Experimental study on ceramic balls impact composite armor

Wei-zhn Wng , Zhi-gng Chen , Shun-shn Feng , Ti-yong Zho

a Underground Target Damage Technology National Defense Key Discipline Laboratory, North University of China, Taiyuan 030051, Shanxi, China

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

Keywords:Impact Penetration Infinitesimal Interface collapse

ABSTRACT Ceramic balls represent a new type of damaging element, and studies on their damaging power of composite armor are required for a comprehensive evaluation of the effectiveness of various types of weapons.The goal of this study was to determine the impact of φ7 mm toughened Al2O3 ceramic balls on a composite ceramic/metal armor. The influences of the ceramic panel and the thickness of the metal backing material on the destroying power of the ceramic balls were first determined. Based on the agreement between numerical simulation, experimental results, and calculation models of the target plate resistance,the response mechanism of the ceramic balls was further analyzed.The results indicate that for a back plate of Q235 steel,with an increasing thickness of the ceramic panel,the piercing speed limit of the ceramic balls gradually increases and the diameter of the out-going hole on the metal back decreases. Different conditions were tested to assess the effects on the piercing speed, the diameter of the out-going hole, the micro-element stress, and the integrity of the recovered ceramic bowl.

1. Introduction

In modern warfare, with the rapid development of warhead technology, lightweight composite armor with strategic value has attracted much attention. The new warhead damage element represented by high-performance and low-cost ceramic balls has emerged as the times require. Compared with ordinary tungsten alloy and steel fragments,ceramic balls have the advantages of high strength, large specific kinetic energy, low-cost and easy production,etc.They can effectively strike at close range armored targets,and the study of damage effect of ceramic balls on armored targets is an important part of weapon system effectiveness evaluation.

In recent years, scholars at home and abroad have made some reports on the damage power and dynamic properties of ceramic damage elements.Nechitatio[1,2]and others have used numerical simulation methods to study the fracture of a ceramic rod impacting metal target, and compared the fracture morphology of the target under different impact velocities.Then Nechitatio carried out in the experimental study of penetrating concrete target with head-mounted polycrystalline diamond penetration projectile at different impact velocities based on the numerical simulation method. Takahashi [3] studied the fracture characteristics of ceramic balls under high pressure by means of an experimental study,theoretical analysis and numerical simulation.Matsuda[4,5]analyzed the fracture characteristics of ceramic balls under thermal shock, established the calculation theory of virtual crack model,and carried out the indentation contrast test on silicon nitride ceramic balls between air and vacuum.Ma[6]used fractal theory to analyze the fracture characteristics of brittle spheres under high velocity impact. The theoretical analysis results are in good agreement with the experimental results.Wang[7-15]pointed out that ceramic balls have the advantages of high strength, high specific kinetic energy and high initial velocity, and have high penetrating efficiency and high aftereffect of close range targets (pine targets, aluminum plates, etc.). Chen [16-25] proposed replacing conventional metal warheads with non-metal ceramic warheads,and found that ceramic composite projectiles can effectively improve the damage power of composite armor.

At present, the damage mechanism and armor-piercing response characteristics of the damage elements of ceramic materials have been preliminaries understood through the research on the damage effectiveness of the damage elements of ceramic materials, but the damage law of light composite armor and the response law of the damage elements of ceramic materials are rarely studied. Based on the ballistic impact test, the influence of ceramic balls on the ultimate penetration velocity and impact breakage threshold of ceramic composite armor under different working conditions is studied in this paper.Based on the numerical simulation and impact pressure calculation theory, the interface breakdown effect of ceramic balls is predicted and analyzed, and the response characteristics of ceramic balls and composite armor are expounded.

2. Experiment preparation

2.1. Laboratory equipment

The experiment was conducted out at the target road of the National Defense Key Discipline Laboratory of Underground Target Damage Technology of the North University of China. The test composite back plate uses a Q235 steel target of 500 mm×500 mm×2 mm in size and a 2024 aluminum target.The ceramic panel includes Al2O3ceramic panels with dimensions of 100 mm×100 mm×1 mm, 100 mm×100 mm×2 mm, and 100 mm×100 mm×3 mm. The middle layer is bonded with glass fibers. In view of the that the loading size of the 7.62 mm ballistic gun to the ceramic ball diameter is less than 7.2 mm and the loading speed is less than 1500 m/s,the penetration power(specific kinetic energy) of the ceramic ball is proportional to the radiusRand the loading speedv,respectively,i.e.a 7.62 mm ballistic gun.There is a loading limit on the size and penetration power of the ceramic ball.In addition, considering that the wavelength of the stress wave of the ceramic ball material is 5.2 mm or less,to explore the breaking law of the ceramic ball effectively it is necessary to ensure that the size of the ceramic ball is at least larger than the wavelength of the ceramic material. A tensile wave and a compression wave were utilized to analyze the breaking law of the ceramic ball. To penetrate the target plate comprehensively and effectively explore the breaking law of the ceramic ball, the fragment is made of a toughened Al2O3ceramic bowl with a size of φ7 mm. The test equipment is shown in Fig.1.

2.2. Ballistic tests

In this study, several effective toughened Al2O3ceramic ball impacting composite target panels was tested. A 7.62 mm caliber ballistic guns were used for testing, and the launch speed was adjusted by modifying the loading of the gunpowder. A laser speedometer is independently developed by the North University of China was used for the speed measurement, within an error of±0.3%.A schematic diagram of the(a)test arrangement and(b)site layout is shown in Fig.2.

Fig.1. Laboratory equipment.

3. Numerical simulation

3.1. Model establishments

A finite element model was built using TRUEGRID software. To reduce the computation time, a three-dimensional finite element model was established based on a 1/2 structure, and symmetric constraints were set on the symmetry plane of the 1/2 model.The ceramic ball and ceramic panel were modeled using the SPH smooth particle algorithm, and the contact between the ball and panel was modeled using a particle-particle contact algorithm.The numerical simulation of the bonding layer utilized the solid joint between the ceramic panel/metal backplane interface.The normal failure force and shear failure force between the finite elements of the particle and the metal back plate were taken as 21 N and 12 N,respectively [25]. Fig. 3 shows the finite element mesh model applied.

3.2. Parameter selections

In this example, the MAT_JOHNSON_HOLMQUIST_CERAMICS material model was used to describe the toughened Al2O3ceramic ball and Al2O3ceramic panel.The Q235 steel back target and 2024 aluminum back target material model were based on the Johnson-Cook material model and the Gruneisen equation of state. To toughen the AL2O3, the ceramic material parameters listed in Table 1 and other material parameters from the literature[16-24]were used in the dynamics software Autodyn to reproduce the process of a ceramic ball impacting a composite armor.

4. Analysis of the results

4.1. Analysis of the response characteristics of ceramic balls

When a ceramic ball impacts a composite target, different influences are imparted on the response characteristics of the ceramic bowl by the thicknesses of the metal back plate material and ceramic plate. Table 2 shows the test results of the impacting and crushing velocity thresholds for some ceramic balls,and Fig.4 shows the relationship between the impacting and crushing velocity thresholds of ceramic balls and the thicknesses of the ceramic plates.Fig.5(a)-(c)present the change curve in the stress applied to the micro units of ceramic balls under two operating conditions.

Fig. 2. Schematic diagram of (a) test arrangement and (b) site layout.

Fig. 3. Finite element mesh model.

As shown in Fig.4,the impacting and breaking threshold of the ceramic balls decreased as the thickness of the ceramic panel increased, where as the impacting and crushing thresholds of ceramic balls on the ceramic plates of the same thickness were less under working condition 1 than those under working condition 2.This is because,compared with the Q235 steel back plate,the 2024 aluminum back plate can deform more readily under the smashing and plugging effect of the ceramic balls and ceramic cones [26];hence, reducing the reaction of the composite ceramic ball to the target surface.At the same impact velocity,the force applied on the micro units of a ceramic ball was smaller than under condition 1(see Fig.5(a)),resulting in increased thresholds for the impact and crushing of the ceramic balls. Under the same impact velocity, the force applied on the micro units of ceramic balls under the two working conditions increased with an increase in the thickness of the ceramic panel (see Fig. 5 (b)). The increase in the thickness of the ceramic panel increased the opportunity to form a ceramic cone. Under the same impact velocity, the force applied on the micro units of a ceramic ball continues to increase(see Fig.5(c)),as does the extension rate of the fracture inside the ceramic ball,causing the ball to break more easily.

As a ceramic ball hits the target plate at a higher speed.A zone of extremely high pressure is formed at the impact point within a rather wide pressure range in which no impact phase changes are present.According to the momentum conservation law at the time of impact and the continuity condition at the interface,the pressure at the hitting point can be expressed by the following formula[27]:

Fig. 4. Relationship between ceramic plate thickness and impact fracture threshold.

According to Newton's third law,

Table 1 Ceramic material parameters.

Table 2 Impact test results of impact crushing speed of some ceramic balls.

Fig. 5. (a) Relationship between ceramic ball micro-element stress σ and t of h=3 mm, (b) Relationship between ceramic ball micro-element stress σ and ceramic

Fig.6. Relationship between unit density ρc of composite target plate and thickness h of ceramic panel.

The real velocity on the impact interface is as follows:

WherePPanduPare the impact pressure and particle velocity acting on the projectile;Pcanducare the impact pressure and particle velocity acting on the ceramic panel;andaPand[fx]andacandbcare the Hugoniot material parameters of the ceramic ball and the ceramic panel and RHA, respectively [16-23], wherevPis the velocity of the ceramic ball, and ρPis the density.

Considering the heterogeneity of the structures, the backplate materials of the composite armor,and the concept of the composite target surface density,the unit body density in the composite target thickness direction is introduced, namely,

where ρccis the surface density of the composite target andhc is the unit thickness of the composite target,the value of which is 1.

By using Eqs. (1)-(5), the impact pressure acting on the projectile when the ceramic ball impacts the ceramic composite target can be solved.Fig.6 shows the relationship between the composite target plate body density and the thickness of the ceramic panel h,and Fig.7 shows the relationship between the force applied on the micro units of the ceramic ball and the thickness of the ceramic panel under two working conditions for an impact velocity of 700 m/s.It can be seen from Fig.6 that the unit body density in the composite target thickness direction increases with an increase in the ceramic panel thickness.The theoretical calculation of the force applied on the micro units (see Fig. 7) and the rules of the numerical simulation (see Fig. 5 (b)) show good consistency.

panel thickness(h) atv=700 m/s, (c) Relationship between the stress σ and t of the ceramic ball micro-element under condition 2.

Fig. 7. Relationship between ceramic ball micro-element stress σ and ceramic panel thickness h.

Under the two working conditions,for the ceramic panels with different thicknesses, the impact energy distribution of a ceramic ball changes under the support of the elastoplastic back plate and the impact property changes.Fig.8(a)and 8(b)present the crushed pattern of the recovered ceramic ball under the condition of limiting piercing. Fig. 8 (c) and 8(d) display the numerically simulated damage cloud diagram of a ceramic ball for the same limit penetration velocity. Fig. 9 shows the relationship between the thickness of the ceramic plate and the residual mass of the ceramic ball for a limited penetration velocity. Fig. 10 presents the relationship between the force applied on the micro units of the ceramic bowl and the thickness of the ceramic panel at the limit penetration velocity.

As can be seen from Fig.8(a)-(b),with an increased thickness of the ceramic panel, the quantity of recovered ceramic bowl fragments gradually reduces and the integrity significantly declines.The fracture cross-section passes through the axis of the impact velocity direction on the ceramic ball. Further, the integrity of the ceramic ball is better under working condition 2 than under working condition 1, because the limit penetration velocity under working condition 1 is greater than that under working condition 2 for the same ceramic panel thicknesses. The pressing and tensile stresses applied to the micro units of the ceramic balls is larger(see Fig.10),with obvious fractures and breakages.From Fig.8(c)-(d),it can be seen that, when increasing the limit of the penetration velocity (see Fig. 12), the fracturing location on the ceramic ball changes, extending from an axial to a radial fracture, with an increase in the fracture area. A comparison of the ceramic ball fractures under the two types of working conditions revealed that the extent of crushing on the ceramic ball under working condition 1 is greater than that under working condition 2,for the same ceramic panel thicknesses. Moreover, its remaining mass is less than that under working condition 2 (see Fig. 9). Comparisons of the numerical simulation results and the experimental phenomena reveal that the two are highly consistent.

Fig.9. Relationship between ceramic plate thickness h and residual mass m of ceramic ball is under ballistic limit.

Fig.10. Relationship between ceramic panel thickness h and micro-element stress σ.

Based on the good consistency demonstrated between the numerical simulation and the experimental results, a further numerical simulation of the fracture characteristics of a ceramic ball during a high-speed impact on the composite target plate was carried out. Fig. 11 shows a cloud diagram of the smashing and damage occurring on the interface of ceramic ball under different impact velocities under working conditions 1 and 2.

As can be seen from Fig.11(a)and 11(b),with an increase in the impact velocity,a more obvious breakage of the ceramic ball occurs,which remains on the side of the positive impact surface. The ability to penetrate the metal back plate became increasingly weaker, with changes in the damaged drums occurring along the direction of the valvular dehiscence, and perforation appearing on the metal back plate.The angle between splashing direction of the fragments of the ceramic panel and ceramic ball and the impact velocity direction increased, gradually changing into transverse splashing. Under a high-speed impact, there is an increased force applied on the ceramic bowl and the fracturing phenomenon becomes even more noticeable. In addition, the kinetic energy is distributed into smaller ceramic fragments causing an increased impact area on the target plate, with most of the kinetic energy consumed by the fracturing of the ceramic panel,the ceramic ball,and the deformation of the metal back plate. The overall piercing power declines, weakening the penetrating ability of the metal back plate.There is increased smashing[28,29]and damage to the interface,without the ability to pierce the metal backboard.This is more obvious for working condition 1 than for working condition 2.Owing the higher backing plate strength under working condition 1 than that under working condition 2.This causes the ceramic ball to be more stressed(see Fig.10), which is made more fragile.

4.2. Analysis of composite armor damage characteristics

Multiple φ7 mm toughened Al2O3ceramic balls were studied for an impact of a composite armor. Some limit penetration velocity test data were obtained and are presented in Table 3 below.Fig.12 shows the relationship between the ceramic panel thickness (h)and the limit penetration velocity (v).

Fig.11. Damaged cloud image of ceramic ball interface under high-speed impact.

Fig.12. Relationship between ceramic panel thickness and h-limit penetration velocity v.

It can be seen from Fig.12 that under the two working conditions, with an increase in the thickness of the ceramic panel, the limit penetration velocity gradually increases.The limit penetration velocity and the increasing rate under condition 1 were greater than those under condition 2. Under the same conditions of the back plate, with an increase in thickness of the ceramic formed cone, more kinetic energy of the ceramic ball is consumed. When the speed of the ceramic ball is higher, the greater the impact pressure on the micro units[30]and the more likely that a crushing will occur (see Figs. 10 and 11). The increase in kinetic energy consumption is not conducive to the influence behavior. For the same ceramic panel thicknesses,when the ceramic balls impact the composite target plate at the constant speed, the ceramic cone forming sizes is almost the same [26], with the same area being affected on the metal backplane. Because the impact strength of a Q235 steel back plate is greater than that of a 2024 aluminum back plate, under the same kinetic energy conditions, the ceramic ball can more easily pierce the ceramic/aluminum composite target plate. This is identical with the experiment results and the published rules for a theoretical model calculation [31].

The damaging characteristics of the ceramic composite targetsunder different impact velocities can differ to a certain extent.Understanding damaging characteristics of the ceramic composite targets will provide guidance for their design. Fig. 13 shows the relationship between the thickness of the ceramic panel and the diameter of the penetrated hole on the back panel. Figs.14 and 15 show the test results and numerical simulation results for a piercing of the composite ceramic target plate under the two working conditions.

Table 3 Test results.

Fig.14 shows that,under working condition 1,when the ceramic panel thickness is 1 mm,the damage on the metal back plate shows a punching and plugging pattern(see Fig.14(a),whereh=1 mm).When the ceramic panel thickness is 2 or 3 mm, the penetrated hole on the Q235 steel metal back plate shows valvular dehiscence and perforation (see Fig.14 (a), whereh=2,h=3 mm, and Fig.14(b)), with identical numerical simulation results. With an increased thickness of the ceramic panel, the broken area around the edge of the penetrated hole on the ceramic panel decreases gradually, and the cracks extend circumferentially from the aperture. Under working condition 2, with an increased thickness of the ceramic panel,the convoluted pattern of the penetrated hole on the 2024 aluminum back plate gradually becomes obvious,transition from a punched and plugged penetrated hole pattern to a ductile penetrated hole pattern,with a good matching between the penetrated hole patterns and the numerical simulation results(see Fig.15).

Fig.13. Relationship between the thickness h of the ceramic plate and diameter r of the back plate at the ultimate penetration speed.

From the results presented in Figs.14 and 15,it can also be seen that as the thickness of the ceramic plate increases, a decrease in the diameter of the penetrated hole gradually occurs,which is not obvious. This is because the ceramic plate is locally fractured and broken after being impacted, and the hole diameter formed by joining the ceramic fragments is incomplete. As the ceramic plate increases in thickness,the overall resistance of the ceramic plate to the shear strength increases, and the compaction effect of the ceramic bowl on the formed ceramic cone decreases until the ceramic ball can no longer pierce the ceramic panel. Hole piercing on the metal back plate mainly occurs by the ceramic core and some of the ceramic bowl fragments; hence, the penetrated hole diameter on the ceramic panel decreases.Considering the results of Fig.13, if the thicknesses of the ceramic panels are the same, the penetrated hole diameter on the back plate under working condition 1 was less than that under working condition 2, and the decreasing rate of the penetrated hole diameter was greater than that for working condition 2. This indicates that the support strength of the Q235 steel back plate is greater than that of the 2024 aluminum back plate, which causes the force applied to the micro units of a ceramic ball to greatly increase (see Fig. 5 (a)-(c)). The quantity of the ceramic fragments decreases during the impact process. But because their volumes are relatively small given the penetrated hole diameter on the metal back plate(see Fig.13).

5. Conclusion

Based on ballistic impact test results, a numerical simulation and a theoretical analysis were used to study the impact of ceramic balls on composite target plates, and the following conclusions were obtained:

Fig.14. Comparison of test results and numerical simulation results under working condition 1.

Fig.15. Comparison of test and numerical simulation results under working condition 2.

1. Increasing the thickness of a ceramic panel and the strength of the metal back plate will lead to an increase in the stress applied to the micro units of a ceramic ball, and the fracturing of the ceramic ball will be more serious under the same impact velocity.For a ceramic panel 3 mm thick and a metal back plate of Q235 steel, the threshold values of the impact and crushing speed of the ceramic ball are at a minimum.With an increase in the impact speed, smashing and damage to the interface are more likely to occur, and there is a decreased chance that the ceramic ball will pierce the metal back plate.

2. When the ceramic panel thickness and strength of the metal back plate are increased, the limit penetration velocity of the ceramic ball increases and its integrity decreases. The holepiercing mode on the ceramic ball changes from overall holepiercing into hole piercing of the local fragments, which causes a change in the hole-piercing pattern on the metal back plate from a punched and plugged penetrated hole pattern to a valvular dehiscing/ductile penetrated hole pattern, with the penetrated hole diameter gradually decreasing in size.

3. Comparing and analyzing the numerical simulation results with the experiment and theoretical calculation results, good consistency was shown, indicating that the numerical simulation method adopted in this paper is reliable (to a certain extent).