Stress Study 04

    
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Firearm Design, Dynamic Strength Analysis:
Compare the stress concentrations in various bolt/receiver, lug, designs for a high-caliber rifle as a function of fillet radius.

Special features:
3D model setup with kinematic-hardening, material model for high-strength, heat-treated steels, contact surfaces with impact, and off-plane symmetry (60 degree symmetry wedge to reduce model size and speed solve).

Output:
Nodal stresses and displacements.

Comments:
The peak pressure event lasts several milliseconds with chamber pressures not exceeding 60,000 psi. The bolt lug with the "extractor" slot is necessarily the weakest of the three so it was modeled for the most conservative result. Shown above is an assembly view of the important components, bolt, bolt-carrier, and receiver (click any of the thumbnails to access).

Regarding the design, notice the "extractor" and slot along with other interesting features, like the ejector pin, bolt carrier charging handle, firing pin hole (pin retracted), bolt-rotation cam-slot and pin, etc. These features carry a very small load compared to the lug interaction during chamber pressurization so can be ignored. Clicking the remaining thumbnails open animations showing the stress concentration levels and displacements as a function of time. The last animation shows the stresses internal to the body using Autodesk Simulation's element "shrink" feature.

Another way to view the surface stresses, or the stress concentration depth, is to use Autodesk Simulation's "slice" feature (not shown), basically a view on a user-selected section plane. Of course, the stresses on any symmetry plane reveal the stress internal to the body along that plane. The radii stresses were compared to various other caliber/bolt designs of similar materials to arrive at a conventional safety factor. The design featured here was 30% more robust than any other retailed, large-caliber, rifle design compared and suggests a very safe, reliable, and functional implementation. Most large caliber designs employ two or three lugs. The smaller caliber designs employ from two to eight lugs. This three lug design is a compromise of many factors with safety and longevity being the most important.