can reach 90% for mm-sized impacts on solar cells [17,21]. Previous studies have also show that there is no spall for submicron sized craters [18]. Our study concerns craters whose sizes are included between 1 and 10 μm. Let us first consider impacts under normal incidence. No ejected spalls are observed (Fig. 2a and d). As the projectile KE does not depends on the incident angle, no spalls should be observed for oblique impacts. Nevertheless for incidence angle around 45° for velocity included between 2 km/s and 5 km/s (Fig. 2i and j), we observe some ejected spalls (for the largest selected craters): the transfer of the incident KE trough the target is concentrated downstream when the incidence is oblique. If the incidence angle is lower than ≈35°, the KE is not too focused downstream. If this angle is greater than ≈55°, part KE is lost by friction. To compute the mass of the ejected spalls, we must:
3.2.3. Depth and shape factor The depth of impact craters depends primarily on the relative mass density of target and projectile and on the impact velocity. Fig. 9 shows the variation of the ratio between crater depth (P) and diameter (Dc) (crater shape factor) as versus impact velocity. With same material for the projectile and the target the hypervelocity regime is reached when the P/Dc ratio (shape factor) is equal to 0.5, for a normal incidence, see for instance, Zukas [19]. As mentioned by Smirnov et al. [8] the shape of the crater is nearly hemispherical for impact velocity larger than 2 km/s. However in our test case with iron projectile and gold-coated glass target, the crater shape factor is significantly larger as shown on Fig. 9. In particular the deepest craters are produced for a velocity close to 5 km/s, instead of a velocity of 2 km/s [8] This factor not only depends on the relative density of the projectile with respect to the strength and the density of the target but also on the incidence angle. (This is compatible with the fact that in the case of oblique incidence, the normal component of the velocity is involved in the cratering process). 3.3. Large impacts
i/obtain the profile of each cavity such as those shown by Fig. 11. ii/find the convergence point of the boundaries of each cavity iii/find the centre of gravity (G) of the cavity profile.
3.3.1. Normal impacts: Spall diameter Ds The diameter of the spall zone (Ds) when present, is also measured. However as the occurrence of a spall zone is not common during the simulation range considered in this paper it is not possible to make a detailed analysis of the variation of Ds with impact velocity.
Then the volume of the cavity is obtained from the second theorem of Pappus-Guldin [19]. Using this method for each spall cavity, we obtain the entire volume of the ejected material. From the fused quartz density we calculate the ejected mass. For example the mass of the ejected spalls shown in Fig. 2i–j.
3.3.2. Oblique impacts For oblique impacts the occurrence of the spallation is more common and it is more pronounced in the downrange direction as shown on Fig. 2e–p. More details on spalls and oblique impacts are given in section 5.
M = 7.42 E−14 kg = 74.2 pg
3.4. Application to the analysis of retrieved impact features
In this example, the velocity v of the projectile is included between 4 km/s and 5 km/s. From Fig. 8, we have.
As an example we have selected some impacts craters found on the solar arrays from HST (see Fig. 10). Observation have been made with a JEOL SEM (20 kV). Craters of interest range from 3 μm to 60 μm diameter (central part, Dc). From a representative data set of 30 craters, about 40% are elliptical and 60% circular. The craters display the various morphologies described earlier: the smallest have only a pit with a raised lip, surrounded by an inner ring of shattered glass; with increasing diameter the spall zone is more and more important. Three oblique impacts are also shown (Fig. 10a Fig. 10e and f).[20,21].
1.50 ≤ 2 b/d ≤ 1.80 as 2.31 μm ≤ 2b ≤ 2.51 μm 1.54 μm ≤ d ≤ 1.67 μm when v = 4 km/s 1.28 μm ≤ d ≤ 1.39 μm when v = 5 km/s These results are consistent with Fig. 5. Then the mass m of the projectile is. 14.9 pg ≤ m ≤ 19.0 pg when v = 4 km/s 8.56 pg ≤ m ≤ 11.0 pg when v = 5 km/s and m < M
4. Spalls ejecta Usually when a solid particle impacts a target, secondary particles, are ejected. Three processes of ejection are observed:
The velocity of the ejected spalls is included between 10 m/s and 100 m/s [17]: the kinetic energy taken away by the spalls is included between 3.71E-5 erg and 3.71E-3 erg. These values are quasi negligible with respect to the KE of the projectile:
- the jetting composed of small and fast liquid particles ejected at grazing angle, - the cone composed of small and fast solid particles ejected at constant elevation angle,
1.19 erg ≤ KE ≤ 1.52 erg when v = 4 km/s 433
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Fig. 2. (continued)
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Fig. 3. Variation of circularity, Ci with impact angle and velocity. Fig. 6. Variation of Dc/d ratio in the 4–5 km/s velocity range.
Fig. 4. Choice of craters for SEM observations and EDX analysis.
Fig. 5. Size distribution of the incident particles.
1.07 erg ≤ KE ≤ 1.38 erg when v = 5 km/s Using this method when the velocity v of the projectile is included between 3 km/s and 4 km/s, the mass of the ejected spalls is M = 6.87 E−13 kg = 687 pg, Fig. 7. a. Variation of Dc/d ratio in the 3–11 km/s velocity range. b. Variation of 2 b/d ratio in the 3–11 km/s velocity range (the curves are the shells of the results for the three oblique incidences).
a value larger than the previous one because the size of the projectile and of the crater is larger than in the previous example. From Fig. 8 we have
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10 × 10 spectrometric measurements were obtained for each crater (Fig. 12). Each spectrum gives the percentage in mass of each atomic element in the sounded volume with an error of a few 2% (see Table 2). Moreover a linear scanning (i.e. a grid of 20 × 1 spectrometric measurements (Fig. 13) of each crater is used to detect far iron ejecta. 5.1. Projectile remnants for normal impacts The projectile remnants depend on the incident kinetic energy. As a general rule. a When the kinetic energy is low (Fig. 2c–d) (KE < 1 erg), a remnant of the projectile is visible inside the crater, generally partially and irregularly broken, sometimes homogeneous and compact, likely incipient fusion, and identified using the EDS spectrometer which detects iron; some hemispherical droplets are found on the walls of the crater, the largest ones near the mouth; EDS spectrometer analysis shows that these droplets are “alloy” of projectile iron-target coating gold. b When the kinetic energy increases (KE = 1.5–2 ergs) small number of rounded hemispherical large droplets of projectile material, apparently melted and afterwards solidified while no iron remnant is found at the bottom of the crater. The largest droplets disappear and many smallest ones, composed of iron-gold, are found on the walls. Occurrence of a layer of molten material around the central cavity and ejected droplets (from projectile or target) are observed. c When KE > 2 ergs a large number of small hemispherical droplets are concentrated on the upper part of the crater walls (Fig. 2a–b) composed of iron-gold; ejected droplets are found outside the crater. The shape of the bottom part of the crater seems to be due to the shock wave, without a direct contact of the projectile. d Large kinetic energies are not within the range of our simulation experiments. It is anticipated that the projectile would melted in the form of very small droplets, probably ejected out of the crater. Also, in this conditions the central part of the crater is mechanically ejected. Consequently the detection of any projectile remnants shall be very difficult when KE > 5 ergs.
Fig. 8. Comparison model (blue) and present data (red). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 9. Evolution of crater depth to crater diameter ratio (P/Dc), with impact velocity.
1.10 ≤ 2 b/d ≤ 1.50 as < 2b > = 2.86 μm d = 2.60 μm when v = 3 km/s d = 1.91 μm when v = 4 km/s
5.2. Projectile remnants for oblique impacts
The mass m of the projectile is
To observe the bottom of the crater, the values of the tilt angles must surround the value of the angle of incidence of the projectile (for example Fig. 2k). But to study the walls of the craters, the stereoscopic views use pictures acquired by tilting the sample at 0°, 7° and 14°. Results differ from normal impacts.
m = 71.8 pg when v = 3 km/s. m = 28.5 pg when v = 4 km/s and
a When KE < 1 erg, we observe an amalgamation of pure iron droplets and iron-gold droplets partly surrounding the bottom of the crater (Fig.2k). On the wall some isolated large hemispherical droplets are observed downstream while a number of small ones are observed upstream. All these droplets are “alloys iron-gold” (Fig. 2l). b When the kinetic energy increases (1 erg < KE < 2 ergs), the amalgamation of droplets near the bottom of the crater disappear. We observed some large droplets downstream and a lot of small ones upstream. Most of the droplets are iron – gold “alloys” but some are pure iron. c When KE > 2 ergs no change is observed downstream but quasi no droplets are observed upstream. It should be recalled that part of the incident energy is used to generate spalls.
KE = 3.23 erg when v = 3 km/s. KE = 2.28 erg when v = 4 km/s The kinetic energy taken away by the spalls is included between 3.44 E−4 ergs and 3.44 E−2 ergs very faint values with respect to the KE of the projectile. The ratio M/m is greater in this case than in the first example because the decrease of the size of the projectile with the increase of its velocity due to experimental constraints. This explains the absence of ejected spalls when the velocity of the projectile is greater than 5 km/s. 5. Identification and evaluation of projectile remnants The study of projectile remnants concerns the craters created from incidence angles whose values are included between 0° and 60°. This preliminary study presents normal impacts and 45° incidence angle impacts as a representative case of oblique impacts. Five craters per velocity interval were analysed. Using the scanning facilities, a grid of
6. Conclusion This study concerns our understanding of the solid component of the space environment, from the morphology and the remnants of the 436
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Fig. 10. Impacts craters observed on solar cells from HST solar arrays.
craters on samples of materials retrieved after exposure to space. Laboratory simulations have been used to support this investigation. Using a SEM we have determined the detailed morphology of the impact craters and with an EDX spectrometer we have identified the remnant material of the projectile. The morphology of the craters depends on the properties of the projectiles and of the targets. In particular we have obtained the diameter and the depth of the craters which are the basis of the determination of projectile parameters. Circularity is useful in determining the angle of incidence of the projectile and consequently the source concerning secondary craters, if the craters are produced by ejecta. Moreover circularity could possibly be used for determining original particle orbits. Then the evolution of the crater size – projectile size ratio with impact velocity, permits to obtain the size of the projectile. If the EDX spectrometer has identified the remnant materials, the mass of the projectile is deduced and consequently leads to a better knowledge of the space environment, i.e. composition, size distribution and space distribution of the impactors. An estimation of the mass of material ejected upon impact is useful for the determination of the amount of orbital debris produced through ejecta process. The results are in agreement with those described by M. Rival [21].
Fig. 11. Profile of the crater and of the cavity left by an ejected spall in the downstream direction. The horizontal line shows the level of the target before impact. 437
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Table 2 Percentage in mass of atomic elements in each selected spectrum. Atomic element
O
Si
Fe
Au
Spectrum 1 Spectrum 2 Spectrum 3
46.36 25.97 49.22
40.00 61.34 48.59
0.00 4.71 0.32
13.64 7.98 1.88
Fig. 13. Line 20 × 1 of EDX spectrometric measurements on a crater and his neighbourhood.
With the EDX spectrometer we have detected the very small amount of projectile remnants. Modelling the sounded volume by the EDX spectrometer, we have been able to estimate the mass of the remnants. We have found that the amount of projectile residue varies with incidence angle and velocity so a better interpretation of oblique impacts is possible. Acknowledgements The authors greatly acknowledge Ralf Srama from the Max Planck Institute (MPI) for Nuclear Physics, Heidelberg (Germany) for the use of dust accelerator, and the support of Christian Durin from the Centre Natinal d'Etudes Spatiales (CNES), Toulouse, France. References [1] Orbital Debris, A Technical Assessment, NRC, National Academy Press, Washington, DC, 1995. [2] V. Adushkin, S. Veniaminov, S. Koslov, M. Silnikov, Orbital Missions safety- A survey of kinetic hazards, Acta Astronaut. (2016) 510–516. [3] N.N. Smirnov, A.B. Kiselev, M.N. Smirnova, V.F. Nitkin, Space traffic hazards from orbital debris mitigation strategies, Acta Astronaut. 109 (2015) 144–152. [4] M. Allbrooks, D. Atkinson, The magnitude of impact damage on LDEF materials, NASA CR (1992) 188258. [5] J.C. Mandeville, Impact detection in space : derivation of physical properties of meteoroids and debris, Adv. Space Res. 17 (12) (1996) 147–153. [6] J.C. Mandeville, J.M. Perrin, Loic Vidal, Experimental hypervelocity impacts: implications for the analysis of material retrieved after exposure to space environment, part I, Acta Astronaut. 81 (2012) 532–544. [7] H. Fechtig, E. Grün, J. Kissel, J.A.M. McDonnell (Ed.), Laboratory Simulations in Cosmic Dust, 1992, pp. 629–630. [8] N.N. Smirnov, K.A. Kondratyev, Evaluation of craters formation in hypervelocity impact of debris particles on solid structures, Acta Astronaut. 65 (2009) 1796–1803. [9] J. Peirs, P. Verleysen, W. Van Paepegem, J. Degrieck, Determining the stress-strain behavior at large strains from high strain rate tensile and shear experiments, Int. J. Impact Eng. 38 (2011) 406–415. [10] E.A. Taylor, Experimental and Computational Study of Hypervelocity Impact on Brittle Materials and Composites, PhD. Thesis University of Kent, Canterbury, 1997.
Fig. 12. Grid 10 × 10 of EDX spectrometric measurement.
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J.-C. Mandeville, et al. [11] C. Denoual, F. Hild, A damage model for the dynamic fragmentation of brittle solids Comput, Methods Appl. Mech. Eng. 183 (2000) 247–258. [12] X. Zhang, H. Hao, G. Ma, Dynamic material model of annealed soda-lime glass, Int. J. Impact Eng. 77 (2015) 108–119. [13] J.C. Mandeville, C.R. Maag, Ch Durin, In situ detection of micrometeoroids and orbital debris : the PIE experiment on MIR, Adv. Space Res. 25 (2) (2000) 329–334. [14] J.M. Zwiener, D.R. Wilkes, M.R. Carruth, R.R. Kamenetzky, F. Hörz, W.H. Kinard, Overview of the space Environmental effects experiments flown on the MIR space station: MEEP, OPM, SPSR, Proc. 8th. Conf. Material in Space Environment, Arcachon, 2000. [15] K. Paul, L. Berthoud, Meteoroid and debris micro-impacts on space exposed solar arrays, Adv. Space Res. 20 (8) (1997) 1441–1445. [16] G. Drolshagen, J.A.M. McDonnell, J.C. Mandeville, A. Moussi, Impact studies of the HST solar arrays retrieved in March (2002), Acta Astronaut. 58 (2006) 471–477.
[17] J.F. Vedder, J.C. Mandeville, Microcraters formed in glass by projectiles of various densities, JGR 79 (23) (1974) 3247–3256. [18] J.C. Mandeville, Etudes de cratères formés sur des surfaces de verre par l’impact de micrométéoroides artificielles, Thèse Université de Toulouse, 1972. [19] J.A. Zukas, et al., Impact Dynamics, Krieger Publishing Company, Malabar, Florida, 1992, pp. 216–219. [20] G.A. Graham, A.T. Kearsley, G. Drolshagen, et al., Microparticles impacts upon HST solar cells, Adv. Space Res. 28 (2001) 1341–1346. [21] M. Rival, J.C. Mandeville, C. Durin, Impact phenomena on brittle materials : analysis of 1μm to 1 mm impact features on solar arrays, Adv. Space Res. 20 (8) (1997) 1451–1456. [22] J.W. Harris, J.R. Bland, Solid Mensuration with Proofs, Wiley, new York, 1948, pp. 110–115.
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Acta Astronautica journal homepage: www.elsevier.com/locate/actaastro
Research paper
Experimental hypervelocity impacts: Implication for the analysis of material retrieved after exposure to space environment
T
Jean-Claude Mandevillea,∗, Jean-Marie Perrinc, Loïc Vidalb a
MANDESPACE, 8 Rue des Régans, 31000, Toulouse, France Institut de Science des Matériaux de Mulhouse (IS2M), CNRS UMR 7361, 15 Rue Jean Starcky, BP2488 68057, Mulhouse, France c SPACENVIR, Observatoire de Haute Provence, 04870, Saint Michel l'Observatoire, France b
ARTICLE INFO
ABSTRACT
Keywords: Meteoroids Orbital debris Hypervelocity impact SEM EDX Brittle material
During the last three decades a wide variety of surfaces have been brought back to Earth after being exposed to the space particulate environment. The impact features found on these surfaces can give clues to the characteristics of the orbital debris and meteoroids that created them. Many investigations have been carried out to derive projectile parameters (size, shape, impact angle, origin) from the morphology of impact features and the analysis of projectile their remnants inside the crater. However, there are still some ambiguities in the interpretation of these results. In this study we have investigated carefully the distinguishing features for craters caused by the impact of high velocity iron particles on thick brittle targets, using a micro-particle accelerator. Up to date instrumentation, SEM and EDX, and in depth results analysis provides an extensive data-set that can be used to improve the scientific return of the investigation of material retrieved after exposure to space.
1. Introduction
materials. However fragmentation and the formation of spalls during impacts on brittle materials increase the difficulties of analytical studies or numerical modelling (see for instance Refs. [10–12]). In this paper we used only phenomenological calculations, the analytical and numerical studies of our impacts on brittle targets will be the subject of a forthcoming publication. The aim of this study is to refine criteria allowing an assessment of size, velocity and origin of a projectile from parameters could be measured on the impact craters. Indeed, samples exposed on MIR (see Refs. [13,14]) and HST solar cells retrieved after more than 8 years of exposure to space [15,16]) showed a large number of elliptical features, mainly for impacts smaller than 100 μm because larger ones have seen their material ejected losing the evidence of ellipticity. These craters are not uncommon, indeed they represent more than one third of impacts smaller than 100 μm observed. Interpretation of this kind of impacts rely on hypervelocity impact tests in the laboratory. In addition, a larger amount of projectile residue is found in oblique craters rather than in circular ones. In section 2 the experimental approach used is described. Section 3 presents the analysis of impact craters to obtain their main geometric parameters in function of a number of projectile masses and size and several impact velocities. Section 4 is devoted to the study of spalls ejecta observed for some oblique impacts while Section 5 presents the analysis of projectile remnants. We conclude in Section 6.
Space environment is hostile to spacecraft [1–3]. Threats come from its components and among them meteoroids and space debris. It is therefore vital to understand the damage they can cause and to take adequate measures to design spacecraft that resist their degrading effects. Accurate debris and meteoroid flux models are crucial for the design of manned and unmanned space missions. For particles sizes smaller than a few millimetres, data on the environment can only gained by in-situ detectors, hardware retrieved after exposure to space and active sensors that provide real-time impact information. Data not only give information on the meteoroid and debris environment but also provide a better knowledge of the hypervelocity impact phenomena on materials exposed to space. However it is still difficult to derive with precision, from the analysis of impact features, the parameters of the projectiles and their possible origin [4,5].In this study we consider the impacts of small size iron particles (< 10 μm) onto brittle glass targets. A study dealing of similar impacts but on ductile material has been published earlier in this journal [6]. In the past many laboratory impact simulations have been done using hypervelocity launchers for large particles and dust particles accelerators for small particles on ductile and on brittle targets [7]. A large number of publications present analytical (see for instance Refs. [6,8]) and numerical (see Ref. [9]) solutions concerning hypervelocity impacts on ductile ∗
Corresponding author. E-mail address: [email protected] (J.-C. Mandeville).
https://doi.org/10.1016/j.actaastro.2019.11.020 Received 20 September 2019; Received in revised form 8 November 2019; Accepted 13 November 2019 Available online 18 November 2019 0094-5765/ © 2019 IAA. Published by Elsevier Ltd. All rights reserved.
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2–12 km/s. In order to study the influence of angle of incidence in the formation of the craters, 4 angles of incidence have been used: 0°, 30°, 45° and 60°, (measured from the normal to the surface). Five velocity ranges were used per incidence angle. 2.4. Observation equipment Impact craters are studied with a scanning electronic microscope (SEM), a IS2M Quanta 400 instrument, with a resolution of about 3.5 nm. The observations were performed at an acceleration voltage of 30 kV. As the samples are conductive (original gold coating), no preparation was needed for the observations. The micrographs were acquired at magnifications ranging from 100× to 100000 X. For depth computation stereoscopic views were done using pictures acquired by tilting the sample at 0°, 7° and 14°, see for details ref. 6. In addition 30°, 37° and 44° tilt angles were used to study the bottom of the craters when the incidence angle is 30° while, 37°, 45° and 52° tilt angles were used when the incidence angle is 45° and 46°, 53°, 60° when the incident angle is 60°. The chemical analysis were performed using an Energy Dispersive X-ray (EDX) spectrometer (Oxford Instruments Inca X-Act) attached to the microscope. Six to eleven representative impacts features per impacted area were analysed (each area correspond to a velocity range). Each impact was observed with SEM, in a secondary electron mode in order to obtain a high resolution picture. Then, analysis of the remnants gives the elemental composition of melted residue. As targets were pure glass and projectiles were carbonyl-iron particles this analysis is rather easy to interpret, the only elements studied being Au, Si, O and Fe. Nevertheless the thin layer of vacuum deposited gold and the depth of the craters could be the cause of artefacts.
Fig. 1. Size distribution of particles.
2. Experimental approach 2.1. Acceleration of microparticles One of the best, if not the only one, method to accelerate small particles to hypervelocities consists in the use of an electrostatic accelerator. The Max Planck Institute for Nuclear Physics in Heidelberg (Germany) has been operating such a facility for many years [7]. Details concerning the experimental process have given in a previous paper [6]. Velocity, charge and mass of the particles are measured by dedicated measuring devices. Due to operational constraints, it was only possible to select several velocity ranges and to allow a large number of particles of different sizes within this velocity interval. The largest particles are accelerated at the lowest velocity and the smallest at the highest velocity. This constraint make the analysis of the impact features a difficult one, as discussed later. Fig. 1 shows the size distribution of particles according to their velocity, as delivered by the accelerator. This dataset correspond to a small number of particles. Upon experimental tests a much larger number of particles are used in each velocity range as shown for instance on Fig. 5.
3. Analysis of impact features 3.1. Energy dependence of craters morphology The constraint above mentioned, i.e. the low resolution of projectiles size in a selected velocity interval, needs comparison with experiments performed with individual projectile whose size and mass are known These experiments permit to link the kinetic energy (KE) of the impact to the morphology of the crater and are useful to understand the results obtained when a number of projectiles simultaneously impact a target. Such simulation experiments were carried out in the past at the NASA Ames Research Centre using a unique electrostatic accelerator [18]. Then it is possible to distinguish the following changes in the morphology of the craters:
2.2. Target material The targets are 1 mm thick, 25 mm in diameter disks of fused quartz. The disks were vacuum-coated with gold before experiments, in order to identify clearly the impact craters. Target can thus be considered as thick as compared to the size of the projectiles. In these conditions the side and rear walls play no significant part in the cratering process.
1. Craters with only a central smooth cavity diameter of the pit Dc for circular craters due to normal incidence, or elliptical axis 2a and 2b for craters due to oblique incidence), surrounded by a circular lip. 2. Central cavity with a small number of radial cracks (2–3) and an inner ring of shattered glass. 3. Central cavity, with an irregular lip (due a beginning of fragmentation), with several radial cracks (5–8), an inner ring of shattered glass and incipient conchoidal fractures; 4. Central cavity partly broken and surrounded by a spallation zone (size Ds), with several (4–6) large spalls either ejected or not. 5. The central cavity disappears and the impact crater has a conical shape, surrounded by conchoidal and radial fractures.
2.3. Impact tests rationale The purpose of experiments was primarily to investigate: - variation in crater morphology with mass (m), velocity (v), kinetic energy (KE) and angle of incidence θ of the projectile. - size of central part of the craters (pit) and size of the spallation zone - depth (P) of the craters as compared to projectile diameter and velocity - assessment of material fragmented and ejected (amount and energy used for the process) - amount of projectile material present in the crater after the impact and its variation with impact parameters (kinetic energy)
For normal impacts, the threshold for the occurrence of the different sequences observed in the analysis of the craters depends mainly on the kinetic energy of the particles. Table 1 summarizes the occurrence the various features. For the experiments described in this paper the kinetic energy is lower than 2.5 erg and it is not possible to see all the features described above. However impacts experiments performed at NASA Ames, with
Projectiles used during the present tests were 0.15–2.3 μm diameter (d) carbonyl-iron spheres accelerated to velocities in the range of 430
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more developed on the far side of the incoming projectile. Fig. 3 shows that the measurement of circularity is a good parameter to assess the angle of incidence of the projectile when the velocity is lower than 10–12 km/s. This is in agreement with crater pictures shown on Fig. 2. In particular the circularity and thus angle of impact are useful in determining the source of secondary impacts observed at many instances on material retrieved after exposure to space.
Table 1 Impact energy and occurrence of spallation. Dc, μm
Ds, μm
E, ergs
Comments
<2 2–3 >3 5–10 10–20 20–50
0 0 10 15–30 30–60 60–150
<1 1–2 >3 3,5–25 30–150 200–500
Central pit only Lip partly broken, inner spall ring Lip broken, inner spall ring, large spalls Spalls not ejected Spalls partially ejected, Halo Spalls ejected
3.2.2. Determination of Dc/d ratio for normal impacts and 2 b/d ratio for oblique impacts The determination of the variation of Dc/d ratio (or 2 b/d ratio) with impact velocity is necessary to derive from the measurement of Dc (or 2b the minor axis of the crater for oblique impacts) the value of the size of the projectile responsible of the impact crater. This determination is primeval for the computation of micrometeoroid or space debris fluxes. In this section we study the Dc/d ratio for normal impacts and the 2 b/d ratio for oblique impacts obtained from experimental simulations in laboratory. More precisely we investigate the variations of these ratios with the velocity of the projectiles and their angle of incidence on the targets. For this determination we use a number of crater sizes Dc (or 2b) measured with an accuracy better than ± 0.05 μm/Dc (or ± 0.05 μm/2b). For each velocity range and for each angle of incidence, we have:
kinetic energies between 1 and 4 ergs display many of the features. This can be seen on the figures of the corresponding publication [15–17]. As it will be shown later the impact craters present on material exposed to space display the various morphologies. 3.2. Impacts geometry One of the objectives of these impact simulations is the interpretation of micro-craters found on material retrieved after exposure to space. In particular to estimate the size of the meteoroids or debris and to assess the possibility to find any residues from the impactor. In this section we consider only the small impacts, such as impacts with a crater diameter Dc (the diameter of the pit), smaller than 5 μm. We were confronted with a difficulty concerning the validity of the choice of impact craters, in particular a possible bias towards selection of large craters (considering that in each velocity interval particles with different size were allowed to strike the target). As mentioned above, a careful analysis of impact diameter measurements and a comparison with previous experiments performed in similar conditions, but using particles with known diameter and velocity [17,18] was necessary to solve this problem. Fig. 2 (a) to 2 (p) show examples of impact craters obtained on the gold-coated fused quartz targets. In general, when the projectile incidence is normal to the target, the impact process produces a central cavity, circular and nearly hemispherical (depth depending on the relative density of projectile and target), surrounded by a zone characterized by radial and conchoidal fractures. When the projectile incidence is oblique, the cavity is hemispheroidal while the radial and conchoidal fractures are concentrated downstream. The conchoidal fractures produce spalls which are ejected or not ejected. For high kinetic energy, the central cavity is fractured and ejected and the shape of the crater is nearly conical (this is not observed with our simulation experiments, but this can be seen on material retrieved from space such on HST solar cells). Thus it is no longer possible to use the Dc/d ratio to characterize the impact, because there is no crater pit. Fig. 2 shows typical impact craters.
- the low magnification pictures (250× and 500x) of the target showing the impact craters due to these projectiles and among them the selected craters whose Dc (or 2b) values are determined (Fig. 4) - the distribution of the sizes of the projectiles (Fig. 5). As an example, let us consider the 4–5 km/s velocity range: Fig. 5 shows the size distribution of the projectiles when the incidence is normal to the target. On this figure, 5 zones are delimited by 4 lines and these zones do not depend on the angle of incidence of the projectiles on the targets. As the size of the craters is an increasing function of the size of the projectiles, using the low magnification pictures ( Fig. 4 for normal incidence), for each incidence angle, we can observe that the largest selected craters are due to projectiles whose size is include within zone 4 while the other selected craters are impacts of projectiles whose size is include within zone 3. If v (km/s) is the velocity of the projectiles, the zone 3 is bounded by:
the bottom line d(µm) = 0.986
the top line
d(µm) = 1.685
0.060( v
0.502( v
4)
(1)
4)
(2)
4)
(3)
While the zone 4 is bounded by: The bottom line equation (2)
3.2.1. Circularity Oblique impacts create elliptical craters and the lip is seen mainly downstream. Furthermore the spallation occurs extensively downstream and sometimes laterally. The spallation occurs for a kinetic energy lower than for a normal impact. It is caused by the horizontal component of the impact velocity vector. Most of the kinetic energy is apparently dissipated in a material volume smaller than for a normal impact when the kinetic energy is dissipated in a larger material hemispherical volume around the point of impact (see Fig. 2e–p). A circularity index, Ci is defined from the semi minor b and semi major an axis ratio, such as:
the top line
d(µm) = 1.819
0.461( v
These equations are derived from Fig. 5. From the largest size value of the selected craters (2.71 μm), we calculate the bounds of the domain of validity of Dc/d ratio using equations (2) and (3). In this same way from the smallest value (2.01 μm), we obtain the bounds of the domain of validity of the ratio using equations (1) and (2). The Dc/d ratio for the overall selected craters are elements of the common part of these domains (Fig. 6). We use the same approach for the other velocity ranges and for oblique incidence (using 2b instead of Dc). The variation of the ratio within the velocity range 3–11 km/s is shown on Fig. 7). On Fig. 7a the craters obtained at Ames are outside the domain of validity. This means that the size of the projectiles are larger than those obtained at Heidelberg. In summary, for the set of selected craters in each velocity range, it is possible to conclude that they are located with the area included into the 2 lines.
Ci = b/a For an impact angle of 0° craters are circular or quasi circular and a lip of deformed and melted material surrounds the central pit. For impact angles of 30°, 45°and 60°, the crater pit is more and more elliptical and the lip has a shape related to the impact direction: the major axis of the crater is aligned with this direction. The edge of the lip is 431
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Fig. 2. a-d. Impacts incidence 0°, Iron particles on gold-coated fused quartz; a: impact at 2–4 km/s, b: impact at 2–4 km/s, c: impact at 4–5 km/s and d: impact at 4–5 km/s e-h. Impacts incidence 30°, Iron particles on gold-coated fused quartz; e: impact at 4–5 km/s, f: impact at 4–5 km/s, g: impact at 5–6 km/s and h: impact at 5–6 km/s i-l. Impacts incidence 45°, Iron particles on gold-coated fused quartz; i: impact at 2–4 km/s, j: impact at 4–5 km/s, k: impact at 5–6 km/s and l: impact at 6–8 km/s m-p. Impacts incidence 60°, Iron particles on gold-coated fused quartz; m: impact at 6–8 km/s, n: impact at 4–6 km/s, o: impact at 10–12 km/s and p: impact at 8–10 km/s.
The ratio Dc/d can be compared with results from Paul and Berthoud model [15], see Fig. 8. In our mathematical expression of Dc, rt (resp. rp) refers to the density of the target (resp. of the projectile) Dc = 1.12 10−4 ρt− 0.5 ρp 0.743 d1.076 v0.65 cos θ0.15 (cgs).
- and, in the case of brittle target, the spalls. The effect on the jetting is indeed visible on Fig. 2i and j, where it is possible to see perforations on the thin gold film torn after impact. The mass of the jetting is lower than 1% of the total ejected mass. The respective mass of cone and spalls depends on the size of the crater: the mass of the ejected spalls increases with the crater size. For craters larger than 100 μm, the mass of the spalls is more than 50% of the total ejected mass and can reach 90% for mm-sized impacts on solar cells [17,21]. Previous studies have also show that there is no spall for submicron sized craters [18]. Our study concerns craters whose sizes are included between 1 and 10 μm. Let us first consider impacts under normal incidence. No ejected spalls are observed (Fig. 2a and d). As the projectile KE does not depends on the incident angle, no spalls should be observed for oblique impacts. Nevertheless for incidence angle around 45° for velocity included between 2 km/s and 5 km/s (Fig. 2i and j), we observe some ejected spalls (for the largest selected craters): the transfer of the incident KE trough the target is concentrated downstream when the incidence is oblique. If the incidence angle is lower than ≈35°, the KE is not too focused downstream. If this angle is greater than ≈55°, part KE is lost by friction. To compute the mass of the ejected spalls, we must:
3.2.3. Depth and shape factor The depth of impact craters depends primarily on the relative mass density of target and projectile and on the impact velocity. Fig. 9 shows the variation of the ratio between crater depth (P) and diameter (Dc) (crater shape factor) as versus impact velocity. With same material for the projectile and the target the hypervelocity regime is reached when the P/Dc ratio (shape factor) is equal to 0.5, for a normal incidence, see for instance, Zukas [19]. As mentioned by Smirnov et al. [8] the shape of the crater is nearly hemispherical for impact velocity larger than 2 km/s. However in our test case with iron projectile and gold-coated glass target, the crater shape factor is significantly larger as shown on Fig. 9. In particular the deepest craters are produced for a velocity close to 5 km/s, instead of a velocity of 2 km/s [8] This factor not only depends on the relative density of the projectile with respect to the strength and the density of the target but also on the incidence angle. (This is compatible with the fact that in the case of oblique incidence, the normal component of the velocity is involved in the cratering process). 3.3. Large impacts
i/obtain the profile of each cavity such as those shown by Fig. 11. ii/find the convergence point of the boundaries of each cavity iii/find the centre of gravity (G) of the cavity profile.
3.3.1. Normal impacts: Spall diameter Ds The diameter of the spall zone (Ds) when present, is also measured. However as the occurrence of a spall zone is not common during the simulation range considered in this paper it is not possible to make a detailed analysis of the variation of Ds with impact velocity.
Then the volume of the cavity is obtained from the second theorem of Pappus-Guldin [19]. Using this method for each spall cavity, we obtain the entire volume of the ejected material. From the fused quartz density we calculate the ejected mass. For example the mass of the ejected spalls shown in Fig. 2i–j.
3.3.2. Oblique impacts For oblique impacts the occurrence of the spallation is more common and it is more pronounced in the downrange direction as shown on Fig. 2e–p. More details on spalls and oblique impacts are given in section 5.
M = 7.42 E−14 kg = 74.2 pg
3.4. Application to the analysis of retrieved impact features
In this example, the velocity v of the projectile is included between 4 km/s and 5 km/s. From Fig. 8, we have.
As an example we have selected some impacts craters found on the solar arrays from HST (see Fig. 10). Observation have been made with a JEOL SEM (20 kV). Craters of interest range from 3 μm to 60 μm diameter (central part, Dc). From a representative data set of 30 craters, about 40% are elliptical and 60% circular. The craters display the various morphologies described earlier: the smallest have only a pit with a raised lip, surrounded by an inner ring of shattered glass; with increasing diameter the spall zone is more and more important. Three oblique impacts are also shown (Fig. 10a Fig. 10e and f).[20,21].
1.50 ≤ 2 b/d ≤ 1.80 as 2.31 μm ≤ 2b ≤ 2.51 μm 1.54 μm ≤ d ≤ 1.67 μm when v = 4 km/s 1.28 μm ≤ d ≤ 1.39 μm when v = 5 km/s These results are consistent with Fig. 5. Then the mass m of the projectile is. 14.9 pg ≤ m ≤ 19.0 pg when v = 4 km/s 8.56 pg ≤ m ≤ 11.0 pg when v = 5 km/s and m < M
4. Spalls ejecta Usually when a solid particle impacts a target, secondary particles, are ejected. Three processes of ejection are observed:
The velocity of the ejected spalls is included between 10 m/s and 100 m/s [17]: the kinetic energy taken away by the spalls is included between 3.71E-5 erg and 3.71E-3 erg. These values are quasi negligible with respect to the KE of the projectile:
- the jetting composed of small and fast liquid particles ejected at grazing angle, - the cone composed of small and fast solid particles ejected at constant elevation angle,
1.19 erg ≤ KE ≤ 1.52 erg when v = 4 km/s 433
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Fig. 2. (continued)
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Fig. 3. Variation of circularity, Ci with impact angle and velocity. Fig. 6. Variation of Dc/d ratio in the 4–5 km/s velocity range.
Fig. 4. Choice of craters for SEM observations and EDX analysis.
Fig. 5. Size distribution of the incident particles.
1.07 erg ≤ KE ≤ 1.38 erg when v = 5 km/s Using this method when the velocity v of the projectile is included between 3 km/s and 4 km/s, the mass of the ejected spalls is M = 6.87 E−13 kg = 687 pg, Fig. 7. a. Variation of Dc/d ratio in the 3–11 km/s velocity range. b. Variation of 2 b/d ratio in the 3–11 km/s velocity range (the curves are the shells of the results for the three oblique incidences).
a value larger than the previous one because the size of the projectile and of the crater is larger than in the previous example. From Fig. 8 we have
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10 × 10 spectrometric measurements were obtained for each crater (Fig. 12). Each spectrum gives the percentage in mass of each atomic element in the sounded volume with an error of a few 2% (see Table 2). Moreover a linear scanning (i.e. a grid of 20 × 1 spectrometric measurements (Fig. 13) of each crater is used to detect far iron ejecta. 5.1. Projectile remnants for normal impacts The projectile remnants depend on the incident kinetic energy. As a general rule. a When the kinetic energy is low (Fig. 2c–d) (KE < 1 erg), a remnant of the projectile is visible inside the crater, generally partially and irregularly broken, sometimes homogeneous and compact, likely incipient fusion, and identified using the EDS spectrometer which detects iron; some hemispherical droplets are found on the walls of the crater, the largest ones near the mouth; EDS spectrometer analysis shows that these droplets are “alloy” of projectile iron-target coating gold. b When the kinetic energy increases (KE = 1.5–2 ergs) small number of rounded hemispherical large droplets of projectile material, apparently melted and afterwards solidified while no iron remnant is found at the bottom of the crater. The largest droplets disappear and many smallest ones, composed of iron-gold, are found on the walls. Occurrence of a layer of molten material around the central cavity and ejected droplets (from projectile or target) are observed. c When KE > 2 ergs a large number of small hemispherical droplets are concentrated on the upper part of the crater walls (Fig. 2a–b) composed of iron-gold; ejected droplets are found outside the crater. The shape of the bottom part of the crater seems to be due to the shock wave, without a direct contact of the projectile. d Large kinetic energies are not within the range of our simulation experiments. It is anticipated that the projectile would melted in the form of very small droplets, probably ejected out of the crater. Also, in this conditions the central part of the crater is mechanically ejected. Consequently the detection of any projectile remnants shall be very difficult when KE > 5 ergs.
Fig. 8. Comparison model (blue) and present data (red). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 9. Evolution of crater depth to crater diameter ratio (P/Dc), with impact velocity.
1.10 ≤ 2 b/d ≤ 1.50 as < 2b > = 2.86 μm d = 2.60 μm when v = 3 km/s d = 1.91 μm when v = 4 km/s
5.2. Projectile remnants for oblique impacts
The mass m of the projectile is
To observe the bottom of the crater, the values of the tilt angles must surround the value of the angle of incidence of the projectile (for example Fig. 2k). But to study the walls of the craters, the stereoscopic views use pictures acquired by tilting the sample at 0°, 7° and 14°. Results differ from normal impacts.
m = 71.8 pg when v = 3 km/s. m = 28.5 pg when v = 4 km/s and
a When KE < 1 erg, we observe an amalgamation of pure iron droplets and iron-gold droplets partly surrounding the bottom of the crater (Fig.2k). On the wall some isolated large hemispherical droplets are observed downstream while a number of small ones are observed upstream. All these droplets are “alloys iron-gold” (Fig. 2l). b When the kinetic energy increases (1 erg < KE < 2 ergs), the amalgamation of droplets near the bottom of the crater disappear. We observed some large droplets downstream and a lot of small ones upstream. Most of the droplets are iron – gold “alloys” but some are pure iron. c When KE > 2 ergs no change is observed downstream but quasi no droplets are observed upstream. It should be recalled that part of the incident energy is used to generate spalls.
KE = 3.23 erg when v = 3 km/s. KE = 2.28 erg when v = 4 km/s The kinetic energy taken away by the spalls is included between 3.44 E−4 ergs and 3.44 E−2 ergs very faint values with respect to the KE of the projectile. The ratio M/m is greater in this case than in the first example because the decrease of the size of the projectile with the increase of its velocity due to experimental constraints. This explains the absence of ejected spalls when the velocity of the projectile is greater than 5 km/s. 5. Identification and evaluation of projectile remnants The study of projectile remnants concerns the craters created from incidence angles whose values are included between 0° and 60°. This preliminary study presents normal impacts and 45° incidence angle impacts as a representative case of oblique impacts. Five craters per velocity interval were analysed. Using the scanning facilities, a grid of
6. Conclusion This study concerns our understanding of the solid component of the space environment, from the morphology and the remnants of the 436
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Fig. 10. Impacts craters observed on solar cells from HST solar arrays.
craters on samples of materials retrieved after exposure to space. Laboratory simulations have been used to support this investigation. Using a SEM we have determined the detailed morphology of the impact craters and with an EDX spectrometer we have identified the remnant material of the projectile. The morphology of the craters depends on the properties of the projectiles and of the targets. In particular we have obtained the diameter and the depth of the craters which are the basis of the determination of projectile parameters. Circularity is useful in determining the angle of incidence of the projectile and consequently the source concerning secondary craters, if the craters are produced by ejecta. Moreover circularity could possibly be used for determining original particle orbits. Then the evolution of the crater size – projectile size ratio with impact velocity, permits to obtain the size of the projectile. If the EDX spectrometer has identified the remnant materials, the mass of the projectile is deduced and consequently leads to a better knowledge of the space environment, i.e. composition, size distribution and space distribution of the impactors. An estimation of the mass of material ejected upon impact is useful for the determination of the amount of orbital debris produced through ejecta process. The results are in agreement with those described by M. Rival [21].
Fig. 11. Profile of the crater and of the cavity left by an ejected spall in the downstream direction. The horizontal line shows the level of the target before impact. 437
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Table 2 Percentage in mass of atomic elements in each selected spectrum. Atomic element
O
Si
Fe
Au
Spectrum 1 Spectrum 2 Spectrum 3
46.36 25.97 49.22
40.00 61.34 48.59
0.00 4.71 0.32
13.64 7.98 1.88
Fig. 13. Line 20 × 1 of EDX spectrometric measurements on a crater and his neighbourhood.
With the EDX spectrometer we have detected the very small amount of projectile remnants. Modelling the sounded volume by the EDX spectrometer, we have been able to estimate the mass of the remnants. We have found that the amount of projectile residue varies with incidence angle and velocity so a better interpretation of oblique impacts is possible. Acknowledgements The authors greatly acknowledge Ralf Srama from the Max Planck Institute (MPI) for Nuclear Physics, Heidelberg (Germany) for the use of dust accelerator, and the support of Christian Durin from the Centre Natinal d'Etudes Spatiales (CNES), Toulouse, France. References [1] Orbital Debris, A Technical Assessment, NRC, National Academy Press, Washington, DC, 1995. [2] V. Adushkin, S. Veniaminov, S. Koslov, M. Silnikov, Orbital Missions safety- A survey of kinetic hazards, Acta Astronaut. (2016) 510–516. [3] N.N. Smirnov, A.B. Kiselev, M.N. Smirnova, V.F. Nitkin, Space traffic hazards from orbital debris mitigation strategies, Acta Astronaut. 109 (2015) 144–152. [4] M. Allbrooks, D. Atkinson, The magnitude of impact damage on LDEF materials, NASA CR (1992) 188258. [5] J.C. Mandeville, Impact detection in space : derivation of physical properties of meteoroids and debris, Adv. Space Res. 17 (12) (1996) 147–153. [6] J.C. Mandeville, J.M. Perrin, Loic Vidal, Experimental hypervelocity impacts: implications for the analysis of material retrieved after exposure to space environment, part I, Acta Astronaut. 81 (2012) 532–544. [7] H. Fechtig, E. Grün, J. Kissel, J.A.M. McDonnell (Ed.), Laboratory Simulations in Cosmic Dust, 1992, pp. 629–630. [8] N.N. Smirnov, K.A. Kondratyev, Evaluation of craters formation in hypervelocity impact of debris particles on solid structures, Acta Astronaut. 65 (2009) 1796–1803. [9] J. Peirs, P. Verleysen, W. Van Paepegem, J. Degrieck, Determining the stress-strain behavior at large strains from high strain rate tensile and shear experiments, Int. J. Impact Eng. 38 (2011) 406–415. [10] E.A. Taylor, Experimental and Computational Study of Hypervelocity Impact on Brittle Materials and Composites, PhD. Thesis University of Kent, Canterbury, 1997.
Fig. 12. Grid 10 × 10 of EDX spectrometric measurement.
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