Post on 31-Mar-2015
transcript
Finite Element Modeling of a 5.56 mm Brass Cartridge
Joseph South & Larry Burton
U.S. Army Research LaboratoryComposites and Lightweight Structures Branch
Outline
• Overview
• Cartridge Challenges
• Brass Cartridge FEA Modeling– Model Generation– Mechanical Results
• Polymer Cartridge FEA Modeling
Overview
• Goal– Development of a baseline thermo-mechanical model for a 5.56 mm
cartridge inside a M16A2 barrel.
• Approach– Creation and validation of a model for the M855 brass cartridge.
– Utilize FEA to assess the feasibility of lightweight polymers in cartridge case applications.
• Technical Barriers– Material functionality is required over the full spectrum of
environmental conditions.
– Strength required to meet all operational functions• propellant gas pressure, primer strike, feed, extraction
Polymer Cartridge Payoff• Polymers have the potential
– to reduce the manufacturing cost• By reducing the number of steps through injection molding
– reduce logistical load
– improve accuracy• Injection mold the bullet in place• Maintain the centerline alignment
Current Basic Issue7 Magazines M855
10 Magazines withPolymer Case Cartridges
Equivalent Weight
Lightweight Cartridge Challenge
• Brass– E ~ 16 Msi
– Tm - 1700°F
– Moisture insensitive
• Polymer– E ~ 0.2 - 1.3 Msi
– Tg - 320°F
– Hygroscopic
Current brass systems require numerous manufacturing steps to produce the final microstructure and hardness gradient
80
100
120
140
160
180
200
220
240
0 2 4 6 8
Location on Cartridge Case
Vic
kers
Hard
ness
2.5
kg L
oad [H
V]
Max Hardness Min Hardness
Brass Cartridge Characteristics
Hardness and microstructure gradient required to accurately model M855 response
0
10000
20000
30000
40000
50000
60000
0 500 1000 1500 2000 2500 3000 3500 4000
• 2D Static Model– Models a 5.56mm brass cartridge
in a M16A2 barrel with barrel extension.
– Incorporates the effect of the hardness gradient along the cartridge length.
– Material models include plasticity.– Primer is assumed have the
same structural characteristics as the cartridge.
– Contact pair between the cartridge and chamber wall and the cartridge and primer.
– Pressure gradient is applied to the inside of the cartridge.
– Thermo-mechanical model.
M855 Pressure Profile
0
20
40
60
80
100
120
140
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
True Strain
True
Str
ess
[ksi
]
0
20000
40000
60000
80000
100000
120000
0.0 0.2 0.4 0.6 0.8 1.0
140 HV 180 HV 200 HV 200 HV
Brass Cartridge Model
Brass Cartridge Modeling
Brass Cartridge Model
• Applied Boundary Conditions– Axisymmetric along y axis– Symmetric BC on primer wall
along the axis.– Zero displacement BC in all
directions applied to the head of the cartridge.
• Assumes continuous intimate contact between the bolt and the cartridge.
• Does not account for rearward motion during firing.
– Zero displacement BC in all directions applied to the barrel extension.
– Total of 31,000 elements.
Thermal FEA Modeling• 2D Axisymmetric Sequential Model• Meshed with 8 node thermal elements
– ID contains surface effect element– 6000 elements
• Calculated from interior ballistics• Thermal loads are applied in a tabular
format to the ID
15
30
55
85
135
195
0
10
00
15
00
20
00
25
00
30
00
35
00
40
00
45
00
50
00
55
00
60
00
65
00
70
00
75
00
80
00
85
00
90
00
95
00
10
00
0
15
00
0
20
00
0
25
00
0
30
00
0
35
00
0
40
00
0
45
00
0
50
00
0
60
00
0
70
00
0
90
00
0
0
50000
100000
150000
200000
250000
300000
350000
400000
450000
Axial Location [in]
Brass Cartridge FEA Results
Failure Criteriault tensile = 120 ksiult comp = 100 ksiult = 0.45
-60
-40
-20
0
20
40
60
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
Length Along Cartridge [in]
Tru
e S
tres
s [k
si]
Radial Stress Axial Stress Hoop Stress Principle Stress
Brass Cartridge FEA Results
-0.0005
0.0000
0.0005
0.0010
0.0015
0.0020
0.0025
0.0030
0.0035
0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00
Length Along Cartridge [in]
Ra
dia
l P
lastic D
isp
lace
me
nt
[in
]
Measured Cartridge DeformationNominal Bore DimensionBore With Maximum Tolerance
Brass Model Summary
• Goal has been to benchmark the M855 brass cartridge with a FEA model.
• The current model incorporates the strength changes in the cartridge due to the variations in the hardness and microstructure.
• The model yields a stress state within the brass that demonstrates 2.0 ultimate factor of safety.
• Measurements from expended cartridges show good correlation with the predicted plastic deformation.
Polymer Cartridge Model
• 2D Axisymmetric Model– Cartridge is entirely polymer– Nylon 612– Internal pressure is loaded in
smaller increments
0
1
2
3
4
5
6
7
8
0 0.1 0.2 0.3 0.4 0.5
True Strain
Tru
e S
tre
ss [
ksi]
-7
-6
-5
-4
-3
-2
-1
0
1
2
0 0.5 1 1.5 2
Location along Cartridge [in]
True
Str
ess
[ksi
]
Radial Stress Axial Stress Hoop Stress Principle Stress
Polymer Cartridge FEA Results
Failure CriteriaNylon 612
ult tensile = 7 ksiult tensile = 0.4
Subjected to an Internal Pressure of 5 ksi
Polymer Cartridge FEA ResultsDisplacement Vector Plot
Polymer Model Summary
• The polymer cartridge model is currently a work in progress.• Due to the mechanical properties of the polymer, modifications
to the case design will be required.• Investigations continue into optimizing the model including
– Parametric assessment of increased wall thickness on survivability of polymer cartridge
– The effect of the cartridge head design on the survivability of the polymer cartridge.
– Alternate materials• Different polymer systems or filled composite systems
Conclusions &Future Direction
• The FEA modeling of the brass M855 cartridge provides a solid foundation to evaluate alternative cartridge materials.
• Future efforts will focus on– Applying the thermal capability to determine in-bore heating
profile.• Allows for investigation of cook-off and thermal softening.
– Use existing model to examine stress state due to• Primer strike, extraction and feed.