Objective:
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Use HyperX to Size the rib as a series of Tech 2 Stiffened Panels.
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Understand the implications of Panel Segments in a traditional Sizing workflow.
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Demonstrate the capability of the Grid-Stiffened Plugin for sizing metallic grid structure.
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Use HyperFEA to Size the stiffener segments for a min buckling EV of 1.0
Create Structure
To create Panel Segments, all panels and beams must be in the same Structure. Note that interface flanges are NOT to be Sized – no need to add them to the Structure.
Create Panel Segments
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Open the automatic Panel Segment generator by clicking on the "Panel Segment - Automatic" button on the Structure tab of the Ribbon.
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Select "Technique 2" from the pop-up menu.
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Select all beam boundaries when indicated and click "Create" to create the Panel Segments.
Review Panel Segments
Notice the change in categories in the Structure Tree. Click on any beam to highlight the entire Panel Segment and Right Click > Edit to understand which elements make up each Panel Segment entity.
Tip
All properties should now be grouped into either the Skin Segment or Stringer Segment node on the Structures tree.
Assign Design Properties
The Design Properties we need are already in the Property Tree. Select "Skin Segments" from the Structure Tree and assign "Skins. Then, select "Stringer Segments - Tech 2" and assign "Bars".
Create and Assign Analysis Properties
Next, we will create two Analysis Properties. One for the skins and another for the bars. In the Property Tree, right-click on "Analyses" and select "Create Panel" from the dropdown.
First, we'll create the Analysis Property for the skins. Name the Property accordingly and press the "Create a new failure mode" button.
Select "Grid Stiffened" and click "Next". This will create a Failure Mode that features methods from the Grid Stiffened Plugin Package.
On the Failure Mode form:
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Name the Failure Mode.
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Enable the Failure Criteria shown in the image below. Note that we are toggling "GS - von Mises-Hill Element Centroid (Yield)" criteria from Ultimate to Limit.
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Click "Apply" to create and save the Failure Mode. Then click apply on the Analysis Property form to save it.
Next, we will create the Analysis Property for the bars. We will roughly follow the same process as above while adding an additional Stiffness Failure Mode to this Property. The Stiffness Failure Mode will be used with HyperFEA starting on Step 17. Make sure to enable/disable the Failure Criteria to match what's shown in the image below and ensure the Limit/Ultimate setting on these criteria matches the image as well.
Tip
This Analysis Property should also be a PANEL Analysis Property. Although it will be assigned to 1D bar elements, those bar elements are considered part of a larger PANEL segment.
Once the Analysis Properties have been created and saved, assign the "Skins" Property to the "Skin Segments" and the "Bars" Property to the "Stringer Segments".
Create and Assign Average Load Property
Create a Load Property using the Average Load Processing Technique and assign it to both the skins and stringers.
Review Property Setup
Next, we have to assign the Properties to the Structures. Select each Structure and assign the Property by dragging it onto the model or right-click > "Assign to Visible Selection" in the Property Tree.
For the Skin Segments:
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Design Property - Skins
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Load Property - Average - 1
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Analysis Property - Skins
For the Stringer Segments:
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Design Property - Bars
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Load Property - Average - 1
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Analysis Property - Bars
Size and Review Results
Select both the Skin and Stringer Segment Structures and run Sizing. You can review the Analysis Results of a particular pocket by selecting that pocket and opening the Analysis Watch Window, as indicated by arrow (3) in the screenshot below.
Note
The Panel Segment sizing algorithm takes longer to run than a traditional HyperX sizing, due to the complex nature of resolving connected sections.
Define a Hole in the Pocket
The Grid Stiffened Orthogrid Buckling method will analytically account for a hole - as long as a diameter value is defined as a setting on the Failure Mode.
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Open the existing GS - Skins Failure Mode from the Property Tree.
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Scroll to the "Orthogrid Buckling Cutouts" settings. Input diameter D = 2.5in.
Note that the other inputs in this section define the knockdown curve, as shown in the image below. For more information, see Grid Stiffened - Adjustments - Cutouts.
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Click "Apply."
Re-analyze the Pocket with As-Size Dimensions and Review Results
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Select the pocket of interest, indicated in the image, and Run Analysis (saved dimensions from previous Sizing run).
Tip
"Lock" the Analysis Watch Window containing the margin results from the first sizing. This way, after Analysis, a new window can be opened to compare the results between the two runs.
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Open an Analysis Watch Window to review the new Orthogrid Buckling margin of safety.
Notice the Orthogrid Buckling margin is now negative for this pocket. This is because the allowable buckling load has been knocked-down to account for the hole.
Caution
After reviewing these results, reset the Cutout Diameter to 0 before moving on to Part 4.
Implement Machining Tolerances
The Grid Stiffened Plugin methods can also write margins with respect to machining tolerances. The tolerance parameters are defined as Analysis Settings on Grid Stiffened Failure Modes.
The tables below outline the meaning of these different Analysis Settings parameters:
|
Nominal Thickness |
Strength |
Buckling |
|---|---|---|
|
\(t < t_{nom}\) |
\(\min \left(t, c_s \left(t-t_t\right)\right)\) |
\(\min \left(t, c_b \left(t-t_t\right)\right)\) |
|
\(t \ge t_{nom}\) |
\(\min \left(t, c_s \left(t-t_l\right)\right)\) |
\(\min \left(t, c_b \left(t-t_l\right)\right)\) |
|
Analysis Setting |
Description |
Default |
|---|---|---|
|
\(t_{nom}\) |
Nominal thickness threshold that determines whether to use the tight or loose tolerance. |
0.125 |
|
\(t_l\) |
Loose tolerance value |
0.01 |
|
\(t_t\) |
Tight tolerance value |
0.007 |
|
\(c_s\) |
Coefficient for strength analysis |
1.10 |
|
\(c_b\) |
Coefficient for buckling analysis |
1.05 |
Update both "Skin" and "Bar" Failure Modes to account for tolerances.
Re-analyze Rib to Account for Machining Tolerance and Review Results
Select the entire rib and Analyze the existing sizing results. This allows you to compare margins both before and after implementing machining tolerances. As you can see, several pockets and ribs may pass margin criteria at nominal thickness, but fail once machining tolerance is taken into account.
Note
If the MS you see is different than the images below, you may not have reset the "Cutout Diameter" set in Part 3. To get back on track, reset that value to 0 and Analyze again.
Use the Data Probe to compare thickness values. Thinner plates are more affected by the thickness tolerance.
Fix Persisting Negative Margin
In some cases, when sizing with panel segments, it is possible that HyperX is unable to find a positive-margin solution because it has "marched itself into a corner" by sizing all connecting segments first. This is the case for the pocket below, which has a persisting negative margin, but is still showing a min-gage thickness.
If this happens, you can "force" HyperX to re-evaluate the pocket by using the Sizing Overrides table to implement a minimum gage that is a step above the current result. So, in the case of this example, the pocket should be set to have a minimum gage of 0.1in.
Re-sizing this pocket results in a positive margin.
Caution
Re-adjust failure modes to no longer account for machining tolerances (un-check option) before proceeding to Part 5.
Set Up Sizing for Eigenvalue Buckling
Caution
Re-adjust failure modes to no longer account for machining tolerances (un-check option) before proceeding.
Say there is a minimum buckling eigenvalue requirement of 1.0 for this part. This is a global requirement, rather than a local (zone-based) requirement that HyperX operates on. Therefore, it requires iterating with HyperFEA to understand the global response of the structure. Implementing a FEA Constraint for global buckling will translate that response in to zone-level stiffness constraints for sizing.
First, we need to create a Set containing only the 1D bar Zones:
In the next step, we will use this Set to tell HyperX to only size the bars to meet the eigenvalue requirement (rather than also sizing the thickness of the pockets).
Next, we need to create and define an FEA Constraint. In this case, we will be creating, defining, and applying a Buckling Constraint.
Define the Manual Constraint as shown in the image. Click "Ok" to save and apply the Constraint.
Note
This is telling HyperFEA to: Meet the required eigenvalue target for the selected buckling load case by adjusting the D11 stiffness of the Bar Zones.