Engineering & Analysis Methods > Material Strength Composites: Ply Approach

Laminate analysis with effective laminates

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**Tom Dragone**:

I'm getting negative strength margins in the design of a composite wing spar cap and would like to understand the failure mechanism better. The spar cap component is modeled as a 2-Stack Unstiffened component: the top stack has 4-80 unidirectional plies (representing the cap) while the bottom stack has 8-12 plies of fabric with close to quasi-isotropic layup (representing skin plies). Both are represented as effective laminates. The failure mode is Composite Strength, Tsai-Wu Interaction in the Top Stack. The optimization has moved to the maximum thickness, even though hand calculations indicate that only a moderate thickness should be required.

Is there an easy way to perform a laminate analysis on this effective laminate to the design-to loads for this component in order to determine ply-by-ply stresses, strains and margins and figure out why it is failing? Would it be possible if there was a discrete laminate for this configuration? If so, how can I generate a discrete laminate for this one component?

**James**:

Hi Tom,

The effective laminate analysis approach is described in this help topic.

http://hypersizer.com/help/#Materials/EffLam/el-using_struct_analysis.php%3FTocPath%3DFeatures%7CMaterials%7CEffective%2520Laminates%7CUsing%2520Effective%2520Laminates%7C_____1

For a two stack, each skin (top and bottom) will be idealized as a pseudo three stack panel. The middle stack makes up 99.99% of the thickness and uses the smeared EA of the laminate to get the correct Aij and approximate Dij terms for the effective laminate. Then the IML and OML strains (which include the bending strain) are localized down into each ply direction. Since the true ply sequence is unknown during the effective analysis, very thin plies are assumed at the IML/OML so the IML/OML laminate strains are analyzed in each ply direction.

To understand why HyperSizer is returning negative margins of safety I would start by checking the reference plane defined for the component. Where is HyperSizer applying the forces? Look at the "stresses" tab, are there high local Mx, My and Mxy bending moments in each stack? If so, these are likely causing high strain at the top and bottom of the stack. I would also check the ply allowables defined in your parent orthogropic materials, perhaps a value was entered incorrectly.

To see the laminate stresses and strains that are used in the analysis, check the analysis details in the "Margin Analysis Detail" Report. See: http://hypersizer.com/help/#ReportMS/report-msdetail-create.php%3FTocPath%3DStress%2520Reports%7C_____4

The controlling pseudo ply number and controlling ply angle will be listed with the corresponding stresses and strains in the fiber direction along with the ply allowables used to generate the strength margin of safety.

I hope this is helpful.

-James

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