Effects of material of metallic frame on the penetration resistances of ceramic-metal hybrid structures

Xuanyi An,Chao Tian,Qitian Sun,Yongxiang Dong

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

Keywords:Hybrid structures Zirconia toughened alumina Penetration resistance Long-rod projectile Metallic frame

ABSTRACT The effects of metallic material on the penetration resistances of ceramic-metal hybrid structures against vertical long-rod tungsten projectiles were studied by artillery-launched experiments and numerical simulation.Hybrid structures with rectangular cores in transverse orthogonal arrangement and slidefitting ceramic inserts of zirconia toughened alumina prisms were fabricated with titanium alloy TC4(Ti6Al4V),AISI 4340 steel and 7075 aluminum alloy panels,respectively.The results showed that the hybrid structure of Ti6Al4V exhibited the highest penetration resistance,followed by that of 7075 aluminum alloy with the same area density.The penetration resistance of the hybrid structure of AISI 4340 steel was the low est.The underlying mechanisms showed that the metallic material of a ceramicmetal hybrid structure can directly affect its energy absorption from the impact projectile,which further affects its penetration resistance.Different metallic frames exhibited different failure characteristics,resulting in different constraint conditions or support conditions for ceramic prisms.The high penetration resistance of the Ti6Al4V hybrid structure was due to its stronger back support to ceramic prisms as compared with that of AISI 4340 steel hybrid structure,and better constraint condition for ceramic prisms by metallic webs as compared with that of 7075 aluminum alloy hybrid structure.The results of mass efficiency and thickness efficiency show ed that the Ti6Al4V hybrid structure has advantages in reducing both the thickness and the mass of protective structure.In addition,because the ceramic-metal hybrid structures in the present work were heterogeneous,impact position has slight influence on their penetration resistances.?2020 China Ordnance Society.Production and hosting by Elsevier B.V.on behalf of KeAi Communications Co.This is an open access article under the CCBY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

1.Introduction

Hybrid sandwich structures have been extensively explored for blast loading and projectile penetration resistance[1-13].It has been reported that,under blast loadings or compressive loadings,the deformations of extruded 6061-T6 aluminum sandwich panels with corrugated cores[1,6]are less significant than those of monolithic alloy panels with the same area densities.How ever,a recent study[4]suggests that the extruded 6061-T6 aluminum sandwich panel without inserting,triangular corrugated cores showed similar penetration resistance against high-speed spherical projectile with mass of about 8.4 g compared with monolithic alloy panel with a thickness approximately that of the two face sheets.Yet the triangular alumina prisms inserted into core channels can significantly affect the penetration resistances of the panels.The targets with alumina filled possess very complicated failure mechanisms,such as ductile hole initiated in the face sheet,as well as the tensile failure and shear failure of the joints between back/face sheet and cores,the cone crack[14-15]initiated in the ceramic prisms separated by metallic webs,and the perforation of back sheet due to shear plugging.In addition,the ballistic limit of hybrid structure is highly sensitive to the impact location of projectile due to its asymmetry.For example,the impact at prism base engages a higher ballistic limit than that at the monolithic alloy panel with the same area density.In contrast,the impact at prism apex engages a lower ballistic limit than that at the monolithic alloy panel with the same area density.

Due to the sensitivity of penetration resistance to impact location,Wadley investigated the effects of the shape of ceramic inserts on penetration resistance against a high-speed of 8.4 g spherical projectile and found that the hybrid structure with rectangular ceramic prisms exhibited the highest penetration resistance,follow ed by those with trapezoidal and triangular ceramic prisms[3].In addition,they also reported that the penetration resistance was related to not only the shape of ceramic prisms beneath the projectile,but also the shape of ceramic prisms of their adjacent ones,and the penetration resistance increased with the width of ceramic prisms increasing.However,wider ceramic prisms can enhance the extent of lateral damage,which decreases the penetration resistance of the hybrid structure in multi-hit scenarios[16].

The hybrid structures with ceramic inserts mentioned above possess high penetration resistances against the high-speed spherical projectile of 8.4 g.In terms of thicker hybrid structures against the high-speed long-rod projectiles with much greater masses,we explored the responses of the hybrid structures with rectangular cores of same area density,yet arranged differently against a vertical 86 g long-rod tungsten projectile[17].The results indicate that the penetration resistance of the targets is significantly improved when the arrangement of ceramic cores changes from upright to transverse parallel or transverse orthogonal(Fig.1)because the hybrid structures with transverse core arrangements show better constraint conditions for ceramic prisms by metallic webs to blunt and erode the projectiles compared to that with upright core arrangement.

Although ceramic-metal hybrid sandwich structures with transverse core arrangements possess high penetration resistances against long-rod projectiles,there are many factors that can affect their penetration mechanisms and the efficacy of ceramic in ballistic performance.The previous research[17]show ed that the constraint condition for ceramic prisms by metallic frames played an important role in the design of hybrid structure.Therefore,it is essential to investigate the effects of material of metallic frame on the penetration resistances of ceramic-metal hybrid structures,which can offer a good reference for the design of next generation of lightweight protection structures.In the present work,transverse orthogonal hybrid structures with ceramic inserts of zirconiatoughened alumina prisms were prepared respectively with titanium alloy TC4(Ti6Al4V),AISI 4340 steel and 7075 aluminum alloy,and their penetration resistances to 86 g long-rod tungsten projectiles were experimentally studied.The detailed penetration process and energy absorption of the substructures were numerically simulated,aiming to explore the related penetration resisting mechanism.

2.Structure design and experimental procedure

2.1.Target preparation

Three ceramic-metal hybrid structures,A,B and C,were fabricated with Ti6Al4V,AISI 4340 steel and 7075 aluminum alloy panels,respectively.Fig.2 shows the detailed fabrication process of hybrid structure A.Brie fly,two mutually perpendicular layers including 10 rectangular prisms(18 mm×18 mm×100 mm)were horizontally removed from a 55 mm thick Ti6Al4V panel.Ten ceramic prisms were inserted into the voids of the Ti6Al4V panel by slid-f t i ting and glued to the panel using epoxy adhesive.A 5 mm thick 603 armor steel sheet was attached to the bottom of the panel using epoxy adhesive to complete the structure.

Hybrid structures B and C were prepared with the same procedure,but using a AISI 4340 steel panel and a 7075 aluminum alloy panel,respectively(Fig.3 and Fig.4).Table 1 shows their area densities.The maximum relative deviation of the area densities of the structures is 5%.Therefore,the area densities of hybrid structures A,B and C were considered as same.

2.2.Ballistic testing and characterization

The penetration resistances of hybrid structures A,B and C were experimentally examined against a pointed long-rod tungsten projectile(Hardness 57 HRC)with the mass of 86 g.Fig.5 shows the experimental arrangement and the parameters of the projectile for ballistic testing.A long-rod 93 tungsten projectile with nylon sabot(Fig.7)was shot using a powder gun with the caliber of 40 mm at normal incidence toward the center of a hybrid structure fixed in a positioning frame(Fig.5).The incident velocity of the projectile to the target face and its penetration characteristics near the target were measured and recorded using a Phantom 710 high-speed camera(Vision Research,Inc.Wayne,New Jersey,USA).In the experiment,the frame rate of the high-speed camera was 30839 fps,and the exposure time was 4.5μs.The exit energy of projectile was evaluated using a witness panel made of AISI 4340 steel if the incident velocity of projectile was higher than the ballistic limit of the hybrid structure.

2.3.Numerical simulation

The penetration process of each hybrid structure was numerically simulated using the nonlinear finite element code LS-DYNA 971 to determine the effect of metal material on the ballistic responses of hybrid structures[18].The Johnson-Cook model is the most popular model for metallic materials subjected to large strains,high strain rates,and high temperature[19-23].In this model,the yield strength depends on the state of equivalent plastic strain,strain rate and temperature.The Johnson-Holmquist 2(JH-2)model is a computational model which is widely used for brittle materials subjected to large strains,large strain rates and high pressures[19,24,25 and 26].This model includes pressure dependent strength,damage and fracture,significant strength after fracture,bulking and strain rate effects.Therefore,the Johnson-Holmquist 2 (JH-2)model was used to simulate zirconiatoughened alumina and the metallic materials including Ti6Al4V,AISI4340 steel,7075 aluminum alloy,tungsten alloy and 603 armor steel were simulated using the Johnson-Cook(JC)model.The JC model constants and JH-2 model constants are listed in Table 2[19-23]and Table 3[24]respectively.The hybrid structure and projectile were modeled with eight node solid elements.Due to the low strength and low thickness of the epoxy adhesive,as compared with the steel and ceramic,“CONTACT-TIED-SURFACE-TO-SURFACE”was used to model the adhesive,instead of constructing an epoxy layer in the model.The tensile stress at failure and the shear stress at failure of the adhesive were set to 120 MPa and 80 MPa,respectively[27].Each part of the system was connected with“CONTACT-ERODING-SURFACE-TO-SURFACE”[18].The contacts were updated once the elements on the free surface of structure were eliminated by the relevant material failure criteria.The minimum and maximum element sizes were set to 0.3 mm and 0.33 mm,respectively.In the damage analyses of hybrid structures,a half model of each armor system was established due to the symmetry of the hybrid structures(Fig.6).

Fig.1.Schematic of hybrid structures with different arrangements[17].

Fig.2.Fabrication process and detailed parameters of hybrid structure A.

Fig.3.Detailed parameters of hybrid structure B and picture of hybrid structure B before impact test.

3.Results and discussion

3.1.Experimental analysis

Fig.4.Detailed parameters of hybrid structure C and picture of hybrid structure C before impact test.

Table 1 Area densities of hybrid structures A,B and C.

Hybrid structures A,B and C with the same structure but different materials were tested to explore the effects of metallic material on penetration resistance.Table 4 summarizes the results of projectile impacts on hybrid structures A,B and C.The incident velocity of the projectile launched by the powder gun was 950 m/s with the variation within 1.6%,and thus was considered same for each structure.All the impacts were normal impacts as shown in Fig.7 obtained from the high-speed camera.Fig.8,Fig.9 and Fig.10 show the pictures of hybrid structures A,B and C after the test,respectively.

As shown in Fig.8[17],hybrid structure A was able to hold the incident projectile.The 603 armor steel attached to hybrid structure A after the impact test,yet a perforated hole was found in the face sheet,and a minor but obvious deformation was observed on both back sheet of the titanium alloy frame and the 603 armor steel plate.In addition,a few cracks were formed on the edges of the first and second layers of the metallic frame in the axial direction of ceramic prisms(Fig.8(c)and Fig.8(d)).After the impact test,the TC4 frame of hybrid structure A was cut open along its impact zone using a wire electrical discharge machine after removing ceramic prisms in its voids,in order to obtain its intact thickness(Fig.8(f)).Some node failures were found between the cells and the face/middle/back sheet of the hybrid structure on the cross section across the penetration hole(Fig.8(f)),similar to the joint failure of the extruded 6061-T6 aluminum sandwich panel caused by the impact of a high-speed spherical projectile[4].The total intact thickness of the impacted hybrid structure A including the 603 armor steel sheet was measured to be 14 mm.The projectile with the mass of 16.5 g was recovered after the impact(Fig.8(e)),and 80.8%of the projectile mass was lost.

Fig.5.Schematic of experimental setup and photo of site layout.

Fig.6.Numerical models of one half of the armor systems(a)Hybrid structure A(b)Hybrid structure B(c)Hybrid structure C.

Fig.7.Illustration of normal projectile impact on the target taken by the high-speed camera.

As shown in Fig.9,hybrid structure B was completely perforated.The 603 armor steel of hybrid structure B was not de-bonded from the structure after the impact either.Although no fractures were found on the edges of hybrid structure B,elliptical ruptures near a ductile hole were initiated on the face sheet due to the movement of shattering ceramic prisms along the inverse direction of the impact(Fig.9(d))and fractures were also observed along the axial direction of ceramic prisms in the back-face of metallic frame made of AISI 4340 steel due to the combined impacts of the projectile and crushed ceramic prisms beneath the projectile(Fig.9(e)).A large permanent deformation with a petaling hole and a bulge of 25 mm was observed on the back of the 603 armor steel sheet(Fig.9(f)).Craters were formed in the witness panel made of AISI 4340 steel(Fig.9(a)).

As shown in Fig.10,hybrid structure C was completely perforated.The hybrid structure C made of 7075 aluminum alloy was almost completely disintegrated by the impact(Fig.10(b)).Most of the ceramic prisms fell off,and a few of ceramic prisms remained in the structure were cracked.The recovered ceramic fragments could remain its size with slight fractures.A ductile hole with a bulge of 16 mm was observed on the back of the 603 armor steel sheet(Fig.10(c)).Craters were also formed in the witness panel made of AISI 4340 steel(Fig.10(d)).

Based on these results,it can be clearly seen that the metallic material of hybrid structure can significantly affect its penetration resistance against long-rod tungsten projectile.The penetration resistances of hybrid structures A,B and C that were prepared with exactly same arrangement of bilayer transverse orthogonal ceramic inserts but different materials(Ti6Al4V,AISI 4340 steel and 7075 aluminum alloy,respectively)at the same area density follow ed the order of A＞C＞B.Hybrid structure A was able to hold the projectile with a thick intact thickness(Fig.8(f)),while hybrid structure B was completely perforated.No fracture was formed on the edges of hybrid structure B,indicating that its ceramic prisms were endow ed good constraint condition by the metallic webs of the AISI 4340 steel hybrid structure.How ever,the density of AISI 4340 steel is higher than that of Ti6Al4V,and thus the back support of hybrid structure B is thinner than that of hybrid structure A with the samearea density.The large fractures formed along the axial direction of ceramic prisms on the back-face of the metallic frame made of AISI 4340 steel and the large permanent deformation with a petaling hole on the back of 603 armor steel sheet of hybrid structure B indicate that it failed to stop the projectile because its back sheet was not strong enough to support the ceramic prisms to erode and resist the projectile.Although hybrid structure C was also completely perforated,the impact produced less scattered debris with lower kinetic energies behind it than behind hybrid structure B,resulting in fewer scattered craters with smaller total area and depth in the witness panel(Fig.10(d)and Table 4).Compared to hybrid structure A,the complete disintegration of hybrid structure C suggests that 7075 aluminum alloy hybrid structure failed to provide a good constraint condition for ceramic prisms to blunt and erode the projectile,consistent with Yang's conclusion that 7075 aluminum alloy was a brittle material under dynamic impact loading conditions[28].These experimental results indicate that different metallic frames exhibited different failure characteristics,resulting in different constraint conditions or support conditions for ceramic prisms.The hybrid structure of Ti6Al4V exhibited the highest penetration resistance,as compared with those of AISI 4340 steel and 7075 aluminum alloy,because of the strong back support to ceramic prisms and good constraint condition for ceramic prisms by metallic webs.

Table 2 JC material model constants for metallic materials[19-23].

Table 3 JH-2 material model constants for zirconia-toughened alumina[24].

3.2.Numerical simulation analysis

The element size in numerical simulation plays an important role to provide accurate outputs.How ever,extremely small size of element makes the numerical process time-consuming.Therefore,the effects of element size on numerical results were investigated in the present work.Fig.11 shows the relative errors between experimental results and numerical results with different element sizes for the intact thickness of hybrid structure A after impact.It can be seen that the relative error varies a lot with the element size increasing.When the element sizes are under 0.35 mm,the relative errors have no big difference and numerical simulation outputs show ed good agreement with the experimental results.Hence,0.3-0.33 mm elements in the present work were enough to provide accurate results.

To further understand the effect of metallic material on penetration resistance,the impact processes of the hybrid structures A,B,and C were numerically simulated.Fig.12 shows the representative simulated material damage contours in the ceramic prisms of hybrid structure A,and deformations of 603 armor steel in hybrid structure B and hybrid structure C.As can be seen,the damage inthe ceramic prisms moves much faster than the penetration velocity of projectile,although the ceramic prisms are separated by the metallic webs.Hybrid structure A is able to hold the incident projectile,while hybrid structures B and C are completely perforated.Table 5 shows the comparison of the numerical and experimental results.The maximum difference between the numerical and experimental results is 10.8%with the average value of 8.97%.Fig.13,Fig.14 and Fig.15 show the numerical and experimental results of failure characteristics of the metallic frames of hybrid structures A,B and C after the test,respectively.It can be clearly seen that numerical simulation outputs show ed good agreement with the experimental results.These results suggest that the numerical models are reliable to simulate the impact processes of the hybrid structures against long-rod projectiles.

Table 4 Impact results of hybrid structures A,B and C.

Fig.8.Pictures of hybrid structure A impacted by a long-rod 93 tungsten projectile[17].(a)Retrieved ceramic debris;(b)Hybrid structure A with perforated face sheet and remained ceramic prisms;(c)Fractures formed in the cellular direction on the first layer of cores;(d)Fractures on the second layer;(e)Residual projectile with the mass of 16.5 g after impact;(f)Cross section of the TC4 frame across the penetration hole.

Fig.9.Pictures of hybrid structure B impacted by the projectile.(a)Craters formed on the witness panel made of AISI4340 steel;(b)Retrieved ceramic debris;(c)Hybrid structure B impacted by the projectile;(d)Elliptic fractures caused by the shear and tearing near a ductile hole on the face sheet;(e)Fractures formed along the axial direction of the ceramic prisms on the back-face sheet of the metallic frame made of AISI 4340 steel near a hole;(f)Permanent deformation with a petaling hole on the 603 armor steel sheet.

Fig.10.Pictures of hybrid structure C impacted by an 8 mm diameter projectile(a)Retrieved ceramic debris and 7075 aluminum alloy debris(b)Broken metallic frame(c)603 armor steel back plate after the impact test(d)Craters formed on the witness panel made of AISI 4340 steel.

Fig.11.Relative errors between experimental results and numerical results with different element sizes for the intact thickness of hybrid structure A.

Fig.16 shows the variation of the projectile's kinetic energy with time for the three hybrid structures.As can be seen,the kinetic energy of projectile in hybrid structure A decreases much more rapidly than those in hybrid structures B and C,indicating that the hybrid structure of Ti6Al4V possesses a higher penetration resistance than those of AISI 4340 steel and 7075 aluminum alloy.In addition,the total energies(sum of kinetic energy and internal energy)(t=220μs)of the metallic frames and 603 armor steel panels of hybrid structures B and C are higher than those of hybrid structure A due to their more significant permanent deformations(Fig.17).How ever,compared with those in hybrid structures B and C,the ceramic prisms in hybrid structure A can absorb more energies from the impact projectile due to its stronger back support and better constraint conditions by metallic webs(Fig.17),consistent with the experimental results described in Section 3.1.These results indicate that the metallic material of hybrid structure can directly affect the energy absorption of the structure from the impact projectile,and a strong back support to ceramic prisms and strong confinement for ceramic prisms by metallic webs are the keys to improve the penetration resistance of hybrid structures against long-rod projectiles.

Fig.12.Simulated material damage contours in the ceramic prisms of hybrid structure A(a)and deformations of 603 armor steel in hybrid structure B(b)and hybrid structure C(c).

Table 5 Comparison of the numerical and experimental results.

Fig.13.Numerical and experimental results of failure characteristics of the TC4 frame of hybrid structure A.

To clearly compare the penetration resistances of the hybrid structures fabricated with different materials,the mass efficiency and thickness efficiency of hybrid structures A,B and C were investigated numerically as shown in Fig.18.The incident velocities of the projectiles were 1200 m/s.The mass efficiency and thickness efficiency compare the ballistic performance of the targets with that of the 603 armor steel target(Quasi static state:Hardness 42 HRC/395 HB,Yield strength 0.91 GPa,Ultimate strength 1.25 GPa,Elongation 24%),and are defined as[29-30]:

Fig.14.Numerical and experimental results of fractures along the axial direction of ceramic prisms in the back-face of metallic frame made of AISI 4340 steel from hybrid structure B.

Fig.15.Numerical and experimental results of the disintegration of metallic frame made of 7075 aluminum alloy from hybrid structure C.

where Fmis the mass efficiency,Fsis the thickness efficiency,Prefis the depth of penetration in 603 armor steel without the hybrid structure,Presis the residual depth of penetration in 603 armor steel by the projectile penetrated through the hybrid structure,δ is the thickness of the hybrid structure,ρ603is the density of 603 armor steel,and ρhsis the average density of the hybrid structure.

Fig.16.Energy histories of the projectiles.

Fig.17.Total energy of the substructures of hybrid structures.

Fig.18.Schematic of depth of penetration test.

The depth of penetration in 603 armor steel without the hybrid structure was 69 mm,and the results of the mass efficiency and thickness efficiency are shown in Table 6.It can be seen that hybrid structure A had the highest mass efficiency,followed by that of hybrid structure C,and the mass efficiency of hybrid structure B was the low est.How ever,the thickness efficiency of hybrid structure B was higher than that of hybrid structure C.All of the mass efficiencies are larger than 1,which means that hybrid structures A,B and C have advantages in reducing the mass of protective structure.How ever,only the thickness efficiency of hybrid structure A is larger than 1,which means that only hybrid structure A has advantages in reducing the thickness of the protective structure.

To further explore the penetration resistance of hybrid structure of Ti6Al4V with ceramic insertions in transverse orthogonal arrangement(hybrid structure A),which exhibited the highest penetration resistance,five representative impact positions were selected to investigate the sensitivity of penetration resistance to impact location as shown in Fig.19.The penetration resistance of the hybrid structure was compared with that of 603 monolithic plate with the same areal density to further understand the antipenetration performance of the hybrid structure.The incident velocities of the projectiles were 1200 m/s,and Fig.20 shows energy histories of the projectiles.It can be seen that the impacts at different positions result in slightly different energy absorptions.The impact at P1 dissipates the energy most,yet which is only 11%higher than the impact at P5,indicating that impact position has only slight influence on the penetration resistance of hybrid structure of Ti6Al4V with ceramic insertions in transverse orthogonal arrangement.The energy dissipated by the hybrid stricture is 24%-35%higher than that dissipated by the 603 monolithic plate with the same areal density,indicating the excellent antipenetration performance of the hybrid structure.

4.Conclusions

Fig.19.Schematic of different impact positions(top view).

Fig.20.Energy histories of the projectiles impacted on the hybrid structure at different positions and on 603 monolithic plate.

The responses of metallic materials of hybrid structures with ceramic insertions in transverse orthogonal arrangement against a vertical long-rod tungsten projectile with mass of 86 g were investigated experimentally and numerically.The hybrid structure fabricated with Ti6Al4V exhibited the highest penetration resistance(with a total intact thickness of 14 mm),while that of AISI 4340 steel and that of 7075 aluminum alloy were completely perforated.The penetration resistance of the AISI 4340 steel hybrid structure was the lowest because the impact of the AISI 4340 steel hybrid structure produced more scattered craters with larger total area(697 mm2)and depth(7 mm)in the witness panel than that of the 7075 aluminum alloy hybrid structure(producing craters with the total area of 550 mm2and the depth of 4 mm in the witness panel).It can be explained that the metallic material of a hybrid structure can directly affect its energy absorption from the impact projectile.Different metallic frames exhibited different failure characteristics,resulting in different constraint conditions or support conditions for ceramic prisms.The ceramic prisms in the hybrid structure of Ti6Al4V were more strongly supported than those in the AISI 4340 steel hybrid structure and were under better constraint conditions than those in the 7075 aluminum alloy hybrid structure,which endowed them the ability to erode and resist the projectile,resulting in higher energy absorption of the ceramic prisms(2.19 times that of ceramic prisms in the AISI 4340 steel hybrid structure,and 1.91 times that of ceramic prisms in the 7075 aluminum alloy hybrid structure).Therefore,a strong back support to ceramic prisms and strong confinement for ceramic prisms bymetallic webs are the key factors to improve the penetration resistances of ceramic-metal hybrid structures against long-rod projectiles.The results of mass efficiency and thickness efficiency showed that the Ti6Al4V hybrid structure has advantages in reducing both the thickness and the mass of protective structure.The results also indicate that the energy dissipated by the Ti6Al4V hybrid stricture is 24%-35%higher than that dissipated by the 603 monolithic plate with the same areal density,indicating the excellent anti-penetration performance of the hybrid structure.In addition,impact position has only slight influence on the penetration resistance of hybrid structure with ceramic insertions in transverse orthogonal arrangement,with no more than 11%difference.

Table 6 Results of the mass efficiency and thickness efficiency.

Acknowledgements

The authors are very grateful for the support received from the National Natural Science Foundation of China(No.11872121).