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

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Brass Cartridge Characteristics

Hardness and microstructure gradient required to accurately model M855 response

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• 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

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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

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Brass Cartridge FEA Results

Failure Criteriault tensile = 120 ksiult comp = 100 ksiult = 0.45

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Brass Cartridge FEA Results

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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

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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.