Journal of the Chinese Institute of Engineers, Vol. 25, No. 1, pp. 99-107 (2002) 99

BALLISTIC PERFORMANCE AND MICROSTRUCTURE OFMODIFIED ROLLED HOMOGENEOUS ARMOR STEEL

Chia-Jung Hu and Pee-Yew Lee*Institute of Materials EngineeringNational Taiwan Ocean University

Keelung, Taiwan 202, R.O.C.

Jium-Shyong ChenMaterials & Electro-Optics Research DivisionChung-Shan Institute of Science &Technology

Tao-Yuan, Taiwan 325, R.O.C.

Key Words: armor, ballistic, adiabatic shear band, penetration mode.

ABSTRACT

The purpose of this research was to evaluate the performance of amodified rolled homogeneous armor (MRHA) steel during ballisticimpact tests. AerMet 100, AISI 1045 and 4130 steels were also testedfor comparison with the MRHA results. The white-etched portionof adiabatic shear bands was found on the front surface around pen-etration cavities in all the impacted armor plates. Scanning electronmicroscopic observation indicates that the adiabatic shear bands foundin 4130, MRHA and AerMet 100 belong to transformed bands whilethose of 1045 steel are deformed bands. Ballistic test results show thatthe ballistic limit velocities of the 2.5-mm thick MRHA steels are higherthan those of AISI 1045 and 4130, and possibly compatible with AerMet100 steel. For 2.5-mm thick armor plates, the penetration modes werefound to be dominated by ductile hole growth and plugging modes.

*Correspondence addressee

I. INTRODUCTION

Armor plate of hardened steel has been used formany years to provide protection of objects againstimpact damage (Doig, 1998a). Applications such astanks, military sites, vaults, and safes, etc. have usedsteel armor plates to provide such protection. Twobasic types of armor plate are conventionally utilizedat the present time. One type is high-hardness armorthat is extremely hard and thus capable of preventingpenetration by projectiles (Raghaven et al., 1969).The other type is rolled homogeneous armor (RHA)that is somewhat softer than high-hardness armor, andis more ductile to prevent brittle fracture (U. S.

Department of Defense, 1984). Due to cost-savingfabrication requirements and more complex vehicleconfigurations, RHA has become the principal mate-rial employed for heavy combat and recovery vehiclessince World War II. In the years 1950 to 1975, manymetallurgical advances have improved the ballisticperformance of RHA (Manganello and Abbott 1972;Wingrove and Wulf, 1973; Woodward, 1976).However, none have been considered successful, andthus RHA material has not been upgraded ballisticallysince 1950.

In order to increase the ballistic performance ofRHA, projects on improved rolled homogeneousarmor (IRHA) were undertaken by the U.S. Army

100 Journal of the Chinese Institute of Engineers, Vol. 25, No. 1 (2002)

Research Laboratory in 1970. The goal was to im-prove ballistic performance and maintain weldabilityof RHA, by increasing armor hardness but notbrittleness. The important facts learned from 15 yearsof investigations include: (1) processed steel armorplates with Rockwell C hardness (Rc) higher than 52can not retain structural integrity when impacted byfull-scale-caliber kinetic energy (KE) rounds; (2) thehardened steel must demonstrate a V-notch Charpy(CHV) impact value greater than 20 ft-lb measuredat -40F (Avyazian and Papetti, 1973; Campbell andAvyazian, 1985; Papetti, 1978). Based on the afore-mentioned information and a consensus of the U.S.Armys armor experts, a suitable composition ofIRHA was 0.25%C-2.05%Ni-1.00%Cr-0.46%Mo-0.34%Mn-0.03%Si, P

C.J. Hu et al.: Ballistic Performance and Microstructure of Modified Rolled Homogeneous Armor Steel 101

penetration velocities. According to U.S.A. militaryspecification: MIL-STD-662E, more than ten shotswere fired for every testing plate vs. 44 grain FSPs,and the V50 is the average of five complete penetra-tion velocities and five partial penetration velocities.Tests were performed at 0 obliquity, i.e., targets werenormal to the trajectory of the projectile.

III. RESULTS AND DISCUSSION

1. Metallograpic Observation

Figure 2 shows representative SEM micrographsfor various heat-treated steels. The tempered mar-tensite structure was observed for all the steel plates.

Fig. 2 Scanning electron micrographs showing microstructure of quenched and tempered steel

102 Journal of the Chinese Institute of Engineers, Vol. 25, No. 1 (2002)

It is an accepted fact that tempered martensite is themost desirable structure for steel armor plates (Doig,1998b). After impact, several plates were cross-sec-tioned perpendicular to the rolling direction. The cor-responding optical micrographs of front surfacesaround penetration cavities in 1045, 4130, MRHA,and AerMet100 steels are shown in Fig. 3. Thewhite-etched portion of adiabatic shear bands(ASB) were found in all the materials. The micro-structure within the band is very hard to elucidateusing optical microscopy. Yet it is possible to ob-serve the large amount of deformation associated with

the ASB using scanning electron microscopy of theetched surface (Fig. 4). For 4130 steel with hardness45 Rc, the width of ASB varied between 1 and1.4 m and the microstructure within the band re-sembles that of the surrounding matrix. Although adistinct interface between the ASB and the matrix isnot observed in this micrograph, martensitic lathsalignments with the band indicates large plastic flow.The width of ASB in the AerMet 100 steel with Rc45 varied from submicron to 2 m. The martensiticlaths adjacent to the band again show some alignmentwith it. For the MRHA steel with hardness Rc 48, a

Fig. 3 Optical micrographs of shear bands formed in 2.5-mm thick plates of AISI 4130, AerMet 100, MRHA (Rc 40), MRHA (Rc 48), andAISI 1045

C.J. Hu et al.: Ballistic Performance and Microstructure of Modified Rolled Homogeneous Armor Steel 103

distinctive ASB with width about 3 m was formedand the martensitic laths characteristic of thequenched and tempered material seem to be absentfrom the interior of the band. The width of ASB forthe MRHA steel with hardness Rc 40 was generallynarrower than that of the MRHA steel with hardnessRc 48. The band also contains voids and smaller

microcracks (Fig. 4). These spherical voids arethought to be produced from tensile stresses actingwithin the band.

Adiabatic shear bands form as a result of athermo-mechanical instability due to the presence ofa local inhomogeneity, including local deformationand heating (Bedford, 1974). This plastic shear

Fig. 4 Scanning electron micrographs of shear bands formed in 2.5-mm thick plates of AISI 4130, AerMet 100, MRHA (Rc 40) andMRHA (Rc 48)

104 Journal of the Chinese Institute of Engineers, Vol. 25, No. 1 (2002)

instability was first studied in high-strain-rate testsby Zener and Hollomon (1944) and termed adiabaticshear. If the thermal conductivity of the material isnot sufficient to conduct the generated heat away, de-formation becomes unstable and is localized on sur-faces of very small thickness (~10 to 50 m). Onmicroscopic observation, these surfaces appear asnarrow bands in which cracks can propagate, indi-cating catastrophic failure of the material. Backmanand Finnegan (1973) first proposed that shear bandsin different metals could be broadly classified as ei-ther transformed or deformed on the basis of theirappearance in metallographic section. Deformedbands are characterized by a very high shear strain(up to 100) in a very thin zone of deformation. In-side the band the grains are highly distorted, but thereis no evident change in the structure of the material.In transformed bands, a crystallographic phase changeoccurs. In steels, they are often called white bandsbecause of their appearance after etching, and arequite different from the matrix.

Based on the above descriptions, examinationof Figs. 3 and 4 indicates that the ASBs found in steels4130, MRHA and AerMet 100 belong to the trans-formed bands while 1045 steel shows deformed bands.

Deformed bands also can be found near the tips ofthe shear bands in AerMet 100 and MRHA steel (Rc48). Generally, the tip of the shear band consisted ofa highly deformed region, narrower than the fullydeveloped band. The observed bands generally wid-ened from the tips toward the tails, which is commonfor all observed bands. It is also noted that bifur-cated shear bands were formed in the MRHA steel(Rc 40). Bifurcation is observed commonly incracking, and the same energies should govern it un-der shear and tension. These observations lend cre-dence to the theory of Curran (1979) that shear bandsnucleate and grow and to the various attempts at mod-eling shear band as mode II cracks (Lee and Swedlow,1984) propagating through materials

2. Ballistic Tests of Thin Armor Plates

Ballistic test results for 2-mm and 2.5-mm thicksteel plates vs. the 44 grain FSP are shown in Fig. 5.The V50 ballistic limit velocity of MRHA plate wassuperior to those of 1045 and 4130 plates with a thick-ness of 2.5mm and that of AerMet 100 with 2 mmthickness. The ballistic limit velocity increased withincreasing hardness for 4130 and MRHA plates, butdecreased for AerMet 100 plate. Hickey and Tho-mas (1985) studied ballistic performance in aquenched-and-tempered ESR/VAR 4340 steel. Theyfound that the V50 ballistic limit velocities of ESR/VAR 4340 steels increased with increasing hardness.The MRHA plate employed in the present study wasmelted by vacuum induction melting (VIM) andvacuum arc remelting (VAR) processes. The meritof VIM/VAR treatment is to reduce undesirable ele-ments such as phosphorus, sulfur, and gases, thus animprovement in toughness can be achieved (Kienel,1988). When the thickness of AerMet 100 plate isincreased to 2.5 mm, the corresponding V50 ballisticlimit velocity is expected to rise to the value of MRHAplate. However, the AerMet 100 is too expensive tobe used for the armor of heavy combat and recoveryvehicles because of its super-high alloy content anda patent right (Hemphill and Wert, 1992). It is alsonoted that the V50 ballistic limit velocity of AerMet100 plate gradually decreases with respect to increas-ing hardness. Examination of Fig. 5 indicates thatmore brittle cracking around penetration cavities isobserved as the hardness is increased from Rc 45 toRc 55.

For 1045 plate, a maximum V50 was obtainedat a hardness Rc 45. It is generally known that theharder the target, the larger the amount of projectileerosion and deformation, and the better the ballisticperformance (Crouch, 1988). However, many stud-ies have also demonstrated that the most energy ab-sorbing mechanism in resisting penetration by an

Fig. 5 Ballistic limit velocities and photographs around penetra-tion cavities for 2-2.5 mm armor plates after impact by 44grain FSP. (a) front surface, (b) rear surface

C.J. Hu et al.: Ballistic Performance and Microstructure of Modified Rolled Homogeneous Armor Steel 105

armor-piercing round is the plastic deformationoccurring in the target material, adjacent to the pointof impact (Manganello and abbott, 1972; Yellop andWoodward, 1980). Therefore it is not surprising thatthe V50 ballistic limit velocity of 1045 armor platewith the highest hardness (Rc 55) did not providebetter protection than plates with lower hardness.This behavior is further shown in Fig. 5 by the oc-currence of catastrophic brittle cracking due to thebrittleness of plate around the penetration cavities.Similar results have also been observed for wroughtaluminum alloys against small-caliber, armor-pierc-ing projectiles (Woodward, 1988).

For RHA armor plates with thickness less than5.6 mm, there are no ballistic testing results reportedin the RHA specification (U.S. Department ofDefense, 1984)[3]. However, in order to evaluate theballistic performance of 8-mm and 12-mm thickMRHA plate prepared in this study, two types of pen-etration tests with 12.7mm APM2 projectiles and20mm APDS projectiles were performed. The bal-listic performance of MRHA plate prepared in thisstudy is much better than that of RHA armor plate(Hu and Lee, 2000).

3. The Penetration Modes

The six penetration modes for ballistic tests

against kinetic energy penetrators include (1)petalling; (2) fragmentation; (3) radial fracture; (4)brittle fracture; (5) plugging; and (6) ductile holegrowth (Backman and Goldsmith, 1978). Fig. 6 showsthe corresponding schematics. Real penetration usu-ally consists of several modes mentioned above. Ifarmor plate is thin and ductile, petalling failure usu-ally occurs. Fragmentation failure appears in thickerarmor plate with enough strength, but brittle. Radialand brittle fractures take place in more brittle armorplates. For medium armor thickness and hardness,plugging and ductile hole growth often occur duringpenetrating. Ductile hole growth takes place, and thenplugging continues to happen if the residual thick-ness of armor plate is just less than the projectilediameter. Adiabatic shear bands, which are the lo-calization of plastic deformation of a material byplugging, occur under high-strain-rate loadingconditions. The plugging occurs when a materialstrain-softens due to a local temperature rise quickerthan it work-hardens (Farrand, 1991).

The penetration mode for 2-mm and 2.5-mmthick steel plates vs. the 44 grains FSP was evaluatedby examining photographs of front and rear surfacesaround penetration cavities as well as the cross-sec-tional views of penetration cavities as shown in Figs.5 and 7. The failure mode of AISI 1045 plates (Rc35-45) was mostly ductile hole growth, Fig. 5. Thedelamination on the rear surface of the 1045 steel wasmore evident as its hardness was increased. The AISI4130 plate with Rc 45 was nearly broken by ductilehole and plugging failure as shown in Fig. 5 and Fig.

Fig. 6 Typical penetration modes of impacted armor plates

Fig. 7 Cross sectional view of each of the impacted targets vs.44 grain FSP

106 Journal of the Chinese Institute of Engineers, Vol. 25, No. 1 (2002)

7(a) & (c). Ductile failure modes, such as tearingand bulging, and plugging exist on the damaged sur-faces of AerMet 100 and MRHA. More brittle crack-ing and raised lips occurred during penetrating whenthe hardness of AerMet 100 was increased. Completeplugging failure was more visible on the harderMRHA plate (Rc 48) than on the Rc 40 plate as shownin Fig. 7(g)-(j). According to the above observations,ductile hole growth and plugging mode occur moreoften than other failure modes in the present study.

IV. CONCLUSIONS

1. The adiabatic shear bands around penetration cavi-ties of 4130, MRHA and AerMet 100 are trans-formed bands while those of 1045 steel are de-formed bands.

2. Ballistic test results show that the V50 ballisticlimit velocities of 2.5-mm thick MRHA steelsagainst the 44 grain fragment-simulating projec-tile are larger than those of AISI 1045 and 4130,and possibly comparable to the AerMet 100 steel.

3. Ductile hole growth and plugging mode occursmore often than other failure modes in the 2 or 2.5mm thick armor plates.

ACKNOWLEDGEMENTS

This work was supported by the National Sci-ence Council of the Republic of China under the GrantNo. NSC88-2623-D-019-002.

NOMENCLATURE

IRHA improved rolled homogeneous armorFSP fragment-simulating projectileAP armor piecing projectileRHA rolled homogeneous armorMRHA modified rolled homogeneous armorHRc rockwell C hardnessKE kinetic energyCHV V-notch charpyDI hardenability indexVIM vacuum induction meltingVAR vacuum arc remeltingSEM scanning electron microscopyAPDS armor piercing discarding sabot penetratorASB adiabatic shear bandsESR electroslag remelting

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Metallurgical Factors Affecting the Ballistic Be-havior of Steel Targets, Journal of Materials,Vol. 7, pp. 231-239.

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20. U. S. Army Materials Technology Laboratory,1987, Military Standard, V50 Ballistic Test forArmor, MIL-STD-662E, Department of theNavy, Defense Printing Service, Philadelphia,PA.

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21. Wingrove, A. L., and Wulf, G. L., 1973, SomeAspects of Target and Projectile Properties onPenetration Behavior, Journal of the AustralianInstitute of Metals, Vol.18, pp. 167-172.

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24. Yellop, J. M., and Woodrward, R. L., 1980, In-vestigations into the Prevention of AdiabaticShear Failure in High Strength Armor Ma-terials, Research in Mechanics, Vol. 1, pp. 41-57.

25. Zener, C., and Hollomon, J. H., 1944, Effect ofStrain Rate upon Plastic Flow of Steel, Journalof Applied Physics, Vol. 15, pp. 22-32.

Manuscript Received: Mar. 23, 2001Revision Received: May 11, 2001

and Accepted: July 04, 2001