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Laminate Allowables - Orthotropic Form


The laminate allowables are entered into the database and displayed as strains and the %age of 45 plies or AML.

Could we include a button that flips the graphical display between stress and strain allowables for LAMINATES. The stress engineers would like to see both stress and strain allowables. This would be a useful feature and consistent with the lamina allowables where both stress and strain allowables are displayed

Also, the software should show if we are using the average modulus Ed or tension and compression modulus Et and Ec

I'm not sure how much sense it would make to have a stress allowable for a laminate. 

First, all you could get would be an average stress allowable over the entire face of the laminate.  It would have no physical meaning.  It would be simply the unit force divided by the thickness, but there would be no location in that laminate where that stress would be valid and it could not be correlated to a true material allowable stress.

Second, this average stress allowable would be different depending on what laminate you are looking at, so if looking at a quasi-iso laminate, for a given strain allowable, the stress allowable would be much different than if looking at a laminate with many zeros.  It seems like in order to establish this display, you would have to pick a laminate at which you want to see the stress allowable.

The more appropriate thing would be to add laminate-based failure modes to the Laminate Analysis form, which we plan to do, and then you can pull up a particular laminate and see the engineering stress allowable for that laminate.

If we observe one of our basic equations for crippling, we see that the design driver is the allowable Fcu (F11c). As you state for a ply basis the ply stress is dependent on the fiber orientation. But for crippling we have calculate the allowable LAMINATE stress of the components in our stiffener section. Perhaps to be politically correct we could call this stress an effective laminate stress. This is not a bogus value, its a fundamental parameter for a fundamental analysis.

Some of our vendors are working with Laminate allowables. The allowables are strain based. If we plot allowable strains v %45 plies the trend is a lower strain allowable with less 45 plies (more zero's), but the laminate with less 45 plies will carry more load and more stress. This is not evident for people not too familiar with composites. A new user will think the laminate with a higher strain allowable will carry more load. The allowable effective laminate stress clarifies this anomaly. For a crippling analysis the user is very interested in the value of Fcu and how this value varies with the percentage of 45 deg plies.

The crippling failure mode is our primary design driver on aerospace structures. It would be useful to see how the "Effective stress allowable" for a laminate changes with the percentage of 45 deg plies. Also, note that we currently quote this laminate stress allowable in our crippling stress reports.

We can use the laminate form to calculate the effective stress allowable of a laminate, but this form is based on user lamina properties. If a customer enter strain allowables on the orthotropic laminate tab, there is a potential for an inconsistency between the entry on  the lamina and laminate form. Ideally these properties should be separated to avoid potential confusion.

Two methods are provided to enter allowables, Lamina and Laminate. A small number of our  customers require both forms to be completed.


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