Finite Element Modeling of a 5.56 mm Brass Cartridge

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

3. 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

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 10 Magazines with Polymer Case Cartridges Current Basic Issue 7 Magazines M855 Equivalent Weight

5. Brass E ~ 16 Msi Tm - 1700°F Moisture insensitive Polymer E ~ 0.2 - 1.3 Msi Tg - 320°F Hygroscopic Lightweight Cartridge Challenge

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.