ballistic performance of steels and aluminums in … · ballistic performance of steels and...
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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
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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]
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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
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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
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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
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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
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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
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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
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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
Equivalent Plastic Strain
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Plastic strain compares favorably with brass
Deformation with modified geometry
• Overall deformation
• Radial primer pocket deformation
Brass
AA-1
Equivalent Plastic Strain
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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
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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.