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By John Evans, September 25, 2013
In the first part of this First Look series on Solid Edge ST6's Simulation environment, I discussed linear static stress validation of a rifle chamber. From the results of the simulation, I observed areas of my design required improvement to maintain the Mil-spec and SAAMI proof loads, specifically to the internal surfaces of the chamber. At the same time, I aimed to maintain feature relationships and to minimize the overall mass of the rifle.
In this First Look article, I describe how I improved the chamber design using Solid Edge's optimization process, as well as validating the bolt engagement design in the assembly file.
After reviewing the linear static stress analysis results, I decided to move to Optimization to get a better estimate of where the chamber thicknesses needed to be adjusted, as well as the mount lengths and so on. I knew that the optimization process would likely dial in a precise variation as I directed; I wasn't sure, however, which direction would work in the final design.
Optimization offers us the ability to propose the following:
In my case, the goal is to limit the amount of mass the barrel contributes to the rifle, while attempting to reach the SAAMI and Mil-Spec proof loads. A couple of factors got in the way, however:
The desired hard-chrome-lined Cro-Mo properties of the barrel material were unable to be validated. Recorded modifications to the material cause it to be stronger and more heat resistant than that of unmodified AISI 4150 barrel steel. The best I could do for the preliminary research was to reach the Mil-spec barrel proof loads by taking these steps:
From these I would have a best reasonable design, knowing that the material will perform at least as good in the configuration, if not better due to the aforesaid material enhancements (see figure 1).
Figure 1: Optimization criteria setup
Each set of variables and limits can be adjusted to suit our needs. In my case, I set on the initial optimization a full max and min range of possibilities to see what aspect of the design would influence the relationships the most (see figure 2).
Figure 2: Editing the stress limits (left), and starting the optimization process from the context menu (right)
The Nastran solver ran out of memory on numerous occasions, and so I completed the optimization by using surface-only results. If the configuration looks to be too intense, Solid Edge will suggest that the overall study options be relaxed to allow only surface results to be calculated. I pursued this option knowing I would come back afterward and evaluate the 3D mesh in a final study.
Once optimization is completed, Nastran reports one of the following responses. With Solid Edge, I experienced three:
The results for whatever number of iterations Nastran solved are cataloged beneath the respective optimization. Each study can contain multiple optimization settings, which can contain multiple iterations. Keep that in mind that each result stored increases the .ssd results file size dramatically.
If the Optimization process does not stop with errors, the option appears to review the summary or see the plots. The latter opens the converged iteration and displays the respective Von Misses stress contours. The remaining iterations can simply be double-clicked upon in order to drill-down until the desired results are displayed (see figure 3).
Figure 3: Optimization results, adjusting the OD mount length and barrel root OD. Notice the organization in the Simulation Panel
Following along with Siemens PLM's wonderful spreadsheet tables, the summary is an attractively arranged spreadsheet report. All factors associated optimization are cataloged, along with the results of each iteration and the variables that drove that solution. These are handed off to any respective study report for automatic inclusion (see figure 4).
Figure 4: Summary table in an Excel spreadsheet - I love this!
The initial study supported the 1.2" OD of the chamber walls with a somewhat short length at the mount. I could have easily worked around this; however, I could see that the neck of the chamber was showing signs of increased stress now that parameters had changed.
I began a new optimization and repeated the process with a fixed 1.2" mount OD and continuing to vary the mount length and root barrel OD variables (see figure 5).
Figure 5: Variables table with the resulting variables used by the analysis after optimization - and a little tweaking from me
Interestingly enough the optimization process moved the barrel nut back to support the chamber neck when the chamber OD was fixed at 1.2". The result was a mount length of 1.4" and a barrel rood diameter of 0.910". The other interesting thing that optimization helped verify is that the concentration of stress was limited to the interior surface of the chamber; increasing the chamber and barrel ODs did not necessarily benefit the design.
To evaluate the 3D mesh applied to the optimized model, I wanted to create a new study. (I could have changed the option and resolved this study, but I wanted to keep it intact.) To do this, I copied and pasted the data in the Simulation panel. While this worked well enough (and indeed copied all the settings I needed), I expected to see a simple Copy Study function in the context menu, but there was none.
I adjusted to the Mil-Spec pressure load and then resolved the model (see figure 6).
Figure 6: ISO contour plot of 3D meshed solution
After using the hardened AISI 4150 barrel material and reviewing the results of the study, I observed the following items:
I feel confident in the chamber design with the hardened AISI 4150 material employing a semi-automatic fire rate. Should a Mil-Spec Cro-Mo-V material be introduced, the barrel should also perform well under continuous full-auto fire rates.
One of the characteristics of this particular rifle design is a rotating bolt delay, and so I decided to revisit an old concept which I thought would be fun to explore. Certain production bolts lock by some form of rotation; all production models of this type delay the cartridge ejection by a gas actuator. To maintain the modular nature and simplicity of this rifle, I like avoiding gas actuators, and instead maintain a simple delayed-blowback design (see figure 7).
Figure 7: Bolt and barrel shown in an assembly
This delay system employs a helical groove milled from the chamber's aft extension. It permits two rollers to guide the bolt from the receiver, and then engage the barrel's chamber directly in a twisting motion, running the cartridge into battery and locking the bolt closed. For the fired cartridge and bolt to blowback after ignition, the bolt must rotate 45° under spring pressure against the ends of the helix.
As these ends must be strong enough to withstand the load, I used Solid Edge's static stress analysis to perform the following tasks (see figure 8):
Figure 8: Static stress ribbon for Solid Edge's assemblies
The setup process is similar to that of LSS (linear static stress) in the part environment, with a few exceptions that I've highlighted below.
The components were simplified individually in their respective part Simplification environments. The barrel was lopped off at the chamber, and the bolt had numerous features removed, including the helical guides, the extractor, and actuator recesses. One interesting aspect was modeling the roller buttons onto the bolt as part of the simplification process. This proved to be advantageous (see figure 9).
Figure 9: Using simplified versions for the study geometry, a nice way to keep the design and study models together. Notice the barrel has been lopped off and the helices and actuator features are gone; I added a simplified roller button, as well. With the simplified geometries added to the study, the meshing completed in short order
One great thing about Solid Edge part simplification is that it presents an alternate model to be evaluated without having to create alternate parts.
To keep the study from taking too much time in a single pass, some refinement of the surface mesh was required; refinement also helps the system from running out of memory. You can see some of that below in figure 10.
To evaluate the strength of the helical engagement lugs, I decided to fix the bolt rotation, yet allow it to translate along its axis. This permits the greatest load transfer to the mesh elements.
I added a Fixed constraint to the barrel and a Cylindrical constraint to the bolt shaft. The latter was tuned to allow axial translation.
Figure 10: Fixed and Cylindrical constraints added to the assembly components (left and right); options bar (above) permits simple tuning
Next, I need to join the two, which I did using Solid Edge's Connectors. These are applied in a two-pick process using the options bar to choose for the behavior either Glue or No Penetration; for my design, I used No Penetration on all three (see figure 11).
Figure 11: No-penetration connectors applied to the bolt and chamber engagement surfaces
I used Solid Edge's standard selection dialog box to weed through overlapping faces. While this works acceptably well, Siemens PLM could introduce better visualization, such as for component/feature methodologies.
Once initiated, the last option to attend to is the Search Distance for the connector, which in this case is a liberal 0.2" to ensure that the surfaces were detected. I found that the Connector's behavior is quite similar to that of that in Siemens PLM's FEMAP analysis software.
An equivalent axial pressure load relative to the chamber pressure (as discussed in pt 1) was applied to the face of the bolt.
At this point the results were hopeful, but not acceptable to me. Solid Edge indicated that while the lugs would hold, the surface would suffer deformation (see figure 12). I judged this to be a future problem, which could lead to failure from fatigue or deformed guide lugs.
Figure 12: While the issue here might be regarded as a singularity, significant stress is being applied to this surface; an adjustment to the design is a wise choice
I could not afford to radially expand the thickness of the engagement lugs, as this would cause the barrel and chamber thicknesses to rise to accommodate the lugs. To spread the load, I instead chose to expand the roller button diameter slightly. I was, however, afraid that due to the helix features in the chamber model, that the part edits would fail in Solid Edge.
But they didn't. In fact, the change took a mere 60 seconds, including the respective adjustment to the button OD. Awesome! This is a perfect candidate for optimization.
I immediately rebuilt the mesh and resolved the study. (The connectors all came through the design changes flawlessly). The results were quite positive: the load spread better, and pointed to some alterations that would strengthen the design further (see figure 13).
Figure 13: The resolved study's Von Misses stress results after design modifications, viewed in a banded contour style
Overall, my model behaved well in Solid Edge ST6 Simulation. The simulation environment performed better than I expected. It held up to changes in the model; changes were fluidly made.
Simplification geometries are the shining feature in the assembly study; nevertheless, I would say that the assembly simulation process in Solid Edge is good, and worth the investment.
Optimization performed wonderfully, and once I balanced the tuning options and locked Smart Dimensions, Solid Edge showed me exactly which relationships needed tweaking to get the design to the next phase.
|John Evans has 30 years experience in the aerospace design, engineering and fabrication, as well as 18 years with MEP and civil engineering. He is certified with AutoCAD Civil 3D and Inventor. More...|
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