Using Finite Element Analysis to Study the Effects of Weapon Shock on Optomechanical Systems

1. Using Finite Element Analysis to Study the Effects of Weapon Shock on Optomechanical Systems McCoy Klink University of Arizona, OPTI 521 14 DEC 06

2. Introduction • Shock is an often-underestimated factor related the failure of optical and mechanical components in weapon-mounted optics. • Broken optics cannot perform. • Few resources on weapon shock exist. • Finite element analysis can improve components if incorporated correctly. • Bottom line up front: Follow these guidelines when modeling weapon-mounted optomechanical systems!

3. Guidelines 1. Take empirical data from the optomechanical system on each weapon with which it will be used. 2. Collect acceleration data from each orthogonal axis. 3. When feasible, collect acceleration data at the component in question. 4. Use acceleration timesteps in milliseconds, in order to provide resolution through the firing event. 5. When designing to withstand weapon shock, consider only the peak acceleration values. 6. Make valid assumptions to facilitate finite element modeling. 7. Be careful with constraint and applied load locations.

4. 1. Empirical Data • Collect your own data. Collect it on the optomechanical system for each weapon with which it will be used. • Weapon shock data is almost never published. • Nor is it accurate… Acceleration values are never the same on the weapon as on the sight. If you want something done right, do it yourself. Your design will thank you.

5. 2. Orthogonal Axes • Collect data along three axes. • Worst-case shock does not always follow the direction along the barrel (intuitive direction of recoil). • Weapon acceleration depends on • Torque • Due to cartridge ejection, etc. • Jump • Resettling • Peak shock can be from the weapon hitting the ground (vertical) just the same as it could be from recoil (longitudinal).

6. 3. Collect at the Component • Mount accelerometers at the component to be modeled, if possible. • As acceleration values are different on the weapon and the system, they are different on the system and the component. • Mass, material, constraints play a role. • When this is not feasible, mount them on the housing (someplace accessible) and pretend it is rigid and transfers all shock.

7. 4. Baby Steps • Use timesteps at least on the order of a millisecond. • This provides resolution through even a single shot firing event. Note the change in Gs!

8. 5. Use Peak Values • Use peak acceleration values. • Worst-case scenario becomes the baseline for FEA and design. • Who cares about low shock values? • You might. There may be a significant frequency or duration correlation. All data has the potential to be significant when it comes to weapon shock. Peak Shock

9. Interjection: The Utility of FEA • Finite Element Analysis is a powerful tool • Linear, static • Stress, Strain, Thermal, Vibration, • Transient, dynamic • Impulse Loading, Cyclic Loading, Fatigue • Provides quick and accurate results at defined locations

10. 6. Make Valid Assumptions • Assume a spherical horse

11. 6. Make Valid Assumptions • Assume a spherical horse • Valid assumptions are necessary to simulate motion • Rigid Housings/components • “Mass-less” components • Coefficient of damping • Total transfer of shock • Same peak shock at same frequencies on different components • Constraints • Timescale

12. 7. Defining Constraints and Loads • You control nodal locations of constraint and load application along the FEA mesh • Make sure to constrain correct DoF’s • Make sure to apply the right loads in the right locations

13. Conclusions • Weapon Shock analysis is no easy task. • Collect your own shock data. Be thorough and precise. • Always use the worst-case to design. • Finite Element Analysis is your friend if the right software is used. Use powerful and flexible software for shock simulation and dynamic analyses. • Model your optical components too! (Before they break). Durability and reliability can be as important as performance.