Gross stress cut-offs in metals

[Paul Wagner]

I'm doing trade studies on metallic uniaxial stiffened skins and I'd like to be able to enforce a gross tension stress cut-off allowable, say for fatigue or damage tolerance.  For this particular study, I'd like to enforce a limit stress cut-off, but an ultimate cut-off would work as well.

These are the things I've considered:

1. Redefine the material allowable strength (Fty) to the gross stress cut-off value (Fdadt).  This won't work because it would corrupt all analysis checks that depend on Fty to be correctly defined.

2. On the failure tab of the sizing form, select "strain limit" for Limit MS and enter Fdadt/E strain cut-off values on the strain allowables tab of the isotropic material property.  This looks like it should work, but HyperSizer doesn't appear to be running this analysis check.  Are the details of the "strain limit" failure mode documented somewhere?  Are there limitations to what this check is applicable to?

3. Define a user-defined failure analysis.  Metallic gross stress checks don't seem to directly fall into any of the user-defined analysis methods: beam buckling, bolted hole, composite strength, crippling, panel buckling, sandwich core, or sandwich face.  Would I be able to adapt the bolted hole check?

I'd appreciate any suggestions you have.

Thanks

[James]

The second option appears to be the best solution. Set a panel strain limit based on the fatigue stress allowable.  To do this activate the 'Strain Limit' analysis method on the failure tab.

The 'Strain Limit' method defines a total panel strain limit. The panel strain limit is independent of the strain allowables entered in the Isotropic Material Form. The panel 'Strain Limit' is set on the computed properties tab (Strain X). Defining a panel strain limit should work for your trade studies if the panel loads are mostly axial and there is little bending in the panel (kx = ky = 0).  Since the panel is isotropic with uniform stiffness then the stress will be constant in all panel objects.

Process: From your 'Stress cut-off value' you can back out an allowable panel strain.  The allowable panel strain is defined on the Computed Properties Tab (Strain X, Strain Y).  The corresponding 'Strain Limit' analysis method (limit or ultimate) should be activated on the Failure Tab. 

Currently there is no formal document to explain this process.

Let me know if this is unclear.