Guo-wen Gao,En-ling Tang,Min-hui Feng,Ya-fei Han,Yue Li,Mei Liu,Ying-liang Xu,Li Wang,Xiao-chu Lin,Rui-zhi Wang,Yi-guang Cheng,Liang-liang Zhao,Zhi-qiang Liang,Jun-ru Wang,Guan-jie Zhao,Qing Gao,Tian-zhi Zheng

School of Equipment Engineering,Shenyang Ligong University,Shenyang,110159,China

Keywords:CFRP/Al HC SPs ABAQUS/Explicit Duration of action between projectile and target Energy absorption

ABSTRACT CFRP/Al HC SPs(Carbon Fiber Reinforced Plastic/Aluminum Honeycomb Sandwich Panels)composite structures are widely used in the aerospace and many other fields due to their excellent performances.In order to reveal profoundly the dynamic response characteristics of CFRP/Al HC SPs composite structure under high-velocity impact loading,the experiments about aluminum honeycomb core and CFRP/Al HC SPs composite structure impacted by high-velocity projectiles have been performed by using a one-stage light gas gun loading and a high-speed camera systems,respectively.The whole physical process of collision is analyzed by using TEMA software which is provided by high-speed camera's manufacturer,and the pictures and related physical parameters of the projectile passing through the different layers at the key moments are extracted based on the set frame rate of the video.Meanwhile,the ABAQUS/Explicit module was used to conduct numerical simulation under the same conditions as the experiments,and extracting the pictures and related physical parameters during the projectile penetrating through aluminum honeycomb core and the sandwich panels at the different key moments.The results show that the durations of action for the projectile impacting the Al HC core and the CFRP/Al HC SPs at the velocity about 280 m/s were 86 μs and 240μs in experiments,respectively,and the durations of action were 90.7 μs and 236.2 μs at the same experimental conditions in numerical simulation,which were basically consistent comparing the experimental to simulated results.Moreover,more than 70%for the kinetic energy of the projectile consumed by the sandwich panels was attributed to the front panel and the aluminum honeycomb core during the entire impact process,and all these have verified the reliability of the numerical simulation.

1.Introduction

CFRP has an excellent properties such as a high specific strength,a big specific stiffness,a small thermal expansion coefficient as well as a large area integral forming easily.Its performance parameters are superior to steel,which has been widely used in many fields[1,2].Aluminum honeycomb core has the characteristics of a light weight,a high specific strength,a big specific stiffness,an excellent impact resistance as well as a strong vibration damping[3].Therefore CFRP/Al HC SPs was widely applied in the aviation and aerospace field due to a good energy absorption and shock absorption characteristics for composite structures.Researchers have also conducted a series of related research.Iva?ez[4]has carried out the experiments by using drop hammers to obliquely impact the carbon fiber/epoxy panels/Nomex honeycomb sandwich panels at a low velocity,and the influence of the different impact angles and impact energies on the contact force,maximum displacement of the impactor,energy absorption and damage area were analyzed.Villanueva[5]found that there were multiple modes of failure for the honeycomb sandwich,but cracking and delamination occur in the skin of the fiber panel in the high velocity impact tests.Wu et al.[6]have conducted a drop hammer test about impacting on CFRP/HC Al SP structure.In their studies,the contact load,energy absorption history and failure modes have been obtained by changing the thickness of the core layer and the punch velocity.Zhang Q.N[7]has performed ballistic impact tests about impacting on aluminum panels/aluminum honeycomb sandwich panels by using one-stage light gas gun to accelerate projectile.Meanwhile,the numerical simulation have been carried out by using ABAQUS software to simulate the experimental process.Finally,the energy dissipation mechanism penetrating the sandwich plate with the different projectile's parameters was analyzed,at the same time the semi-empirical energy absorption model of the aluminum honeycomb sandwich plate was established when the aluminum honeycomb sandwich plate subjected to projectile’impact.Alavi[8]has performed plenty of experiments about columnar steel projectile impacting on aluminum honeycomb below the velocities of 150 m/s.The ballistic limits of aluminum honeycomb with different apertures were obtained,and the energy absorption characteristics of aluminum honeycomb was revealed.However,Buitrago et al.[9]have carried out experiments by using light gas gun to load the spherical projectile with the diameter of 7.5 mm impacting on aluminum honeycomb sandwich structure at the velocity of 92 m/s～548 m/s.The experimental results showed that the impact energy was mostly absorbed by the skin of the sandwich structure.Meanwhile,the residual velocity and ballistic limit in experiments were compared to simulated results.Ryan et al.[10]have conducted an ultra-high velocity impact test on CFRP/Al HC SPs,which was used in satellite structure.The damage mechanism of the sandwich structure was investigated under ultra-high velocity impact,and comparing the numerical simulation with experimental results.In the paper,the experiments have been carried out about 2A12 aluminum projectile with high velocity impacting on single aluminum honeycomb core and CFRP/Al HC sandwich panels.The experimental photographs of the entire collision process were acquired by a high-speed camera combining with one-stage light gas gun loading experiments.Meanwhile,ABAQUS/Explicit module was used to conduct the numerical simulation at the same experimental conditions.The durations of action and the energy distributions in each layer were obtained during the projectile-impacted single aluminum honeycomb core and the CFRP/Al HC sandwich panels,respectively.The simulated results were basically consistent with the experimental results,and all these have verified the reliability of the numerical simulation.

2.Experiment

2.1.Loading and testing system

The experiments have been performed at Intense Dynamic Research Center of Shenyang Ligong University,China.The Research center have one-stage light gas gun with caliber of 16mm,which can accelerate 2A12 aluminum projectile with the diameter of 15.3 mm to 1000 m/s.The method of projectile blocking the laser beam is adopted to record the start and end moments of the laser velocity measurement system during the projectile flight,then the velocity of the projectile can be determined based on the distance between the two laser beams.The laser velocity measurement system consists of two sets of light-emitting diodes,two corresponding sets of receiving diodes and a detonation velocity meter.The continuous images in the process of impact were acquired by PCO.dimax HS4 high-speed camera,produced by PCO company,Germany.Fig.1 shows the schematic diagrams of the loading and measurement systems for one-stage light gas gun.

2.2.Experimental materials and parameters

(1)Carbon fiber reinforced plastic(CFRP)

The CFRP used in experiment was produced by Shandong ITL Advanced material Co.Ltd.,China,in which the carbon fiber filament was produced by Toray company,Japan.And laid in a staggered method of 0o/90o,which are immersed in epoxy resin and compressed by press machine.The geometric size of CFRP is 120 mm×120 mm×3 mm.

(2)Aluminum honeycomb core

Aluminum honeycomb core is supplied by Suzhou Beecore Cellular Technology Co.,Ltd.,China.The geometric parameters of aluminum honeycomb core are displayed in Table 1.The basic mechanical property parameters of 3003H18 aluminium alloy are displayed in Table 2.

In experiments,2A12 aluminum alloy columnar projectile is adopted,and its diameter,length and mass areφ15.3 mm,15.3mm and 7.47 g,respectively.The targets with different composition are impacted by projectiles with velocities of 260.5 m/s,276.4 m/s,281.3 m/s and 290.6 m/s,respectively.And the whole physical process is acquired continuously by using high-speed camera.

3.Experimental results and analysis

3.1.Damage analysis of the target

(1)The damage of aluminum honeycomb core

Fig.2 shows the damage of a projectile impacting on an aluminum honeycomb core at a speed of 276.4 m/s.

According to Fig.2(a),the area of projectile hole is approximately 2.064 cm2,which is almost the same as the cross-sectional area of the projectile.Meanwhile,there is a serious tensile deformation at the edge of the projectile hole,and a large depression around the projectile hole.In Fig.2(b),the projectile hole is trumpet-shaped in general,and the area is significantly larger than that in Fig.2(a),which is approximately 2.941 cm2,the edges are severely torn and many large cracks appear in the longitudinal direction,which the maximum length of crack is about 25 mm,and the edges of the projectile hole have serious compression deformation.Fig.2(c)shows the aluminum honeycomb core plugs and flyers picked up after the experiment,and most of them are plugs with the size of about 18.3 mm long and 7.5 mm wide.The other part is spatter that is followed by tearing when the aluminum honeycomb core was penetrated.

(2)Damage of each component of CFRP/Al HC sandwich panels

Fig.3 lists the damage of each component for CFRP/Al HC sandwich panels.

As can be seen from Fig.3(a)and(b),the geometric size of the projectile hole is approximately the cross-sectional area of the projectile.In Fig.3(b),there is a small amount of tear,but no large damage area due to the supporting effect of the aluminum honeycomb core.The area of the aluminum honeycomb core is significantly increased by comparing Fig.3(c)with Fig.2(a),which damage area is approximately 3.50 cm2.The CFRP in CFRP/Al HC sandwich panels was impacted by projectile,the aluminum honeycomb core can provide support and resulting in some aluminum honeycomb core compressed.When the projectile penetrates through the CFRP,the resin and the carbon fiber plug with the same velocity impact on the aluminum honeycomb core,and the fracture of CFRP is indeterminate,therefore,the projectile hole of the aluminum honeycomb core appears an irregular round hole.Fig.3(d)and(e)is the rear panel of the sandwich panels,there is significant delamination,cracking,and forming a cross-shaped cracking around the surface.In addition,the burrs formed by the adhesion of carbon fibers to the resin are also intertwined and the damage is quite complicated.When the projectile penetrates the steel plate,the process is divided into four consecutive stages,bulging deformation,dishing deformation,ductile hole enlargement,and projectile exit[11].The process is similar to the projectile penetrating CFRP.

Table 1 Geometric parameters of aluminum honeycomb core.

Table 2 The basic mechanical property parameters of 3003H18 aluminium alloy.(3)Projectile's nature and experimental parameters

3.2.Pictures acquisition and processing by high-speed camera

The entire physical process of 2A12 aluminum projectile impacting on the aluminum honeycomb core can be recorded clearly by high-speed camera,the instantaneous velocity, flight attitude of the projectile before collision the target,the interaction time between the projectile and the target,the residual velocity after the projectile penetrating through the target,the scattering of the plug and fragment can be obtained by acquiring the pictures,combining with the video's own software-TEMA,at the key moments during the impact process.The paper takes 2A12 aluminum projectile impacts on single aluminum honeycomb core at a speed of 276.4 m/s and 2A12 aluminum projectile impacts on CFRP/Al HC sandwich panels at a speed of 281.3 m/s as examples to analyze the key physical phases of the collision process.

(1)Analysis of the physical process for a projectile impacting on aluminum honeycomb core at a speed of 276.4 m/s

According to the number of frames,and the whole physical process and pictures at the key moments extracted are analyzed through using the video captured by a high-speed camera and TEMA software.Fig.4 shows the experimental pictures extracted at different key moments.

Fig.4(a)～(f)are experimental pictures obtained by taking a picture for every two frames(approximately 42.78μs).However,the final two pictures are separated by 107.0 μs and 213.9μs,respectively.It can be clearly seen from Fig.4 that the attitude of the projectile and target before and after projectile colliding with the aluminum honeycomb core.According to the video,one can know that the projectile penetrating through the aluminum honeycomb core last about 4 frames(about 86μs),then the projectile and the crushing aluminum honeycomb core plug flew away at the same velocity,accompanying with some sheared debris to fly out.Combining the video analysis with TEMA software,the instantaneous velocity and the collision angle before the projectile impacting on the aluminum honeycomb core,and the residual velocity after collision can be obtained easily.Fig.5 shows the correlation results of the projectile flight attitude obtained by the TEMA software.

Based on Fig.5(a),one can know that the velocity of the aluminum projectile before collision is approximately 276.4 m/s(the velocity is approximately equal to the laser measurement velocity).From Fig.5(b),one can see that the angle between the projectile and the target is about 87.5o,which is almost perpendicular to the target.According to Fig.5(c)and(d),it can be seen that the velocity after collision is about 240.5m/s,and the angle with the target about 85o,which shows that the variation of angle is not obvious during the collision.It can also be expressed that the aluminum honeycomb core is a homogenous material in the axial direction.When the aluminum projectile coll ides with the aluminum honeycomb core,the force acting on the projectile is relatively uniform,and the aluminum honeycomb core consumes a large part of kinetic energy of projectile during penetration,which is approximately 69.31J.

(2)Analysis of the physical process for a projectile impacting on CFRP/Al HC sandwich panels at a speed of 281.3 m/s

Because the interaction time of the projectile colliding with the sandwich panel is longer than that of impacting single aluminum honeycomb core,the frame rate of the picture acquiring is increased.Fig.6 shows the acquisition of experimental images at different key moments.

Fig.6(a)～(g)are the pictures captured by TEMA software for every three frames(about 56.49μs).However,Fig.6(g)and(h)shows the experimental pictures are separated by 94.1μs,respectively.One can see from Fig.6 that the attitude of projectile and target before and after colliding with the CFRP/Al HC sandwich panels.From the analysis of the video image,one can see that the aluminum projectile impacting on the sandwich panel last about 13 frames(240μs).The panel is torn from Fig.6(e)and(f).Fig.6(g)and(h)show that significant spattering of the splinters occurred when the projectile penetrated through the sandwich panel,which interspersed with broken filaments,resin matrix and aluminum honeycomb core chips.Based on the video processing from TEMA software,one can know that the initial velocity of 2A12 aluminum projectile is about 281.3 m/s,and the incidence angle nearly vertical target,which is about 89.5o.Meanwhile,the residual velocity is approximately 173.3m/s.After the projectile penetrating through the sandwich panel,the projectile reverse mostly due to the effect of cracking CFRP.

Table 3 The comparison between laser measurement velocity and the velocity obtained by a high-speed camera.

3.3.Comparison of laser velocity measurement and high-speed camera acquisition velocity

In experiments,the single aluminum honeycomb core and the CFRP/Al HC sandwich panels were impacted by aluminum alloy projectiles,respectively.The initial velocity of the impact was measured by using a laser velocity measurement system and acquired by a high-speed camera during the process.Table 3 lists the velocities from the laser velocity measurement and a high-speed camera systems.

It can be seen from Table 3 that the initial velocity can be obtained by laser velocity measurement and acquired by a high-speed camera.However,the residual velocity can be only obtained by the video.By comparing the velocity of the laser measurement with the velocity obtained by the video,the former is slightly higher than the latter.The main reason is that the laser velocity measurement system is close to the muzzle of the launch tube,it's not affected by the air resistance of the flight.Moreover,the distance between the two laser beams is relatively close,and the velocity obtained can be considered as an instantaneous velocity within a short distance,thus the velocity value obtained by the laser velocity measurement is higher than that acquired by a high-speed camera.

4.Numerical simulation

Due to the extremely short response time of high-velocity impact,it has undergone many complicated physical processes within a short time.Many of the changes take place during the interaction process between the projectile and the target,so there is no effective characterization method for the details of each physical process.However,numerical simulation provides an effective means to solve this problem.Among the materials in simulation,CFRP is a kind of complex anisotropic flat woven material,and the weaving direction of 0o/90oalternately.At the same time aluminum honeycomb core is also an anisotropic material.Therefore two kinds of anisotropic materials are bound to bring challenges for the establishment of material models in numerical simulations.However,the ABAQUS/Explicit module offers possibilities for numerical simulation aiming at composite materials.Therefore,ABAQUS/Explicit module was adopted to solve the dynamics problem of the composite structure caused by high-velocity impact.

4.1.Modeling

Firstly,three components are needed to create,the first is a 120 mm×120 mm×3 mm solid plate,defined as CFRP and divided into 12 layers,alternating with 0o/90o,Meanwhile a coordinate system corresponding to the weaving direction was established.The second is an aluminum honeycomb core with a geometric size and an edge length of 120 mm×120 mm×20 mm and 1 mm,respectively.The third is a 2A12 aluminum alloy columnar projectile with geometric size ofφ15.3 mm×15.3 mm.

Secondly,the material of CFRP,aluminum honeycomb core and aluminum projectiles were defined,respectively.Among them,the mechanical performance parameters of aluminum honeycomb core are listed in Tables 1 and 2.The density,elastic modulus and poisson's ratio of aluminum projectile are 2700 kg/m3,70GPa,0.33,respectively.Table 4 shows the mechanical properties of CFRP Laminates,partial material parameters are taken from Ref.[12].

Grid partition is an important part of numerical simulation,C3D8R cell is used as finite element grid for CFRP and 2A12 aluminum projectile,which the grids are divided as a three dimensional and eight-node reduced integral solid element.However,the shell cell of S4R is adopted for the aluminum honeycomb core.In the definition of contact,each part is defined as universal contact.In order to apply a displacement load,A reference point must be set on the aluminum projectile and the CFRP laminate.Meanwhile,encastre U1=U2=U3=UR1=UR2=UR3=0 are used as the boundary condition of the CFRP/Al HC sandwich panels.

4.2.Constitutive equations

(1)CFRP composite material

The orthotropic constitutive model is used for the CFRP composite material[13].The orthotropic constitutive model is defined in the elastic stage,and the stress and strain are satisfied with alinear relationship,which 9 elastic constants are included.

Table 4 Mechanical properties of CFRP Laminates.

(2)Aluminum honeycomb core

A Johnson-Cook constitutive model is adopted as constitutive equation for the aluminum honeycomb core[14],which takes into account the influence of strain,strain rate and temperature on stress.The relationship can be expressed as following

Where,σis Von Mises equivalent stress,A is the quasi-static yield strength of the material at room temperature,B is the strain enhancement coefficient,and n is the enhanced index.However,ε is equivalent plastic deformation,˙ε is the plastic strain rate,˙ε0is the initial strain rate,C is the strain rate sensitivity coefficient,m is the temperature softening coefficient,Tmis the melting point of the material,Tris the reference temperature(normal temperature)and T is the current temperature.

4.3.Failure criteria

(1)CFRP laminates

Four kinds of damage failure modes will be produced in composite materials during the collision process,specifically expressed as the tensile fracture of the fiber,the failure of the fiber,the tensile and compressive failure of the matrix when the material is compressed.Fig.7 shows a simplified model of material damage.

It can be seen from Fig.7 that the OA segment is a linear elastic zone when the condition is satisfied with δ＜δθ,and point A reached the critical point of destruction when the condition is satisfied with δ= δθ.However,when the condition is satisfied with δθ≤ δ≤ δmaxand unloading in this zone,the AC segment and D are the damage softening zone and damage factor(0≤D≤1),respectively,and the stress and strain will change along the OB direction.Finally,When the condition meetδ＞δmax,the material will lose its load-bearing capacity at point C,and lamination of the CFRP laminate maybe occur.Therefore,the Hashin criterion[15]is used as the initial failure criterion for CFRP composite laminates.Specifically the relationships can be expressed as

①Fiber fracture criteria

②Matrix tensile cracking criterion

③Matrix extrusion fracture criteria

(2)Aluminum honeycomb core

A Johnson-Cook failure model is used as failure criteria for aluminum honeycomb cores[16],which can be expressed as

Where,Δε is the increment of the equivalent plastic strain when the deformation occur,and εfis the instantaneous failure strain at the current increment.In formula(6),the first parenthesis indicates that the strain of fracture decreases with increasing of static water stress tensor,the second bracket indicates the effect of the increased strain rate to the failure strain,and the third bracket indicates the thermal softening effect of the material ductility.

5.Numerical simulation results and analysis

5.1.Comparison of target topography between simulated and experimental results

In Fig.8,the pictures of the physical process for the aluminum projectile impacting on the single aluminum honeycomb core at a speed of 276.4 m/s are given in simulation.

Fig.8(a)shows that the projectile touches the aluminum honeycomb core exactly,and Fig.8(d)shows that the aluminum honeycomb core is just forming a plug and pushed out by the projectile,the physical process undergoes about 90μs,which is similar to the experimental measurement results.The plug formed by the collision is a mushroom-head shape,with a thickness of about 3.8 mm,which is also close to the experimental results.Fig.9 is a numerical simulation of the physical process for the aluminum projectile striking the CFRP/Al HC sandwich panels at a speed of 281.3 m/s.

The sandwich panel penetrated through byaluminum projectile experienced about 235μs.In Fig.9(b),the rear panel of CFRP showed a serious damage,around which the hole was partially enlarged.Serious stratifications,cracks,fractures and cross-like diffusion to the surroundings occur in the physical process by comparing Fig.9(c)with Fig.3(e).It can be seen that the numerical simulation is basically consistent with the experimental results.In view of the serious damage of the rear panel of CFRP,and further understanding the scope of damage to the rear panel after the projectile penetrating through the sandwich panel,the ABAQUS slicing function is used to observe the sandwich panel at different positions when the composite structure is penetrated,which is shown in Fig.10.Fig.10(a)～(f)show sliced images that the distance from the center of projectile hole-5 mm,0mm,5 mm,10mm,15 mm and 30 mm,respectively.

From Fig.10(b),it can be clearly seen that the front panel is sheared,the aluminum honeycomb core is compressed completely to form the plug,and the rear panel is delaminated,broken and lifted outside.In Fig.10(d),no serious damage was found on the front panel and the aluminum honeycomb core,but the destruction of the rear panel was still serious.The apparent destruction of the rear panel disappears until a distance of 30 mm from the center of the projectile hole,and it can be approximately consider that the significant damage to the rear panel is approximately a circular area with a diameter of 60 mm when the aluminum projectile collides with the sandwich panel.

5.2.Comparison of simulated and experimental results in residual velocity and duration of action during projectile impacting on target

The variable curve of the velocity with time can be plotted by obtaining the relationship between the energy of the projectile and duration during the impact process.Fig.11 is a time history curve of a projectile colliding against an aluminum honeycomb core.

It can be seen from Fig.11 that the projectile touches the aluminum honeycomb core at point A,and the projectile penetrates through the aluminum honeycomb core at point B,the coordinates of the two points are A(15.1,276.3)and B(105.8,239.6),respectively.In numerical simulation,the residual velocity of the projectile penetrating through the aluminum honeycomb core is about 239.6 m/s,which is basically the same as the experimental measurement of 240.5m/s.The duration of the projectile interacting the target is about 90.7μs,which is near to 86 μs acquired by the video.Fig.12 shows the time history curve of a projectile impacting on a composite CFRP/Al HC sandwich panels.

In Fig.12,point A(12.1,281.5)indicates that the projectile justly contacted with the sandwich panel,point B(35.6,243.5)indicates that the CFRP front panel was penetrated through,and point C(107.6,205.8)indicates that the aluminum honeycomb core was completely cut and crush.However,Point D(248.3,175.5)indicates that the projectile begins to move at a constant velocity after passing through the target.The duration of the projectile penetrating through the target is about 236.2μs,which is basically consistent with the 240μs obtained by the high-speed camera.And the residual velocity after the projectile penetrating through the target is about 175.5 m/s,which is close to the result of 173.3m/s obtained by the high-speed camera.The residual velocities after the projectile penetrating through each part of the sandwich panel are 243.5 m/s,205.8 m/s and 175.5m/s,respectively.And the percentage of energy consumed by each part is 41%,35%and 24%in total consumption of energy during the projectile penetrating the sandwich panel.It can be seen that the front panel and the aluminum honeycomb core consume most of the energy during the projectile penetrating the target.

Fig.13 is an enlargement of the BC segment in Fig.12,and the BC segment is a case where the projectile striking the aluminum honeycomb core with the porous material of the sandwich panel.

As can be seen from Fig.13,there are seven more prominent embossing points,which means that compression,deformation and energy absorption are interrelated when the projectile strikes the aluminum honeycomb core.The coordinates of each point in Fig.13 are a (46.2,239.6),b (56.5,234.6),c (66.1,230.2),d (77.4,225.8),e(87.5,220.3),f(100.2,211.4)as well as g(108.7,207.5),and each point releases energy for each interval 10μs once.Each pair of aluminum honeycomb cores is squeezed once by projectile,and the aluminum honeycomb core will buffer and release energy once.When the interaction of projectile reach the point g,the aluminum honeycomb core breaks,the energy is released instantaneously,then the projectile has a rebound phenomenon.It will take time to perform the physical process,which is consistent with the interaction mechanism of“space-for-time"for the porous cellular material.

6.Conclusions

In the paper,the experiments have been performed about high velocity impact on single aluminum honeycomb cores and CFRP/Al HC sandwich panels by using one-stage light gas gun loading and high-speed camera acquisition systems.And the entire physical process of the impact was analyzed by using high-speed camera combining with software TEMA.Meanwhile,the ABAQUS/Explicit module was used to conduct numerical simulation under the same conditions as the experiments.The following conclusions can be drawn:

(1)The initial and residual velocities obtained by numerical simulation are basically consistent with the experimental results.

(2)In experiments,the duration of action for the projectile impacting the single aluminum honeycomb core and the CFRP/Al HC sandwich panels at a speed of about 280 m/s was 86 μs and 240μs,respectively.Under the same conditions,the numerical simulation results of the duration of action were 90.7μs and 236.2μs,respectively.The simulated results are basically consistent with experimental results.

(3)Based on the numerical simulation,the time history curve of the aluminum projectile passing through the sandwich panels was obtained,and the physical explanation of the energy absorption process for aluminum honeycomb core have been performed.

(4)More than 70%of the kinetic energy lost in the collision was consumed by the front panel and the aluminum honeycomb core.Meanwhile,the reliability of the numerical simulation was verified by experiments.

Acknowledgments

The authors would like to acknowledge National Natural Science Foundation of China(Grant No.11472178),Open Foundation of Hypervelocity Impact Research Center of CARDC(Grant No.20180201)and Open Project of State Key Laboratory of Explosion Science and Technology in Beijing Institute of Technology(Grant No.KFJJ18-04M)to provide fund for conducting experiments.