GRP/Fibreglass info needed

[irving2008]

I know there are a few here who know something about fibreglass/GRP. What I need help with is how to embed a mounting into a GRP sheet and work out how much load it can take.

[EddyCurrent]

I don't know the answer but I imagine the strength would be increased if you also put rivets or bolts through the whole thickness each side of the lug, if that's possible in your application.

[irving2008]

Can't as need to preserve water tightness of lower/inner bit so can't penetrate it. If I could and know it was 100% guaranteed watertight I'd go that route and it'd be trivial. I need to figure out what size (area) of ali plate I need for a given force. So far haven't come up with a foolproof way to do this.

[EddyCurrent]

What about making some test pieces and applying weights to the mounting ?

[irving2008]

in an ideal world, but no time nor facilities.... was trying to model it with FEA but cant seem to get a meaningful answer...

[mekanik]

What is the substrate and the application ?
If substrate is fairly substantial the plate could be drilled and countersunk to take woodscrews, edges of plate need a good chamfer possibly 30 deg so that you can get a decent layup when bonding the cover layers of glass(will prevent air pockets you might get with 90 deg interface). bond the plate to existing fiberglass using your epoxy and screw it to substrate, then apply your epoxy and glass to cover the assembly.

[Neale]

This is the kind of thing that boat-builders are doing all the time, but I can't think of case where I've ever seen a plate embedded in laminate. For one thing, polyester resin isn't a very good adhesive so it won't bond to the plate - I suppose that you could put holes in so that the laminate is bonded through the plate, or use an epoxy laminate? Fixings to take rigging loads on a GRP boat would generally be through-bolted and use a sealant like Sikaflex which would give a good working life and if properly made, guaranteed waterproof.

[GEOFFREY]

There are purpose made fixings for this job called "big foot". They are available with male or female (stud or tube nut) protrusions and I think stainless would be a better option. The flange plate on these fixings is usually perforated to assist adhesion. I do not know how to calculate pull out strength, but would suggest the better way might be to rebate the the outer skin for the footplate. drill through for the stud bond the fixing in from the outside and then glass over the rebated area. The second skin of fibreglass applied to a precured skin does not adhere as well as a single skin laid up in several layers. If the studs you use are M6, I may be able to give you a few in stainless steel. If you google Bigfoot fixings they may be able to give you some technical advice. Good luck.G.

[charlieuk]

whats the application? if its taking any real load you are going to need a fair size plate and a good thick laminate to do it that way, i think you would be better off coming from behind if you can and then making water tight which should not be too hard and will give you far more strength.

[GEOFFREY]

Sorry Irving, "bigfoot" is just my nickname for these fixings, they are called "big heads"!!! G.