Study on energy release characteristics of reactive material casings under explosive loading

Ning Du ,Wei Xiong ,To Wng ,Xin-feng Zhng ,* ,Hi-hu Chen ,Meng-ting Tn

a School of Mechanical Engineering,Nanjing University of Science & Technology,Nanjing,Xiaolingwei 200,Jiangsu,210094,China

b School of Materials Science and Technology,Nanjing University of Aeronautics and Astronautics,Nanjing,Yudaojie 29,Jiangsu,211106,China

Keywords: Reactive materials Explosive loading Shock-induced chemical reaction Energy release characteristics Fragmentation

ABSTRACT Reactive Materials(RMs),a new material with structural and energy release characteristics under shockinduced chemical reactions,are promising in extensive applications in national defense and military fields.They can increase the lethality of warheads due to their dual functionality.This paper focuses on the energy release characteristics of RM casings prepared by alloy melting and casting process under explosive loading.Explosion experiments of RM and conventional 2A12 aluminum alloy casings were conducted in free field to capture the explosive fireballs,temperature distribution,peak overpressure of the air shock wave and the fracture morphology of fragments of reactive material(RM)warhead casings by using high-speed camera,infrared thermal imager temperature and peak overpressure testing and scanning electron microscope.Results showed that an increase of both the fireball temperature and air shock wave were observed in all RM casings compared to conventional 2A12 aluminum ally casings.The RM casings can improve the peak overpressure of the air shock wave under explosion loading,though the results are different with different charge ratios.According to the energy release characteristics of the RM,increasing the thickness of RM casings will increase the peak overpressure of the near-field air shock wave,while reducing the thickness will increase the peak overpressure of the far-field air shock wave.? 2021 China Ordnance Society.Publishing services by Elsevier B.V.on behalf of KeAi Communications

1.Introduction

RMs are broadly defined as energetic compounds that will release large amount of combustion energy after impact or dynamic loading [1-3].RMs have many potential applications such as in reactive shaped charge liners,reactive material projectiles and fragmentation due to their dual functionality.The application of reactive materials has been extended to warhead casings,which can enhance the power of the warhead.In order to expand the application fields of RMs (e.g.improvement of warhead power),many studies have been conducted mainly on the mechanical properties [2,3] and energy release behavior of RMs under shock loading [4,5].At medium and low strain rates,Taylor test [6] and Split Hopkinson Pressure Bar [7] have been widely used to study the dynamic mechanical behavior of RMs.At high strain rates,the explosive loading device,powder gun and compressed gas gun are commonly used to explore the energy release characteristics of RMs under impact reaction [8,9].

The strength of RMs can reach hundreds of MPa that they can replace the conventional inert materials for warhead casings to enhance the energy output of weapons,such as peak overpressure and explosion pulse [10,11].The destructive effect of warhead explosion on the target includes the direct effect of explosive products,the destructive effect of air shock waves and the killing effect of fragments formed by warhead casing rupture.The conventional aluminum casing hardly consumes the energy stored in the material under explosive loading.In addition,large fragments (about mm) produced by the aluminum casing are not able to ignite and damage long-distance targets,which has no additional strengthening effect on the initial explosion.Increasing the thickness of the inert casing will increase the energy (released by explosives)consumed by the deformation and fragmentation of the casing as well as fragment dispersion,but reduces the energy consumed by the expansion of explosive products and the formation of shock waves.Therefore,increasing the thickness of the inert casing will reduce the peak overpressure of the air shock wave[12].Compared with the inert casings,fragments produced by the RM casings can not only produce kinetic energy damage,but also couple this kinetic energy with the secondary energy release generated by the rapid chemical reactions during fragment impact,which has an additional strengthening effect on the air shock wave [13].The greater the thickness of the RM casings is,the higher the chemical energy contained in the reactive casings is.Meanwhile,more explosive detonation energy are consumed for fragment forming and reactive reaction activation.At present,the complex response behavior and energy release characteristics of RM casings under explosive loading are worth studying.

The study of energy release characteristics of RMs mainly focuses on the impact release behavior [14,15].Clemenson [16]studied the energy release law of implosion of RMs by measuring the transient and quasi-static pressures,determining the effectiveness of reaction enhancement of each alloy and discussing the influence of end confined explosion and impact-induced fragment reaction.In addition,the size and distribution of casing fragments were obtained by analyzing the coarse and small fragments recovered from the test.Fabignon et al.[17] added reactive metals to high explosives and propellants from the perspective of energy release of RMs to improve the energy density and energy output capacity of materials.Guadarrama et al.[18] did research on the energy output characteristics of implosion of powder RMs with different formulations and measured the quasi-static pressure and transient pressure generated by explosion.Compared with inert materials,the quasi-static pressure and peak overpressure of the air shock wave increased.Koga et al.[19-21] prepared magnesium aluminum alloy powder with controllable particle size and improved the combustion performance.Part of aluminum was replaced by magnesium,which reduced the total theoretical volume or combustion enthalpy and improved the reaction efficiency.Ames et al.[22,23]designed the secondary impact test in an airtight container.It was found that the impact velocity had a major influence on the energy release behavior of RMs,and the impact velocity range of initiating reaction was obtained.A higher impact velocity could improve the pressure and chemical reactivity in the airtight container.The scholars mentioned above studied the energy release characteristics of RMs under explosive loading in a closed condition by experimental methods,and compared the differences of warhead energy output for different formulations and different casing geometries,but paid less attention to the influence of RM casings with the same formulation on warhead energy output in free field.At present,the research on the energy release characteristics of RM casings in the air explosion are still in the exploratory stage,and there are few literature reports.Therefore,it is important to analyze the energy release characteristics of RM casings under explosive loading,and determine the temperature of the explosion field,the propagation characteristics of air shock waves of RM casings as well as the strengthening effect of air shock waves at different distances,which can provide valuable theoretical guidance.

In this paper,the temperature of the infrared thermal image and the overpressure of RMs were tested for the conventional 2A12 aluminum alloy (similar to 6061 Al alloy) casings and RM casings with different thicknesses were measured,and the parameters of explosion fireballs were observed using high-speed camera.The fracture morphology of recovered fragments for different materials was observed by scanning electron microscope,followed by the analysis of the fracture morphology characteristics and the average oxygen content on surface.The characteristics and fireball temperature of the RM casings and conventional 2A12 aluminum alloy casings with typical thicknesses under explosive loading were analyzed,and the attenuation law of air shock waves under explosive loading of RMs was studied.This study provides reference for the engineering application of RMs.

2.RM casing preparation

The authors referred to the preparation process of typical RMs[24,25] in many studies.In this study,the RM casings were prepared by the alloy smelting and casting process.The absolute combustion heat of the RM casings measured by the oxygen bomb calorimeter in a pure oxygen atmosphere,was about 23.2 kJ/g.

2.1.Test materials

The main raw material used in the test is industrial pure aluminum,sponge zirconium and Mg-Zn master alloy with a purity of more than 99 wt%.Before batching,the surface of the raw material are polished to remove the oxide skin,and then degreased,cleaned and dried for further use.

2.2.Preparation of RM casings

Pure aluminum was placed in the CaO crucible of the ZG-10 vacuum induction melting furnace (Shanghai Chenhua Technology Co.,Ltd.),and the sponge zirconium,Mg-Zn master alloy and metal calcium were added into the feeder.Before smelting,the furnace was vacuumed to reduce the interference of oxygen in the air with the test results.The vacuum degree was controlled at 0.01 pa.Argon (Ar) was flushed into the furnace until the air pressure was not less than 0.5 atm.The pure aluminum was melted at 750°C by electric induction heating.After the aluminum solution was clear,sponge zirconium and Mg-Zn master alloy were added in turn and the solution was stirred.When the alloy solution became clear,the alloy liquid was stirred and vacuumed until the pressure in the furnace was not higher than 0.09 atm.More argon was flushed into the furnace to make the air pressure in the furnace not less than 0.5 atm.The alloy liquid was stirred again.Then,1%metal calcium was added for deoxidization.When the metal calcium was melted up,the alloy liquid was stirred and vacuumed until the pressure in the furnace was less than or equal to 0.09 atm.Then the power was cut off to reduce the temperature to 680°C and keep it for 15 min.Argon was again flushed into the furnace and the pressure in it was kept not less than 0.8 atm.The power was turned on and heated up the solution to 780°C.When the alloy became clear,it was recast to obtain the required casting.Finally,three kinds of casings with different thicknesses were machined on a lathe to the appropriate dimensions,with masses of 158.6 g,253.1 g and 347.3 g respectively.The RM casings prepared by the alloy smelting and casting process is shown in Fig.1.

The conventional 2A12 aluminum alloy was purchased from commercial vendors and machined on a lathe to the appropriate dimensions,which was used for comparative study to ensure that the quality,inner diameter and other parameters of the inert casings was basically the same as those of the RM casings.The properties of casings from actual measurements are shown in Table 1.Among them,dis the unilateral wall thickness of the casing;Dis the outer diameter of the casing;D0is the inner diameter of the casing;his the height of the casing;msis the mass of the casing made of different materials;η is the mass ratio of casing to explosive charge.

Table 1 Properties of casings from actual measurements.

Fig.1.RM casings.

Fig.2.Structure schematic diagram of confined charge.

Fig.3.Explosive loading device of the RM casing.

The RM used in this paper is a kind of low-density metal oxidation material,which is inert and non-reactive in general.For the selection of RMs,they can be thermite,metal matrix composites(aluminum/nickel),polymer matrix composites (aluminum/polytetrafluoroethylene)and flammable metals(aluminum,zirconium),etc.However,the strength of the material after going through the pressing-sintering process is low,which is not adequate to replace the conventional inert material casings in warheads.The total energy output can be increased using the RM casings made of aluminum,sponge zirconium and Mg-Zn master alloy.In this paper,they are studied as ingredients of RM casings.

2.3.Explosive loading device

The structure schematic diagram of the confined charge is shown in Fig.2,and the explosive loading device of the RM casing is shown in Fig.3,which includes the JH-2 charge,detonator,booster,explosive charge,casing and end cover with a diameter of 60 mm.The parameters of detonator,booster,explosive charge and end cover were stay the same in each experimen,and the casing thickness was the only variable.Therefore,the author believes that the changes observed were caused by the changes of the casing(composition or thickness).The height of the charge was 110 mm,which was initiated from the center of the upper end.The energy release characteristics of the conventional 2A12 aluminum alloy and RM casings with different mass ratios of casing to explosive charge (η=0.30,0.48,0.66) under explosion loading were compared.

2.4.Experimental setup

The schematic diagram of the experimental layout is shown in Fig.4.The charge was placed on a PVC pipe support,1.5 m from the ground.Four dynamic pressure gauges were used,and the distance from the sample to each pressure gauge was 2.5 m,3.5 m,4.5 m and 6 m,respectively.The pencil blast gauge was the 6233AA series piezoelectric blast pressure pencil probe of Kistler Company.A high-speed camera and an infrared thermal imager were used to test the explosion parameters and temperature field distribution characteristics of fireballs.The horizontal distance between the RM explosive loading device,high-speed camera and the infrared thermal imager was 25 m.Among them,the explosive loading process was shot by the high-speed camera at the speed of 5000 frames/s and with a resolution of 1280(H)×240(V)to capture the transient evolution process of the firelight structure in the explosive loading process of different materials.The IRS 669 infrared thermal imager produced by Shanghai Thermal Imaging Electromechanical Technology Co.,Ltd.was used to monitor the temperature characteristics of the explosion fireballs with the 640(H) × 480 (V) resolution and the temperature range of -40°C to 2000°C were selected.According to the previous test experience and the range of emissivity of explosive products given in Refs.[26],the emissivity of the infrared thermal imager was set to 0.42.In order to measure the diameter of fireballs,the steel plate was taken as reference,and the length and width of the steel plate were both 600 mm.In addition,in order to analyze the size and distribution of fragments of the RM casings,the coarse and fine fragments recovered from the test were analyzed.The test recovery device was made up of a 100 mm thick pearl cotton EPE foam and a 10 mm thick rubber sheet.The length and width of the recovery device were 500 mm and it was installed in a wooden box.The front side of the foam board was a nitrile rubber sheet to ensure that no large fragments would escape from the back of the foam board.The foam board was removed from the fixture and a tweezer was used to collect fragments from the foam.

Fig.4.Schematic diagram of the experimental layout.

3.Analysis and discussion

The modes of damage caused by explosive explosion are mainly air shock waves and thermal damage.The thermal damage is mainly characterized by the parameters of the explosion fireballs,which mainly include the diameter,duration and temperature of fireballs.In addition,the peak overpressure of the air shock wave can reflect the characteristics of explosion in air[27].Based on this,three kinds of casings with different thicknesses were tested to analyze the energy release characteristics of RMs.

3.1.Analysis of the growth and reaction process of the explosion fireballs

3.1.1.High-speed camera observation results of the explosion process

The process and evolution of fireball expansion under explosive loading for different casings recorded by the high-speed camera are shown in Fig.5.

The results showed that obvious firelight was generated after explosions.With the passage of time,the firelight generated first intensified and then gradually weakened.When η=0.3 and η=0.66,the duration of the firelight of the RM casings were 24.8 ms and 43 ms(43/24.8=1.7)and gray smoke was found in the process of explosion loading.For the conventional 2A12 aluminum alloy casings,after 6 ms’ duration of the firelight,it began to weaken and finally disappeared,and a large amount of black smoke was produced in the end.With the increase of the η value,the duration of the firelight gradually increased.Comparing various durations,it was found that the duration for different materials increased as the thickness of the casing increased.Among them,when η for the conventional casings rose from 0.3 to 0.48,the duration increased by 8.2 ms (31.8 ms minus 23.6 ms).When η went up from 0.48 to 0.66,the duration increased by 6.4 ms(38.2 ms minus 31.8 ms).The two increases had little difference.However,for the RM casings,the rising trend of the duration of the firelight was different.The increase in duration went up rapidly to 17.8 ms(42.6 ms minus 24.8 ms),and then decreased significantly to only 0.4 ms(43 ms minus 42.6 ms).In addition,each high-speed photo in the figure was scaled at the same scale,and fireball diameters at different moments could be measured in proportion with the steel plate size as reference.When η increased from 0.3 to 0.66,at 6 ms,the diameters of fireballs generated by the conventional casings were 2.30 m,2.00 m and 1.42 m,respectively.This means the diameters decreased gradually by 15% and 40.8%,respectively.In comparison,the diameters of fireballs for the RM casings were 2.45 m,3.57 m and 3.87 m,respectively,meaning the diameter increased significantly by 45.7% and 8.4%,respectively.When η rose from 0.3 to 0.48,for the conventional material,the decrease in diameter reduced slightly,while for the RM,the diameter increased significantly.When η went up from 0.48 to 0.66,for the conventional mateiral,the diameter decreased significantly,while for the RM,the increase in diameter rose significantly.The reason is:for the conventional material,as the thickness of the casings increased,the energy consumed by casing fracture and fragment dispersion increased,resulting in a significant reduction in the fireball diameter and firelight duration.The thickness of the RM casings affected the reaction degree of the RM.As the casing thickness increases,the increase in the firelight duration and in the freball diameter rose rapidly first,and then significantly reduced.This is because with the increase of the RM casing thickness,the reaction degree first increased rapidly (η increased from 0.3 to 0.48),and then slowly (η increased from 0.48 to 0.66),but not significantly.

In order to compare the reaction degree of different materials,the fireball diameter and firelight duration diagram under different conditions are supplemented and compared,so as to show the differences in different situations more clearly.Fig.6 shows a comparison of the fireball diameter and firelight duration for different materials after explosion.The test results showed that the fireball diameter and firelight duration for the RM was greater than those for the conventional 2A12 aluminum alloy,because the RM was broken under strong explosive loading,and the fractured material reacted with the detonation products and air to release energy.In addition,the kinetic energy generated by fragments form the RM casings was coupled with the energy released by the rapid chemical reactions caused by fragment impact,which prolonged the firelight duration,enhanced firelight brightness and increased the diameter of the fireballs.

Within the detonation products of a high explosive,as is fitting for this study,RM with various oxidizing species can typically follow some of these“global”reactions [28]：

Fig.5.Forming and evolution of fireballs for the two charges with different materials.

Fig.6.A comparison of explosion fireball diameter and firelight duration for different materials.

The energy release reaction under explosive loading of the RM casings can be roughly divided into three processes:(1)The oxygen free explosion reaction in the initial stage.The oxygen in the air does not participate in the reaction;it is mainly the reaction of the molecular compounds of the JH-2 explosive charge,and the duration is within 1 μs [27].(2) The non-oxygen combustion reaction after the explosive charge explosion.The products CO2,CO and H2O are in high temperature and pressure after the explosion,which can react with part of the RM casing.In this stage,no external air is required to participate in the reaction,and the duration is less than 1 ms.The difference can be seen from the 0.4 ms fireball forming in Fig.5a and b,5c and 5d,5e and 5f.In addition,the energy of air shock waves is coupled with the energy released by the reaction of part of the RM casing,which enhances the air shock wave.Eqs.(1)-(3)show that the reaction between aluminum/carbon dioxide and aluminum/water and aluminum/carbon monoxide vapor produces energy.(3) The oxygen combustion reaction after the explosive charge explosion is mainly the fast combustion reaction of explosive products (C,CO,H,etc.).RM casing fragments and oxygen in the air,lasting for tens of milliseconds.Eqs.(4)-(7)show that theRM fragment reacts with oxygen in the air.

3.1.2.Analysis of the explosion driven reaction behavior of RM

Fig.7 shows the fragment dispersion at typical moments during the explosive process of different materials.It can be seen from Fig.7(a)and Fig.7(b)that at 1.2 ms,the fragments have scattered around and obvious firelight can be seen at the explosion center and at the dispersed RM as well,while no firelight was seen at the dispersed conventional material,and obvious firelight can be seen at the right side of the material,which was generated due to the impact of small conventional material on the steel plate.The result of Fig.7(b)shows that some dispersed fragments showed obvious firelight,indicating that some RM continues to react during the dispersing process.The test results of RM fragments impacting on the steel plate are shown in Fig.5(f).The RM fragments have hit the steel plate on the right at 6 ms,and obvious firelight can be observed at the right side of the explosion,but no firelight appears at the same position on the left side (without steel plate).This phenomenon shows that chemical reactions in the RM under explosive loading are not complete.At the moment of explosion,only part of the RM participates in the reaction and releases energy under the action of detonation products,and the visible firelight at the explosion center is obviously enhanced.On the other hand,the fragments burst out of the detonation products and disperse around.In this process,the RM reacts continuously and follow-up reaction occurs after impacting on the steel plate.

Test results showed that the casing of the conventional material ruptured,and formed fragments which dispersed around under explosive loading.Chemical reactions took place in the RM casings under explosive loading,releasing some energy and enhancing the firelight produced by explosive explosion.In addition,after the RM casing ruptured,the detonation products rushed out and surrounded the casing.Meanwhile,the fragments were still driven by the detonation products,and finally rushed out of the detonation products and dispersed around.At this time,chemical reactions continued in the RM in the dispersion process,accompanied by obvious firelight.

Fig.7.Experimental results of fragments dispersing at specific moments for different materials.

3.2.Distribution characteristics of the temperature field in the process of explosion

In order to further compare the influence of the RM and conventional 2A12 aluminum alloy casings on the temperature field distribution and radiation performance of explosion fireballs,the infrared thermogram of the relative maximum temperature at typical moments for η=0.66 was analyzed.The process and evolution of fireball expansion under explosive loading for different casings recorded by the high-speed camera are shown in Fig.5.And the smoke was found in the process of explosion loading for different casings.The frame frequency of infrared thermal imager is limited,only one infrared thermogram is captured.Although the time is not clear,the effect of the RM and conventional 2A12 aluminum casings on the explosive fireball temperature can be compared under the same test method.

The thermal image can be divided into three zones from the inside to the outside:high temperature zone,transition zone and low temperature zone.In the figure,the red is high temperature zone,which is mainly concentrated in the explosion center,the outermost pink part is the low temperature zone,and between the two is the transition zone.At the highest temperature,four lines are drawn on the surface of the fireball,including two horizontal lines and two vertical ones.The temperature distribution on the corresponding lines was analyzed.The comparison of Fig.8(a)and Fig.8(b) shows that the surface temperature of the fireball for the conventional material has two peaks in the horizontal direction and one peak in the vertical direction,while the surface temperature of the fireball for the RM has one peak in both the horizontal and vertical directions.This is due to the coupling of energy released by the explosive and the RM casings under explosive loading,which enhances the central temperature of the fireball and prolongs the diffusion time of the central high temperature zone to the surrounding area.The results show that the cracking and reaction of the RM occur first after the explosive detonation,whose energy coupled with the energy released by the RM casing enhances the central temperature.In the longitudinal and horizontal directions,straight lines passes through the high temperature zone directly from the edge of the low temperature zone,and finally passes through the low temperature zone,forming a single peak.For the conventional material,in the longitudinal direction,a straight line passes through the high temperature zone from the edge of the low temperature zone,forming a single peak.In the horizontal direction,a straight line passes through the high temperature zone from the edge of the low temperature zone,and then enters the high temperature zone through the transition zone again,which formed two peaks.The radiation zone of the RM(3.297 m2) is 1.44 times that of the conventional material(2.296 m2),and the high temperature zone of the RM is 4.14 times that of the conventional material(0.2585/0.06237).This is because the RM participates in chemical reactions and releases energy,which enhances the radiation and high temperature zones produced by explosive explosion.The maximum temperatures for the RM and the conventional material are 959.9°C and 847.7°C,respectively.The former is 13.2% higher than the latter.This is because at the moment of explosive explosion,the energy released by the chemical reactions in the RM under the action of detonation pressure has a greater contribution to the temperature rise.

Fig.8.Maximum temperature of explosive fireballs and corresponding temperature variation curves.

3.3.Analysis of propagation characteristics of air shock waves

3.3.1.Results of the air shock wave test

In the air explosion test,during the period of time before the air shock wave generated by the charge explosion is transmitted to the free-field pressure sensor,the ambient pressure of the sensor is the ambient air pressurep0.When the air shock wave reaches the sensor,the pressure rises rapidly top,and then the overpressure slowly decays to the ambient air pressurep0.△p=p-p0is called the air shock wave overpressure.The pressure measured by the sensor in this test is the air shock wave overpressure △p.When the pressure is measured at the distance R,the air shock wave overpressure-time △p(t) curve at this point will be acquired after the air shock wave passes through.The air shock wave overpressure-time curves for the conventional material and RM casings at different distances measured by pressure sensor are shown in Fig.9.It can be seen that the trend of pressure changes with time for the conventional material and RM casings is the same.In addition,the decay rate in the air shock wave overpressure-time curve for the RM is slower than that of the conventional material,which is related to the RM participating in the coupling reaction of detonation products of the explosive charge.With the propagation of the air shock wave,the pressure and other parameters decrease rapidly.This is because the wave front of the air shock wave expands as the propagation distance increases,and the unit area energy on the wavefront decreases rapidly.

The generation of the secondary shock wave is caused by the successive implosion of the sparse wave toward the center from the contact interface between the air and the detonation products.This phenomenon is initially obtained by calculating the detonation process of the explosive charge with the method of characteristics[30].The typical positive pressure duration and peak overpressure under explosive loading are shown in Fig.10.It can be seen that the air shock wave overpressure curve obtained from the test decays rapidly in the initial stage and then slowly,followed by the small amplitude shock wave in the positive pressure area or negative pressure area.By comprehensive analysis,it is determined that the small amplitude shock wave is the secondary shock wave.

The test results in Fig.10 show that the secondary shock wave will appear in the later attenuation stage of the air shock wave overpressure-time curve.If the arrival time of the secondary shock wave is prior to the negative pressure duration,it will have a greater impact on the positive pressure duration,which will extend the positive pressure duration but have a small impact on other positive pressure characteristics,as shown in Fig.10 (a).If the arrival time of the secondary shock wave is after the negative pressure duration,the positive pressure duration will be greatly reduced,and then the positive pressure impulse will be reduced,as shown in Fig.10 (b).

3.3.2.Analysis of the peak overpressure and positive pressure duration of the air shock wave

Parameters related to the description of the air shock wave include the peak overpressure (peak or trough) and propagation speed of the air shock wave.The arrival time,peak overpressure and positive pressure duration of the air shock wave can be used to quantify the instantaneous energy release of the explosive [29].Positive pressure durationt+is a characteristic parameter of the air shock wave,and also one of the important parameters that affect the damage to the target.When the air shock wave is transmitted to the pressure sensor,the pressure suddenly rises to a certain peak value,which is usually referred to as the peak overpressure of the air shock wave.and then in the timet+,the pressure slowly decays to the ambient pressure.The part of the time history when the pressure is greater than the initial ambient pressure becomes the positive pressure durationt+.The peak overpressure △pand the positive pressure durationt+measured at different distances are as shown in Table 2.In Fig.10 (a)t+is the duration of two air shock waves,and in Fig.10(b)t+is the duration of a single air shock wave.

Table 2 Measured results of air shock wave overpressure and positive pressure duration.

The distribution of peak overpressure and positive pressure duration at different distances is shown in Fig.11.Generally,the peak overpressure and positive pressure duration of the air shock wave at different distances for the RM casings with different thicknesses under explosive loading are greater than those for the conventional 2A12 aluminum alloy casings.When the test distance of the air shock wave overpressure sensor is less than 3.5 m,the peak pressure of the air shock wave for the RM decays faster than that for the conventional material,and the positive pressure duration for the RM rises slower than that for the conventional material.When the distance exceeds 3.5 m,the situation is just the opposite.

When η=0.30,the peak overpressure of the air shock wave for the RM is lower than that for the conventional casing in the near field (2.5 m).As the distance increases to the far field (6 m),the opposite happens.This is because when the casing is thin,the RM casings will rupture rapidly and form fragments under the high pressure explosive loading.In this process,a part of energy is needed to activate the chemical reactions in the RM.However,due to the short duraion and low chemical reaction efficiency of the RM,the pressure of the RM is lower in the near-field region (2.5 m).Further drvien by the detonation products,the RM gradually releases chemical energy,which plays an obvious role in strengthening the far-field air shock wave pressure.When η rises from 0.3 to 0.66,at the same distance,the peak overpressure of the air shock wave decreases under the explosion loading of the conventional casings.In this η range,the peak overpressure of the air shock wave decreases significantly under the explosion loading of the RM casings in the near zone (2.5 m) by 42.9 kPa (122.8 kPa minus 79.9 kPa) and in the far field (6 m),it decreases from 24.1 kPa to 19.7 kPa,a small decrease of 18%.The peak overpressure of the air shock wave is similar at 3.5 m and 4.5 m.The experimental results show that the increasing thickness of the RM casings can increase the peak overpressure of the near-field air shock wave,and the decreasing thickness can slightly increase the peak overpressure of the far-field air shock wave.

On the other hand,as the distance increases,the positive pressure duration for the RM and conventional material increases.This is because in the process of air shock wave propagation,the front of the wave propagates at supersonic speed,while the tail of the positive pressure zone propagates at sonic speed corresponding to the ambient air pressure,so the positive pressure zone is continuously widened,prolonging the positive pressure duration.In addition,as the positive pressure zone of the air shock wave widens with the increase of the propagation distance,the amount of compressed air increases,making the average energy of unit mass air decrease.When η is between 0.3 and 0.66,after the explosion loading of the RM and conventional material,the decrease in their positive pressure duration is fairly small and obviously large,i.e.,1.1085 ms (1.5987 ms minus 0.4902 ms) in the near field (2.5 m).When the distance increases to the far field (6 m),the positive pressure duration increases as well.The decrease in the positive pressure duration for the RM under explosive loading in the far field (6 m) is greater than that in the near field (2.5 m),which is 0.8685 ms (6.1565 ms minus 5.288 ms),while the decrease in the positive pressure duration for the conventional material is smaller than that in the near field (2.5 m),which is 1.006 ms (4.5644 ms minus 3.5584 ms).The results show that the thickness of the conventional material casings has a great influence on the positive pressure duration.As the thickness of the conventional material casing increases,the positive pressure duration shortens.On the conrary,the thickness of the RM casings has little effect on the positive pressure duration.Compared with the thicker casings,reducing the thickness of the RM casings is conducive to extending positive pressure duration.

Fig.9.The air shock wave overpressure as a function of time.

It can be seen from Fig.11 that when η values are the same,the arrival time of the secondary shock wave for the RM is less than that for the conventional material,and the overpressure amplitude of the secondary shock wave for the RM is higher than that for the conventional material.When η increases from 0.3 to 0.66 and the RM is 2.5 m away from the explosion center,the secondary shock wave appears behind the negative pressure zone.As the distance increases to 4.5 m,the secondary shock wave appears behind the positive pressure zone,which indicates that the arrival time of the secondary shock wave is related to the overpressure amplitude and the test distance.

3.3.3.Analysis of propagation velocity of the air shock wave

Fig.10.Diagram of the explosive air shock wave overpressure phenomenon showing the typical positive pressure duration and peak overpressure.

Through the arrival time of the wave,the instantaneous energy released under the explosive loading of the RM casings can be quantified[29].The arrival time difference of the wave is shown in Table 3,and the average velocity of the air shock wave versus distance is shown in Fig.12,where,t1,t2,t3andt4are the time when the air shock wave reaches the sensor at 2.5 m,3.5 m,4.5 m and 6 m respectively,Δtis the arrival time difference between the two adjacent sensors,and ΔLis the distance between the two adjacent sensors.The average velocity of the air shock wave between each sensorv21,v32,v43can be calculated.It can be seen from Table 3 that the arrival time difference of the air shock wave of the RM is less than that of the conventional material,and that the air shock wave velocity of the RM is greater than that of the conventional material.From the calculation,it can be seen that during the air shock wave propagation,within 6 m,the front of the wave propagates at supersonic speed.This phenomenon shows that under the action of detonation pressure,the RM participates in the reaction and release energy,which is coupled with the explosion energy,enhancing the air shock wave velocity.

Table 3 Time difference of wave arrival.

It can be seen from Fig.12 that the average velocity of the air shock wave for different materials gradually decreases with the increase of distance.When η is 0.3,the decreasing range of the average velocity of the air shock wave for the RM casings decreases first and then increases,while that for the conventional material casings increases first and then decreases.When η increases from 0.48 to 0.66,the decrease in the average velocity of the air shock wave increases first and then decreases.When η goes up from 0.3 to 0.66,the average velocity of air shock wave increases at the same distance for the RM casings but decreases at the same distance for the conventional material casings.The results show that the average velocity can be increased by increasing the thickness of the RM casings.

Fig.11.Measured results of the air shock wave overpressure and positive pressure duration.

Fig.12.The average velocity of the air shock wave as a function of distance.

3.4.Fracture characteristics of recovered fragments

The previous study shows that the thickness of the RM casings has a significant effect on the peak overpressure and positive pressure duration.In order to further study the relationship between the thickness of the RM casings and the reaction degree,fragments with different thicknesses were recovered.Scanning electron microscope (SEM) coupled with energy dispersive X-ray spectroscopy (EDX) was used to observe the fracture morphology and compositional distribution of fragments with similar sizes collected from different samples,and the fragment sizes were calculated,as shown in Fig.13 and Fig.14.

Fig.13.Fracture characteristics of fragments under explosive loading for the conventional 2A12 aluminum alloy material.

Fig.14.Fracture characteristics of fragments under explosive loading for the RM.

Fig.15.The average oxygen content distribution for different kinds of fragments.

Fig.13a～c shows the dimple like pattern.The fracture surface of the conventional material fragment generally features ductile fracture,and the image quality of local small white particles and edges is poor.This indicates that there is non-metallic material at this place,leading to the decrease of conductivity,which may be oxide of the material.The granular and vacuolar material with a surface particle size of 2-15 μm in Fig.14a is the instantaneous oxidation products of metals in high-temperature atmosphere,and the thick film material with cracks on the surface in Fig.14b～ c is the instantaneous oxidation products of metals in hightemperature atmosphere.Because the conductivity of oxides is poor,the charge effect will be produced when the samples are bombarded by the SEM electron beam,and the partial discharge phenomenon will appear on the surface,leading to the poor SEM image quality.The average oxygen contents of fragments were plotted in Fig.15 based on the results of EDX mapping analysis.The typical morphology characteristic regions of fragments under explosive loading for the RM and the conventional 2A12 aluminum alloy material were selected for analysis,where the white particles were not included.Under the same test conditions,quantitative analysis can be carried out to compare the reaction degree of the RM and the conventional 2A12 aluminum alloy casings under explosive loading.When η increases from 0.3 to 0.66,the average oxygen content on the surface of the conventional material decreases gradually,while on the surface of the RM,it increases first and then decreases.The average oxygen content on the surface of the RM is 4.75,12.49 and 24.8 times higher than that of the conventional material.Compared with the conventional material,the average oxygen content on the fracture surface of the recycled RM fragments increases with the increase of η,and the reaction degree increases as well.This is because As the thickness of the casings increases,the time for the casings to fracture and form fragments after explosion is longer.That is to say,the detonation loading time for the RM is longer,thus improving the reaction efficiency.

4.Conclusions

A set of 6 experiments were conducted in order to study the energy release characteristics of RM casings prepared by the alloy melting and casting process under explosive loading.The established RM casings were compared to the conventional 2A12 aluminum alloy casings.The following conclusions are drawn from analysis of experimental results:

(1) When the mass ratio of casing to explosive increased from 0.3 to 0.66,the firelight duration,fireball diameter,temperature distribution and average oxygen content on the fracture surface of the recovered fragments increased obviously.The cracked thick film on the surface is the instantaneous oxidation products of metals in high-temperature atmosphere.Increasing the casing thickness is conducive to the increase of reaction degree.

(2) The explosion loading of the RM casings can improve the peak overpressure of the air shock wave,but the results are different with different charge ratios.According to the energy release characteristics of RMs,increasing the thickness of the RM casings are conducive to the increase of the peak overpressure of the near-field air shock wave,and reducing it is beneficial to the increase of the peak overpressure of the far-field air shock wave.In addition,the secondary shock wave helps to increase the positive pressure duration for the RM.

(3) The average velocity of the air shock wave can be increased by increasing the thickness of the RM casings.The average oxygen content on the surface of the reactive material fragments compared to the conventional 2A12 aluminum alloy material increases significantly with the increase of the casing thickness,and the reaction degree increases as well.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work is funded by the Fundamental Research Funds for the Central Universities (No.30920021108) and Open Foundation of Hypervelocity Impact Research Center of CARDC (20200106).