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|ADI MATERIALS FOR BALLISTIC PROTECTION |

|Sebastian BALOS1, Leposava SIDJANIN1, Dragan RAJNOVIC1, Olivera ERIC2 |

|1 Department of Production Engineering, Faculty of Technical Sciences, University of Novi Sad, Trg Dositeja Obradovica 6, 21000 Novi Sad, |

|Serbia |

|2 Institute „Kirilo Savic“, Vojvode Stepe 51, 11000 Belgrad, Serbia |

|sebab@uns.ac.rs, lepas@uns.ac.rs, draganr@uns.ac.rs, olivera66eric@ |

Abstract: Perforated plates offer a very convinient combination of properties: ballistic protection, combined with a high multi – hit resistance, making them very attractive for mounting at some distance from the basic armour of the armoured vehicle. Although the traditional material for making perforated plates has been the high strength steel, there are some alternatives. One of them is the ADI (Austempered Ductile Iron) material. ADI posessed a unique microstructure – ausferrite, obtained after heat treating of ductile iron. This material is an attractive replacement for steels for a number of reasons: adequate mechanical properties for a number of applications and lower cost of production and machining. Mechanical properties of ADI are similar to high strength steels, except for ductility, which is lower to some extent, but the specific geometry of perforated plates allow the arrest of the propagated crack, overcoming this drawback. On the other hand, ADI is cheaper to produce as well as machine than high strength steels, due to a smaller volume of material removed and higher machinability due to the recence of graphite nodules.

Key words: ballistic protection, add-on armour, ADI materials

1. INTRODUCTION

The end of Cold war influenced the dramatic diminishing of military funds, leading to an ever increasing need for modernization of various armoured vehicles in order to keep them updated [1]. However, their armour protection levels proved inadequate in many occasions, which led to the wide adoption of add – on armour protection. This type of armour is mounted on top of the basic armour, offering a higher protection level. Many types of add – on armour have been used, where the majority is based on metallic or ceramic components. The simplest way of increasing ballistic protection is to bolt a high hardness rolled homogenous armour, however, this type of armour although cheap, offers a less convinient mass efficiency [2, 3]. Therefore, ceramics were used, offering twice the hardness and mass efficiency. On the other hand, it is well known that ceramics in general suffer from a relatively low fracture toughness and ductility, leading to problems regarding multi – hit resistance. This drawback refers to all types of advanced ceramics: although the first shot is stopped, other subsequent shots, if hit near the first may penetrate due to the cracked and weakened ceramic material. This influences a careful optimization of ceramic tile size, which, in turn, if is small enough may offer a longer overall edge length [4 - 6]. If the projectile impacts the edge, mass efficiency drops by the free edge effect [7]. However, free egde effect may be turned in favor of the add – on armour, by applying a different type of armour, non – homogenous armour. This comprises the use of fences and perforated plates. Namely, when the projectile impacts the edge, or near the edge, a non – homogenous stess develops, which can induce yaw, or even fracture the projectile, as projectiles or penetrating cores tend to have relatively high hardness, but low ductility as well. A yawed or fractured penetrating core has a dramatically lowered penetration, in some cases lower than the basic armour protection level [8]. To achieve this, a certain distance between the basic and add-on armour must exist, to allow the yaw to induce, or, in case of penetrating core fragmentation, to allow a sufficient fragment separation to spread their kinetic energy on a larger area of basic plate [9].

In this paper, some results of ballistic testing perforated plates made from steel are shown, with a special attention to some aspects that may influence the application of a novel generation of ADI (Austempered Ductile Iron) materials for the same purpose.

ADI material is a heat treated ductile iron. By heat treating, a special microstructure is obtained, called ausferrite. Ausferrite is a mixture of ausferritic ferrite, and carbon enriched retained austenite [10 - 12]. ADI material posseses a wide range of mechanical properties, combining high strength and fracture toughness similar to high strength steels, while its ductility is notably higher that that of other cast irons, albeit lower than high strength steels. Furthermore, one of ADI material’s advantages over steel is its lower density, which may influence a higher mass efficiency, or similar mass efficiency at lower cost, which is another benefit.

2. PERFORATED PLATES

2.1 Ballistic testing

The basics of the free-edge-effect has been defined in the excellent work of Chocron et.al. [8]. They proved the existance of bending stresses as the penetrating core impacts the add-on armour plate edge, Fig.1. Furthermore, the minimum perforated plate thickness for a given armour piercing ammunition was defined as absolutely neccessary for inducing sufficient bending stresses for core fracture in two parts. This effectively lowers the kinetic energy approximately in half, lowering the cores penetrating performance.

[pic]

Fig.1. Penetrating core bending at edge impact – maximum stress is marked with an arrow[8].

In Balos [9] and Balos et. al. [1, 13], where the experimental setup shown in Fig.2 was used, it was found that when an optimized material and geometry of an add-on perforated plate is used, a larger, heavy machinegun/sniper rifle armour piercing incendiary (API) ammunition can be defeated by the application of the basic armour only half the usual thickness. The results for an optimized perforated plate are shown in Table 1.

[pic]

Fig.2. Ballistic testing target: add-on perforated plate mounted on the basic plate by a frame [9].

The result shown in Table 1, show the influence of perforated plate thickness, which, if sufficiently thin, may not result in a successful induction of yaw or penetrating core bending stresses to cause fracture. Furthermore, it can be seen that penetrating core can fracture not only in two, but in three, four and even five parts. Such phenomenon is documented by the number of impacts on the basic plate, Fig.3, dramatically lowering the kinetic energy of the penetrating core. Although the basic plate thickness has not been varied, it is likely that a thinner than the used 13 mm plate would certainly stop those fragments. This result proves that perforated plate is a highly efective add-on armour.

Table 1 Ballisic testing results of perforated plates made from Hardox 450 steel (0.22C-0.69Si-1.62Mn-0.8Cr), hole diameter 9 mm, plate thickness 6 and 4 mm [9].

|No. |v10 [m/s] |Description of basic plate damage |

|6 mm peforated plate |

|1 |867.7 |Smooth bulge – core fractured in 4 parts |

|2 |859.2 |Smooth bulge |

|3 |881.7 |Smooth bulge – core fractured i 3 parts |

|4 |862.7 |Smooth bulge – core fractured in 5 parts |

|5 |863.0 |Smooth bulge |

|4 mm perforated plate |

|1 |865.6 |Hole normal |

|2 |866.7 |Hole normal |

|3 |879.2 |Hole normal |

|4 |880.5 |Hole normal |

|5 |872.0 |Cracked bulge (two cracks) |

[pic]

Fig.3. Five penetrating core fragments made smooth bulges on the basic plate [9].

2.2 Macroscopic characterization

After impact, it was noted that the damaged area near the impact point is fairly limited. This phenomenon is in contrast to the already menationed free-edge-effect that degrades the protection level if used on an armour added directly (without air gap) on top of the basic plate. Namely, even ductile materials such as steels, after a certain number of impacts catastrophically shatter due to the crack propagation and linking. Although crack nucleation in peforated plates does exist as well, their propagation is arrested by the nearest hole, where the crack sinks. This behavior is shown in Fig.4. The arrest of the crack prevents a further crack propagation and crack linking, leading to a high multi hit resistance of perforated plates, Fig. 4.

[pic]

Fig.4. Perforated plate impact damage showing cracking between holes [9].

3. ADI materials for perforated plates

3.1 Mechanical properties of ADI

Mechanical properties of ADI make this novel material sililar to steel in some aspects. Namely, in accordance with ASTM A897M-03 standard [14], there are five grades of ADI materials, Table 2.

Table 2 Standard ADI material grades according to ASTM A897/M-03 and Hardox 450 (H450) mechanical properties as shown in [9].

|Grade |Min.Rm [MPa]|Min.Rp0.2% |A [%] |K0 [J] |BHN |

| | |[MPa] | | | |

|1 |900 |850 |9 |100 |269-341 |

|2 |1050 |750 |7 |80 |302-375 |

|3 |1200 |850 |4 |45 |341-444 |

|4 |1400 |1100 |2 |25 |388-477 |

|5 |1600 |1300 |1 |15 |402-512 |

|H450 |1450 |1255 |11 |60* |445 |

*V-notch specimen (KV)

From Table 2, it can be seen that ultimate tensile stress (Rm), yield strength (Rp0.2%) and hardness of Hardox 450 steel closely corresponds to Grade 4 ADI. On the other hand, its elongation (A) and charpy impact strength are higher. However, these mechanical properties may be of secondary importance for this particular application, due to specific geometrical characteristics of perforated plates, where, as shown in Fig.4, crack propagation is stopped by the nearest hole. Crack nucleation may be initiated only by another impact, while damaged area remains relatively limited to five or six interconnected holes [9].

3.2 Density of ADI materials

In addition to convinient key mechanical properties, which are similar to some kind of steels, ADI materials posess lower density. Namely, as mass percent of carbon amounts to 3,5 – 4 % [15]. The typical microstructure of ADI showing spgeroidal graphite is shown in Fig.5. As a result, an ADI component will be 10 % lighter than steel, if the geometry will be retained [16]. This means that a perforated plate may be lighter if made from ADI, having a higher mass effectiveness when made from steel.

[pic]

Fig.5. A typical ADI microstructure containing spheroidal graphite and ausferrite metal matrix [17].

3.3 Technological advantages

As the ADI material is in effect a heat treated ductile iron, it is fundamentally obtained by casting. It is well known that per given mass of product, cast parts are cheaper [11]. Furthermore, hole shape and provile may not be cyllindrical, but rather specially suited for various types of ammunition. Namely, hole dimensions must be finely tuned to various types of penetrating core dimensions to obtain sufficient ballistic protection of the armour system [9]. As the hole becomes smaller, the free – edge effect is diminished or totally lost since such plate behaves like a homogenous plate, while if the hole size is too large, the projectile may pass through it, without contact, or with an insufficient contact to induce bending stresses and cause fracture. The application of cores of various shape and dimensions (step or conical shape), may make such a perforated plate effective against different ammunition types and calibers. Although hole sides will nevertheless have to be finished by machining, the volume of the material removed is much smaller than in the case od drilling the whole hole. In addition to this, machinability of ADI material is higher than that of heat treated steels, due to a lubricating effect of graphite, resulting in further savings in time and cost [18]. This way, the machining of ADI perforated plate may be both shorter and cheaper compared to steel, resulting in a cheaper product. For crankshafts, if forged steel is replaced by ADI, an overall cost may be lowered by 30 % [19]. However, for the purpose of perforated plates, an even higher savings may be expected.

Testing of ADI materials for this application is already undergone in UK [20, 21] and Serbia on the behalf of Ministry of Defence [22].

4. Final Remarks

For common engineering purposes, it can be said that more ductile grades of ADI can compete with medium-strength steels, while high strength grades of ADI compete less effectively with the high-strength steels. These high strength grades may replace high strength steels for purposes that demand primarily high wear resistance and moderate ductility. Furthermore, ballistic protection in form of perforated plates may be another attractive application of high strength ADI for a number of reasons:

• Lower density offers higher mass effectivness, that can be utilized in two basic ways: lowering the weight of add-on armour and increasing ballistic resistance.

• Optimizing the geometry to cover more different calibers with one perforated plate.

• Lowering cost of production due to cheaper production process and machining.

Therefore, ADI material may be a feasible alternative to high strength steels for production of an advanced perforated plates for ballistic protection.

ACKNOWLEDGEMENT

This study is supported by the Ministry of Education and Science of the Republic of Serbia, through technology development project TR34015.

REFERENCES

[1] Balos, S., Grabulov, v. Sidjanin, l. (2010) Future armoured troop carrying vehicles, Defence Science Journal, Vol. 60, pp. 483-490

[2] Ogorkiewitz, R., (2002) Armor for Light Combat VehiclesAdvances in armour materials, International Defence Review July Vol. 6, pp. 41-45

[3] Ogorkiewitz, R., (1991). Advances in Armour Materials, International Defence Review, Vol. 4, pp. 349-352

[4] HAZELL, P., Roberson, C.,Moutinho, M. (2008) The design of mosaic armour: The influence of tile size on ballistic performance, Materials & Design 29, 2008, pp. 1497-1503

[5] BLESS, S., JURICK, D. (1998) Design for multi-hit capability, International Journal of Impact Engineering 21, 1998, pp. 905–908

[6] DE ROSSET, W. (2005) Patterned armor performance evaluation, International Journal of Impact Engineering 31, pp. 1223–1234

[7] GOLDSMITH, W. (1999), Review: Non-ideal Projectile Impact on Targets, International Journal of Impact Engineering Vol. 22, pp. 95-395

[8] CHOCRON, C., ANDERSON JR, C., GROSCH, D., POPELAR, C. (2001), Impact of the 7.62-mm APM2 projectile against the edge of a metallic target. International Journal of Impact Engineering Vol. 25, pp. 423-437

[9] Balos, S. (2010) Nehomogeni dodatni razmaknuti metalni oklop za oklopna vozila, PhD Thesis, Faculty of technical sciences, Novi Sad, Serbia

[10] SIDJANIN, L., SMALLMAN, R.E. (1992) Metallography of bainitic transformation in austempered ductile iron, Materials Science and Technology, Vol.8, pp 1095-1103

[11] HARDING, R.A. (2007) The production, properties and automotive applications of austempered ductile iron, Kovove Materialy, Vol.45, pp 1-16

[12] ERIC, O., RAJNOVIC, D., ZEC, S., SIDJANIN, L., JOVANOVIC, M.T. (2006) Microstructure and fracture of alloyed austempered ductile iron, Materials Characterization, Vol.57, pp 211-217

[13] Balos, S., Grabulov, v. Sidjanin, l. (2009) 50CrV4 steel as a material for perforated plates in ballistic аpplication, Proceedings of the 10th International Scientific Conference on Flexible Technologies - MMA 2009, Novi Sad, Serbia, October 9-10, 2009, pp. 274-277

[14] ASTM A897M-03 standard, West Conshohocken, USA

[15] Smallman, R., Ngan, A. (2007) Physical Metallurgy and Advanced Materials, Elsvier Ltd., Oxford, UK, 458

[16] RIMMER, A., (2004), ADI solutions aid vehicle design, FTJ, pp. 54-56

[17] Rajnović, D., Erić, O., Sidjanin, L. (2008) Transition temperature and fracture mode of as-cast and austempered ductile iron, Journal of Microscopy Vol. 232, pp. 605-610

[18] Kovač, P., Sidjanin, L., Fišl, J., Rajnovic, D., Stojaković, D. (2003) Machinability of ADI Materials on Microstructure - Level, 2nd International Congress of Precision Machining, Prague, Czech Republic, pp. 175-179

[19] Sidjanin, L. (1996), Cast Iron – A Real Advanced Material, Towards the Millenium – A Materials Perspecive, The University Press, Cambridge, UK, 335-350

[20]

[21]

[22] BALOS, S, (2008) Add-on Armorfor Light Combat Vehicles, Y Report Vol. 25, pp. 47 - 53

Correspondence

Sebastian Balos, Docent, Department of Production Engineering, Faculty of Technical Sciences, University of Novi Sad, Trg Dositeja Obradovica 6, 21000 Novi Sad, Serbia

phone: +381 21 485-2339, fax: +381 21 454-495

e-mail: sebab@uns.ac.rs

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