1. Finite Element Modeling of a 5.56 mm Brass Cartridge
Joseph South & Larry Burton
U.S. Army Research Laboratory
Composites and Lightweight Structures Branch

2. Brass Cartridge FEA Modeling
Outline
Overview
Cartridge Challenges
Brass Cartridge FEA Modeling
Model Generation
Mechanical Results
Polymer Cartridge FEA Modeling

3. Overview Goal Approach Technical Barriers
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

4. 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 Issue
7 Magazines M855
10 Magazines with
Polymer Case Cartridges
Equivalent Weight

5. Lightweight Cartridge Challenge
Brass
E ~ 16 Msi
Tm °F
Moisture insensitive
Polymer
E ~ Msi
Tg - 320°F
Hygroscopic

6. Brass Cartridge Characteristics
Current brass systems require numerous manufacturing steps to produce the final microstructure and hardness gradient
Hardness and microstructure gradient required to
accurately model M855 response

7. Brass Cartridge Model 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

8. Brass Cartridge Modeling

9. 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.

10. 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

11. Brass Cartridge FEA Results
Failure Criteria
sult tensile = 120 ksi
sult comp = 100 ksi
eult = 0.45

12. Brass Cartridge FEA Results

13. 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.

14. Polymer Cartridge Model
2D Axisymmetric Model
Cartridge is entirely polymer
Nylon 612
Internal pressure is loaded in smaller increments

15. Polymer Cartridge FEA Results
Subjected to an Internal Pressure of 5 ksi
Failure Criteria
Nylon 612
sult tensile = 7 ksi
eult tensile = 0.4

16. Polymer Cartridge FEA Results
Displacement Vector Plot

17. 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

18. 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.