Embedding steel plate in carbon fibre laminate

[RPM7]

Hi guys, i have an application where i am bolting a structure to a CF laminate.
I need to use a plate (drilled & tapped); embedded fasteners won't work for my application.
I have other features moulded on the rear face, so ideally i would rather not mount the plate on that face.
Are there any issues with embedding the plate within the laminate at the mid plane?
Is it just inter-laminar shear that needs to be considered?
NB: the plate will be tapered to allow for drop offs.

Thanks

 

[Compositepro]

There are CTE mismatch issues, which depend very much on the size of the parts. Adequate taper of the edges of the plate will minimize peeling stresses and stress concentration due to sudden stiffness change. A carbon ply that wraps around the edges of the plate will help to stop delamination of the carbon plies at this point. While what you propose maybe doable it is, in general, not a robust design.

 

[SWComposites]

In addition to CTE, you need to sort out,
- surface prep of the plate for bonding
- how you will tap the plate once it is cured into the laminste
- how you will get the load out of the plate into the laminate with initiating a disbond or delam

 

[RPM7]

Thanks for the replies guys, really appreciate it.
I was actually a little worried about delam but forgot about CTE.
I think this will be a no go for me; there are too many uncertainties involved.
I'm going to have to look at an alternative solution.

@Compositespro, quick question with regards to tapering edges for the drop-off.
In these instances, I would assume that it wouldn't be appropriate to use a flat plate with machined foam for the tapered edges in lieu of machining the plate itself?

Thanks

 

[Compositepro]

Machined foam would increase the sudden change in stiffness from steel to foam, which increases the stress concentration. The purpose of tapering is to gradually change from the stiffness of steel to that of composite, and to increase the surface area through which these forces flow. When tapering for composite repair patches, where the stiffnesses match, the taper is 20:1 to 50:1.

You may be interested in looking at how stud plates for composites are designed.

http://www.npfasteners.com/composites/master-plate-stud-pr.htm

The plates are made from relatively thin steel and have holes that increase bonding area and reduce the stiffness of the steel. The adhesive is also much lower modulus than the matrix resin, which allows for much higher strain before cracking and thus the stresses can be spread over a larger bond area. These would be better than an embedded plate.