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Ballistic Performance of Steels and Aluminums in FE Firing Simulations

Justin Mach, Ph.D.Finite Element Analysis EngineerATK Small Caliber Systems(816) [email protected]

May 25, 2011

Mark Lee, P.E.Finite Element Analysis EngineerATK Small Caliber Systems(816) [email protected]

OSR #11-S-2170

mailto:[email protected]�

mailto:[email protected]�


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Alternative materials reduce case weight

Why steel and aluminum alloys as alternative cartridge case materials?

• Reduced case weight

– 8% density reduction for steel

– 67% density reduction for aluminum

• Material cost

General mechanical property differences

• Stiffness

• Strength

0%

20%

40%

60%

80%

100%

Brass Steel Aluminum

% o

f Bra

ss C

ase

Wei

ght

05

101520253035

Brass Steel Aluminum

E [x

106

psi]


3

Finite element modeling

Firing simulation

• Axi-symmetric, transient dynamic simulation of firing (case pressurization) event

• Solver – Abaqus/Explicit (v6.9-EF1)

• Nominal chamber, case and bolt face geometry represented in all models

• Primer cup geometry is included in Aluminum 5.56 mm case models


4

Brass case (baseline) model setup

00.10.20.30.40.50.60.70.80.9

1

0 0.001 0.002 0.003

Nor

mal

ized

Pre

ssur

e (P

/Pm

ax)

[psi

/ ps

i]

Time [s]

.50 Cal. brass

5.56 mm brass

0123456789

10

0 0.1 0.2 0.3 0.4

Nor

mal

ized

Tru

e St

ress

[p

si /

psi]

True Plastic Strain [in / in]

0.00-0.21

0.20-0.32

0.32-0.45

0.45-0.58

0.58-0.86

0.86-1.00

Normalized case height [in/in] (.50 Cal)

Geometry

• Nominal case geometry for .50 Cal. and 5.56 mm brass cartridge cases

Material model

• Based on an extensive material evaluation of LakeCity 5.56 mm cartridge cases (Tew, 2003)

Loading

• No explicit modeling of energetic material − uniform pressure history is applied

• .50 Cal. – calculated

• 5.56 mm – measured mid-case pressure


5

Steel .50 Cal. case model setup

0

1

2

3

4

5

6

7

8

9

10

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

Nor

mal

ized

Tru

e St

ress

[psi

/ ps

i]

True Plastic Strain [in / in]

LCS Hard 0.00-0.33

LCS Annealed 0.33-1.00

SS Hard 0.00-0.33

SS Annealed 0.00-0.33

Geometry

• .50 Cal. steel case geometry is unaltered from brass case

Material model

• Two candidate steels:

• LCS – a low-carbon steel

• SS – a stainless steel

• Case is divided into two sections, representative of the division in brass flow stress curves

Loading – peak pressure

• 65 ksi

• 85 ksi

LCS

SS


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Acceptable deformation with increased residual force

Plastic strain

• Overall deformation similar for brass and LCS cases

• SS shows clear advantage for resisting deformation in case head

Average residual force

• Low-carbon steel

• 2x brass force at 65 and 85 ksi

• Stainless steel

• 1.5x brass force at 65 ksi

• Similar to brass force at 85 ksi

Brass

LCS

SS

Equivalent Plastic Strain

0

0.5

1

1.5

2

2.5

Before bolt unlock

After bolt unlock

Before bolt unlock

After bolt unlock

65 ksi 85 ksi

F / F

bras

s[lb

f. / l

bf.] Brass LCS SS

Average Residual Force


7

Less pocket deformation with reduced spring-back

Radial displacement of primer pocket (65 ksi)

• Peak – LCS similar to brass – SS much less than brass

• Residual - LCS 2x brass deformation – SS much less than brass


8

Steel can replace brass, reduce weight and cost

Residual force

• LCS – 2x residual force of brass case at 65 and 85 ksi

• SS – 1.5x residual force of brass case at 65 ksi

Deformation

• LCS – similar to brass case performance

• SS – clear advantage over brass and LCS cases in unsupported (case head) region

Geometry

• LCS – Not likely that further weight reduction is attainable in current material state

• SS – Likely candidate for further weight reduction

Material cost

• Low-carbon steel < Stainless steel < Cartridge brass

• LCS requires coating for corrosion resistance

• Coating may also benefit extraction for LCS and SS


9

Aluminum 5.56 mm case model setup

0

1

2

3

4

5

6

7

8

9

10

0 0.02 0.04 0.06 0.08

Nor

mal

ized

Tru

e St

ress

[psi

/ ps

i]

True Strain

AA-1

AA-2

AA-3

260 Brass Extra Spring

Geometry

• 5.56 mm aluminum case geometry is altered from brass case to accommodate lower strength materials

Material model

• Three candidate aluminum alloys: AA-1, AA-2, AA-3

• Entire case is in hardened condition necessary for adequate performance

Loading – peak pressure

• 65 ksi


10

Aluminum 5.56 mm case design

Deformation with brassgeometry

• Aluminum case (AA-1) radial primer pocket deformation is approximately 8 times that of brass case

Modified geometry

• Dimensions changed that do not affect case/bullet or case/weapon interface

Brass

AA-1



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Plastic strain compares favorably with brass

Deformation with modified geometry

• Overall deformation

• Radial primer pocket deformation

Brass

AA-1



12

Primer pocket deformation compares favorably with brass

Radial deformation of primer pocket

• AA-1 and AA-2 compare favorably with the brass case

• AA-3 is unstable prior to reaching design pressure

• All alloys’ performance is limited to design pressures less than brass case


13

Aluminum alloys better for extraction

Residual force

• Reduced residual contact force between case and chamber

• Potential benefit for weapon extraction

Brass AA-1 (mod4) AA-1 (mod5)


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Aluminum 5.56 mm Results

Case design

• AA-1 – 8x deformation of brass case with same geometry

• Dimensional changes made to increase AA case performance

– No effect on case/bullet or case/weapon interface

– Tradeoff with internal case volume

Performance

• AA-1 – Best candidate

• AA-2 – Has potential to work well

• AA-3 – Not likely to work for the estimated material state


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References

• Abaqus v6.9-EF1. Simulia – Dassault Systemes.

• Tew, B. W. 2003. "Material and Structural Evaluation of Lake City 5.56 Cartridge Case," ATK Lake City Small Caliber Ammunition.

ballistic performance of steels and aluminums in … · ballistic performance of steels and...

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