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International Journal of Impact Engineering journal homepage: www.elsevier.com/locate/ijimpeng

Experimental impact study using an explosive driven projectile accelerator and numerical simulation T. Saburi a, *, S. Kubota a, M. Yoshida b, Y. Wada a, Y. Ogata a a

Application and Environmental Protection Team, Research Core for Explosion Safety, National Institute of Advanced Industrial Science and Technology, Onogawa 16-1, Tsukuba, Ibaraki 305-8569, Japan b Explosion Research Institute Inc., Ishiyoshi Bldg. 103, Ushiku 5-18-2, Ibaraki 300-1234, Japan

a r t i c l e i n f o

a b s t r a c t

Article history: Available online 25 July 2008

This paper presents a projectile impact experiment using a compact accelerator driven directly by explosives, and a numerical simulation of the impact. The compact projectile accelerator has been developed to evaluate the perforation resistance of structural materials. Projectile shooting tests were conducted and the relationship between the explosive weight and the injected projectile velocity was obtained. A series of impact tests on the targets, with varying projectile velocity, was examined using the developed accelerator. The projectile was made of SNCM (nickel–chromium–molybdenum special steel of the Japanese Industrial Standard) and the targets were aluminum 5052S alloy plates. The projectile track and the impact process on the targets were observed with a SHIMAZU HPV-1 high-speed video camera and the velocity of the projectile and interactive behavior were evaluated. A numerical simulation study was conducted using the parallel version of the non-linear ﬁnite element code of LS-DYNA to follow the impact experiments and determine the ballistic limit of the projectile for the targets. Ó 2008 Elsevier Ltd. All rights reserved.

Keywords: Projectile accelerator High-speed video camera Penetration Numerical simulation

1. Introduction For the purpose of safe counter-terrorism, a portable explosive processing chamber was developed, which can deal with improvised explosive devices on site. The chamber should be able to withstand ballistic fragments as well as blast pressure, subject to the restrictions imposed by its weight and size. Therefore, an important issue is to evaluate the ballistic characteristics of structural materials by means of projectile impact testing. There are many different techniques for accelerating projectiles, such as a two-stage light gas gun, a powder gun, a plasma accelerator, a rail gun and so on. The application of these techniques depends on speciﬁc requirements, such as projectile size, achieving the necessary velocity, possessing a certain amount of equipment, and the ability to be maintained. Accordingly, the requirements for our experiments were as follows: a compact device that can be utilized under various experimental conditions; a device that can be used repeatedly for a certain experimental period; and a device that has the ability to control velocities of more than 1000 m/s. The ease-ofmaintenance requirement was met by developing a disposable accelerator. Based on this, a direct explosive drive method [1,2] was chosen as the acceleration technique and the compact accelerator was a novel design. In this study, the performance of the accelerator was evaluated by conducting projectile shooting tests and the * Corresponding author. Tel./fax: þ81 29 861 8760. E-mail address: [email protected] (T. Saburi). 0734-743X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijimpeng.2008.07.012

relationship between the explosive weight and the projectile’s velocity was evaluated. A series of impact experiments was examined, using the accelerator. Currently, impact analysis is typically done by numerical simulation in ﬁelds such as ballistics [3–7]. A non-linear ﬁnite element code of LS-DYNA [8] was used to follow the experimental results and the ballistic limits of target materials were investigated. 2. Experiment Fig. 1 shows the accelerator, which mainly comprises an acceleration muzzle and an explosive vessel. The stainless steel acceleration muzzle is 70 mm in length and 10 mm in inner diameter, and has a tapered structure at the base of the muzzle for gas compression. A 0.5 mm PMMA partition plate is attached to the bottom of the muzzle to separate projectiles from explosives with an air gap. The muzzle is attached to an aluminum explosive vessel, which is 100 mm in length and 30 mm in inner diameter, and the explosive material is placed in the vessel in contact with the partition plate. An instantaneous electric detonator is held in an aluminum holder and installed in the vessel in contact with the explosive. The typical size of the accelerator is 160 mm in length. Projectile testing and impact experiments on targets were examined with the accelerator. The projectile was made of SNCM 439 (nickel–chromium–molybdenum steel of the Japanese Industrial Standard), and had a columnar body of 10 mm diameter and 10 mm depth and a weight of about 6 g. Aluminum 5052S alloy

T. Saburi et al. / International Journal of Impact Engineering 35 (2008) 1764–1769

Electric detonator holder

Explosive PMMA plate SCNM projectile Stainless muzzle

50mm

10mm

60mm

100mm Electric detonator

Aluminum vessel

Fig. 1. Schematic drawing of the compact accelerator.

plates, 200 mm wide and 5, 10 and 15 mm thick, respectively, were used as targets. Each of these plates was placed approximately 30 cm from the accelerator and was ﬁxed at the four corners with bolts. The explosives used to accelerate projectiles were 5–35 g of Emulsion and Composition C4. Projectile velocity was controlled by adjusting the weight of explosives. The projectile’s track and impact on the targets were observed using a SHIMAZU HyperVision HPV-1 high-speed video camera that can capture one million frames per second at a resolution of 312 260 pixels. Trigger timing for the video camera and a ﬂash lamp were controlled by using the ion gap method. The projectile velocity and interactive behavior were evaluated visually with recorded images. The relationship between the projectile’s initial velocity and the penetration status at each target thickness was obtained in a series of impact experiments. In the case of penetration, the residual velocity after penetration was also evaluated. 3. Experimental results and discussion Projectile shooting tests were performed. The projectile attained a constant velocity immediately upon exiting the accelerator and the velocities ranged from 200 to 1000 m/s, as shown in Fig. 2. We found that there is an approximate linear correlation between the weight of explosives and the velocity achieved in our experimental range. For example, the projectile achieved about 400 m/s with 5 g of emulsion, and 800 m/s with 15 g of emulsion. However, the velocity predicted by Gurney’s equation [8] will show asymptotic tendency. Deviation of data was conspicuous at higher explosive

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weights, so there is a possibility that the break of the linearity is covered in the dispersive region. The projectile velocity did not rise, even when the weight of the explosive exceeded 35 g. These results are considered to be due to the tensile strength of the aluminum vessel because most of the explosion’s energy will be consumed in the destruction of the accelerator and therefore the accelerator’s driving force will not increase. As shown in Fig. 3, the accelerator was destroyed by random fragmentation, and Fig. 4 shows the accelerator after the shot. Consequently, the aluminum vessel was replaced with a stainless steel vessel, in the expectation of achieving a higher velocity. The results show that replacing the aluminum vessel with a stainless steel one did not improve velocity, but did contribute to the stability of the projectile’s yaw and pitch angles. Fig. 5 shows the accelerator after the shot, with the stainless steel vessel before the shot shown on the left. It can be seen that the stainless steel vessel fragmented into several regular pieces, in contrast to that shown in Fig. 4, and the end of the muzzle was also fractured. This suggests that the explosion’s energy was propagated towards the muzzle effectively and evenly, and that this contributed to the stable injection of the projectile. On the other hand, a higher projectile velocity could not be achieved with this arrangement. To rectify this, other improvements such as the length of the muzzle and so on seem to be necessary. We conducted experiments with 5–15 mm aluminum targets, varying the projectile’s velocity. Fig. 6 shows a typical high-speed video image for the perforation of 15-mm thick aluminum. The projectile impacted the aluminum surface, accompanied by an oxidation reaction of the aluminum, and ejected a plug of the target with a residual velocity of 120 m/s. The repetition of such experiments enabled us to obtain detailed data concerning the evaluation of ballistic limits, but we also obtained data with the support of numerical simulation. 4. Numerical simulation A numerical simulation study was conducted using the nonlinear ﬁnite element code of LS-DYNA [9] to follow the impact experiments. The projectile and target plate were modeled as a solid mesh of about 400,000 elements and 450 elements, respectively, as shown in Fig. 7. The typical mesh size was 1 mm for the projectile and 0.25–2.5 mm for the aluminum plate. The target model had at least 10 meshes in the thickness direction. A ﬁne 0.25mm mesh was generated around the contact surface of the aluminum plate with the projectile. The model for the projectile had a pitch angle of up to 3 , to match the experimental results. The Johnson–Cook constitutive material model [10] was used for both materials, and the Mie–Gruneisen equation of a state model [11,12] was applied in conjunction with the Johnson–Cook model. The Johnson–Cook model is expressed as

sy ¼ A þ B3p

n

1 þ c ln 3*

1 T *m

(1)

where A, B, C, n and m are input constants, 3p is effective plastic strain, 3* ¼ 3p =30 is effective plastic strain rate for 30 ¼ 1.0 s1 and T* is homologous temperature. The strain at fracture 3f is given by

ih

h

ih

3f ¼ D1 þ D2 exp D3 s* 1 þ D4 ln 3* 1 þ D5 T *

i

(2)

where D1 to D5 are input constants and s* is the ratio of pressure divided by effective stress. Fracture occurs when the damage D is 1.0.

D ¼ Fig. 2. Experimental results for the relationship between projectile velocity and explosive weight.

X D3p

3f

(3)

Typical values of Johnson–Cook and Mie–Gruneisen equation of state model for aluminum 5052S alloy and SNCM 439 steel are

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T. Saburi et al. / International Journal of Impact Engineering 35 (2008) 1764–1769

Fig. 3. Example of high-speed video images of fragmentation of the accelerator.

given in Table 1. Fracture parameters D1–D5, which determine the strain at fracture, were determined from typical reference data and trial-and-error with experimental results and D1 parameter was mainly adjusted for each plate thickness. A constraint condition was applied to the four corners of the plate. Initial velocity conditions of up to 1000 m/s were applied to the projectile material. A segment based eroding surface to surface was used for the contact condition between the projectile and the target. The series of impact simulations, including EOS calculations, caused a considerable CPU load, so the calculation was executed using a parallel version of LSDYNA970 on a cluster of PCs (128 2.8-GHz Intel Xeon processors, 192 GB of memory and a myrinet2000 interconnection). In the current LS-DYNA970 version, there are inoperative functions equal to a single version, such as a ﬂuid-structure interaction problem, but it was conﬁrmed that the program works well for structure– structure impact interaction and material failure problems. We improved the calculation efﬁciency by using 16–20 processors,

Fig. 4. Picture of the accelerator after the shot with the aluminum vessel.

which was considered to be a favorable CPU size to maintain good scalability in our calculation model and problem size. 5. Numerical results and discussion Fig. 8 shows the results of the numerical simulation for perforation of the target. When the projectile impacted the target surface, the target elements were compressed and distorted, and this led to the fracture. It has been reported that there are at least three different stress states in the perforation process [13]. In Fig. 9,

Fig. 5. Picture of the accelerator with the stainless steel vessel after the shot. Parts on the left are components of the original accelerator.

T. Saburi et al. / International Journal of Impact Engineering 35 (2008) 1764–1769

Fig. 6. High-speed video images of the perforation of a 15-mm thick aluminum plate (projectile velocity: 745 m/s).

Fig. 7. (a) Full view of the computational model of the projectile and the target plate. (b) Close-up view of the model around the impact area.

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Table 1 Typical model parameters for Johnson–Cook and Mie–Gruneisen EOS

900

Aluminum 5052S alloy

800

Strength constants A (Pa) 3.689Eþ8

Young’s modulus E (Pa) 7.03Eþ10

B (Pa) 6.839Eþ8

Fracture constants D2 D1 0.2 0.123

n 0.73 D3 1.5

Mie–Gruneisen EOS constants C S1 5328.0 1.338

Poisson’s ratio n 0.33

C 0.0083 D4 0.007

g0

m 1.7 D5 0.0

A 0.48

2.0

600

Caluculation for AL Plate t 05mm t 10mm t 15mm

500 400 300 200

SNCM 439 steel Johnson–Cook constants Shear modulus Density r (kg/m3) G (Pa) 7750.368 8.412Eþ10

Young’s modulus E (Pa) 2.040Eþ11

Strength constants A (Pa) 1.539Eþ9

n 0.18

Fracture constants D1 0.0

AL 05mm AL 10mm AL 15mm

700

Residual Velocity (m/sec)

Johnson–Cook constants Shear modulus Density r (kg/m3) G (Pa) 2680.0 2.59Eþ10

Experiment

100

Poisson’s ratio n 0.29

0 200

B (Pa) 4.770Eþ8 D2 0.0

Mie–Gruneisen EOS constants C S1 4569.0 1.490

D3 0.0

C 0.012 D4 0.0

g0 2.17

400

m 1.0 D5 0.0

A 1.4

the simulation results for the projectile’s impact were obtained from the initial velocity, from the residual velocity relation for each plate thickness, as well as from the experimental results. The ballistic limit velocities for 5, 10 and 15 mm target plates were found to be about 220, 460 and 700 m/s, respectively. Some deviation was obtained between experimental and numerical results

600

800

1000

Initial Velocity (m/sec) Fig. 9. Relationship between initial and residual velocities of the projectile for 5, 10 and 15 mm aluminum 5052S target plates obtained by experiments and numerical simulation.

for higher initial velocities, especially for the 5-mm thick target. There are many model constants for considering the effects of strain hardening, strain rate hardening, thermal softening and material fractures, and some of them need to be determined from experimental tests. Further experiments and calibration analysis are capable of improving the deviation from experimental behavior. It is also believed that mesh quality is sensitive to this kind of impact problem. Regarding the 5-mm thick target, the number of meshes in the thickness direction is small compared to the other targets. It would be valuable to examine the correlation between mesh quality and the constitutive model for a wide range of the projectile velocities. As shown in Fig. 10, the results enable us to

Projectile Velocity (m/sec)

800

600

400

200

Penetration Non Penetration Ballistic limit 4

6

8

10

12

14

16

18

Target Plate Thickness (mm) Fig. 8. Computational results for the perforation process of a 15-mm thick aluminum target (projectile velocity: 800 m/s).

Fig. 10. Ballistic limits of the projectile for 5, 10 and 15-mm thick aluminum 5052S target plates.

T. Saburi et al. / International Journal of Impact Engineering 35 (2008) 1764–1769

draw a line, which indicates the ballistic limit as a function of target thickness. There is an approximately linear relationship between the aluminum target thickness and the projectile velocity for our experimental conditions. 6. Conclusion We have developed a compact projectile accelerator, which controls projectile velocity by varying the weight of the explosive, and an approximately linear relationship between the explosive weight and injected projectile velocity was obtained. However, Gurney equation will predict the existence of the non-linear relationship in the dispersive region. More precise analysis is necessary to clarify this tendency. A series of impact experiments and numerical simulation enabled the perforation resistance of the structural materials of the chamber to be evaluated. We scheduled experiments using the designed prototype chamber and the numerical simulation for projectile accelerator, which are expected to provide a better understanding of the acceleration mechanism. Acknowledgements The authors acknowledge the support of the Special Coordination Fund for Promoting Science and Technology, Ministry of Education, Culture, Sports, Science and Technology (MEXT).

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