Yu-xing Sun ,Xin Wng ,Chong Ji ,,Chng-xio Zho ,Pei-li Liu ,Lei Meng ,Kun Zhng ,To Jing

a College of Field Engineering,Army Engineering University of PLA,Nanjing,210007,Jiangsu,China

b Qingdao Advanced Marine Material Technology Co.,Ltd,Qingdao,266000,Shandong,China

c Department of International Training,Army Engineering University of PLA,Nanjing,210042,Jiangsu,China

Keywords: Polyurea ASTM1045 steel plate High velocity impact Penetration SHPB test Damage mechanism

ABSTRACT In this study,the anti-penetration performance of polyurea/ASTM1405-steel composite plate subjected to high velocity projectile was analyzed.Two kinds of modi fied polyurea material(AMMT-053 and AMMT-055)were selected and a ballistic impact testing system including speed measuring target system and high-speed camera was designed.This experiment was conducted with a ri fle and 5.8 mm projectile to explore the effects by the polyurea coating thickness,the polyurea coating position and the glass-fiber cloth on the anti-penetration performance of polyurea/ASTM1405-steel composite plate.The result showed that the effects of polyurea coating position were different between two types of polyurea,and that the effects of glass-fiber position were disparate between two types of polyurea as well.For AMMT-053 polyurea material,it was better to be on front face than on rear face;whereas for AMMT-055 polyurea,it was better to be on rear surface although the difference was very subtle.Additionally,formulas had been given to describe the relationship between the effectiveness of polyurea and the thickness of polyurea coating.In general,AMMT-055 had better anti-penetration performance than AMMT-053.Furthermore,five typical damage modes including self-healing,crack,local bulge,spallation and local fragmentation were defined and the failure mechanism was analyzed with the results of SHPB test.Additionally,the bonding strength played an important role in the anti-penetration performance of polyurea/steel composite plate.

1.Introduction

In recent years,with the spread of terrorism,increasingly frequent bomb attacks pose signi ficant threats to people’s lives and property.In addition,explosion accidents,such as explosions of petroleum and natural gas equipment,also result in great losses.In these explosion accidents,explosive shock waves and fragments are the two main causes of casualties.Therefore,for applications in both military and civilian fields,it is crucial to find a new material with good explosion and penetration resistance performances.Polyurea is an elastomeric material that can form high-strength and high-elastic coatings on the surfaces of structures.In recent years,scholars have noticed that polyurea can improve the protective performances of structures in explosion shock waves and fragment penetration.Consequently,the dynamic mechanical properties of polyurea materials and the research of explosions and impact protection have been paid much attention.However,most studies have been conducted to explore the dynamic responses of polyurea-coated plates subjected to blast loading.A limited number of papers has been published on the behaviors of polyureacoated plates when subjected to localized loads generated from ballistic impacts.

At room temperature,polyurea is a typical microphasedispersed,thermoplastic,crosslinked polymer.Its highly complex internal microstructure provides polyurea with excellent comprehensive mechanical properties.Sarva et al.[1]studied the compressive stress-strain behaviors of polyurea materials at different strain rates,and the experimental results showed that the stress-strain behaviors were closely related to the strain rate.Zhai[2]studied the mechanical properties of a new type of explosionproof and shock-resistant polyurea.Sayed et al.[3]analyzed the mechanical characteristics of polyurea materials,such as the pressure hysteresis,strain-rate sensitivity,and Mullins effect.The material parameters were calibrated through uniaxial tensile tests to simulate and verify the effect of polyurea materials on improving the impact resistances of high-strength steel plates.Shim et al.[4]used an improved split Hopkinson pressure bar(SHPB)device to conduct a series of compression tests on polyurea,conducting experiments at low,medium,and high strain rates,focusing on the strain-rate sensitivity characteristics of polyurea materials.In conclusion,in the static and quasi-static experiments,the properties,tensile strength,elongation at break,and tear strength of polyurea were excellent.The mechanical behavior of polyurea becomes more complex under dynamic loading,which is summarized as follows:(1)the stress-strain curve is nonlinear,(2)the mechanical behaviors are highly sensitive to strain rate and temperature effects,and (3)the mechanical behaviors are highly correlated to the pressure.These properties are bene ficial for the improvement of the impact resistance of polyurea during explosions.Additionally,the material composition has a signi ficant effect on the mechanical properties of polyurea,and it has been shown that the mechanical behaviors of polyurea are related to the ratio of hard segments to soft segments.

Some scholars have found that the polyurea layer can effectively reduce the residual velocity of a projectile and improve the antipenetration performance of a target plate.Mohotti et al.[5-7]conducted experimental and partial simulation studies to investigate the effect of polyurea layers with different thicknesses and different coating positions on the impact resistance of aluminum/polyurea composite structures.Bogoslovov et al.[8]found that if a 10-mm-thick polyurea coating was applied to the surface of a steel plate,the glass transition of the polyurea occurred when impacted,and the anti-penetration performance of the structure was significantly improved.Fowler et al.[9]believed that the polyurea layer could improve the ballistic resistance during penetration.Roland[10]tested the bulletproof effect of polyurea as a protective coating for steel plates.It was found that if the glass transition temperature of polyurea was high enough or the range was wide enough,the glass transition occurred when the projectile penetrated,and the anti-penetration performance was improved.Xue et al.[11]conducted experimental and numerical simulation studies on the impact resistance of three structures:a DH-36 steel plate,a steel/polyurea plate,and a sandwiched steel/polyurea/steel plate.The test results showed that a polyurea coating on the back of the steel plate could effectively improve the penetration resistance of the structure,but the sandwich con figuration had little contribution to the improvement of the penetration resistance of the structure.Cai et al.[12]also carried out experimental and simulation studies on the bulletproof performances and self-sealing behaviors of reinforced polyurea/steel structures under the impact of high-speed projectile bodies.Zhao et al.[13]used 3.3-g cube fragments to penetrate a polyurea-coated glass-fiber/aramid composite target plate and carried out numerical simulations.The results showed that a polyurea coating on the front surface could effectively improve the anti-penetration performance of the glass-fiber/aramid composite plate,while a coating on the back surface reduced the energy absorption effect of the composite structure.

There are also some research results on the dynamic responses of polyurea-coated structures to blast loading.Baylot et al.[14]carried out an experimental study on the damage of a concrete brick masonry structure under the blast loading,aiming to explore the damage mode and failure mechanism of polyurea-reinforced walls.Wang et al.[15]experimentally studied the explosive damage of a MU20 brick wall reinforced by polyurea under different working conditions.The polyurea-reinforced coating could effectively restrain the displacement,deformation,and crack propagation of the wall under blast loading and could play an important role in keeping the masonry wall structure intact under blast loading.A wall with both sides reinforced by polyurea had the largest blast resistance capacity.Based on the Amirkhizi linear viscoelastic constitutive model,Samiee et al.[16]carried out a numerical simulation study on the dynamic response characteristics of a polyurea-coated DH-36 steel circular plate subjected to blast loading.The results showed that a steel/polyurea composite structure had the best resistance to explosive deformation.The thicker the polyurea layer was,the better the effect became.Ha[17]combined a high-strength carbon-fiber-reinforced polymer(CFRP)with a high strength and polyurea(PU)with a high toughness to obtain a new type of reinforced composite with better stiffness,ductility,and adhesion properties,and this composite was proven to exhibit better anti-explosion performances by explosion tests.Based on explosive experiments of a polyurea-glass fiber composite plate structure,Tekalur et al.[18]suggested that the nonlinear constitutive relationship,the extremely high strain-rate correlation,and the failure of the bond layer were the main reasons for the energy absorption of polyurea based on macroscopic and microscopic analyses of the structure failure.

The studies on polyurea layers subjected to impact loading are limited,and these studies mainly focused on the anti-blasting performances of polyurea layers.At present,there are insufficient experimental data to support the research on the anti-penetration performances of polyurea materials.Additionally,the formula system of polyurea is very complex,and the mechanical properties of polyurea with different formulations vary greatly.Therefore,two types of modi fied polyurea were selected,and ballistic impact tests using a velocity measurement system and a high-speed camera were performed.Five damage modes were defined to describe the damage mechanism of the polyurea layer,which was supported by the results of SHPB tests.The residual velocity reduction was analyzed to discuss the anti-penetration performances of polyurea materials.Furthermore,a formula was also fitted to describe the relationship between the residual velocity and the thickness of the polyurea layer.

2.Characteristics of target plates

2.1.Polyurea layer

Polyurea is fabricated by the rapid chemical reaction between isocyanate and amine groups.Due to the various formula systems of polyurea and the complex chemical reaction process,the mechanical properties of polyurea with different formulations vary greatly.To explore the anti-penetration performances of polyureacoated ASTM1045 steel plates experimentally,two kinds of modified polyurea(AMMT-53 and AMMT-55)with different chemical formulas were selected,and polyurea/ASTM1045 composite plates were fabricated in advance.Table 1 shows the parameters of the AMMT-53 and AMMT-55 polyurea.The hardness of the AMMT-53 polyurea(black)was higher,while the strength of the AMMT-55(grey)was higher.

Table 1 Parameters of AMMT-53 and AMMT-55 polyurea.

The manufacturing process of the polyurea/steel composite plates is shown in Fig.1(a).To improve the interfacial bonding strength between the polyurea coating and steel plate,the steel plate was subjected to abrasive blasting,and the surface of the steel plate was covered with a speci fic binder.The bonding strength of a speci fic binder was about 2 MPa.In addition,all of the polyurea was sprayed with Graco H-XPs spraying equipment and a Fusion AP spraying gun.In process of making polyurea/steel composite plates,the deviation between the measured and designed thicknesses of the polyurea layer were controlled to within 0.2 mm.For brevity, “polyurea A” and “polyurea B” will hereinafter be used to denote the AMMT-53 and AMMT-55 polyurea,respectively.

Fig.1.Preparation of AMMT-53 and AMMT-55 polyurea coated steel plate.

Fig.2 shows the stress-strain curves of AMMT-53 and AMMT-55 polyurea under different strain rates,which is obtained by compressive test of SHPB(Split-Hopkinson Pressure Bar).Polyurea A was in the elastic stage under low strain rates.When the strain rate increased,it exhibited characteristics of the elastic-plastic stage.Polyurea B exhibited elastic characteristics,but under the condition of high strain rates,yield slip and strain hardening occurred.

2.2.ASTM1045 steel plate

The material of the square plate(250 mm×250 mm×4 mm)was ASTM1045 steel.Table 2 shows the chemical composition of ASTM1045 steel,and Table 3 shows the mechanical performance parameters of ASTM1045 steel.

2.3.Glass-fiber cloth

To fabricate a glass-fiber-enhanced composite plate,a glassfiber cloth was fixed to the steel plate surface with a speci fic binder in advance,and the excess cloth at the edges was removed to obtain the glass-fiber/steel plate,which was subsequently sprayed with polyurea(Fig.1(b)).The parameters of the glass-fiber cloth are shown in Table 4.Yarn linear density is used to characterize the linear density of a single yarn;warp/weft density is the number of yarns within 1 inch in the direction of warp or weft;breaking strength is the greatest force that the glass fiber cloth specimen,with size of 5 cm×20 cm,can bear during tensile test;leno is a weave in which the warp yarns are twisted together in pairs between the weft or filling yarns.

Table 2 Chemical composition of ASTM1045 steel.

Table 3 Mechanical performance parameters of ASTM1045 steel.

Table 4 Parameters of glass-fiber cloth.

3.Experimental setup

The ballistic impact testing system included a speed measurement target system and a high-speed camera.Additionally,ballistic experiments were conducted using a ri fle to launch a 5.8-mm ordinary bullet.The results were analyzed to study the effects of the polyurea coating thickness,the position of the polyurea layer,and the glass-fiber cloth on the projectile penetration resistance of polyurea/ASTM1045 composite plates.

Fig.2.Results of SHPB test.

3.1.Ballistic impact test experimental setup

This experiment included 40 projectiles with sizes of 5.8 mm to test the bulletproof effect of the polyurea-coated steel plate.Fig.3(a)shows the general view of the projectile.The projectile had a diameter of 5.8 mm,a length of 24 mm,and a weight of 4.2 g.There was a steel core with a large aspect ratio inside the warhead with a total length of 20.3 mm and a diameter of 3.8 mm(Fig.3(b)).

As shown in Fig.4,the ballistic impact testing system included two main parts:a speed measurement target system and a highspeed camera.The speed measurement target system was composed of two sets of speed measurement targets and a NLG202G-2 time recording instrument(Fig.4(e)).The speed measurement target in front of the polyurea-coated steel plate was a photoelectricity test system(Fig.4(d))with a high accuracy,while the other speed measurement target behind the polyurea/steel composite plate was an on-off target made of tin foil(Fig.4(b)).The on-off target maintained an open circuit under normal conditions,and it became a closed circuit only when the projectile penetrated the target paper.When the circuit became closed,the NLG202G-2 time recording instrument(Fig.4(e))received an electrical signal and recorded the time.The two targets were kept 1.2 m apart,and the velocity of the projectile could be obtained based on the distance and time recorded.Additionally,Fig.4(c)shows the fixing device of the composite plate.The plate was fixed diagonally with two clamps.Fig.4(g)shows a schematic diagram of the test system.

In addition,a high-speed camera was also set to measure the speed of the projectile,as shown in Fig.4(f).The frame rate of the high-speed camera was 10000 fps,the exposure time was 1/50000 s,and the image resolution was 1024×512 px.Additionally,manual trigger and mid-point trigger modes were used.In these modes,the high-speed camera continually captured images under normal conditions.Once the high-speed camera received an electrical signal from the manual trigger,it recorded the images from 1 s before and 1 s after the electrical signal.Fig.5 shows the results of the high-speed camera.A minus sign indicates times before the electrical signal.Additionally,the grid of the background plate was 10 cm×10 cm,which was used as a reference to measure the velocity of the projectile.The combination of the high-speed camera and on-off target ensured that the measured residual velocity was that of the projectile and not the fragments.

Fig.4.(a)Ballistic impact test system;(b)On-off target with tin foil paper;(c)Fixing device;(d)Photoelectricity test system;(e)NLG202G-2 time recording instrument;(f)Highspeed camera;(g)Schematic diagram of the test system.

3.2.Experimental scheme

In Table 5,the yellow and grey rectangles depict the polyurea layer and steel plate,respectively,and the red rectangles in category III depict the glass-fiber cloth.Moreover,categories I,II,and III represent the polyurea coating on a single side of the steel,on both side of the steel,and with an added glass-fiber cloth,respectively.In all the schematic representations,the left surface represents the surface exposed to the projectile.The term “plate arrangement” means the thickness of each layer in the composite structure.

4.Experimental results

Based on the results of ballistic impact tests,the failure mode and residual velocity reduction were analyzed.Five failure modes-cracking,self-healing,local bulging,spallation,and local fragmentation-were observed.The residual velocity reduction is discussed in terms of three aspects:the effect of the polyurea layer thickness,the effect of the coating position and glass-fiber cloth,and the comparison between the two polyurea types.

Viis the velocity of the projectile before penetrating the polyurea/ASTM1045-steel composite plate,andVjis the residual velocity of the projectile after penetrating the polyurea/ASTM1045-steel composite plate.All the composite plates were pierced by 5.8-mm ordinary projectiles launched by the ri fle.Table 6 shows the test results ofViandVj.The damage modesa,b,c,d,anderepresent cracking,self-healing,local bulging,spallation,and local fragmentation,respectively,as shown in Fig.7.

The measured thickness columns in Table 6 show the designed thicknesses of the polyurea layers and the maximum deviations.For example,for A-I-1,the designed polyurea layer thickness was 2 mm,the minimum measurement thickness was 1.92 mm,and the maximum thickness was 2.06 mm.Therefore,the maximum deviation was 0.8 mm,and the measurement thickness was expressed as 2±0.08 mm.

A/B-I/II/III-X denotes polyurea A/B coating the steel plate corresponding to category I/II/III with serial number X in Table 5.Failure mode X/Y corresponds to the damage mode of the front/rear face.

Table 5Different arrangements of polyurea and ASTM1045 composite plates.

Table 6 Testing results.

Fig.5.Images of high-speed camera.

4.1.Analysis of failure mechanism of polyurea layer

4.1.1.Five damage modes of polyurea layer

Referring to the eight perforation mechanisms theory of Backman and Goldsmith[19],the failure mechanisms of the ASTM1045 steel plate subjected to sharp-nosed projectiles in this experiment mainly included ductile craters and fragmentation,as shown in Fig.6 (a).When the high-velocity projectile impacted the ASTM1045 steel plate,a small puncture occurred near the impact area.Due to the ductility of the steel plate and the projectile continuously exerting pressure on the bullet hole during penetration,the bullet hole expanded continuously.This process is commonly called “ductile crater enlargement” (Fig.6(d)).The average diameter of the bullet hole on the front and rear faces was 9-10 mm.The 5.8-mm projectile consisted of a high-strength steel core and a softer copper skin.In the penetration process,the highstrength steel core played a major destructive role on the steel plate,and the softer copper skin detached from the steel core and attached to the bullet hole after penetration.On the front face,the petal shape of the copper skin was visible(Fig.6(c)).In addition,there were many fragments when the projectile went through the steel plate,which were evident in the high-speed camera images(Fig.6(b)).Fig.6(e)shows part steel core after penetrating the target plate.In this experiment,sandbags were set behind the on-off target to collect the steel core of the projectile.The third steel core had a higher residual velocity and more residual kinetic energy,and it broke through the sandbag.After the steel core broke through the sandbag,it continued to move to the rear and collided with hard objects,such as the wall and steel plate behind the target.In the impact process,deformation occurred.Thus,the third hard core deformed signi ficantly.

Taking the eight perforation mechanisms of pointed projectiles penetrating steel plates as a reference,five damage modes were proposed on the basis of the target plate damage.The five damage modes were cracking,self-healing,local bulging,spallation,and local fragmentation,as shown in Fig.7.When polyurea coatings were applied on both sides,one or two damage modes occurred,as shown in Fig.7.

?Damage mode a(cracking):A circumferential tensile stress was present in the polyurea coating after the bullet impacted the composite plate.Once the tensile stress exceeded the critical failure stress of the polyurea,the circumferential tensile stress formed a crack tip and continued to expand the crack.

?Damage mode b(self-healing):After the projectile penetrated the composite plate,there was only a tiny hole on the surface of polyurea layer,and the diameters of the holes tended to be less than 1 mm.The polyurea was perforated due to the pressure of the projectile when it was penetrated.However,as an elastic body,the polyurea shrank at the breach,forming a small wound.

?Damage mode c(local bulging):Because of the superposition of the compression wave and the re flected tensile wave,there was tensile stress in the polyurea.If the tensile stress exceeded the bonding strength on the interface between polyurea and steel without reaching the critical failure stress,the polyurea layer separated from the steel plate in the zone near the impact hole.There was no break in the polyurea layer,so the polyurea formed a bulged surface externally.

?Damage mode d(spallation):Due to the combination of shear and tensile failure,the polyurea layer spalled.There was a relatively complete thin polyurea wafer that separated from the composite plate.The spallation included two conditions:complete spallation and partial spallation.Complete spallationmeans that the polyurea layer separated from the steel plate.Partial spallation means that part of the polyurea separated and part was still attached to the steel plate.

Fig.6.High velocity projectiles penetrated the bare steel plate.

?Damage mode e(local fragmentation):If the tensile stress in the polyurea layer exceeded the bonding strength and the critical dynamic tensile strength,the polyurea layer near the impact hole would separate from the steel plate and break into several pieces.Similar to the spallation damage mode,the local fragmentation mode could be divided into complete local fragmentation and partial local fragmentation.Complete local fragmentation means that no polyurea appeared on the steel in the damage zone,and partial local fragmentation means that not all of the thick polyurea was separated in the damage zone.

Fig.7.Schematic diagram of five damage modes.

The impact hole could have more than one typical pattern of damage at a time.For instance,A-II-1 had both crack and local fragmentation,no matter on the front face or rear face.

4.1.2.Influence of polyurea layer thickness on the damage mode

To explore the in fluence of the polyurea layer thickness on the failure mechanism,the composite plates with polyurea coated on a single surface were examined,as shown in Fig.8.

Fig.8 shows the failure modes of the polyurea/steel composite plate corresponding to different polyurea thicknesses.As shown in Fig.8(a),if polyurea Awas coated on the front face,the main failure mode was cracking.The details of the cracking failure mode are presented in Table 8.When the thickness of the polyurea layer was 2 mm,not only radial cracks appeared.As the layer was very thin,the strength of the polyurea layer was not as large as others.Therefore,due to the radial tensile stress,circular cracks also appeared on the face.Four fan-shaped failure zones formed due to the radial and circular cracks.When the layer thickness was 2 or 4 mm,in addition to cracks,a crater formed around the bullet hole in the penetration process.Additionally,with the increase in the thickness of the polyurea layer,the number of cracks showed a general decreasing trend.Abnormal results were obtained when the thickness was 4 mm.The process of forming the crater absorbed more energy,which reduced the crack generation.When the polyurea layer thickness reached 10 mm,no cracks formed on the surface.As a result,for polyurea A on the front face,the failure mode of polyurea layer was mainly cracking.However,with the increase in the thickness in the polyurea layer,the number of cracks decreased,and the failure mode became self-healing for the 10-mm-thick polyurea layer.As shown in Table 7,when polyurea coated the rear face,the failure mode was local fragmentation.However,the failure mode varied with thickness.The failure mode was complete local fragmentation when the polyurea layer thickness was 2 or 4 mm,while the failure mode became partial local fragmentation when the polyurea layer thickness reached 6 mm.As shown in Table 7,the sizes of the damage areas were similar.However,the volume of the damage area increased with increasing depth,which means the fragmentation would absorb more kinetic energy of the projectile and reduce the velocity of the projectile more.

For polyurea B,as shown in Fig.8(b),the failure mode on front face was self-healing,while the failure mode on rear face was spallation.The polyurea B on the front face had strong “self-healing” ability under the condition of high-speed impacts,which could effectively reduce the out flow of liquid from the impact hole.When the thickness of the polyurea layer was 2 mm,the polyurea layer was slightly swollen in the vicinity of the bullet hole.This was due to a tensile wave in the polyurea layer.When the tensile stress was greater than the bonding strength,the polyurea would separate from the steel plate.However,the tensile stress was lower than the failure stress,and the polyurea layer was not torn.Therefore,the failure mode of B-I-6 was not solely self-healing but both selfhealing and local bulging.When polyurea B was coated on the rear face,there was a relatively complete thin polyurea wafer separated from the composite plate(Fig.9).Part of kinetic energy of the projectile was converted to deformation energy of the polyurea and the kinetic energy of the thin polyurea wafer.The failure mode changed from complete spallation to partial spallation with increasing polyurea thickness.As shown in Table 9,the size of the damage area increased with increasing layer thickness.

Table 7 Size of the damage area(AMMT-53).

Table 8 Cracking details(AMMT-53).

Table 9 Size of the damage area(AMMT-55).

Comparing polyurea A and B on the front face,the damage mode for polyurea A was mainly cracking and the damage mode for polyurea B was mainly self-healing,as shown in Fig.10.To explore the dynamic mechanical performance,the stress-strain curves of two polyurea materials at different strain rates were obtained by SHPB experiments,as shown in Fig.2.For polyurea A,when the strain rate was 1000 s-1,it was in the elastic stage with an elastic modulus of 1358 MPa.However,if the strain rate reached or exceeded 2000 s-1,the material underwent not only an elastic stage but also an elastic-plastic stage.As a result,the stiffness of the polyurea A increased and brittleness began to appear.Normally,polyurea exhibits high elasticity,and this state is called a high elasticity or rubber state.However,under a high-speed impact,polyurea no longer exhibits elasticity,and its brittleness is signi ficantly enhanced.This state is called the glass state[8,10].The elastic-plastic stage means that polyurea no longer only shows properties of the highly elastic state but has begun to exhibit the characteristics of the glass state.In the elastic-plastic stage,the polyurea undergoes not only elastic deformation but also plastic deformation,which is consistent with the macroscopic brittle failure modes,such as cracking and local fragmentation.Polyurea B retained elastic phase characteristics,regardless of whether the strain rate was 1000,2000,3000,or 4000 s-1.Therefore,the damage modes of polyurea B were mainly self-healing and spallation when it was subjected to impact.

Fig.8.In fluence of the polyurea layer thickness on the failure mode of the polyurea/steel composite plate.

Fig.10.The comparison of the damage mode corresponding to polyurea A and polyurea B.

As shown in Fig.8,the rear faces of polyurea A and B underwent a process from complete damage to partial damage.The damage mode was local fragmentation for polyurea A and spallation for polyurea B.Due to the complex damage mechanisms of spallation and local fragmentation,the polyurea underwent not only tensile failure but also shear failure.After the projectile impacted the composite plate,the compression stress wave was launched into the target plate along the direction of the projectile motion.When these compression waves reached the interfaces of the different materials in the target,they were partially re flected back in the opposite direction of the original wave and partially transmitted.As the wave re flected back,a tensile wave formed in the polyurea.Because of the superposition of compression and tensile waves,a strong tensile stress was generated in the polyurea.Once the tensile stress reached or exceeded the critical failure stress,namely,the critical dynamic tensile strength,the material broke and cracked.Regardless of which side of the steel plate the polyurea was coated,the mechanisms of wave propagation in the polyurea were similar,as shown in Fig.11(a)~(c).In three dimensions,the wave system is very complex and dif ficult to analyze.A triangular pulse wave in the one-dimensional field is considered as an example[20,21],as shown in Fig.11(d).It is assumed that the strength of compression wave isσm,the wavelength isλ,the wave speed isc,and the strength of the re flected unloaded tensile waveRis fixed.When the shock wave reaches the free surface and then propagates for a period of timet,the pressure of the compression wave at the positiond（d≤ct）away from the free surface is as follows:

The strength of the re flected unloaded tensile waveRis as follows:

The stress of the material at a speci fied location is as follows:

The tensile stress is related to the distance to the free surface,which can explain why the spallation and local fragmentation were complete spallation and complete local fragmentation,respectively,when the polyurea layer was thin.When the polyurea layer was not thick enough,according to Eq.(3),the tensile stress in the polyurea did not reach the dynamic tensile strength.However,the bonding strength was less than the tensile stress,so the polyurea separated from the steel completely.If the bonding strength could be enhanced,more kinetic energy of the projectile would be converted.Therefore,the bonding strength is very important for the anti-penetration performance of polyurea layers.

In addition to the tensile failure described above,shear failure also occurred.When the projectile penetrated the target plate,due to the signi ficant impact effect,the projectile had a certain shear effect on the polyurea layer.For samples A-I-3,B-I-3,and B-I-4 shown in Fig.8,the polyurea layer was damaged through spallation or local fragmentation,which was caused by tensile failure,and a larger damage area formed on the upper face.In addition,in the zone close to the impact hole,there was a smaller damage area with a slightly larger diameter than the impact hole.This smaller damage area was caused by shear failure.The damage area of the polyurea layer was examined.The breach showed good texture characteristics macroscopically.The shape of the broken area was regular,and delamination was evident.

4.1.3.Discussion on self-healing damage mode

Fig.11.Wave propagation in the composite plate subjected to projectile impact.

Some elastic fillers appeared in some of the polyurea/steel composite plates.These were mainly concentrated in the selfhealing and cracking damage modes.The filler was off-white in appearance.Moreover,these fillers had good elasticity.This phenomenon might have been related to the thermal effect of the highspeed projectile penetrating the target plate.The steel core of the projectile had a high stiffness,and the projectile traveled at a high speed and remained stable by rotation during the flight.When the projectile penetrated the composite target plate,some kinetic energy of the projectile was converted to internal energy,and the temperature of the steel plate and polyurea material increased.Polyurea materials have low melting points.They begin to become soft at around 90°C and melt at 160°C.Therefore,due to the thermal effect of the impact process and the dynamic action of the penetration of the rotating projectile,the loose and elastic filler was formed by the melted polyurea in the impact hole.This phenomenon helped to enhance the self-healing effect of the polyurea materials,which could effectively reduce the out flow of liquid from the impact hole if it were used in a container(see Fig.12).

4.1.4.Failure modes of polyurea coating on both sides of bare plate and glass-fiber enhanced plate

For samples with polyurea coated on both sides,the damage mode was slightly different from that with polyurea coated on a single side(either front or rear face).The damage mode of the front face for samples A-II-1,A-II-2,and A-II-3 were local fragmentation,and the damage modes of the rear face were a combination of cracking and local fragmentation.The results were similar for polyurea B.The rear faces of B-II-2 and B-II-5 not only showed spallation,but circular cracks also formed around the failure zone.The front face of B-II-2 was different from the others,as shown in Fig.13(b),in that the polyurea separated from the surface.This occurred because the polyurea layer in fluenced the wave reflection and superposition and the tensile stress was enhanced.Therefore,more projectile kinetic energy was converted into other forms in this process.As shown in Fig.14,when the glass-fiber cloth was added to the rear face,compared with the same polyurea layer thickness but without glass-fiber cloth under the same conditions,the size of damage area was signi ficantly reduced.However,as the damage area decreased,the number of cracks increased.For polyurea B,the surface of B-III-2 and B-III-4 had circumferential and radial cracks,respectively.

4.2.Analysis of anti-penetration performance of polyurea layer based on residual velocity reduction

The damage mechanism of the polyurea/steel composite plate was analyzed qualitatively based on the five damage modes.To explore the anti-penetration performance of the polyurea/steel composite plate quantitatively,the residual velocity reduction was analyzed.

Fig.12.Elastic filler in impact hole of polyurea layer.

Fig.13.The failure mode of polyurea/steel composite plate corresponding to polyurea coated on both sides.

4.2.1.Influence of coating thickness on velocity reduction of projectile

When the projectile penetrated the polyurea/ASTM1045-steel composite plate,the effectiveness of polyurea coating was mainly re flected in the velocity.Fig.15 shows the variation of the velocity.

In Fig.15,the velocity of the projectile before penetrating the target,Vi,ranged from 880 to 900 m/s.In this experiment,the difference inViwas less than 2.2%,which meant thatVicould be treated as constant.Additionally,as the thickness of the polyurea coating increased,the velocity of the projectile after penetration showed a downward trend.As shown in Fig.15(a)and(b),when the thicknesses of the polyurea A coatings on the front and rear faces of the steel plate reached 10 mm,Vjdecreased to 576 and 581 m/s,respectively,corresponding to velocity reductions of 34.6%and 40.7%.Similarly,as shown in Fig.15(c)and Fig.15(d),when the thicknesses of the polyurea B coatings on the front and rear faces of the steel plate reached 10 mm,Vjdecreased to 438.2 and 455.5 m/s,respectively,corresponding to velocity reductions of 50.4%and 48.5%.The data showed that the polyurea coating could enhance the anti-penetration performance of the steel plate.To study the in fluence of the polyurea coating thickness on the anti-penetration performance of the composite structure,the results were numerically fitted with the following equation:

where the parameters were determined to be:

Fig.14.The failure mode of polyurea/steel composite plate enhanced by glass-fiber.

For the fitting process,the type of substrate,type of polyurea,and the coating position were the same.The only variable was the polyurea layer thickness.Thus,the fitting formulas only included the polyurea layer thickness.

Table 10 shows the coef ficient of association of the numerical fitting results,which shows that the fits were in good agreement with theVjdata.Therefore,the formulas were accurate and effective.In Fig.16,the curves decreased slowly at first and then more rapidly.The kinetic energy,which is the main indicator of the projectile motion,is defined as follows:

If the velocity drops by the same amount,a projectile with a high velocity loses more kinetic energy than a projectile with a low velocity.For example,for two identical projectiles with initial velocitiesViof 900 and 450 m/s,if both projectile velocities decreased by 450 m/s,the projectile with a velocity of 900 m/s would lose three times the kinetic energy than the projectile with a velocity of 450 m/s.Therefore,the general trend of the curve fitted would be gradual in former part and then steep in the latter part.However,samples A-I-1-5 exhibited a different waveform from the other samples.According to the analysis results,as shown in Fig.19,the anti-penetration performances of the samples with polyurea A coating the rear faces were much weaker than those under other conditions,especially A-I-1,A-I-2,and A-I-3.Due to the poor antipenetration performances under these conditions,the waveform in Fig.15(a)was slightly different from the others.Finally,it must be emphasized that the formulas are suitable only for the conditions of this experiment.

4.2.2.Influence of coating position and glass-fiber on the polyurea effectiveness

Fig.17 shows the comparison of projectile velocity reductions of different coating positions.By analyzing the results in Fig.17,the in fluence of the coating position on the polyurea effectiveness was evident.For polyurea A(AMMT-53),which is slightly hard and brittle,the polyurea coating on the front face was better than that on the rear face.However,polyurea B(AMMT-55)has better strength and ductility.Thus,the polyurea acted as a buffer layer to reduce the velocity on the front face,and it easily formed a plug to absorb the remaining kinetic energy of the projectile when the polyurea was on the rear face,as shown in Fig.9.The effectiveness of polyurea B on the rear face was slightly better than that of polyurea B on the front face,although the difference was subtle.Table 11 shows a comparison of the velocity reductions for the different coating positions.The relative difference is the difference of the velocity reductions when polyurea coated the front and rear faces.For polyurea A,polyurea on the front face produced better anti-penetration performances.When the coating thickness was less than 6 mm,the relative difference was 45%-65%.The relative difference decreased when the coating thickness reached 8 or 10 mm.The velocity reduction when polyurea coated the rear face was greater than that when it coated the front face,but the relative difference for polyurea B was no more than 20%.

Fig.9.The relatively complete thin polyurea wafer in spallation.

Fig.15.The velocity of projectile corresponding to different thickness of the polyurea coating.

Table 10 Coef ficients from numerical fitting.

In Table 12,the thickness of the coating is not only the thickness of the polyurea;it includes the polyurea and glass-fiber cloth.The glass-fiber cloth was 0.6 mm thick.The relative difference is the difference in the results when the glass-fiber cloth was present and with a pure polyurea coating of the same thickness.For example,AIII-1 and A-III-2 were compared with A-II-1.For AMMT-53,the glass-fiber cloth enhanced the anti-penetration performance,and the effect of the glass-fiber cloth on the rear face was better than that on the front face.Additionally,as the thickness of the coating increased,the enhanced effect of the glass-fiber cloth on the polyurea began to diminish.However,for the AMMT-55 polyurea material,the in fluences of the glass-polyurea position on the composite structure were different.The effect of the glass-fiber cloth on the front face was better than that on the rear face.When the glass-fiber cloth was on the rear face for AMMT-55,it did not enhance the performance of the composite structure.The antipenetration performance was weakened in this condition.

Table 11 Comparison of velocity reductions(different positions).

Table 12 In fluence of glass-fiber cloth on the anti-penetration performance of composite plate.

4.2.3.Analysis of velocity reduction per unit area density of two kinds of polyurea

Fig.18 and Fig.19 show the velocity reductions per unit areal density with only a polyurea coating and with a composite structure including polyurea and steel.In Fig.18,the data show that the velocity reductions per unit areal density corresponding to A-I-1,BI-1,B-I-2,A-I-6,and B-I-6 were higher than those under other conditions.Under the above five conditions,the thickness of the polyurea was small,2 or 4 mm,which means that the velocity reduction of the projectile was not linearly correlated with the thickness of the polyurea coating in general.However,for polyurea A,when the thickness of the polyurea coating was higher than 6 mm(≥6 mm),the velocity reduction per unit areal density of the polyurea coating changed slightly,whether the polyurea coating was on one(category I)or two(category II)sides.This means that the variation of the projectile velocity was linearly correlated with the increase in the polyurea A layer thickness when the polyurea coating reached a certain thickness.Polyurea B did not show similar features.For polyurea B,when the thickness of the polyurea coating increased,the velocity reduction per unit areal density continually declined.

In Fig.19,numbers 1-5 represent polyurea coatings on the rear face,6-10 represent polyurea coatings on the front face,and 11-17 represent polyurea coatings on both sides.Compared with the experiment results for the bare steel plate(number 0 in Fig.19),the velocity reduction of the projectile per unit areal density of the composite plate was higher. Thus, the anti-penetration performance of the composite plate was improved.For the same weight,the composite plate could withstand greater ballistic limits than the bare steel plate.By analyzing the data in groups 1-5,6-10,and 11-17,all the velocity reductions per unit areal density of the composite plate showed an upward trend.Thus,if the thickness of polyurea coating was in the rage of 0-10 mm,the thicker the polyurea coating was,the better the anti-penetration performance of the composite plate became.

Fig.16.Numerical fits of the effectiveness and thickness of the polyurea coating.

Fig.17.Comparison of velocity reductions for different positions of the polyurea coating.

Fig.18.Velocity reduction per unit areal density(considered only the polyurea coating).

In Figs.18 and 19,the velocity reduction per unit areal density of polyurea B was higher than that of polyurea A,no matter it is for only the polyurea coating or the whole composite plate.For two types of polyurea coatings with the same unit areal density,the ballistic limit of polyurea B was better,because the strength of polyurea B was much higher than that of polyurea A.The tensile strength of polyurea B reached 35 MPa,and the elongation at break of polyurea B was six times greater than that of polyurea A.Whether the coating was a buffer layer on the front face or formed a plug on the rear face,the polyurea absorbed more kinetic energy and further reduced the velocity.

5.Conclusions

In this study,experiments were conducted with a ri fle and 5.8-mm projectiles to explore the anti-penetration performances of polyurea/ASTM1045-steel composite plates subjected to high velocity impacts.By analyzing the damage mechanism and comparing the results based on the velocity of the projectile,the following conclusions were drawn:

(1)Five damage modes,including cracking,self-healing,local bulging,spallation,and local fragmentation,were identi fied to analyze the damage of polyurea/steel composite plate.The relationship between damage modes and polyurea layer thickness was summarized.The results of SHPB tests were utilized to explain the damage mechanisms.Additionally,the elastic filler was discovered,and the difference in the damage modes between single-sided and double-sided polyurea were analyzed.

(2)The polyurea material coating on the surface of the steel plate could improve the anti-penetration performance,and the effect of AMMT-55 was better than that of AMMT-53.As the thickness of the polyurea coating increased,the velocity of the projectile after penetration showed a decreasing trend.Additionally,formulas were provided to describe the relationship between the effectiveness of polyurea and the thickness of the polyurea coating,which are suitable only for the conditions of these experiments.

(3)The in fluences of the polyurea coating position were different for two kinds of polyurea.The effect of AMMT-53 on the front face was greater than that on the rear face,and the difference of the anti-penetration performance could reach 65%.The effect of AMMT-55 on the rear face was greater than that on the front face,but this difference was very subtle and usually no more than 20%.

(4)The in fluences of glass-fiber cloth positions were different for the two kinds of polyurea.The glass-fiber cloth on the rear face was better than that on the front face for AMMT-53,while the glass-fiber on the front face was better than that on the rear face for AMMT-55.Additionally,as the thickness of the coating increased,the enhanced effect of the glass fiber on the polyurea began to weaken.The glass-fiber cloth on rear face for AMMT-55 did not enhance the performance of the composite structure,and the anti-penetration performance was weakened in this case.

(5)The bonding strength was very important for the antipenetration performance of the polyurea layer.If the bonding strength were enhanced,more kinetic energy of the projectile would be converted to other forms in the process of penetration.

Declaration of competing interest

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

Acknowledgements

This research was supported by the National Natural Science Foundation of China(Nos.51978660).The authors would like to gratefully acknowledge this support.