bonding stainless steel to fiberglass

I'm looking for some advice on how to properly design an adhesive connection between a stainless plate and fiberglass.

specifically, I want to attach a clevis bracket to the underside of a catamaran bridgedeck. I have no way to get access to through-bolt, so I'm considering epoxy or such. The stainless clevis will be welded to a plate which I'd like to bond to the underside of the bridgedeck (which is glass reinforced polyester). The clevis bracket will attach to one end of a hydraulic cylinder to tilt a drive leg.

I'm wondering if anyone could suggest a suitable epoxy, as well as maximum shear loading and tension loading, and recommended adhesive layer thickness? Any practical concerns about differential thermal expansion in a stainless to fiberglass connection?

Any practical advice on making a long-term reliable connection would be appreciated.

Thanks

Doug M

 

Reliance on epoxy could tend to delamination with high enough load. You might like to explore options for "blind" fastening, but if the load is going to be high, you really need a backing plate. Maybe you can cut a hole for a circular, flush screw-in port on the deck, and get inside that way. Another way might be to epoxy a much bigger plate on first, say thick ply your plate is already attached to with counter-sunk through fastenings.

 

The solution might be Sikaflex. They have a brochure in the internet with instructions and product characteristics for all kinds of marine applications.

 

The key will likely be surface area under a bearing plate the clevis bracket. I'd weld as big a plate as necessary to the bracket, to provide enough surface area to insure it stays put. How much load are you estimating this will need to tolerate?

 

Plexus adhesives?

 

the absolutely maximum working load will be around 400 lbs, but i want to leave lots of safety factor for shock loads. If I use a 4" x 10" plate the load is spread over 40 sq.in., so there is 10 psi shear, and most epoxies can handle a couple thousand psi max. So i'm thinking this should be extremely conservative, but that's just theory so I'd really like to hear from anyone who has experience with such joints.

CDK, sikaflex properties look great but my concern is that it is one part and would only cure around the edges.

 

If you want the strongest structural adhesive you can get for bonding dissimilar materials you should look at the 2-part Plexus MA530 methacrylate product.

 

3M Aerospace Sealant AC-130-2 Surface Pre-Treatment

Originally developed by Boeing as Boegel for aircraft field repairs to prep metals for epoxy bonding.

 

3M has a marine department that I have called over the years and found very helpful. I suggest you call them and present your plan.

 

you'd need to know more about the FRP laminate it is to be bonded to, mainly the thickness, and proximity of any framing.

 

If it has a core of honeycomb or foam you could use Aircraft style inserts. Here is a website that has the inserts and instructions with a video, it can explain it better than I can. Where it shows a "L" shaped tool to remove the core around the insert, I always used an allen wrench in a drill.
http://www.clipnuts.com/products/potted-in-inserts.html