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Hypervelocity impact of TiB2-based composites as front bumpers for space shield applications
Materials and Design 97 (2016) 473–482
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Hypervelocity impact of TiB2-based composites as front bumpers for space shield applications Xuegang Huang a,⁎,1, Chun Yin b,1, Jie Huang a, Xuezhong Wen a, Zhongmin Zhao c, Junyan Wu c, Sen Liu a a b c
Hypervelocity Aerodynamics Institute, China Aerodynamics Research and Development Center, No. 6 Erhuanlu Nanduan, Mianyang 621000, PR China School of Automation Engineering, University of Electronic Science and Technology of China, No. 2006 Xiyuan Road, Chengdu 611731, PR China Department of Vehicle and Electrical Engineering, Mechanical Engineering College, No. 97 Heping Xilu, Shijiazhuang 050003, PR China
a r t i c l e
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Article history: Received 28 January 2016 Received in revised form 25 February 2016 Accepted 28 February 2016 Available online 3 March 2016 Keywords: TiB2-based composites Hypervelocity impact Whipple shield Debris cloud
a b s t r a c t Based on the typical Whipple-type shield, a novel shield configuration with a front bumper of TiB2-based composite is designed in this paper, and the hypervelocity impact tests are conducted to compare the protection performance of this new composite bumper with that of the conventional aluminum bumper at the predicted impact velocities of about 3 km/s, 5 km/s and 7 km/s. The protection efficiency of different bumper materials is evaluated by the bumper perforation as well as the damage pattern of the rear wall and the witness plate. The postmortem observations of macroscopic and microscopic damage morphology reveal that the TiB2-based composite bumper can diminish the damage to the rear wall and exhibits better protective performance than the aluminum bumper, especially when the impact velocity reaches 5 km/s or more. Analyses of the shadowgraph of debris indicate that the kinetic energy of debris cloud is distributed among a large number of ultrafine particles, therefore, the wellfractured projectile fragment and widespread impact area can be taken as the main reasons for the better performance of TiB2-based composite bumpers. Besides, preliminary researches demonstrate some positive application of this new bumper material in protective systems for micrometeoroid and orbital debris shielding. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Since humans have always been aspiring to explore the space, numerous spacecrafts and payloads have been launched into space over the years, leading to the creation of orbital debris around the Earth as well as the meteoroid hazard [1]. Nowadays spacecrafts are threatened by the increasing probabilities of micro-meteoroid and orbital debris (MMOD) impact damage, which can potentially degrade performance, shorten the service life, or result in catastrophic accidents [2]. Some specific protection systems have been developed to ensure that spacecrafts meet desired safety and mission success goals [3]. Whipple shield, as a typical protective structure was first proposed by the well-known astrophysicist Fred Whipple in the 1940s, which have been widely applied to protecting spacecrafts from MMOD during the last decade [4–6]. The conventional Whipple shield consists of a thin front bumper followed at a standoff distance from the rear wall. The function of the front bumper is to break up the projectile into a debris cloud within both the projectile and bumper fragments. Because the impact momentum of the projectile can be distributed into an expanding cloud, the extensive impact area of debris cloud can diminish the damage to the rear wall [7]. ⁎ Corresponding author. E-mail address: [email protected] (X. Huang). 1 Xuegang Huang and Chun Yin contributed equally to this work.
http://dx.doi.org/10.1016/j.matdes.2016.02.126 0264-1275/© 2016 Elsevier Ltd. All rights reserved.
Based on similar protection principles of Whipple shield, some novel shield configurations, such as Stuffed Whipple and Multi-Shock shield, have been developed accordingly [6]. A series of hypervelocity impact experiments have been performed to evaluate the potential of various enhanced configurations (monolithic plates, double-bumper, multibumper, stuffed layer, oblique plate, rigid meshes) for MMOD shielding [8–11]. At present a larger number of researches indicate that the protection efficiency of Whipple shield can be significantly influenced by its geometry configuration (e.g. standoff distance, oblique angle, bumper thickness) and material properties (e.g. density, modulus of elasticity, acoustic velocity) [3]. Because the front bumper has a great effect on the fragmentized degree of the projectile at the initial hypervelocity impact as well as the characteristic of debris cloud formation, it is crucial to choose proper bumper material for shield configuration with better protective performance. The aluminum alloy is recognized as a conventional bumper material of Whipple shield for its high strength-toweight ratio [12]. However, with the rapid development of space exploration activities, high-performance shielding materials will be crucially needed to meet higher protection requirements in the future [13,14]. To achieve this, some innovative materials have been developed, such as advanced ceramic materials, polymers materials, amorphous alloy and composite materials [15–19]. Meanwhile, the ballistic performance and the potential application of these innovative shielding materials have also been discussed [20–25].
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Compared with the conventional single-phase ceramics, TiB2-based composites exhibit not only higher hardness and better chemical stability at high temperatures, but also possess enhanced fracture toughness and bending strength [26–29]. Moreover, the density of TiB2-based composites is about 4.3 g/cm3, which is much lower than that of ordinary metal and alloy materials, and just slightly higher than that of aluminum alloy. In this experiment, a thin plate of novel TiB2-based composite is proposed as front bumper of Whipple shield, and several hypervelocity impact tests are conducted to evaluate the performance of this new bumper material. In order to better understand the protection efficiency and damage mechanism of this new bumper material, comparison tests with aluminum alloy bumper are also arranged. The damage characteristics of the shields of different bumper materials are discussed. For a further understanding of the relationship between debris cloud formation and damage mechanism, a Hypervelocity Sequence Laser Shadowgraph Imager is carried out to record the formation process of debris cloud. 2. Experimental procedure 2.1. Material preparation Combustion synthesis in high-gravity fields is a novel and economical preparation technology which has been applied to prepare refractory metals and ceramic composites with high densification [9,10]. The TiB2-based composites used as front bumper in the experiment are fabricated by combustion synthesis in high-gravity fields. SEM and XRD results show that the TiB2-based composite is composed of fine TiB2 platelets as the primary phase, irregular TiC grains as the secondary phase and Ni inter-metallic compound as inter-crystalline phase, as is shown in Fig. 1. The molar ratio of TiB2 to TiC is measured to be 2:1. About 5 wt% Ni is added as the toughener to increase densification and toughness. The density, hardness, fracture toughness and flexural strength of the TiB2-TiC ceramic composite measure respectively 4.3 g/cm3, 21 ± 1.5 GPa, 11.5 ± 1.8 MPa·m0.5 and 800 ± 25 MPa. 2.2. Hypervelocity impact experiments Spacecraft impact tests are carried out on a ballistic range of China Aerodynamics Research and Development Center (CARDC), the
launcher of the range is a 7.62 mm calibre two-stage light-gas gun (LGG). The high-energy propellant powder in powder chamber is driven by gunpowder, and then the combustion gas of the propellant powder pushes a piston to compress hydrogen gas in the first stage. Then the highly compressed hydrogen gas accelerates a projectile (which mounted in a sabot) in the second stage right after the high-pressure gas blast a diaphragm between the stages. The projectile can be launched up to 7.36 km/s by this LGG. Impact velocity of the projectile can be derived by the three laser-photodetector stations installed at various locations along the flight direction. Passage of the projectile through a laser beam leads to a momentary drop in the output signal of the photodetector, then the time interval between the electrical pulses is obtained to calculate the projectile velocity between any pair of laser-photodetector stations. Measurement accuracy of the impact velocity is better than ±0.3%. The LY12 aluminum alloy spheres with a constant diameter of 3 mm are used as the projectiles, the density of aluminum alloy projectile is 2.79 g/cm3. The spherical projectiles strike the bumper at a normal angle (θ = 0°, θ is the impact angle), and the expected launch speeds are set as 3 km/s, 5 km/s and 7 km/s. The expansion process of the debris cloud produced behind the bumper is recorded by Hypervelocity Sequence Laser Shadowgraph Imager. A detailed listing of the materials used in the tests and parameters of the experimental procedures is given in Table 1. The Al-6061 aluminum alloy is selected as bumper plates and rear wall plates, the density and the yield strength of Al-6061 are 2.73 g/cm3 and 40 ksi. Fig. 2(a) is a photograph (top view) of a typical Whipple shield structure with aluminum bumper for our hypervelocity impact experiments, while a monolithic Al-6061 bumper is used as the front bumper in this protective configuration, and the aluminum bumper is 1 mm thick. The relative distances between the plates are d1 and d2, as is shown in Fig. 2(a). In all the impact experiments, d1 = 80 mm, and d2 = 40 mm. The rear wall is 2 mm thick, and the witness plate is 1 mm thick. The rear wall and the witness plate are 200 mm × 200 mm squares, and all aluminum plates are Al-6061 alloy. The arrow in Fig. 2(a) indicates the impact direction of projectile relative to the target plate. Fig. 2(b) is the photograph (front view) of a TiB2-based composite enhanced Whipple Shield structure for our hypervelocity impact experiment. This experimental configuration is similar to the aluminum bumper configuration mentioned in Fig. 1, but the monolithic Al-6061 front bumper is replaced by a TiB2-based composite bumper in this assemble device. Other aluminum plates measuring 200 mm (width) × 200 mm (length) are Al-6061 alloy too. The TiB2-based composite discs are cut and ground into thin plates measuring 60 mm (width) × 60 mm (length) × 0.65 mm (thickness). Then the TiB2-based composite plate is fixed into a pair of clamping plates within a centre hole of 55 mm in diameter. The size of the clamping plate is 200 mm (width) × 200 mm (length). The TiB2-based composite plate with a thickness of 0.65 mm and the aluminum plate with a thickness of 1 mm are chosen as the front bumper respectively, for the purpose of comparison. Compared with the monolithic aluminum bumper, the TiB2-based composite bumper proves having nearly the same areal density, which will enhance the credibility of the comparison of TiB2-based composite bumpers and aluminum bumpers for space debris protection. 3. Result and discussion 3.1. Macroscopic damage morphology
Fig. 1. The SEM microstructure of the TiB2-based composite.
In all hypervelocity impact experiments, both the aluminum bumper and the TiB2-based composite bumper are penetrated by spherical projectiles. The penetrated holes are found in the rear wall behind the aluminum bumper and the composite bumper at the predicted impact velocity of 3 km/s. However, no penetrated holes are found in the rear
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Table 1 Hypervelocity impact test parameters. Shot no.
1-1
1-2
1-3
2-1
2-2
2-3
Bumper material Impact velocity (km/s) Projectile weight (g) Projectile diameter (cm) Bumper thickness (mm) Rear wall thickness (mm) Witness panel thickness (mm)
Al-6061 3.00 0.0393 0.30 1 2 1
Al-6061 5.11 0.0394 0.30 1 2 1
Al-6061 7.32 0.0395 0.30 1 2 1
Composite 3.15 0.0392 0.30 0.65 2 1
Composite 5.2 0.0390 0.30 0.65 2 1
Composite 7.22 0.0391 0.30 0.65 2 1
walls of other tests done at the predicted impact velocity of 5 km/s and 7 km/s. Based on the postmortem observation of damages to the front bumpers, rear walls and witness plates, it can be concluded that the novel TiB2-based composite bumper exhibits better performance than aluminum bumpers at any predicted impact velocity of 3 km/s–7 km/ s, and the protection efficiency of the Whipple shields with TiB2-based composite bumpers is significantly improved with the impact velocity increasing. The differences between the performances of the two shields are apparent. Fig. 3 shows the damage features of the bumpers made of different materials after hypervelocity impact tests. The penetrated holes produced in the aluminum bumpers are larger than the ones in TiB2based composite bumpers at predicted impact velocities of 3 km/s– 7 km/s. As a result of the plastic deformation of aluminum alloy, there is curling formed around the penetrated hole of aluminum bumpers. However, such curling cannot be spotted in TiB2-based composite bumpers. The diameter of the penetrated hole in aluminum bumper increases from the low impact velocity to higher ones, and the curling around the penetrated hole is more obvious with the increasing impact velocities too. In the case of TiB2-based composite bumper (Fig. 3(d, e, f)), a penetrated hole surrounded by circular fracture zone is observed in the experiment. But all TiB2-based composite bumpers remain structurally integrate except for these penetrated holes. The diameter of the penetrated hole in TiB2-based composite bumper increases slightly from the low impact velocity to higher ones. Moreover, the width of the circular fracture zone around the penetrated hole is calculated as 2.31 mm at 3.15 km/s, 2.13 mm at 5.2 km/s, and 2.17 mm at 7.22 km/ s, respectively. There is no obvious change when the impact velocity increases, and those slight differences in width can be ascribed to measurement errors. After the hypervelocity impact tests, a debris cloud containing fragments of the projectile and bumper materials forms behind the bumper, which results in the damages of penetrations or craters to the rear wall. The protection efficiency of different bumper materials can be evaluated
by the damage pattern of the rear wall, such as the penetrated hole, the characteristics of bumps, the depth of maximum craters and the size distribution of the craters. Fig. 4 shows the front surface of the rear wall obtained in each impact test, while Fig. 5 shows the back surface of it. Table 2 presents the statistical data of the damage patterns on the rear wall and the witness plate. Damage patterns on the rear wall can be divided into three categories: (1) Penetrated hole, which is always in the centre of the crater zone; (2) Main crater (diameter N 1 mm), which is obviously larger than other craters and is always in the centre region of the crater zone; (3) Micro-crater (diameter b 1 mm), which is widespread among the main craters and outside the region of the crater zone. The statistical results indicate that the damage degree of the rear wall behind composite bumpers is lower than that of the rear wall behind the aluminum bumpers. At predicted impact velocity of 3 km/s, the complete penetration on the rear wall is observed in each test, the penetrated hole in the rear wall behind Al-6061 bumper is larger than the one in the rear wall behind TiB2-based composite bumper. Meanwhile, the damage state of the witness plates behind the rear walls also confirms the better protection efficiency of shielding configuration with TiB2based composite bumpers. According to Table 1, the number of main craters (diameter N 1 mm) in the rear wall behind composite bumpers is obviously less than the ones in the rear wall with Al-6061 bumpers at the predicted impact velocity of 5 km/s or 7 km/s. Compared with the back of the rear wall with Al-6061 bumpers, the bump number of the back of the rear wall with TiB2-based composite bumpers is significantly reduced at any impact velocity. When it comes to the damage pattern of the rear wall with TiB2based composite bumpers at the hypervelocity impact of 3.15 km/s, lots of micro-craters (diameter b 1 mm), on the other hand, are observed apart from some major craters on the front surface of the rear wall. And the diameter of the micro-crater distribution zone is measured as 100 mm. Similar phenomenon can also be observed on the rear wall with composite bumpers at the impact velocity of 5.2 km/s
Fig. 2. Experimental configurations with different bumpers.
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Fig. 3. Penetrated holes in the front bumper.
or 7.2 km/s, only, the distribution zone of micro-craters is larger than the ones formed at a lower impact velocity of 3.15 km/s. Increased number of micro-craters are observed at higher impact velocities. By examining these micro-craters with microscope, it can be found that a little composite bumper element exist at the bottom of some micro-craters. This phenomenon indicates that these micro-craters are mainly produced by the impact of composite bumper debris. These micro-craters are also found on the surface of the rear wall with Al-6061 bumpers at
the impact velocity of 5.11 km/s to 7.32 km/s. However, the number of micro-craters on the rear wall with Al-6061 bumpers is much less than that on the rear wall with composite bumpers at the same impact velocity. Moreover, on the surface of the rear wall with Al-6061 bumpers at the impact velocity of 3 km/s, micro-craters are not spotted. Fig. 6 shows the damage pattern on the rear wall of the compositeenhanced shield used for the 7.2 km/s impact test, which presents three concentric circular zones with different crater characteristics.
Fig. 4. Front surface of the rear wall after impact (plate size: 200 mm × 200 mm).
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Fig. 5. Rear surface of the rear wall after hypervelocity impact (plate size: 200 mm × 200 mm).
There are very clear boundaries among the centre damage region, the intermediate circular band, and the outer circular band, as marked in Fig. 6. The radius of each region is measured as about 25 cm, 50 cm and 90 cm, respectively. In the case of the centre damage region (Fig. 6(a)), the whole centre surface of the rear wall is covered by a lot of contiguous craters. Meanwhile, melting feature within a smooth inner surface and bottom surface of the crater is evident at the centre circular region, which indicates that these contiguous craters are mainly produced by the impact of the molten aluminum droplets from the broken projectile. In the intermediate circular band (Fig. 6(b)), the damage pattern is mainly composed of a large number of micro-craters formed by the impact of solid composite bumper fragments as well as a small amount of main-craters formed by the impact of molten aluminum droplets. When it comes to the outside circular band, a few scattered micro-craters are observed, which result from the impact of the solid composite bumper powders. Based on the above postmortem observations, it can be concluded that, at similar impact velocity, TiB2-based composite bumpers can break up the projectile adequately and exhibit better protective performance, especially when the impact velocity reaches 5 km/s or more. These damage patterns also clearly illustrate the effect of a change in the bumper material on the damage behavior produced by the impact of debris clouds.
3.2. Microscopic damage morphology The microscopic damage patterns produced both on the front and the rear surfaces of TiB2-based composite bumpers under hypervelocity impact are shown in Fig. 7 and Fig. 8. At the impact velocity of 3.15 km/s, the front fracture morphology of the penetrated hole exhibits a mixed fracture mode of the pull-out grooves of TiB2 grains and the transgranular fractures of the TiC/Ni phases, as is shown in Fig. 7(a). And the whole fracture surface remains clear and clean, except for a few of scattered debris sticking on the inner surface of the penetrated hole, which are identified as aluminum projectile fragments based on EDS mapping results in Fig. 8. As the impact velocity increases from 3.15 km/s to 5.2 km/s and 7.22 km/s, the fracture morphology of the penetrated hole becomes more granular and rougher, and obvious intergranular cracks are produced between TiB2 crystals. In addition, a lot of refined debris and particles are obviously observed on the fracture surface of the penetrated hole, which is also considered to be the initial impact product of aluminum projectile on the composite bumper. As is shown in Fig. 9 (a), the rear fracture surface of the penetrated hole is clear and clean at the impact velocity of 3.15 km/s. However, when the impact velocity amounts to 5.2 km/s (Fig. 9 (b)), the back fracture surface of the penetrated hole is covered by solid particles and molten products, such as spherical and discoid solidification products.
Table 2 The statistical data of damage patterns on the rear wall and the witness plate. Shot no.
1-1
1-2
1-3
2-1
2-2
2-3
Bumper material Impact velocity (km/s) Perforation number Perforation diameter (mm) Main crater number (ϕ N 1 mm) Max crater diameter (mm) Max crater depth (mm) Bump number of back surface Witness panel crater number
Al-6061 3.00 2 2.8/2.4 86 – – 13 5
Al-6061 5.11 0 – 425 1.8 1.5 103 –
Al-6061 7.32 0 – 533 1.1 0.75 287 –
Composite 3.15 1 2.9 44 – – 5 3
Composite 5.2 0 – 135 1.9 1.2 32 –
Composite 7.22 0 – 0 0.9 0.4 3 –
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Fig. 6. Views of sections of damage patterns formed on front rear-wall surface for the composite bumper test (V = 7.22 km/s).
When the impact velocity reaches 7.22 km/s, the number and distribution of solidification covering will be increased greatly. Moreover, some larger discoid solidification products are also observed in this area, as is shown by the arrows in Fig. 9(c), which are identified as the splashed patterns of the molten aluminum droplets by EDS in Fig. 10. In order to identify the origin of these molten aluminum droplets covered the surface of the penetrated hole at the impact velocity of 7.22 km/s, EDS is conducted to examine the Al element distribution on the rear surface of the composite bumper, as is shown in Fig. 11. EDS result reveals that Al elements deposit mainly on the fracture zone around the penetrated hole, so it can be inferred that the solidification aluminum production covering the fracture zone is mainly from the molten fragments of aluminum projectile under the initial impact on the composite bumper. Moreover, some larger discoid solidification products are observed both on the fracture zone and undamaged surface of the composite bumper, which are mainly produced by the secondary impact of molten aluminum droplets bouncing back from the rear wall.
SEM and EDS results of the composite bumper after impact tests indicate that as the impact velocity increases, the composite bumper material around the penetrated hole is more severely fractured, and the aluminum projectile can be broken or even molten sufficiently. It may indicate higher temperature rise of aluminum projectile during the penetration progress in TiB2-based composite bumper at the velocity of N5 km/s, and the high impact temperature causes the molten products of the aluminum projectile, but it doesn't result in visible molten products of composite bumper due to the extreme high melting temperature of TiB2 (~2980 °C). 3.3. Debris cloud formation The damage characteristics of the rear wall behind the bumper depend on the morphological feature of the debris cloud produced by hypervelocity impact [30]. Fig. 12(a) presents a typical feature of debris clouds produced by the impact of Al sphere with Al-6061 bumper at
Fig. 7. The fracture morphology of the front perforation surface in TiB2-based composite bumper.
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Fig. 10. The Al element distribution on the fracture zone of the perforation backside (V = 7.22 km/s). Fig. 8. The Al element distribution on the front surface of penetrated holes in TiB2-based composite bumper (V = 3.15 km/s).
the impact velocity of 7.32 km/s. According to the results and analyses in reference [31], the debris cloud can be described as possessing three major structural features: (1) the ejecta veil located ahead of the bumper is entirely composed of bumper fragments from the impact fracture surface of the front bumper; (2) the external bubble of debris is of a kind of significant thin shell structure, which mainly consists of the bumper debris produced from the rear fracture surface of the bumper; (3) the internal structure located inside and at the front of the expanding bubble of bumper debris is mainly composed of the projectile debris. As shown by the laser shadowgraph images of the debris cloud in Fig. 12(a), a large number of visible solid slivers from broken aluminum bumper are observed in the ejecta veil and the external bubble, and the bulk of the broken projectile fragments mass appears to be concentrated in the internal structure which results in the primary serious damage to the rear wall. The effect of a change in the bumper material on the morphology of debris cloud is shown in Fig. 12(b). The debris cloud shown in this laser shadowgraph image is produced by the impact of an aluminum sphere with a TiB2-based composite bumper at an impact velocity of 7.22 km/s. Significant changes in debris-cloud properties are observed when the bumper material is changed from Al-6061 to TiB2-based composite. This debris cloud in Fig. 12(b) can be described as having three major structural features too. However, instead of the solid slivers of the ejecta veil and the external bubble of debris in the configuration with aluminum bumper, the obvious powdery cloud within a large number of ultrafine particles is formed in this composite-enhanced configuration, which is identical to the severe fractured perforation surface of the composite bumper in Fig. 7(c). Moreover, the internal structure located both
inside and at the front of the expanding bubble of bumper debris is obviously extended in Fig. 12(b), as contributes to make the bulk of the broken projectile fragment mass appear to be fairly well-dispersed in a wider shell of the internal structure. Taking into consideration the microscopic damage morphology of the composite bumper material and the impact characteristic of the craters on the rear wall in Fig. 6, it can be inferred that the observed centre damage region on the rear wall is the result of the impact of the large fragment concentration. Because the kinetic energy of debris cloud scatters among a large number of ultrafine particles, the primary damage of the powdery cloud to the rear wall is accordingly diminished. The same protection mechanism can also be applicable to the impact tests of protection configuration with TiB2-based composite bumper at the impact velocity of 5.2 km/s. At impact velocity about 3 km/s, the front head of debris cloud is composed of bulk fragments of aluminum projectile, as is shown in Fig. 13(a). The main projectile mass shown by the arrows is viewed to be concentrated in the cloud head, resulting in the remarkable impact craters or penetrated holes on the rear wall. For the shielding configuration with TiB2-based composite bumper, the size of main fragments in the front head of debris cloud appears to diminish in Fig. 13(b). The external bubble of debris cloud comprises of lots of fine bumper powder and does not appear to have the ability to produce any significant crater on the rear wall. When the impact velocity decreases to about 3 km/s, the improvement of the protection efficiency of TiB2-based composite bumper to the rear wall is very limited. This is because the TiB2-based composite bumper can't break the projectile into smaller fragments or powders, even though its protection efficiency is slightly better than that of Al-6061 bumper. As has been shown above, the shape and characteristics of debris cloud are determined by bumper material and impact velocity, as is
Fig. 9. The fracture morphology of the rear perforation surface in TiB2-based composite bumper.
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regime, the critical projectile diameter dc can be calculated as: " dc ¼
t w ðσ =40Þ1=2 þ t b
#18=19 ð1Þ
1=2
0:6ð cos θÞ5=3 ρp V 2=3
where ( V LV ¼
1=3 ; for t b =dp b0:16 1:436 t b =dp 2:60; for t b =dp ≥0:16
ð2Þ
and dc is the critical projectile diameter (cm), tb is the bumper thickness (cm), tw is the rear wall thickness (cm), ρp is the density of the projectile material (g/cm3), σ is the yield strength of the rear wall material (ksi), and dp is the diameter of the projectile. The ballistic limit equation in the hypervelocity regime is given by: Fig. 11. The Al element distribution on the rear perforation surface of composite bumper (V = 7.22 km/s).
crucial to the damage pattern of the rear wall. For the traditional allaluminum Whipple-type shield, the ballistic limit evaluation can be described as three parts, each of which corresponds to a projectile state following impact velocity on bumper plate [32,33]. The low-velocity regime (V ≤VLV/ cos θ ) is defined for impacts in which the projectile perforates the bumper plate without fragmenting, resulting in the impact of an intact (albeit deformed) projectile on the shield rear wall. Once the shock pressure of increased impact velocity is sufficient to induce projectile fragmentation (and eventually melting), these impacts are termed the shatter regime (or intermediate regime). Within the shatter regime, increases in impact velocity could fragment the projectile sufficiently, which lead to a more equally dispersed debris cloud of smaller fragments and particles, thus the damage to the rear wall is mitigated accordingly, and the shielding performance is promoted progressively. The hypervelocity regime (V ≥ VHV/ cos θ) is defined for impacts in which the increased impact velocity of projectile would bring about the further damage to the rear wall shield due to the increased lethality of fragment cloud. The ballistic limit equation of Whipple shield (single Al-bumper) can be established using the JSC Whipple Equation [33]. In the low velocity
dc ¼ 3:918F 2
1=3 2=3 t w s1=3 σ y =70 1=3 1=9
2=3
ρp ρb ðV cos θÞ
ð3Þ
where ρb is the density of the bumper material (g/cm3) and S is the standoff between the bumper and rear wall (cm). The term F2⁎ represents a de-rating factor for configurations with insufficiently thick bumpers. For the shatter regime, the ballistic limit can be calculated by linear interpolation: dc ¼ dc ðV LV Þ þ
½dc ðV HV Þ dc ðV LV Þ ðV V LV Þ V HV V LV
ð4Þ
For conventional aluminum bumper, impact regime transition velocities (VLV, VHV) are dependent on the outer bumper and projectile material, defined as about 3 km/s and 7 km/s [32]. Due to the actual impact velocities in this paper exceed the low-velocity regime based on the Eq. 2 (i.e. V N VLV =2.6, for tb/dp ≥ 0.16), all the aluminum projectiles could be fragmented as a result of sufficient shock pressure, this explains why there is no intact projectiles are found in the laser shadowgraph images of the debris cloud produced by hypervelocity impacts. Meanwhile, according to the above equations, we can also obtain the critical projectile diameter dc of the all-aluminum configuration under
Fig. 12. Laser shadowgraph images of the debris cloud formed by hypervelocity impact. (a) Al-6061 bumper, V = 7.32 km/s (b) TiB2-based composite bumper, V = 7.22 km/s.
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Fig. 13. Laser shadowgraph images of the debris cloud formed by hypervelocity impact. (a) Al-6061 bumper, V = 3.0 km/s (b) TiB2-based composite bumper, V = 3.15 km/s.
the impact tests in this paper: 8 < 0:16 cm; for V ¼ 3:00 km=s dc ¼ 0:29 cm; for V ¼ 5:11 km=s : 0:46 cm; for V ¼ 7:32 km=s
ð5Þ
Since the projectiles with a constant diameter of about 3 mm are used in the impact tests, which is far exceeds the critical projectile diameter dc under the impact velocity of 3 km/s, it pretty easy to understand why the rear wall is penetrated by the debris cloud under the impact velocity of 3 km/s. On the other hand, the actual size of the projectile is far less than the critical projectile diameter dc under the impact velocity of 7.32 km/s, so the shield configuration represents appropriate protection performance. When it comes to the impact velocity of 5.11 km/s, due to the calculated critical projectile diameter dc is much closed to the actual size of the projectile, which means that the rear wall of shield configuration may be perforated or near perforated, the results of impact test also confirm the above-mentioned discuss. For non-aluminum plate bumpers, an equivalent plate thickness tb,eq is calculated from the areal densities of the actual bumper plate, i.e.: t b;eq ¼ t Comp
ρComp ρAl
ð6Þ
As the areal density of the actual TiB2-based composite bumper plate has been concerned in this paper, the TiB2-based composite bumper with 0.65 mm thick is equivalent to the aluminum bumper with 1 mm thick, therefore, the critical projectile diameter d⁎c of the novel configuration with TiB2-based composite bumper could also be calculated as follows: dc
8 < 0:17 cm; for V ¼ 3:15 km=s ¼ 0:29 cm; for V ¼ 5:20 km=s : 0:46 cm; for V ¼ 7:22 km=s
ð7Þ
We can found that d⁎c of enhanced shielding configuration is roughly equivalent to dc of all-aluminum configuration under the similar impact velocity. Despite the critical projectile diameter and the similar actual projectile size, the TiB2-based composite enhanced shield configuration could reduce the damage to the rear wall efficiently, especially for that the actual projectile diameter reach the critical projectile diameter, the TiB2-based composite bumper obviously relieve the damage to the rear wall. In conclusion, since TiB2-based composite bumper can
sufficiently break up the projectile under hypervelocity impact, the impactor momentum of the projectile fragment can be distributed into the widespread small-size debris or particles, resulting in a wider impact area of the rear wall, so that the configuration with TiB2-based composite bumper displays a better protective performance. 4. Conclusion The potential application of single-shield front bumpers made of TiB2-based composites is discussed for a Whipple-type shield in this paper. Hypervelocity impact experiments are conducted to compare the protection efficiency of front composite bumpers with that of conventional aluminum bumpers by normal impact of spherical aluminum projectile with the predicted velocity of 3 km/s, 5 km/s and 7 km/s. The 2 mm-thick rear-wall plate and 1 mm-thick witness plate are mounted behind the bumper, and the size and distribution of craters on the rearwall plate are taken as a criterion for evaluating different bumper materials. TiB2-based composite bumpers prove to display better protection efficiency than traditional aluminum bumpers when the impact velocity increases. The debris cloud propagation behavior induced by a hypervelocity impact is recorded by Hypervelocity Sequence Laser Shadowgraph Imager. Based on the shadowgraphs of debris clouds generated behind the bumpers as well as the fracture morphologies of the penetrated holes in the bumpers, the debris clouds with the powdery microstructure are found to be generated behind the composite bumpers, and the fragments and spalls will be broken into much smaller pieces than by traditional aluminum bumpers. That's why the impact damage on the rear wall behind composite bumpers can be obviously reduced. Therefore, these preliminary experiments just demonstrate the potential applications of TiB2-based composites in the space systems for the impact protection of micro-meteoroid and orbital debris. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51502338 and 61503064). References [1] J.C. Liou, N.L. Johnson, Risks in space from orbiting debris, Science 311 (2006) 340–341. [2] J. Hyde, E.L. Christiansen, R.P. Bernhard, J.H. Kerr, D.M. Lear, A history of meteoroid and orbital debris impacts on the space shuttle, Proceedings of the Third European Conference on Space Debris, ESA SP-473, 2001.
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Hypervelocity impact of TiB2-based composites as front bumpers for space shield applications Xuegang Huang a,⁎,1, Chun Yin b,1, Jie Huang a, Xuezhong Wen a, Zhongmin Zhao c, Junyan Wu c, Sen Liu a a b c
Hypervelocity Aerodynamics Institute, China Aerodynamics Research and Development Center, No. 6 Erhuanlu Nanduan, Mianyang 621000, PR China School of Automation Engineering, University of Electronic Science and Technology of China, No. 2006 Xiyuan Road, Chengdu 611731, PR China Department of Vehicle and Electrical Engineering, Mechanical Engineering College, No. 97 Heping Xilu, Shijiazhuang 050003, PR China
a r t i c l e
i n f o
Article history: Received 28 January 2016 Received in revised form 25 February 2016 Accepted 28 February 2016 Available online 3 March 2016 Keywords: TiB2-based composites Hypervelocity impact Whipple shield Debris cloud
a b s t r a c t Based on the typical Whipple-type shield, a novel shield configuration with a front bumper of TiB2-based composite is designed in this paper, and the hypervelocity impact tests are conducted to compare the protection performance of this new composite bumper with that of the conventional aluminum bumper at the predicted impact velocities of about 3 km/s, 5 km/s and 7 km/s. The protection efficiency of different bumper materials is evaluated by the bumper perforation as well as the damage pattern of the rear wall and the witness plate. The postmortem observations of macroscopic and microscopic damage morphology reveal that the TiB2-based composite bumper can diminish the damage to the rear wall and exhibits better protective performance than the aluminum bumper, especially when the impact velocity reaches 5 km/s or more. Analyses of the shadowgraph of debris indicate that the kinetic energy of debris cloud is distributed among a large number of ultrafine particles, therefore, the wellfractured projectile fragment and widespread impact area can be taken as the main reasons for the better performance of TiB2-based composite bumpers. Besides, preliminary researches demonstrate some positive application of this new bumper material in protective systems for micrometeoroid and orbital debris shielding. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Since humans have always been aspiring to explore the space, numerous spacecrafts and payloads have been launched into space over the years, leading to the creation of orbital debris around the Earth as well as the meteoroid hazard [1]. Nowadays spacecrafts are threatened by the increasing probabilities of micro-meteoroid and orbital debris (MMOD) impact damage, which can potentially degrade performance, shorten the service life, or result in catastrophic accidents [2]. Some specific protection systems have been developed to ensure that spacecrafts meet desired safety and mission success goals [3]. Whipple shield, as a typical protective structure was first proposed by the well-known astrophysicist Fred Whipple in the 1940s, which have been widely applied to protecting spacecrafts from MMOD during the last decade [4–6]. The conventional Whipple shield consists of a thin front bumper followed at a standoff distance from the rear wall. The function of the front bumper is to break up the projectile into a debris cloud within both the projectile and bumper fragments. Because the impact momentum of the projectile can be distributed into an expanding cloud, the extensive impact area of debris cloud can diminish the damage to the rear wall [7]. ⁎ Corresponding author. E-mail address: [email protected] (X. Huang). 1 Xuegang Huang and Chun Yin contributed equally to this work.
http://dx.doi.org/10.1016/j.matdes.2016.02.126 0264-1275/© 2016 Elsevier Ltd. All rights reserved.
Based on similar protection principles of Whipple shield, some novel shield configurations, such as Stuffed Whipple and Multi-Shock shield, have been developed accordingly [6]. A series of hypervelocity impact experiments have been performed to evaluate the potential of various enhanced configurations (monolithic plates, double-bumper, multibumper, stuffed layer, oblique plate, rigid meshes) for MMOD shielding [8–11]. At present a larger number of researches indicate that the protection efficiency of Whipple shield can be significantly influenced by its geometry configuration (e.g. standoff distance, oblique angle, bumper thickness) and material properties (e.g. density, modulus of elasticity, acoustic velocity) [3]. Because the front bumper has a great effect on the fragmentized degree of the projectile at the initial hypervelocity impact as well as the characteristic of debris cloud formation, it is crucial to choose proper bumper material for shield configuration with better protective performance. The aluminum alloy is recognized as a conventional bumper material of Whipple shield for its high strength-toweight ratio [12]. However, with the rapid development of space exploration activities, high-performance shielding materials will be crucially needed to meet higher protection requirements in the future [13,14]. To achieve this, some innovative materials have been developed, such as advanced ceramic materials, polymers materials, amorphous alloy and composite materials [15–19]. Meanwhile, the ballistic performance and the potential application of these innovative shielding materials have also been discussed [20–25].
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Compared with the conventional single-phase ceramics, TiB2-based composites exhibit not only higher hardness and better chemical stability at high temperatures, but also possess enhanced fracture toughness and bending strength [26–29]. Moreover, the density of TiB2-based composites is about 4.3 g/cm3, which is much lower than that of ordinary metal and alloy materials, and just slightly higher than that of aluminum alloy. In this experiment, a thin plate of novel TiB2-based composite is proposed as front bumper of Whipple shield, and several hypervelocity impact tests are conducted to evaluate the performance of this new bumper material. In order to better understand the protection efficiency and damage mechanism of this new bumper material, comparison tests with aluminum alloy bumper are also arranged. The damage characteristics of the shields of different bumper materials are discussed. For a further understanding of the relationship between debris cloud formation and damage mechanism, a Hypervelocity Sequence Laser Shadowgraph Imager is carried out to record the formation process of debris cloud. 2. Experimental procedure 2.1. Material preparation Combustion synthesis in high-gravity fields is a novel and economical preparation technology which has been applied to prepare refractory metals and ceramic composites with high densification [9,10]. The TiB2-based composites used as front bumper in the experiment are fabricated by combustion synthesis in high-gravity fields. SEM and XRD results show that the TiB2-based composite is composed of fine TiB2 platelets as the primary phase, irregular TiC grains as the secondary phase and Ni inter-metallic compound as inter-crystalline phase, as is shown in Fig. 1. The molar ratio of TiB2 to TiC is measured to be 2:1. About 5 wt% Ni is added as the toughener to increase densification and toughness. The density, hardness, fracture toughness and flexural strength of the TiB2-TiC ceramic composite measure respectively 4.3 g/cm3, 21 ± 1.5 GPa, 11.5 ± 1.8 MPa·m0.5 and 800 ± 25 MPa. 2.2. Hypervelocity impact experiments Spacecraft impact tests are carried out on a ballistic range of China Aerodynamics Research and Development Center (CARDC), the
launcher of the range is a 7.62 mm calibre two-stage light-gas gun (LGG). The high-energy propellant powder in powder chamber is driven by gunpowder, and then the combustion gas of the propellant powder pushes a piston to compress hydrogen gas in the first stage. Then the highly compressed hydrogen gas accelerates a projectile (which mounted in a sabot) in the second stage right after the high-pressure gas blast a diaphragm between the stages. The projectile can be launched up to 7.36 km/s by this LGG. Impact velocity of the projectile can be derived by the three laser-photodetector stations installed at various locations along the flight direction. Passage of the projectile through a laser beam leads to a momentary drop in the output signal of the photodetector, then the time interval between the electrical pulses is obtained to calculate the projectile velocity between any pair of laser-photodetector stations. Measurement accuracy of the impact velocity is better than ±0.3%. The LY12 aluminum alloy spheres with a constant diameter of 3 mm are used as the projectiles, the density of aluminum alloy projectile is 2.79 g/cm3. The spherical projectiles strike the bumper at a normal angle (θ = 0°, θ is the impact angle), and the expected launch speeds are set as 3 km/s, 5 km/s and 7 km/s. The expansion process of the debris cloud produced behind the bumper is recorded by Hypervelocity Sequence Laser Shadowgraph Imager. A detailed listing of the materials used in the tests and parameters of the experimental procedures is given in Table 1. The Al-6061 aluminum alloy is selected as bumper plates and rear wall plates, the density and the yield strength of Al-6061 are 2.73 g/cm3 and 40 ksi. Fig. 2(a) is a photograph (top view) of a typical Whipple shield structure with aluminum bumper for our hypervelocity impact experiments, while a monolithic Al-6061 bumper is used as the front bumper in this protective configuration, and the aluminum bumper is 1 mm thick. The relative distances between the plates are d1 and d2, as is shown in Fig. 2(a). In all the impact experiments, d1 = 80 mm, and d2 = 40 mm. The rear wall is 2 mm thick, and the witness plate is 1 mm thick. The rear wall and the witness plate are 200 mm × 200 mm squares, and all aluminum plates are Al-6061 alloy. The arrow in Fig. 2(a) indicates the impact direction of projectile relative to the target plate. Fig. 2(b) is the photograph (front view) of a TiB2-based composite enhanced Whipple Shield structure for our hypervelocity impact experiment. This experimental configuration is similar to the aluminum bumper configuration mentioned in Fig. 1, but the monolithic Al-6061 front bumper is replaced by a TiB2-based composite bumper in this assemble device. Other aluminum plates measuring 200 mm (width) × 200 mm (length) are Al-6061 alloy too. The TiB2-based composite discs are cut and ground into thin plates measuring 60 mm (width) × 60 mm (length) × 0.65 mm (thickness). Then the TiB2-based composite plate is fixed into a pair of clamping plates within a centre hole of 55 mm in diameter. The size of the clamping plate is 200 mm (width) × 200 mm (length). The TiB2-based composite plate with a thickness of 0.65 mm and the aluminum plate with a thickness of 1 mm are chosen as the front bumper respectively, for the purpose of comparison. Compared with the monolithic aluminum bumper, the TiB2-based composite bumper proves having nearly the same areal density, which will enhance the credibility of the comparison of TiB2-based composite bumpers and aluminum bumpers for space debris protection. 3. Result and discussion 3.1. Macroscopic damage morphology
Fig. 1. The SEM microstructure of the TiB2-based composite.
In all hypervelocity impact experiments, both the aluminum bumper and the TiB2-based composite bumper are penetrated by spherical projectiles. The penetrated holes are found in the rear wall behind the aluminum bumper and the composite bumper at the predicted impact velocity of 3 km/s. However, no penetrated holes are found in the rear
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Table 1 Hypervelocity impact test parameters. Shot no.
1-1
1-2
1-3
2-1
2-2
2-3
Bumper material Impact velocity (km/s) Projectile weight (g) Projectile diameter (cm) Bumper thickness (mm) Rear wall thickness (mm) Witness panel thickness (mm)
Al-6061 3.00 0.0393 0.30 1 2 1
Al-6061 5.11 0.0394 0.30 1 2 1
Al-6061 7.32 0.0395 0.30 1 2 1
Composite 3.15 0.0392 0.30 0.65 2 1
Composite 5.2 0.0390 0.30 0.65 2 1
Composite 7.22 0.0391 0.30 0.65 2 1
walls of other tests done at the predicted impact velocity of 5 km/s and 7 km/s. Based on the postmortem observation of damages to the front bumpers, rear walls and witness plates, it can be concluded that the novel TiB2-based composite bumper exhibits better performance than aluminum bumpers at any predicted impact velocity of 3 km/s–7 km/ s, and the protection efficiency of the Whipple shields with TiB2-based composite bumpers is significantly improved with the impact velocity increasing. The differences between the performances of the two shields are apparent. Fig. 3 shows the damage features of the bumpers made of different materials after hypervelocity impact tests. The penetrated holes produced in the aluminum bumpers are larger than the ones in TiB2based composite bumpers at predicted impact velocities of 3 km/s– 7 km/s. As a result of the plastic deformation of aluminum alloy, there is curling formed around the penetrated hole of aluminum bumpers. However, such curling cannot be spotted in TiB2-based composite bumpers. The diameter of the penetrated hole in aluminum bumper increases from the low impact velocity to higher ones, and the curling around the penetrated hole is more obvious with the increasing impact velocities too. In the case of TiB2-based composite bumper (Fig. 3(d, e, f)), a penetrated hole surrounded by circular fracture zone is observed in the experiment. But all TiB2-based composite bumpers remain structurally integrate except for these penetrated holes. The diameter of the penetrated hole in TiB2-based composite bumper increases slightly from the low impact velocity to higher ones. Moreover, the width of the circular fracture zone around the penetrated hole is calculated as 2.31 mm at 3.15 km/s, 2.13 mm at 5.2 km/s, and 2.17 mm at 7.22 km/ s, respectively. There is no obvious change when the impact velocity increases, and those slight differences in width can be ascribed to measurement errors. After the hypervelocity impact tests, a debris cloud containing fragments of the projectile and bumper materials forms behind the bumper, which results in the damages of penetrations or craters to the rear wall. The protection efficiency of different bumper materials can be evaluated
by the damage pattern of the rear wall, such as the penetrated hole, the characteristics of bumps, the depth of maximum craters and the size distribution of the craters. Fig. 4 shows the front surface of the rear wall obtained in each impact test, while Fig. 5 shows the back surface of it. Table 2 presents the statistical data of the damage patterns on the rear wall and the witness plate. Damage patterns on the rear wall can be divided into three categories: (1) Penetrated hole, which is always in the centre of the crater zone; (2) Main crater (diameter N 1 mm), which is obviously larger than other craters and is always in the centre region of the crater zone; (3) Micro-crater (diameter b 1 mm), which is widespread among the main craters and outside the region of the crater zone. The statistical results indicate that the damage degree of the rear wall behind composite bumpers is lower than that of the rear wall behind the aluminum bumpers. At predicted impact velocity of 3 km/s, the complete penetration on the rear wall is observed in each test, the penetrated hole in the rear wall behind Al-6061 bumper is larger than the one in the rear wall behind TiB2-based composite bumper. Meanwhile, the damage state of the witness plates behind the rear walls also confirms the better protection efficiency of shielding configuration with TiB2based composite bumpers. According to Table 1, the number of main craters (diameter N 1 mm) in the rear wall behind composite bumpers is obviously less than the ones in the rear wall with Al-6061 bumpers at the predicted impact velocity of 5 km/s or 7 km/s. Compared with the back of the rear wall with Al-6061 bumpers, the bump number of the back of the rear wall with TiB2-based composite bumpers is significantly reduced at any impact velocity. When it comes to the damage pattern of the rear wall with TiB2based composite bumpers at the hypervelocity impact of 3.15 km/s, lots of micro-craters (diameter b 1 mm), on the other hand, are observed apart from some major craters on the front surface of the rear wall. And the diameter of the micro-crater distribution zone is measured as 100 mm. Similar phenomenon can also be observed on the rear wall with composite bumpers at the impact velocity of 5.2 km/s
Fig. 2. Experimental configurations with different bumpers.
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Fig. 3. Penetrated holes in the front bumper.
or 7.2 km/s, only, the distribution zone of micro-craters is larger than the ones formed at a lower impact velocity of 3.15 km/s. Increased number of micro-craters are observed at higher impact velocities. By examining these micro-craters with microscope, it can be found that a little composite bumper element exist at the bottom of some micro-craters. This phenomenon indicates that these micro-craters are mainly produced by the impact of composite bumper debris. These micro-craters are also found on the surface of the rear wall with Al-6061 bumpers at
the impact velocity of 5.11 km/s to 7.32 km/s. However, the number of micro-craters on the rear wall with Al-6061 bumpers is much less than that on the rear wall with composite bumpers at the same impact velocity. Moreover, on the surface of the rear wall with Al-6061 bumpers at the impact velocity of 3 km/s, micro-craters are not spotted. Fig. 6 shows the damage pattern on the rear wall of the compositeenhanced shield used for the 7.2 km/s impact test, which presents three concentric circular zones with different crater characteristics.
Fig. 4. Front surface of the rear wall after impact (plate size: 200 mm × 200 mm).
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Fig. 5. Rear surface of the rear wall after hypervelocity impact (plate size: 200 mm × 200 mm).
There are very clear boundaries among the centre damage region, the intermediate circular band, and the outer circular band, as marked in Fig. 6. The radius of each region is measured as about 25 cm, 50 cm and 90 cm, respectively. In the case of the centre damage region (Fig. 6(a)), the whole centre surface of the rear wall is covered by a lot of contiguous craters. Meanwhile, melting feature within a smooth inner surface and bottom surface of the crater is evident at the centre circular region, which indicates that these contiguous craters are mainly produced by the impact of the molten aluminum droplets from the broken projectile. In the intermediate circular band (Fig. 6(b)), the damage pattern is mainly composed of a large number of micro-craters formed by the impact of solid composite bumper fragments as well as a small amount of main-craters formed by the impact of molten aluminum droplets. When it comes to the outside circular band, a few scattered micro-craters are observed, which result from the impact of the solid composite bumper powders. Based on the above postmortem observations, it can be concluded that, at similar impact velocity, TiB2-based composite bumpers can break up the projectile adequately and exhibit better protective performance, especially when the impact velocity reaches 5 km/s or more. These damage patterns also clearly illustrate the effect of a change in the bumper material on the damage behavior produced by the impact of debris clouds.
3.2. Microscopic damage morphology The microscopic damage patterns produced both on the front and the rear surfaces of TiB2-based composite bumpers under hypervelocity impact are shown in Fig. 7 and Fig. 8. At the impact velocity of 3.15 km/s, the front fracture morphology of the penetrated hole exhibits a mixed fracture mode of the pull-out grooves of TiB2 grains and the transgranular fractures of the TiC/Ni phases, as is shown in Fig. 7(a). And the whole fracture surface remains clear and clean, except for a few of scattered debris sticking on the inner surface of the penetrated hole, which are identified as aluminum projectile fragments based on EDS mapping results in Fig. 8. As the impact velocity increases from 3.15 km/s to 5.2 km/s and 7.22 km/s, the fracture morphology of the penetrated hole becomes more granular and rougher, and obvious intergranular cracks are produced between TiB2 crystals. In addition, a lot of refined debris and particles are obviously observed on the fracture surface of the penetrated hole, which is also considered to be the initial impact product of aluminum projectile on the composite bumper. As is shown in Fig. 9 (a), the rear fracture surface of the penetrated hole is clear and clean at the impact velocity of 3.15 km/s. However, when the impact velocity amounts to 5.2 km/s (Fig. 9 (b)), the back fracture surface of the penetrated hole is covered by solid particles and molten products, such as spherical and discoid solidification products.
Table 2 The statistical data of damage patterns on the rear wall and the witness plate. Shot no.
1-1
1-2
1-3
2-1
2-2
2-3
Bumper material Impact velocity (km/s) Perforation number Perforation diameter (mm) Main crater number (ϕ N 1 mm) Max crater diameter (mm) Max crater depth (mm) Bump number of back surface Witness panel crater number
Al-6061 3.00 2 2.8/2.4 86 – – 13 5
Al-6061 5.11 0 – 425 1.8 1.5 103 –
Al-6061 7.32 0 – 533 1.1 0.75 287 –
Composite 3.15 1 2.9 44 – – 5 3
Composite 5.2 0 – 135 1.9 1.2 32 –
Composite 7.22 0 – 0 0.9 0.4 3 –
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Fig. 6. Views of sections of damage patterns formed on front rear-wall surface for the composite bumper test (V = 7.22 km/s).
When the impact velocity reaches 7.22 km/s, the number and distribution of solidification covering will be increased greatly. Moreover, some larger discoid solidification products are also observed in this area, as is shown by the arrows in Fig. 9(c), which are identified as the splashed patterns of the molten aluminum droplets by EDS in Fig. 10. In order to identify the origin of these molten aluminum droplets covered the surface of the penetrated hole at the impact velocity of 7.22 km/s, EDS is conducted to examine the Al element distribution on the rear surface of the composite bumper, as is shown in Fig. 11. EDS result reveals that Al elements deposit mainly on the fracture zone around the penetrated hole, so it can be inferred that the solidification aluminum production covering the fracture zone is mainly from the molten fragments of aluminum projectile under the initial impact on the composite bumper. Moreover, some larger discoid solidification products are observed both on the fracture zone and undamaged surface of the composite bumper, which are mainly produced by the secondary impact of molten aluminum droplets bouncing back from the rear wall.
SEM and EDS results of the composite bumper after impact tests indicate that as the impact velocity increases, the composite bumper material around the penetrated hole is more severely fractured, and the aluminum projectile can be broken or even molten sufficiently. It may indicate higher temperature rise of aluminum projectile during the penetration progress in TiB2-based composite bumper at the velocity of N5 km/s, and the high impact temperature causes the molten products of the aluminum projectile, but it doesn't result in visible molten products of composite bumper due to the extreme high melting temperature of TiB2 (~2980 °C). 3.3. Debris cloud formation The damage characteristics of the rear wall behind the bumper depend on the morphological feature of the debris cloud produced by hypervelocity impact [30]. Fig. 12(a) presents a typical feature of debris clouds produced by the impact of Al sphere with Al-6061 bumper at
Fig. 7. The fracture morphology of the front perforation surface in TiB2-based composite bumper.
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Fig. 10. The Al element distribution on the fracture zone of the perforation backside (V = 7.22 km/s). Fig. 8. The Al element distribution on the front surface of penetrated holes in TiB2-based composite bumper (V = 3.15 km/s).
the impact velocity of 7.32 km/s. According to the results and analyses in reference [31], the debris cloud can be described as possessing three major structural features: (1) the ejecta veil located ahead of the bumper is entirely composed of bumper fragments from the impact fracture surface of the front bumper; (2) the external bubble of debris is of a kind of significant thin shell structure, which mainly consists of the bumper debris produced from the rear fracture surface of the bumper; (3) the internal structure located inside and at the front of the expanding bubble of bumper debris is mainly composed of the projectile debris. As shown by the laser shadowgraph images of the debris cloud in Fig. 12(a), a large number of visible solid slivers from broken aluminum bumper are observed in the ejecta veil and the external bubble, and the bulk of the broken projectile fragments mass appears to be concentrated in the internal structure which results in the primary serious damage to the rear wall. The effect of a change in the bumper material on the morphology of debris cloud is shown in Fig. 12(b). The debris cloud shown in this laser shadowgraph image is produced by the impact of an aluminum sphere with a TiB2-based composite bumper at an impact velocity of 7.22 km/s. Significant changes in debris-cloud properties are observed when the bumper material is changed from Al-6061 to TiB2-based composite. This debris cloud in Fig. 12(b) can be described as having three major structural features too. However, instead of the solid slivers of the ejecta veil and the external bubble of debris in the configuration with aluminum bumper, the obvious powdery cloud within a large number of ultrafine particles is formed in this composite-enhanced configuration, which is identical to the severe fractured perforation surface of the composite bumper in Fig. 7(c). Moreover, the internal structure located both
inside and at the front of the expanding bubble of bumper debris is obviously extended in Fig. 12(b), as contributes to make the bulk of the broken projectile fragment mass appear to be fairly well-dispersed in a wider shell of the internal structure. Taking into consideration the microscopic damage morphology of the composite bumper material and the impact characteristic of the craters on the rear wall in Fig. 6, it can be inferred that the observed centre damage region on the rear wall is the result of the impact of the large fragment concentration. Because the kinetic energy of debris cloud scatters among a large number of ultrafine particles, the primary damage of the powdery cloud to the rear wall is accordingly diminished. The same protection mechanism can also be applicable to the impact tests of protection configuration with TiB2-based composite bumper at the impact velocity of 5.2 km/s. At impact velocity about 3 km/s, the front head of debris cloud is composed of bulk fragments of aluminum projectile, as is shown in Fig. 13(a). The main projectile mass shown by the arrows is viewed to be concentrated in the cloud head, resulting in the remarkable impact craters or penetrated holes on the rear wall. For the shielding configuration with TiB2-based composite bumper, the size of main fragments in the front head of debris cloud appears to diminish in Fig. 13(b). The external bubble of debris cloud comprises of lots of fine bumper powder and does not appear to have the ability to produce any significant crater on the rear wall. When the impact velocity decreases to about 3 km/s, the improvement of the protection efficiency of TiB2-based composite bumper to the rear wall is very limited. This is because the TiB2-based composite bumper can't break the projectile into smaller fragments or powders, even though its protection efficiency is slightly better than that of Al-6061 bumper. As has been shown above, the shape and characteristics of debris cloud are determined by bumper material and impact velocity, as is
Fig. 9. The fracture morphology of the rear perforation surface in TiB2-based composite bumper.
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regime, the critical projectile diameter dc can be calculated as: " dc ¼
t w ðσ =40Þ1=2 þ t b
#18=19 ð1Þ
1=2
0:6ð cos θÞ5=3 ρp V 2=3
where ( V LV ¼
1=3 ; for t b =dp b0:16 1:436 t b =dp 2:60; for t b =dp ≥0:16
ð2Þ
and dc is the critical projectile diameter (cm), tb is the bumper thickness (cm), tw is the rear wall thickness (cm), ρp is the density of the projectile material (g/cm3), σ is the yield strength of the rear wall material (ksi), and dp is the diameter of the projectile. The ballistic limit equation in the hypervelocity regime is given by: Fig. 11. The Al element distribution on the rear perforation surface of composite bumper (V = 7.22 km/s).
crucial to the damage pattern of the rear wall. For the traditional allaluminum Whipple-type shield, the ballistic limit evaluation can be described as three parts, each of which corresponds to a projectile state following impact velocity on bumper plate [32,33]. The low-velocity regime (V ≤VLV/ cos θ ) is defined for impacts in which the projectile perforates the bumper plate without fragmenting, resulting in the impact of an intact (albeit deformed) projectile on the shield rear wall. Once the shock pressure of increased impact velocity is sufficient to induce projectile fragmentation (and eventually melting), these impacts are termed the shatter regime (or intermediate regime). Within the shatter regime, increases in impact velocity could fragment the projectile sufficiently, which lead to a more equally dispersed debris cloud of smaller fragments and particles, thus the damage to the rear wall is mitigated accordingly, and the shielding performance is promoted progressively. The hypervelocity regime (V ≥ VHV/ cos θ) is defined for impacts in which the increased impact velocity of projectile would bring about the further damage to the rear wall shield due to the increased lethality of fragment cloud. The ballistic limit equation of Whipple shield (single Al-bumper) can be established using the JSC Whipple Equation [33]. In the low velocity
dc ¼ 3:918F 2
1=3 2=3 t w s1=3 σ y =70 1=3 1=9
2=3
ρp ρb ðV cos θÞ
ð3Þ
where ρb is the density of the bumper material (g/cm3) and S is the standoff between the bumper and rear wall (cm). The term F2⁎ represents a de-rating factor for configurations with insufficiently thick bumpers. For the shatter regime, the ballistic limit can be calculated by linear interpolation: dc ¼ dc ðV LV Þ þ
½dc ðV HV Þ dc ðV LV Þ ðV V LV Þ V HV V LV
ð4Þ
For conventional aluminum bumper, impact regime transition velocities (VLV, VHV) are dependent on the outer bumper and projectile material, defined as about 3 km/s and 7 km/s [32]. Due to the actual impact velocities in this paper exceed the low-velocity regime based on the Eq. 2 (i.e. V N VLV =2.6, for tb/dp ≥ 0.16), all the aluminum projectiles could be fragmented as a result of sufficient shock pressure, this explains why there is no intact projectiles are found in the laser shadowgraph images of the debris cloud produced by hypervelocity impacts. Meanwhile, according to the above equations, we can also obtain the critical projectile diameter dc of the all-aluminum configuration under
Fig. 12. Laser shadowgraph images of the debris cloud formed by hypervelocity impact. (a) Al-6061 bumper, V = 7.32 km/s (b) TiB2-based composite bumper, V = 7.22 km/s.
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Fig. 13. Laser shadowgraph images of the debris cloud formed by hypervelocity impact. (a) Al-6061 bumper, V = 3.0 km/s (b) TiB2-based composite bumper, V = 3.15 km/s.
the impact tests in this paper: 8 < 0:16 cm; for V ¼ 3:00 km=s dc ¼ 0:29 cm; for V ¼ 5:11 km=s : 0:46 cm; for V ¼ 7:32 km=s
ð5Þ
Since the projectiles with a constant diameter of about 3 mm are used in the impact tests, which is far exceeds the critical projectile diameter dc under the impact velocity of 3 km/s, it pretty easy to understand why the rear wall is penetrated by the debris cloud under the impact velocity of 3 km/s. On the other hand, the actual size of the projectile is far less than the critical projectile diameter dc under the impact velocity of 7.32 km/s, so the shield configuration represents appropriate protection performance. When it comes to the impact velocity of 5.11 km/s, due to the calculated critical projectile diameter dc is much closed to the actual size of the projectile, which means that the rear wall of shield configuration may be perforated or near perforated, the results of impact test also confirm the above-mentioned discuss. For non-aluminum plate bumpers, an equivalent plate thickness tb,eq is calculated from the areal densities of the actual bumper plate, i.e.: t b;eq ¼ t Comp
ρComp ρAl
ð6Þ
As the areal density of the actual TiB2-based composite bumper plate has been concerned in this paper, the TiB2-based composite bumper with 0.65 mm thick is equivalent to the aluminum bumper with 1 mm thick, therefore, the critical projectile diameter d⁎c of the novel configuration with TiB2-based composite bumper could also be calculated as follows: dc
8 < 0:17 cm; for V ¼ 3:15 km=s ¼ 0:29 cm; for V ¼ 5:20 km=s : 0:46 cm; for V ¼ 7:22 km=s
ð7Þ
We can found that d⁎c of enhanced shielding configuration is roughly equivalent to dc of all-aluminum configuration under the similar impact velocity. Despite the critical projectile diameter and the similar actual projectile size, the TiB2-based composite enhanced shield configuration could reduce the damage to the rear wall efficiently, especially for that the actual projectile diameter reach the critical projectile diameter, the TiB2-based composite bumper obviously relieve the damage to the rear wall. In conclusion, since TiB2-based composite bumper can
sufficiently break up the projectile under hypervelocity impact, the impactor momentum of the projectile fragment can be distributed into the widespread small-size debris or particles, resulting in a wider impact area of the rear wall, so that the configuration with TiB2-based composite bumper displays a better protective performance. 4. Conclusion The potential application of single-shield front bumpers made of TiB2-based composites is discussed for a Whipple-type shield in this paper. Hypervelocity impact experiments are conducted to compare the protection efficiency of front composite bumpers with that of conventional aluminum bumpers by normal impact of spherical aluminum projectile with the predicted velocity of 3 km/s, 5 km/s and 7 km/s. The 2 mm-thick rear-wall plate and 1 mm-thick witness plate are mounted behind the bumper, and the size and distribution of craters on the rearwall plate are taken as a criterion for evaluating different bumper materials. TiB2-based composite bumpers prove to display better protection efficiency than traditional aluminum bumpers when the impact velocity increases. The debris cloud propagation behavior induced by a hypervelocity impact is recorded by Hypervelocity Sequence Laser Shadowgraph Imager. Based on the shadowgraphs of debris clouds generated behind the bumpers as well as the fracture morphologies of the penetrated holes in the bumpers, the debris clouds with the powdery microstructure are found to be generated behind the composite bumpers, and the fragments and spalls will be broken into much smaller pieces than by traditional aluminum bumpers. That's why the impact damage on the rear wall behind composite bumpers can be obviously reduced. Therefore, these preliminary experiments just demonstrate the potential applications of TiB2-based composites in the space systems for the impact protection of micro-meteoroid and orbital debris. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51502338 and 61503064). References [1] J.C. Liou, N.L. Johnson, Risks in space from orbiting debris, Science 311 (2006) 340–341. [2] J. Hyde, E.L. Christiansen, R.P. Bernhard, J.H. Kerr, D.M. Lear, A history of meteoroid and orbital debris impacts on the space shuttle, Proceedings of the Third European Conference on Space Debris, ESA SP-473, 2001.
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