Finite Element Modeling of a 5 56 mm

Finite Element Modeling of a 5. 56 mm Brass Cartridge Joseph South & Larry Burton U. S. Army Research Laboratory Composites 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 M 16 A 2 barrel. • Approach – Creation and validation of a model for the M 855 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 Issue 7 Magazines M 855 Equivalent Weight 10 Magazines with Polymer Case Cartridges

Lightweight Cartridge Challenge • Brass • Polymer – E ~ 16 Msi – E ~ 0. 2 - 1. 3 Msi – Tm - 1700°F – Tg - 320°F – Moisture insensitive – Hygroscopic

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 M 855 response

Brass Cartridge Model • 2 D Static Model – Models a 5. 56 mm brass cartridge in a M 16 A 2 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. M 855 Pressure Profile

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 • 2 D 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

Brass Cartridge FEA Results Failure Criteria sult tensile = 120 ksi sult comp = 100 ksi eult = 0. 45

Brass Cartridge FEA Results

Brass Model Summary • Goal has been to benchmark the M 855 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 • 2 D Axisymmetric Model – Cartridge is entirely polymer – Nylon 612 – Internal pressure is loaded in smaller increments

Polymer Cartridge FEA Results Subjected to an Internal Pressure of 5 ksi Failure Criteria Nylon 612 sult tensile = 7 ksi eult tensile = 0. 4

Polymer Cartridge FEA Results Displacement 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 M 855 cartridge provides a solid foundation to evaluate alternative cartridge materials. • Future efforts will focus on – Applying 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.