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Investigation on energy output structure ofexplosives near‑ground explosion

Xu, Wen‑long; Wang, Cheng; Yuan, Jian‑ming; Goh, Wei‑liang; Li, Tao

2019

Xu, W., Wang, C., Yuan, J., Goh, W., & Li, T. (2019). Investigation on energy output structureof explosives near‑ground explosion. Defence Technology, 16(2), 290‑298.doi:10.1016/j.dt.2019.08.004

https://hdl.handle.net/10356/145129

https://doi.org/10.1016/j.dt.2019.08.004

© 2020 China Ordnance Society. Production and hosting by Elsevier B.V. on behalf of KeAiCommunications Co. This is an open access article under the CC BY‑NC‑ND license(http://creativecommons.org/licenses/by‑nc‑nd/4.0/).

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Defence Technology 16 (2020) 290e298

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Defence Technology

journal homepage: www.keaipubl ishing.com/en/ journals /defence-technology

Investigation on energy output structure of explosives near-groundexplosion

Wen-long Xu a, Cheng Wang a, *, Jian-ming Yuan b, Wei-liang Goh b, Tao Li a

a State Key Lab of Explosion Science and Technology, Beijing Institute of Technology, Beijing, 100081, Chinab Temasek Laboratories, Nanyang Technological University, 50 Nanyang Drive, 637553, Singapore

a r t i c l e i n f o

Article history:Received 6 May 2019Received in revised form23 July 2019Accepted 5 August 2019Available online 10 August 2019

Keywords:Near-ground blastReflected waveMarcher steamAluminized explosives

* Corresponding author.E-mail address: wangcheng@bit.edu.cn (C. Wang)Peer review under responsibility of China Ordnan

https://doi.org/10.1016/j.dt.2019.08.0042214-9147/© 2020 China Ordnance Society. ProductioND license (http://creativecommons.org/licenses/by-n

a b s t r a c t

In order to give the energy output structure of typical explosives near-ground explosion in real groundconditions, the free-field shockwave, ground reflection shockwave and Mach wave overpressure timehistory of composition B explosive, RDX explosive and aluminized explosive were measured by airpressure sensors and ground pressure sensors. The shape of the free-field shock wave, ground reflectionshock wave, and Mach wave and explosion flame were captured by high-speed camera. The experi-mental results show that, at the same horizontal distance from the initiation point, the peak over-pressure of explosive shock wave of composition B explosive, both in the air and on the ground, is lessthan that of RDX and aluminized explosives. At a distance of 3.0m from the initiation point, the peakoverpressure of aluminized explosives is slightly less than that of RDX explosives. Owing to theexothermic effect of aluminum powder, the pressure drop of aluminized explosives is slower than that ofRDX explosives. At 5.0m from the initiation point, the peak overpressure of aluminized explosives islarger than that of RDX explosives. At the same position from the initiation point, among the three kindsof explosives, the impulse of aluminized explosives is the maximum and the impulse of composition Bexplosives is the minimum. With the increase of the horizontal distance from the initiation point, theheight of Mach triple-points (Mach steam) of the three explosives increases gradually. At the samehorizontal distance from the initiation point, there is poorly difference in the height of Mach triple-pointsbetween aluminized explosive and RDX explosive, and the height of Mach triple-points of composition Bexplosive is much smaller than that of other two explosives. The maximum diameter and duration of thefireball formed by aluminized explosives are the largest, followed by composition B explosive, and themaximum diameter and duration of the fireball formed by RDX explosive are the smallest.

© 2020 China Ordnance Society. Production and hosting by Elsevier B.V. on behalf of KeAiCommunications Co. This is an open access article under the CC BY-NC-ND license (http://

creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

Blast wave is one of the main damage models of the explosionon personnel, equipment and protective structure. Therefore, thestudy of energy output structure of the explosion wave is of greatsignificance to the safety protection in weapon test, blasting engi-neering, explosive industry and other areas [1e5].

In the past, the characteristics of energy output of various ex-plosives near-ground explosion were studied. The parameters ofTNT, PBX and Hexel blasting in the air explosion were tested by

.ce Society

n and hosting by Elsevier B.V. on bec-nd/4.0/).

Wang et al. [6]. The peak overpressure and impulse values of thethree explosives at different positions were given. The resultsshowed that, at the same proportional distance, the peak over-pressure and impulse of TNT are significantly lower than those ofPBX and Hexel explosives. Ripley et al. [7] studied the effect ofafterburning energy release and ground reflection on near-fieldimpulse of metalized explosives (TBX and IEF). Due to the after-burning of detonation products and metal particle additives, theTBX and IEF explosive impulse performed as well as or better thanthe C4 explosive in the near field. Feng et al. [8] experimentallyinvestigated the effects of adding ammonium perchlorate (AP) toRDX, HMX and aluminized explosives on the shockwave over-pressure, duration and explosion temperature of air explosion.Zhong et al. [9] built a high resolution and high precision pressuremeasurement system. Based on this system, the explosion

half of KeAi Communications Co. This is an open access article under the CC BY-NC-

Fig. 1. Explosives used in the experiments.

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overpressure of TNT with different mass was obtained and a newempirical formula between peak overpressure and proportionaldistance of shockwave was proposed.

To convenient investigate the energetic output of all kinds ofexplosives, the TNT equivalence was widely concerned. Rigby et al.[10] investigated the TNT equivalence of PE4 by simulations andexperiments. Experimental results were compared to a series ofnumerical analyses conducted with different masses of TNTexplosive. The results indicated that a TNT equivalence of 1.2 bestdescribes the blast waves produced from PE4 detonations. Acomprehensive study of equivalent mass factors for far-field deto-nations have been conducted but almost no equivalent mass factorsare available for near-field detonations. Shin et al. [11] proposedTNT equivalency mass factors for four explosives (PETN, Composi-tion B, Pentolite and Tetryl) considering incident and normally re-flected peak overpressure and scaled impulse, in the near-fielddetonations. Grisaro and Edri [12] studied the TNT equivalencyfactors by verified one-dimensional numerical simulations. A newapproach was presented to calculate the equivalency factor forimpulse and overpressure through a single gauge measurement.The results show that the equivalency factor strongly depends onthe internal energy ratio of the explosive.

In recent years, the influence of the ambient environment on

Fig. 2. Experime

explosive energy output has also been widely concerned. Silnikovet al. [13] investigated the effect of ambient pressure on the blastwave parameters resulted from high explosive. They found thatwhen the surrounding pressure was reduced, both pressure andimpulse resulted from explosions decreased at a certain distancefrom the blast source. Veldman et al. [14] studied the effect ofambient pressure on reflected blast impulse and overpressure onspherical C4 charges of 226.8 g. The results indicated that, at 61 cmstandoff, variation of ambient pressure had no significant effect onpeak reflected overpressures. However, with the increase ofambient pressure, the reflected impulses increased. To studied theeffect of rapid afterburn on the shock development, Tyas et al. [15]conducted a series of tests to measure the reflected pressure actingon a rigid target in inert atmospheres and oxygen-rich atmo-spheres. The results show that early-stage afterburn has a signifi-cant influence on the reflected shock parameters in the near-field.The expanding detonation product cloud remains luminous formuch longer durations in air than in nitrogen, indicating thatafterburn reactions are ongoing at the cloudeair interface. Duanet al. [16] tested aluminized explosives with different aluminum tooxygen (Al/O) ratio and studied the influence on pressure proper-ties of confined explosion. Jiba et al. [17] investigated the effects of afine water mist environment on the detonation of PE4 explosive

ntal setup.

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

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charges in a semi-confined blast chamber. They discussed thedetonation parameters including arrival time of the shock waves,peak overpressures, specific impulse of the positive phase, period ofthe negative phase and the specific impulse of the multiple

Fig. 4. Typical overpressure curves of composition B, RDX an

reflections in the mist condition compared to the atmosphericcondition.

Previous studies mainly focused on the two parameters ofexplosive shock wave pressure and impulse. In fact, Mach wave

d aluminized explosive at different measuring positions.

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plays an important role in the energy output structure of explosivesnear-ground explosion. However, up to now, few studies on Machwave propagation characteristics of large equivalent explosion inreal ground conditions have been published. Because of thecomplexity of reflection of a shock wave in real ground condition,experimental study is the most reliable method to analyze theenergy output structure of near-ground explosion [18e20]. In thispaper, the time history of the free-field shock wave, groundreflection shock wave and Mach wave overpressure of compositionB, RDX and aluminized explosives blasting near the ground weremeasured using air pressure sensors and ground pressure sensors.The shape of the free-field shock wave, ground reflection shockwave, Mach wave and explosion flame were captured by high-speed camera. The peak overpressure and impulses of air andground shock wave at different horizontal distances from the

Fig. 5. Error bar of peak overpressure of the three expl

initiation point of three explosives blasting near the ground werecompared. The propagation law of the quantified height of Machtriple-points with the horizontal distance from the initiation pointof the three explosives was calculated, and the characteristics of theexplosion flame size and duration of the three explosives wereanalyzed.

2. Experimental methodology

2.1. The samples of explosives

As shown in Fig. 1, the explosives used in the experiments arecomposition B explosive casted by a mixture of 60.0% RDX and40.0% TNT with density of 1.68 g/cm3. The density of pressed RDXexplosive and aluminized explosive is 1.72 g/cm3 and 1.80 g/cm3,

osives vs. horizontal distance from initiation point.

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respectively. The aluminized explosives are composed by pressing65.0% RDX explosives, 30.0% aluminum powder and 5.0% adhesives.The weight of cylindrical explosive with diameter of 100.0mm is2.0 kg. Three groups of repeated tests were conducted for eachexplosive.

2.2. Experimental setup

The experimental setups are illustrated in Fig. 2. The groundcondition is natural soil and the explosive is fixed on the woodenbench with the height of 1.5m. Two types of sensors are used, oneis the air pressure sensor at a height of 1.57m, and the horizontaldistance from the initiation point is R¼ 3.0m, 5.0m, 7.0m, 9.0mand 11.0m respectively. The other is ground pressure sensor with adistance from the initiation point of R¼ 3.0m, 4.0m, 5.0m, 6.0m,7.0m, 9.0m and 11.0 m. The explosives are located at the center ofthe connecting line between two marker rods. The high-speedcamera is perpendicular to the plane of the marker rods, and thedistance between the marker rods is 15.0m. At R¼ 3.0m, an airpressure sensor produced by Kistler company of 6233A0050(0e0.34MPa) is used. At other air positions, air pressure sensors of6233A0025 (0e0.17MPa) is placed. At R¼ 3.0m and 4.0m, theground pressure sensors produced by Kistler company of 211B4(0e1.40MPa) are used. At other ground positions, the groundpressure sensors of 211B6 (0e0.34MPa) are adopted. A dataacquisition instrument of TraNET 404 with 16 channels is used torecord the messages from air and ground pressure sensors. Thesampling rate of the data acquisition instrument is 10MS/s/CH. TheFASTCAM SA5 high-speed camera produced by Photron Companyof Japan is used. The shooting speed is set to 7000 frames persecond 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 isshown in Fig. 3. After the detonation of an explosive at a certainheight from the ground, the explosive shock wave propagatesoutward with spherical wave (purple line). Then it contacts theground (Fig. 3 A) and generates reflected wave (green line). Thereflected shock waves gradually catch up with the initial shockwaves. The intersection points B, C and D of the initial shock waveand reflected wave rising continuously and causing Mach wavebelow the intersection point. The intersection points of the initialshock wave, reflected shock wave and Mach wave (Fig. 3 B, C, D) isknown as the Mach triple-point. If the height of the air pressure

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

sensor exceeds that of Mach triple-point, the overpressure ofdouble peak will be measured. The first peak is the initial shockwave and the second peak is reflected shock wave. With the in-crease of the distance between the gauge position and the initiationpoint, the height of the Mach triple-point increases gradually.When the height of the Mach triple-point is higher than that of theair pressure sensor, the air pressure sensor will measure a singlepeak wave, that is, Mach wave. After Mach wave is formed, theoverpressure measured by ground sensor is the bottom pressure ofMach stem.

Typical overpressure curves of composition B, RDX and alumi-nized explosive measured in the air and on the ground are pre-sented in Fig. 4. The initial shock pressures of the three explosives issignificantly different. At R¼ 3.0m, the air peak overpressure ofRDX explosive (174.61 kPa) is 12.3% higher than that of compositionB explosive (155.53 kPa). And, the ground peak overpressure of RDXexplosive (372.91 kPa) is 26.4% higher than that of composition Bexplosive (294.94 kPa). Different initial shock pressures of the ex-plosives result in the difference of reflection pressure. At R¼ 3.0m,there are two peaks of overpressure in the air shock wave formedby each explosive, and the first peak overpressure is much largerthan that of the second. When the second peak overpressure de-creases, the negative peak overpressurewhich is lower than normalatmospheric pressure appears, and the value of negative peakoverpressure is slightly larger than that of the second peak over-pressure. Then the air pressure at R¼ 3.0m remains below thenormal atmospheric pressure for a long time. At R¼ 5.0m, there aretwo peaks of overpressure in the air of the three explosives, and thevalue of the second peak is larger than the first one. The first peakvalue of overpressure is the initial shock wave propagating in theair, and the second peak value is the reflection shock wave pro-duced on the ground. Because the intensity of reflected shock waveis greater than that of initial shock wave, for the same explosive, thetime interval between the peak value of reflected shock wave andthe peak value of initial shock wave at R¼ 7.0m decreasescompared with that at R¼ 5.0m. At R¼ 7.0m, the time intervalbetween the peak overpressure of reflected shock wave and thepeak overpressure of initial shockwave generated by composition Bexplosives is the largest, followed by RDX explosives, and thealuminized explosives is the smallest. At R¼ 9.0m, the reflectedshock wave caught up with the initial shock wave and combinedinto a single peak wave (Mach wave). At R¼ 9.0m, the height of theMach stem is larger than that of the air pressure sensor (1.57m).The shock waves of the three types of explosives measured by

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

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ground pressure sensors are all single peak shock waves.

3.2. Analysis of peak overpressure and impulses

The spread for each location of experimental measured valuesand their mean ones are shown in Figs. 5 and 6, the peak over-pressure in the air at R¼ 3.0m, 5.0m and 7.0m is the initial shockwave pressure. For all the cases, the maximum deviation betweenthe measured values and their mean ones is 6.8%. In the range ofR¼ 3.0e7.0m, the ground peak overpressure of the same explosiveis larger than that of the initial shock wave measured by the airsensor. When the distance from the detonation point is larger than9.0m, the shock wave measured by the air sensor is Mach wave,and its peak overpressure is basically consistent with the groundpeak overpressure. Overpressure difference of RDX and aluminizedexplosives compared to Composition B are illustrated in Fig. 7. At

Fig. 8. Error bar of impulse of the three explosives

the same position from the initiation point, the peak overpressureof composition B explosive is less than that of RDX and aluminizedexplosives both in the air and on the ground. At R¼ 3.0m, the peakoverpressure of aluminized explosives is slightly less than that ofRDX explosives. Because of the exothermic effect of aluminizedexplosives, the pressure drop of aluminized explosives is slowerthan that of RDX explosives. Therefore, the peak overpressure ofaluminized explosives is larger than that of RDX explosives at thedistance of R¼ 5.0m.

Variation in impulse of the three explosives with horizontaldistance from initiation point are shown in Figs. 8 and 9. In all thecases, the maximum deviation of the positive impulses betweenthe measured values and their mean ones is 2.9%. For R¼ 3.0m, thepositive impulses of the same explosive measured by the groundsensor is much larger than that measured by the air sensor. In therange of R¼ 3.0e4.0m, the ground impulse decreases rapidly.

vs. horizontal distance from initiation point.

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

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

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Therefore, when the distance from the detonation point is largerthan 4.0m, the difference of the impulses between the air and theground is small. Impulse difference of RDX and aluminized explo-sives compared to Composition B are illustrated in Fig. 10. At thesame position from the initiation point, both in the air and on theground, among the three kinds of explosives, the impulse ofaluminized explosives is the maximum and the impulse ofcomposition B explosives is the minimum.

Fig. 10. Impulse difference of RDX and aluminized explosives compared to Composi-tion B.

Fig. 11. Propagation of shock wave

3.3. Propagation of shock wave and flame

Fig. 11 shows the initial shock wave, reflected shock wave andMach wave of explosive B at different distances from the detona-tion point by solving the difference between two adjacent photo-graphs of high-speed camera by Photoshop (PS) software. As theactual distance between the marker rods (15.0m) is known, theheight of Mach triple points at different positions can be calculatedby comparing the image distance with the actual distance. Theheight of Mach triple points of the three kinds of explosives varieswith the distance from the initiation point are shown in Fig. 12. Inall the cases, the maximum deviation of the height of Mach triplepoints between the calculated values and their mean ones is 4.8%.With the increase of the horizontal distance from the initiationpoint, the height of Mach triple points of the three explosives in-creases. At the same horizontal distance from the initiation point,there is little difference in the height of Mach triple points causedby aluminized explosive and RDX explosive, and the height of Machtriple points of composition B explosive is much smaller than thatof other two explosives. At R¼ 7.0m, the mean height of Machtriple points obtained by high-speed camera (1.49m for aluminizedexplosive) is less than that of the air sensor (1.57m). Meanwhile,the double peaks measured by the air sensor at R¼ 7.0m prove thatthe height of Mach triple points is less than the height of the airsensor. The results demonstrate that the measurement techniquesof both pressure sensors and high-speed camera have highreliability.

front caused by composition B.

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

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Fireball shape of the three kinds of explosives exploded atdifferent time is shown in Fig. 13. The state of fireball formed byexplosion of aluminized explosive is quite different from that ofcomposition B explosive and RDX explosive. The fireball formed bythe explosion of aluminized explosive is bright white, and there area lot of powdery substances on the edge of the fireball, presumablyto be aluminum powder. The mean maximum diameter of thefireball formed by aluminized explosive is 7.13m, which is largerthan that of composition B explosive (6.15m) and RDX explosive(6.09m). Meanwhile, the maximum diameter duration of anexplosive fireball formed by aluminized explosive is the longest,followed by composition B explosive, and fireball formed by RDXexplosive has the shortest duration.

4. Conclusions

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

1. At the same horizontal distance from the initiation point, thepeak overpressure of explosive shock wave of composition Bexplosive, both in the air and on the ground, is less than that ofRDX and aluminized explosives. At R¼ 3.0m, the peak over-pressure of aluminized explosives is slightly less than that ofRDX explosives. Due to the exothermic effect of aluminized ex-plosives, the pressure drop of aluminized explosives is slowerthan that of RDX explosives. At R¼ 5.0m, the peak overpressure

of aluminized explosives is larger than that of RDX explosives. Atthe same position from the initiation point, among the threekinds of explosives, the impulse of aluminized explosives is themaximum and the impulse of composition B explosives is theminimum.

2. With the increasing horizontal distance from the initiationpoint, the height of Mach triple points of the three explosivesincreases gradually. At the same horizontal distance from theinitiation point, there is little difference in the height of Machtriple points between aluminized explosive and RDX explosive,and the height of Mach triple points of B explosive is muchlower than the other two explosives.

3. The maximum diameter and duration of the fireball formed byaluminized 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 ScienceFoundation of China (No. 11732003), Beijing Natural ScienceFoundation (No. 8182050), Science Challenge Project (No.TZ2016001) and National Key Research and Development Programof China (No. 2017YFC0804700).

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