INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 12, No. 1, pp. 85-90 FEBRUARY 2011 / 85

DOI: 10.1007/s12541-011-0010-2

NOMENCLATURE

d = diameter of tube specimen

t = thickness of tube and plate specimen

= angle of attack to live rounds test

D = penetration damage diameter

TD = total damage

ni = number of cycles at the i th stress level

Ni = number of cycles to failure corresponding at the i th stress level

L = safety life

eq = equivalent stress

N = number of cycles

S = stress

1. Introduction

In military aircrafts, the ballistic damage tolerance design

against the bullet hit is very important as it is related with its

survivability. However, in most countries having advanced aircraft

technology, such information keeps being undisclosed. Therefore,

extensive studies are required on this area when developing a new

military aircraft. Hence, in terms of survivability for aircraft

structure, it is necessary to examine the enemy threats and assess

the vulnerable points to such threats based on the past combat

experiences.1-3 Following such assessments, analysis need to be

performed on the vulnerability from ballistic damage. Only then,

the survivability of aircraft structure will be secured by

incorporating the results from such analysis onto the design.

A series of experimental studies were carried out concerning the

normal and oblique impact of armor piercing bullets on single and

layered plates made of mild steel, RHA steel and Al alloy.4-6 In the

studies, target damage and measurement of residual velocities and

the angles in both normal and oblique impact were observed.

However, the demands and standards for ballistic damage are dealt

in a very limited manner for structure design in military aircraft

specifications as it is not easy to quantify and apply them as an

analytical tool.7-9

Therefore, in the current study, as a method to meet the

requirement for a specific level of survivability of a military aircraft

structure, fundamental data on ballistic damage tolerance design

will be obtained through testing the live rounds fire, suggesting a

procedure and an example of applying this data to the ballistic

damage tolerance design.

2. Experimental Method

2.1 Specimens and Fixtures

Tube and plate configurations of test specimens are shown in

Fig. 1. To establish statistically, the specimens penetration damage

diameter, live rounds tests were repeated numerous times. Table 1

A Study on the Ballistic Damage Tolerance Design of Aircraft Structure from Armor Piercing Bullet Hits

Jang-Wook Hur1,#

1 Defense Acquisition Program Administration, 2-3 Yongsango-gil, Yongsan-dong, Yongsan-gu, Seoul, South Korea, 140-841# Corresponding Author / E-mail: hhjw88@hanmail.net, TEL: +82-2-6497-4362, FAX: +82-2-6497-4362

KEYWORDS: Armor Piercing Bullet, Angle of Attack, Damage Shape, Ballistic Damage Tolerance Design, Penetration Damage Diameter, Minors Rule

A damage reference database from armor piercing bullet hits was established for tube and plate specimens with

different thicknesses. The penetration damage diameters of the tube specimens showed larger at the center than the

periphery in the front, but they resulted larger at the periphery than the center in the rear. As the angle of attack of

the plate specimens increased, the penetration damage diameters increased as well, with the penetration damage

diameters becoming larger in the rear than the front. Using the damage reference database, the fatigue analysis was

performed to determine whether the safety requirements for the military aircraft could be met.

Manuscript received: April 19, 2010 / Accepted: October 24, 2010

KSPE and Springer 2011

86 / FEBRUARY 2011 INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 12, No. 1

and 2 show the diameter and thickness of the specimens, angle

ofattack and number of rounds employed in the tests. Specifically to

the tube specimens, in order to examine the damage shapes incurred

by penetrations through the path (center or periphery of tube) of

projectile when a bullet hit the target, tests were performed only at

0 angle of attack. Fig. 2 shows the specimen locking fixtures

employed in the tests. They were designed not only to firmly

support the specimens but also to prevent them from breaking away

from the bullet impact force and to maintain the preset angle of

attack (0, 30, 60).

2.2 Live Rounds Test

Live rounds tests were performed at a domestic professional

testing facility and armor piercing bullets were used. Experimental

arrangement to the live rounds tests is shown in Fig. 3. Machine

gun(Cal. 50) was used in the tests and impact velocity to the target

was about 520 m/s. In order to measure the bullet velocity at impact,

photo screen sensors were placed at 2 m intervals in front of the

target, and a universal counter (5335A) was used. In addition, high

speed camera (15,000 frames/s) was used to observe the behavior at

impact. Target configurations after live rounds tests are shown in

Fig. 4. The shape of penetration damage hole with angle of attack

presented an ellipse. Therefore, the penetration damage diameter

was defined by the length (D1) of major axis of Fig. 4(b).

Furthermore, the yaw angles defined by the differences in bullets

lengthwise path of flight angles were measured. To secure the tests

reliability, the actual tests were performed only after the measured

reentering angles within the tolerance range of 5.6

(a) Section view (b) Front view

Fig. 4 Target configurations after live rounds tests

(a) Tube (b) Plate

Fig. 1 Specimen configurations

Table 1 Tube specimens to the live rounds tests

Diameter,

d (mm)

Thickness,

t (mm)

Angle of attack,

()

Number of rounds

Al alloy Stainless steel

39.0 2.7

0 24 -

3.5 20 -

50.0 4.0 0 - 6

Table 2 Plate specimens to the live rounds tests

Thickness,

t (mm)

Angle of attack,

()

Number of rounds

Al alloy Stainless steel

3.2

0 5 -

30 5 -

60 5 -

12.7

0 5 2

30 5 2

60 5 2

25.4

0 5 2

30 5 2

60 5 2

Total 45 12

(a) 0 (b) 30 (c) 60

Fig. 2 Specimen locking fixture configurations

Fig. 3 Experimental arrangement to the live rounds tests

INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 12, No. 1 FEBRUARY 2011 / 87

3. Test Results and Discussions

Photographs taken at the moments of bullet impact on the Al

alloy with a high speed camera are shown in Fig. 5. Fig. 5(a) shows

the bullet penetrating the tube through its center in a straight line.

Fig. 5(b) shows the bullet penetrating the tube through its periphery,

which looks as if it has been divided in two due to the difference in

light intensity of camera direction. Fig. 6 displays the front and rear

photographs of the tube specimens after live rounds tests. In case of

the Al alloy specimen, due to its low strength property, the rear side

appeared as if it had been torn while the stainless steel specimen

having relatively high strength property showed a much cleaner cut.

Table 3 and Fig. 7 show the average penetration damage

diameter in the front and rear side of the tube specimens. In both

tube thicknesses of 2.7 mm [Fig. 7(a)] and 3.5 mm [Fig. 7(b)], the

penetration damage diameters were greater at the rear than the front.

Furthermore, the penetration damage diameters showed larger at the

center than the periphery in the front but they resulted larger at the

periphery than the center in the rear. Also, the penetration damage

diameters at the rear of tube specimens showed a tendency to

increase in the thickness of 3.5 mm compared to the thickness of

2.7 mm. These phenomena are seen in case of spaced armor where

ballistic testing is carried out on two plates with some distance kept

between them.10 The first plate attempts to resist and break the

bullet. In doing so the path of the projectile is changing. So instead

of hitting the second plate at 0 angle of attack, the projectile hits at

some angle. This angle of attack increases the damage area at the

back plate. Therefore, the cylindrical hollow tube has an equivalent

effect as seen in the spaced armor. And irregular edge effect caused

large standard deviation at the periphery in the front. However,

spaced armor effect was not presented at the stainless steel having

relatively high strength property. From Table 3, maximum

penetration damage diameters of the Al alloy tubes of 2.7 mm and

3.5 mm thick are respectively 21.9 mm and 25.7 mm while that of

the stainless steel tube of 4.0 mm thick is 14.8 mm.

Fig. 8 shows the front and rear photographs of the plate

specimens after live rounds tests. At 0 angle of attack, the

penetration limit was approximately 25.4 mm (1.0 inch) to the Al

alloy plate while it was approximately 12.7 mm (0.5 inch) to the

stainless steel plate because the bullets got stuck in the mentioned

thickness of both plates. Therefore, the bullets were either

penetrated or stuck up to the thickness of 25.4 mm of the Al alloy

plate as well as the thickness of 12.7 mm of the stainless steel plate.

But at more than 30 angle of attack, the bullets were just bounced

off the same thickness plate specimen surface.

(a) Tube center

(b) Tube periphery

Fig. 5 Photographs depicting Al alloy tube specimens at bullet

impact

Type Front Rear

Center Periphery Center Periphery

Al alloy

d : 39mm

t : 3.5mm

Stainless steel

d : 50mm

t : 4.0mm

Fig. 6 Photographs showing the front and rear sides of tube

specimens after live rounds tests (0 angle of attack)

Table 3 The results of penetration damage diameter to the tube

specimens

Material Thickness,

t (mm)

Shape of

penetration

Front (mm) Rear (mm) Number

of

testingAve. Standard

deviation Ave.

Standard

deviation

Al

alloy

2.7 Center

Periphery

17.7

15.5

0.9

8.5

20.9

21.9

4.2

2.3

18

6

3.5 Center

Periphery

18.8

16.8

1.3

5.4

22.6

25.7

4.1

3.0

16

4

Stainless

steel 4.0

Center

Periphery

14.8

13.9

1.8

3.8

14.3

15.1

0.7

0.7

3

3

Fig. 7 Graph showing the change in penetration damage diameter

at different Al alloy tube thicknesses

Type Front Rear

t : 12.7mm t : 25.4mm t : 12.7mm T : 25.4mm

Al alloy

Stainless

steel

Fig. 8 Photographs showing the front and rear side of plate

specimens (0 angle of attack)

88 / FEBRUARY 2011 INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 12, No. 1

Table 4 and Fig. 9 display the relationship between the angle of

attack and penetration damage diameters to the plate specimens. As

the angle of attack increased, the penetration damage diameters

increased as well, with the penetration damage diameters larger in

the rear than the front. This phenomenon is deemed to occur due to

the angle effect, and as the angle of attack increased, the bullets

penetration irregularity did as well, increasing the standard

deviations in the penetration damage diameter. As shown in Fig. 9,

the penetration damage diameters were slightly larger in the

specimens of 3.2 mm thick than those of 12.7 mm thick. The

influence of the thickness became more pronounced as the angle of

attack increased. At 60 angle of attack, the bullet failed to

completely penetrate the 12.7 mm thick specimen and thus, data

was not obtained. And, maximum penetration damage diameters of

the Al alloy plates of 3.2 mm, 12.7 mm and 25.4 mm thick are

respectively 37.8 mm, 47.0 mm and 65.4 mm and that of stainless

steel plate of 12.7 mm thick is 15.6 mm.

4. Application of Ballistic Damage Tolerance Design

4.1 Procedure for ballistic damage tolerance design

Fig. 10 shows the general flow chart for the ballistic damage

tolerance design. For the aircraft safety critical structures, the

configurations and materials vulnerable to bullet hit need to be

selected. Thereafter, through the live rounds tests, damage reference

database is constructed for use in determining the damage shape

and size, and completing the ballistic damage tolerance sizing by

iteration of analysis up to the certain level satisfying the specific

requirements. In this study, damage reference database for

application of design was obtained through 3. Test Results and

Discussions.

Such ballistic damage tolerance design application will be

performed to the aircraft structures whose safety is threatened under

the fire of heavy machine gun from the enemy. One example would

be the engine support structure shown in Fig. 11.

4.2 Application Model and Stress Analysis

The engine support structure, as shown in Fig. 11, is supported

at 3 points on the left, right and end points. However, because the

end point is not exposed to the outside, the engine only requires

ballistic damage tolerance designs on the left and right support

Fig. 10 Flow chart for the ballistic damage tolerance design

Fig. 11 Configuration of engine support structure

Table 4 The results of penetration damage diameter to the plate

specimens

Material Thickness,

t (mm)

Angle of

attack,

()

Front (mm) Rear (mm) Number

of

testingAve.

Standard

deviation Ave.

Standard

deviation

Al

alloy

3.2

0

30

60

12.7

18.3

37.8

0.5

1.0

1.7

13.8

19.0

30.9

1.0

1.0

1.4

5

5

5

12.7

0

30

60

11.1

13.0

47.0

0.3

2.2

10.1

13.4

14.1

-

1.5

4.8

-

5

5

5

25.4

0

30

60

11.0

18.4

65.4

0.4

2.4

6.4

-

-

-

-

-

-

2

2

2

Stainless

steel 12.7

0

30

60

10.4

-

-

0.4

-

-

15.6

-

-

0.7

-

-

2

2

2

Fig. 9 Relation between the angle of attack and penetration damage

diameter to the Al alloy plate specimens

INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 12, No. 1 FEBRUARY 2011 / 89

Table 5 Damage to the engine support structure

Material Penetration damage

diameter, D(mm) Angle of attack, ()

Stainless steel 15.6 0

Fig. 12 A model of the engine support structure with ballistic

damage

Fig. 13 Stress analysis results for engine support structure with

ballistic damage

Fig. 14 Relation between number of cycles and equivalent stress of

engine support structure with ballistic damage

structures. In the ballistic damage analysis, the considered damage

size was, as shown in Table 5, the penetration damage diameter

obtained from the live rounds test at 0 angle of attack to the 12.7

mm thickness of stainless steel plate which is similar to the engine

support structure in view of thickness and material property.

The 3-dimensional FEM analysis was performed with the

ballistic damage located at the edge of stress concentration area of

engine support structure as it was the worst condition for the fatigue

analysis and ballistic damage tolerance design in terms of aircraft

safety, as shown in Fig. 12. The analysis model in Fig. 12 was

established using CHEXA, CBUSH and RBE2 model of the

NASTRAN program. Here, the engine support structures design

load was divided into static and dynamic load spectrums and most

of the loads worked as the compressive load to the engine support

structure. The stress distribution obtained from FEM analysis is

shown in Fig. 13. It resulted that the maximum von Mises stress of

168 MPa occurred at the damaged area and such stresses less than

90.8 MPa were distributed to the surrounding areas.

4.3 Fatigue Analysis and Ballistic Damage Tolerance Design

The safety of a military aircraft is judged by the minimum flight

time (required life time) for the safe return to base after having been

hit from the enemy heavy machine gun, and this can be verified

with the fatigue life expectancy curve (S-N curve) that shows the

ingredient property of the structure.

The equivalent stress can be calculated from the static and

dynamic loads of design load spectrums. Total damage (TD) can be

obtained as shown in Eq. (1) by using Minor's Rule and S-N curve.

1 2 3

1 2 3

i

D

i

n n n nT

N N N N

= = + + +

(1)

Where, ni is number of cycles at the i th stress level, Ni is

number of cycles to failure corresponding to the i th stress level and

ni /Ni is damage ratio at the i th stress level. Therefore, the safety life

(L) can be calculated as shown in Eq. (2) from the required life time

(minimum flight time) and Eq. (1).11,12

1

D

L Required Life TimeT

= (2)

For the engine support structure, the mean S-N curve, and the

working S-N curve that takes in consideration the safety factor are

shown in Fig. 14. Generally, the working S-N curve for predicting

the aircraft structures fatigue expectancy considers the number of

testing and reliability level to apply the reduction factors that falls

approximately ~ of mean S-N curve.13,14 The signs in Fig.

14 show the equivalent stresses, the results of fatigue analysis of

engine support structure with ballistic damage. Because the

equivalent stresses under the design load spectrums were very low,

the safety life expectancy was evaluated to meet the required level.

From such fatigue analysis results, the safety of the military aircraft

structure having ballistic damage was verified, and these results

were applied to ballistic damage tolerance design.

90 / FEBRUARY 2011 INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 12, No. 1

5. Conclusion

To establish the damage design concept of the military aircraft,

study of the damage shape by armor piercing bullet was performed

and its design application was examined. The summary of findings

is as follows:

1) The penetration damage diameters of the tube specimens

showed larger at the center than the periphery in the front, but

they resulted larger at the periphery than the center in the rear.

2) The maximum penetration damage diameters of the Al alloy

tubes of 2.7 mm and 3.5 mm thick are respectively 21.9 mm

and 25.7 mm, and that of stainless steel tube of 4.0mm thick is

14.8 mm.

3) As the angle of attack of the plate specimens increased, the

penetration damage diameters increased as well, with the

penetration damage diameters becoming larger in the rear than

the front.

4) The maximum penetration damage diameters of the Al alloy

plates of 3.2 mm, 12.7 mm and 25.4 mm thick are respectively

37.8 mm, 47.0 mm and 65.4 mm, and that of stainless steel

plate of 12.7mm thick is 15.6 mm.

5) The damage reference database was constructed through the

live rounds tests in this study, and its fatigue analysis results

could make it possible to verify whether the military aircraft

met the specific required level of survivability.

REFERENCES

1. John, G. E., Design Manual for Impact Damage Tolerant

Aircraft Structure, AGARD-AG-238-ADD, 1981.

2. Jensen, J. E., The Ballistic Damage Characteristics and

Damage Tolerance of Wing Structural Elements, Damage

Tolerance in Aircraft Structures, ASTM, pp. 215-229, 1971.

3. Donald, F. H., Damage Tolerance of Semi-monocoque Aircraft:

Specialists Meeting on Impact Damage Tolerance of

Structures, AGARD, 1976.

4. Anderson Jr., C. E., Hohler, V., Walker, J. D. and Stilp, A. J.,

The influence of projectile hardness on ballistic performance,

International Journal of Impact Engineering, Vol. 22, No. 6, pp.

619-632, 1999.

5. Gupta, N. K. and Madhu, V., An experimental study of normal

and oblique impact of hard-core projectile on single and layered

plates, International Journal of Impact Engineering, Vol. 19,

No. 5-6, pp. 395-414, 1997.

6. Gupta, N. K. and Madhu, V., Normal and oblique impact of a

kinetic energy projectile on mild steel plates, International

Journal of Impact Engineering, Vol. 12, No. 3, pp. 333-343,

1992.

7. U.S. Department of Defense, Military Handbooks:

Survivability, Aircraft, Nonnuclear, General Criteria - Vol. 1,

MIL-HDBK-336-1, 1982.

8. U.S. Department of Defense, Military Handbooks:

Survivability, Aircraft, Nonnuclear, Airframe - Vol. 2, MIL-

HDBK-336-2, 1983.

9. U.S. Department of Defense, Military Standard: V50 Ballistic

Test for Armor, MIL-STD-662F, 1997.

10. Jena, P. K., Ramanjeneyulu, K., Kumar, K. S. and Bhat, T. B.,

Ballistic studies on layered structures, Materials and Design,

Vol. 30, No. 6, pp. 1922-1929, 2009.

11. Park, T. G., Choi, C. H., Won, J. H. and Choi, J. H., An

Efficient method for fatigue reliability analysis accounting for

scatter of fatigue test data, Int. J. Precis. Eng. Manuf., Vol. 11,

No. 3, pp. 429-437, 2010.

12. Lee, S. H., Park, T. W., Park, J. K., Yoon, J. W., Jun, K. J. and

Jung, S. P., A fatigue life analysis of wheels on guide-way

vehicle using multi-body dynamics, Int. J. Precis. Eng. Manuf.,

Vol. 10, No. 5, pp. 79-84, 2009.

13. Bannantine, J. A., Comer, J. J. and Handrock, J. L.,

Fundamentals of Metal Fatigue Analysis, Prentice Hall, pp. 6-

15, 1990.

14. U.S. Department of Defense, Military Handbooks: Metallic

Materials and Elements for Aerospace Vehicle Structure, MIL-

HDBK-5H, 1998.