Investigation on energy output structure of explosives near-ground explosion

Wen-long Xu , Cheng Wng ,*, Jin-ming Yun , Wei-ling Goh , To Li

a State Key Lab of Explosion Science and Technology, Beijing Institute of Technology, Beijing,100081, China

b Temasek Laboratories, Nanyang Technological University, 50 Nanyang Drive, 637553, Singapore

Keywords:Near-ground blast Reflected wave Marcher steam Aluminized explosives

ABSTRACT In order to give the energy output structure of typical explosives near-ground explosion in real ground conditions, the free-field shockwave, ground reflection shockwave and Mach wave overpressure time history of composition B explosive, RDX explosive and aluminized explosive were measured by air pressure sensors and ground pressure sensors.The shape of the free-field shock wave,ground reflection shock wave, and Mach wave and explosion flame were captured by high-speed camera. The experimental results show that, at the same horizontal distance from the initiation point, the peak overpressure of explosive shock wave of composition B explosive, both in the air and on the ground, is less than that of RDX and aluminized explosives. At a distance of 3.0 m from the initiation point, the peak overpressure of aluminized explosives is slightly less than that of RDX explosives. Owing to the exothermic effect of aluminum powder,the pressure drop of aluminized explosives is slower than that of RDX explosives. At 5.0 m from the initiation point, the peak overpressure of aluminized explosives is larger than that of RDX explosives.At the same position from the initiation point,among the three kinds of explosives, the impulse of aluminized explosives is the maximum and the impulse of composition B explosives is the minimum. With the increase of the horizontal distance from the initiation point, the height of Mach triple-points (Mach steam) of the three explosives increases gradually. At the same horizontal distance from the initiation point,there is poorly difference in the height of Mach triple-points between aluminized explosive and RDX explosive,and the height of Mach triple-points of composition B explosive is much smaller than that of other two explosives.The maximum diameter and duration of the fireball formed by aluminized explosives are the largest, followed by composition B explosive, and the maximum diameter and duration of the fireball formed by RDX explosive are the smallest.

1. Introduction

Blast wave is one of the main damage models of the explosion on personnel, equipment and protective structure. Therefore, the study of energy output structure of the explosion wave is of great significance to the safety protection in weapon test, blasting engineering, explosive industry and other areas [1-5].

In the past, the characteristics of energy output of various explosives near-ground explosion were studied. The parameters of TNT, PBX and Hexel blasting in the air explosion were tested by Wang et al. [6]. The peak overpressure and impulse values of the three explosives at different positions were given. The results showed that, at the same proportional distance, the peak overpressure and impulse of TNT are significantly lower than those of PBX and Hexel explosives. Ripley et al. [7] studied the effect of afterburning energy release and ground reflection on near-field impulse of metalized explosives (TBX and IEF). Due to the afterburning of detonation products and metal particle additives, the TBX and IEF explosive impulse performed as well as or better than the C4 explosive in the near field. Feng et al. [8] experimentally investigated the effects of adding ammonium perchlorate (AP) to RDX, HMX and aluminized explosives on the shockwave overpressure, duration and explosion temperature of air explosion.Zhong et al.[9]built a high resolution and high precision pressure measurement system. Based on this system, the explosion overpressure of TNT with different mass was obtained and a new empirical formula between peak overpressure and proportional distance of shockwave was proposed.

Fig.1. Explosives used in the experiments.

To convenient investigate the energetic output of all kinds of explosives,the TNT equivalence was widely concerned.Rigby et al.[10] investigated the TNT equivalence of PE4 by simulations and experiments. Experimental results were compared to a series of numerical analyses conducted with different masses of TNT explosive. The results indicated that a TNT equivalence of 1.2 best describes the blast waves produced from PE4 detonations. A comprehensive study of equivalent mass factors for far-field detonations have been conducted but almost no equivalent mass factors are available for near-field detonations. Shin et al. [11] proposed TNT equivalency mass factors for four explosives (PETN, Composition B, Pentolite and Tetryl) considering incident and normally reflected peak overpressure and scaled impulse, in the near-field detonations. Grisaro and Edri [12] studied the TNT equivalency factors by verified one-dimensional numerical simulations. A new approach was presented to calculate the equivalency factor for impulse and overpressure through a single gauge measurement.The results show that the equivalency factor strongly depends on the internal energy ratio of the explosive.

In recent years, the influence of the ambient environment on explosive energy output has also been widely concerned. Silnikov et al. [13] investigated the effect of ambient pressure on the blast wave parameters resulted from high explosive. They found that when the surrounding pressure was reduced, both pressure and impulse resulted from explosions decreased at a certain distance from the blast source. Veldman et al. [14] studied the effect of ambient pressure on reflected blast impulse and overpressure on spherical C4 charges of 226.8 g.The results indicated that,at 61 cm standoff,variation of ambient pressure had no significant effect on peak reflected overpressures. However, with the increase of ambient pressure, the reflected impulses increased.To studied the effect of rapid afterburn on the shock development,Tyas et al.[15]conducted a series of tests to measure the reflected pressure acting on a rigid target in inert atmospheres and oxygen-rich atmospheres. The results show that early-stage afterburn has a significant influence on the reflected shock parameters in the near-field.The expanding detonation product cloud remains luminous for much longer durations in air than in nitrogen, indicating that afterburn reactions are ongoing at the cloud-air interface. Duan et al.[16]tested aluminized explosives with different aluminum to oxygen (Al/O) ratio and studied the influence on pressure properties of confined explosion.Jiba et al.[17]investigated the effects of a fine water mist environment on the detonation of PE4 explosive charges in a semi-confined blast chamber. They discussed the detonation parameters including arrival time of the shock waves,peak overpressures,specific impulse of the positive phase,period of the negative phase and the specific impulse of the multiple reflections in the mist condition compared to the atmospheric condition.

Fig. 2. Experimental setup.

Fig. 3. Shock wave distribution of near-ground explosion.

Fig. 4. Typical overpressure curves of composition B, RDX and aluminized explosive at different measuring positions.

Previous studies mainly focused on the two parameters of explosive shock wave pressure and impulse. In fact, Mach wave plays an important role in the energy output structure of explosives near-ground explosion. However, up to now, few studies on Mach wave propagation characteristics of large equivalent explosion in real ground conditions have been published. Because of the complexity of reflection of a shock wave in real ground condition,experimental study is the most reliable method to analyze the energy output structure of near-ground explosion [18-20]. In this paper, the time history of the free-field shock wave, ground reflection shock wave and Mach wave overpressure of composition B, RDX and aluminized explosives blasting near the ground were measured using air pressure sensors and ground pressure sensors.The shape of the free-field shock wave, ground reflection shock wave, Mach wave and explosion flame were captured by highspeed camera. The peak overpressure and impulses of air and ground shock wave at different horizontal distances from the initiation point of three explosives blasting near the ground were compared. The propagation law of the quantified height of Mach triple-points with the horizontal distance from the initiation point of the three explosives was calculated,and the characteristics of the explosion flame size and duration of the three explosives were analyzed.

2. Experimental methodology

2.1. The samples of explosives

Fig. 5. Error bar of peak overpressure of the three explosives vs. horizontal distance from initiation point.

As shown in Fig.1, the explosives used in the experiments are composition B explosive casted by a mixture of 60.0% RDX and 40.0% TNT with density of 1.68 g/cm3. The density of pressed RDX explosive and aluminized explosive is 1.72 g/cm3and 1.80 g/cm3,respectively. The aluminized explosives are composed by pressing 65.0%RDX explosives,30.0%aluminum powder and 5.0%adhesives.The weight of cylindrical explosive with diameter of 100.0 mm is 2.0 kg. Three groups of repeated tests were conducted for each explosive.

2.2. Experimental setup

The experimental setups are illustrated in Fig. 2. The ground condition is natural soil and the explosive is fixed on the wooden bench with the height of 1.5 m.Two types of sensors are used,one is the air pressure sensor at a height of 1.57 m, and the horizontal distance from the initiation point isR=3.0 m, 5.0 m, 7.0 m, 9.0 m and 11.0 m respectively.The other is ground pressure sensor with a distance from the initiation point ofR=3.0 m, 4.0 m, 5.0 m, 6.0 m,7.0 m,9.0 m and 11.0 m.The explosives are located at the center of the connecting line between two marker rods. The high-speed camera is perpendicular to the plane of the marker rods, and the distance between the marker rods is 15.0 m. AtR=3.0 m, an air pressure sensor produced by Kistler company of 6233A0050(0-0.34 MPa)is used.At other air positions,air pressure sensors of 6233A0025 (0-0.17 MPa) is placed. AtR=3.0 m and 4.0 m, the ground pressure sensors produced by Kistler company of 211B4(0-1.40 MPa) are used. At other ground positions, the ground pressure sensors of 211B6 (0-0.34 MPa) are adopted. A data acquisition instrument of TraNET 404 with 16 channels is used to record the messages from air and ground pressure sensors. The sampling rate of the data acquisition instrument is 10 MS/s/CH.The FASTCAM SA5 high-speed camera produced by Photron Company of Japan is used. The shooting speed is set to 7000 frames per second at 1024×1024 resolution.

3. Experimental results and discussion

3.1. Propagation of explosive shock wave near the ground

Shock wave propagation process of explosive near the ground is shown in Fig. 3. After the detonation of an explosive at a certain height from the ground, the explosive shock wave propagates outward with spherical wave (purple line). Then it contacts the ground (Fig. 3 A) and generates reflected wave (green line). The reflected shock waves gradually catch up with the initial shock waves. The intersection points B,C and D of the initial shock wave and reflected wave rising continuously and causing Mach wave below the intersection point. The intersection points of the initial shock wave,reflected shock wave and Mach wave(Fig.3 B,C,D)is known as the Mach triple-point. If the height of the air pressure sensor exceeds that of Mach triple-point, the overpressure of double peak will be measured. The first peak is the initial shock wave and the second peak is reflected shock wave. With the increase of the distance between the gauge position and the initiation point, the height of the Mach triple-point increases gradually.When the height of the Mach triple-point is higher than that of the air pressure sensor, the air pressure sensor will measure a single peak wave, that is, Mach wave. After Mach wave is formed, the overpressure measured by ground sensor is the bottom pressure of Mach stem.

Fig.6. Mean value of peak overpressure of the three explosives vs.horizontal distance from initiation point.

Typical overpressure curves of composition B, RDX and aluminized explosive measured in the air and on the ground are presented in Fig.4.The initial shock pressures of the three explosives is significantly different. AtR=3.0 m, the air peak overpressure of RDX explosive(174.61 kPa)is 12.3%higher than that of composition B explosive(155.53 kPa).And,the ground peak overpressure of RDX explosive (372.91 kPa) is 26.4% higher than that of composition B explosive (294.94 kPa). Different initial shock pressures of the explosives result in the difference of reflection pressure.AtR=3.0 m,there are two peaks of overpressure in the air shock wave formed by each explosive, and the first peak overpressure is much larger than that of the second. When the second peak overpressure decreases,the negative peak overpressure which is lower than normal atmospheric pressure appears, and the value of negative peak overpressure is slightly larger than that of the second peak overpressure. Then the air pressure atR=3.0 m remains below the normal atmospheric pressure for a long time.AtR=5.0 m,there are two peaks of overpressure in the air of the three explosives,and the value of the second peak is larger than the first one.The first peak value of overpressure is the initial shock wave propagating in the air, and the second peak value is the reflection shock wave produced on the ground.Because the intensity of reflected shock wave is greater than that of initial shock wave,for the same explosive,the time interval between the peak value of reflected shock wave and the peak value of initial shock wave atR=7.0 m decreases compared with that atR=5.0 m. AtR=7.0 m, the time interval between the peak overpressure of reflected shock wave and the peak overpressure of initial shock wave generated by composition B explosives is the largest, followed by RDX explosives, and the aluminized explosives is the smallest. AtR=9.0 m, the reflected shock wave caught up with the initial shock wave and combined into a single peak wave(Mach wave).AtR=9.0 m,the height of the Mach stem is larger than that of the air pressure sensor (1.57 m).The shock waves of the three types of explosives measured by ground pressure sensors are all single peak shock waves.

Fig. 7. Overpressure difference of RDX and aluminized explosives compared to Composition B.

3.2. Analysis of peak overpressure and impulses

The spread for each location of experimental measured values and their mean ones are shown in Figs. 5 and 6, the peak overpressure in the air atR=3.0 m,5.0 m and 7.0 m is the initial shock wave pressure. For all the cases, the maximum deviation between the measured values and their mean ones is 6.8%. In the range ofR=3.0-7.0 m,the ground peak overpressure of the same explosive is larger than that of the initial shock wave measured by the air sensor.When the distance from the detonation point is larger than 9.0 m, the shock wave measured by the air sensor is Mach wave,and its peak overpressure is basically consistent with the ground peak overpressure.Overpressure difference of RDX and aluminized explosives compared to Composition B are illustrated in Fig. 7. At the same position from the initiation point, the peak overpressure of composition B explosive is less than that of RDX and aluminized explosives both in the air and on the ground.AtR=3.0 m,the peak overpressure of aluminized explosives is slightly less than that of RDX explosives. Because of the exothermic effect of aluminized explosives, the pressure drop of aluminized explosives is slower than that of RDX explosives. Therefore, the peak overpressure of aluminized explosives is larger than that of RDX explosives at the distance ofR=5.0 m.

Fig. 8. Error bar of impulse of the three explosives vs. horizontal distance from initiation point.

Fig. 9. Mean value of impulse of the three explosives vs. horizontal distance from initiation point.

Variation in impulse of the three explosives with horizontal distance from initiation point are shown in Figs.8 and 9. In all the cases, the maximum deviation of the positive impulses between the measured values and their mean ones is 2.9%.ForR=3.0 m,the positive impulses of the same explosive measured by the ground sensor is much larger than that measured by the air sensor. In the range ofR=3.0-4.0 m, the ground impulse decreases rapidly.Therefore, when the distance from the detonation point is larger than 4.0 m,the difference of the impulses between the air and the ground is small. Impulse difference of RDX and aluminized explosives compared to Composition B are illustrated in Fig.10. At the same position from the initiation point, both in the air and on the ground, among the three kinds of explosives, the impulse of aluminized explosives is the maximum and the impulse of composition B explosives is the minimum.

Fig.10. Impulse difference of RDX and aluminized explosives compared to Composition B.

Fig.11. Propagation of shock wave front caused by composition B.

Fig. 12. Height of Mach triple point vs. horizontal distance from initiation point(composition B, RDX and aluminized explosives).

3.3. Propagation of shock wave and flame

Fig.11 shows the initial shock wave, reflected shock wave and Mach wave of explosive B at different distances from the detonation point by solving the difference between two adjacent photographs of high-speed camera by Photoshop (PS) software. As the actual distance between the marker rods (15.0 m) is known, the height of Mach triple points at different positions can be calculated by comparing the image distance with the actual distance. The height of Mach triple points of the three kinds of explosives varies with the distance from the initiation point are shown in Fig.12. In all the cases, the maximum deviation of the height of Mach triple points between the calculated values and their mean ones is 4.8%.With the increase of the horizontal distance from the initiation point, the height of Mach triple points of the three explosives increases. At the same horizontal distance from the initiation point,there is little difference in the height of Mach triple points caused by aluminized explosive and RDX explosive,and the height of Mach triple points of composition B explosive is much smaller than that of other two explosives. AtR=7.0 m, the mean height of Mach triple points obtained by high-speed camera(1.49 m for aluminized explosive) is less than that of the air sensor (1.57 m). Meanwhile,the double peaks measured by the air sensor atR=7.0 m prove that the height of Mach triple points is less than the height of the air sensor. The results demonstrate that the measurement techniques of both pressure sensors and high-speed camera have high reliability.

Fig.13. Typical explosive fireball shape of the three kinds of explosives.

Fireball shape of the three kinds of explosives exploded at different time is shown in Fig.13. The state of fireball formed by explosion of aluminized explosive is quite different from that of composition B explosive and RDX explosive.The fireball formed by the explosion of aluminized explosive is bright white,and there are a lot of powdery substances on the edge of the fireball,presumably to be aluminum powder. The mean maximum diameter of the fireball formed by aluminized explosive is 7.13 m, which is larger than that of composition B explosive (6.15 m) and RDX explosive(6.09 m). Meanwhile, the maximum diameter duration of an explosive fireball formed by aluminized explosive is the longest,followed by composition B explosive, and fireball formed by RDX explosive has the shortest duration.

4. Conclusions

In this paper, the characteristics of the free-field shock wave,ground reflection shock wave,Mach wave and flame propagation of composition B explosive, RDX explosive and aluminized explosive under near-ground explosion are experimentally investigated.The main findings are summarized as follows:

1. At the same horizontal distance from the initiation point, the peak overpressure of explosive shock wave of composition B explosive,both in the air and on the ground,is less than that of RDX and aluminized explosives. AtR=3.0 m, the peak overpressure of aluminized explosives is slightly less than that of RDX explosives.Due to the exothermic effect of aluminized explosives, the pressure drop of aluminized explosives is slower than that of RDX explosives.AtR=5.0 m,the peak overpressure of aluminized explosives is larger than that of RDX explosives.At the same position from the initiation point, among the three kinds of explosives,the impulse of aluminized explosives is the maximum and the impulse of composition B explosives is the minimum.

2. With the increasing horizontal distance from the initiation point, the height of Mach triple points of the three explosives increases gradually. At the same horizontal distance from the initiation point, there is little difference in the height of Mach triple points between aluminized explosive and RDX explosive,and the height of Mach triple points of B explosive is much lower than the other two explosives.

3. The maximum diameter and duration of the fireball formed by aluminized explosive are the largest, followed by explosive B,and explosive fireball formed by RDX explosive is the shortest.

Acknowledgments

This research is supported by the National Natural Science Foundation of China (No. 11732003), Beijing Natural Science Foundation (No. 8182050), Science Challenge Project (No.TZ2016001)and National Key Research and Development Program of China (No. 2017YFC0804700).