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Author Topic: Stiffness Requirements and HyperFEA FEA constraints  (Read 3357 times)

mraedel

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Stiffness Requirements and HyperFEA FEA constraints
« on: November 13, 2014, 11:37:48 AM »
Hello,

I use HyperSizer and import an ANSYS model. 4 load cases are used (101-104, Attachment 1+2).

I define load-case dependent displacement constraints for loadcase 2 (=load set 102), 3 (=load set 103) and 4 (=load set 104) in HyperFEA (Attachment 3).

To consider these constraints I activate Stiffness Requirement, Membrane and Stiffness Requirement, Bending in the Failure Tab of the Project Sizing Window.

If I calculate the model in HyperFEA, the Project Sizing windows shows the load set number 101 for the minimum margin of safety for the stiffness requirement for some components (Attachment 4).

How is this possible? I did not define displacement constraints for load step (load case). What am I missing?

Thanks for a hint
Martin

Ryan

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Re: Stiffness Requirements and HyperFEA FEA constraints
« Reply #1 on: November 13, 2014, 11:51:30 AM »
Hi Martin,

The "Apply to load cases" option refers to which load cases are used to pull the FEA displacements.

This option does not affect the required stiffness margin of safety. The stiffness requirements from HyperFEA are always applied to all load cases.

In the Failure tab it's flagging load case 1/load set 101 as the "critical" case/set probably because this is the first load case in the project.

Thanks,
Ryan

mraedel

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Re: Stiffness Requirements and HyperFEA FEA constraints
« Reply #2 on: November 13, 2014, 12:01:50 PM »
Ok, Ryan, thanks. I get that.

I use the same failure tab definitions for all components. All the other relevant user inputs are the same for all components as well.

Why are there other components which show LS=102 or LS=104 for the stiffness requirements?

Ryan

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Re: Stiffness Requirements and HyperFEA FEA constraints
« Reply #3 on: November 14, 2014, 02:18:50 PM »
There may be some load case dependence on stiffness. The may be due to temperature or postbuckling. There might also be some numerical rounding effects that cause different load cases to be flagged as critical.

If you run a single load case at a time and look at the stiffness in the Computed Properties tab, you might be able to see what's happening.

mraedel

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Re: Stiffness Requirements and HyperFEA FEA constraints
« Reply #4 on: November 17, 2014, 11:21:36 AM »
Hi Ryan,

seems I still understand the Stiffness Requirements wrong.

I unchecked all FEA Constraints in HyperFEA and calculated only load case 101 by deselecting load case 102-104 in the Load Cases tab of the Project Setup window. In the Failure Tab of the Project Sizing window, the Stiffness Requirements (Membrane & Bending) remain active, but should have no input to evaluate the condition from.

As I understood you, HyperFEA takes the nodal displacement of load cases in the active FEA constraints to compute the necessary stiffness. Since no constraints are active anymore, this criterion should give no results.

However, it is still the sizing criterion for the single calculation of load case 101 (Minimum MoS). How can this be?

Ryan

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Re: Stiffness Requirements and HyperFEA FEA constraints
« Reply #5 on: November 17, 2014, 01:58:04 PM »
The required stiffness values still remain even though HyperFEA constraints are not changing them. This is by design since required stiffness values can come from sources other than HyperFEA.

You can view the required stiffness values per component in the Computed Properties tab. You an also view them for the entire model in the FEM Viewer with Data | Input Computed Properties.

To clear out the required stiffness values, use the Computed Properties tab or HyperFEA using FEA Constraints | Removed Required Stiffnesses.