I usually glass my boards with 3x6oz. + 2x6oz.
Question- if I glass with 4x4oz. + 4x4oz. or 5x4oz. + 3x4oz. (in order to be as strong as my usual 6oz. schedule) will the board be strong or brittle?
I’ve glassed a Marko 2.5# with 3x4oz on both sides of the board. It felt a bit stiff, but quite hard to dent. Might of been the 2.5# that made it feel a bit stiff. Either way, 3x4oz feels stronger than 2x6oz if that helps.
Thanks Deanbo. That’s interesting that the 3x4oz. feels stronger than the 2x6oz. But with a 4x4oz. or a 5x4oz. deck will the board be strong or brittle? I know there are people who can help me. Please do! Oh, I’m using 1# EPS.
More glass means more strength. more resin means more weight. BIG difference between strength and brittleness. As a rule I’d go for maximum amount of glass and least resin. Therefore if using heavier cloth means fewer layers (and less resin) go for the heavy cloth. Trouble is that then you have weave filling issues and may have to resort to more filler coats, more weight and the result is no better. No simple answer. Sorry, the wine’s kicking in, I’ve just had a great week off with some amazing waves in Cornwall and Pembrokeshire (home) so things might be a bit blurrrred!
So then, what causes a board to be brittle?
If you were using polyester, then too much catalyst would do it. Otherwise are you sure you’re not confusing brittleness with stiffness?
I’m, not really sure. I was reading some posts that said sometimes new builders confuse strength with brittleness. They’ll load on the glass thinking it will be strong but in reality it is brittle. Maybe I’m just confused. So it shouldn’t be brittle with either schedule, if I’m using epoxy?
Thanks
If you are using epoxy, the hardness is already determined by the manufacturer. RR epoxy has some different resins that all have different hardnesses, depending on the impact resistance you want. The ones with higher impact resistance have more “give” or flexibility.
The previous post regarding the glass / resin proportions was correct. To illustrate with an exageration: If you glassed a board with a thick layer of epoxy resin and no fiberglass cloth, it would be fairly weak when put under flex. You’d probably get some cracks and breakage just before the whole thing went kerflooey. But if you glassed with four layers of 4 oz. fiberglass, and just enough resin to get the job done, the board would be quite strong. It could flex and not break because of fiberglass fabric.
I don’t think brittleness comes into play here, only strength. Just use the proper proportion of glass to resin. Three layers of 4 oz. is stronger that two layers of 6 oz.
Doug
Brittleness is a measure of how far something can deflect (stretch or bend) without failing. Polyester resin is more brittle than epoxy. If you take a sample of each without glass and cure it, the epoxy will take more abuse than the polyester. This is because it deflects to adsorb the bending/stretching/impact energy. The polyester will deflect less but will break earlier.
This explains why epoxy boards are stronger. With these composites the idea is that the resin keeps the fibres together, keeps the water out and SPREADS THE LOADS TO THE FIBRES which are the strongest component. Both resins stretch a bit and the load is then taken by the fibres BUT polyester will stretch least before it cracks (because it’s brittle). Thus the resin begins to fail long before the glass has taken the maximum load it can.
Epoxy stretches more before it cracks (less brittle, or tougher) and therefore more of the load is taken by the fibres which is what they are there for.
Kevlar is stretchier than glass and so it can deflect more without breaking which is why it is so good at impact resistance, it also happens to be stronger.
Carbon does not deflect much even under high loads but it fails suddenly, it is very brittle.
Hope this makes sense, it does to me but I know what I mean!
Quote:
If you are using epoxy, the hardness is already determined by the manufacturer. RR epoxy has some different resins that all have different hardnesses, depending on the impact resistance you want. The ones with higher impact resistance have more “give” or flexibility.
The previous post regarding the glass / resin proportions was correct. To illustrate with an exageration: If you glassed a board with a thick layer of epoxy resin and no fiberglass cloth, it would be fairly weak when put under flex. You’d probably get some cracks and breakage just before the whole thing went kerflooey. But if you glassed with four layers of 4 oz. fiberglass, and just enough resin to get the job done, the board would be quite strong. It could flex and not break because of fiberglass fabric.
I don’t think brittleness comes into play here, only strength. Just use the proper proportion of glass to resin. Three layers of 4 oz. is stronger that two layers of 6 oz.
Doug
Along the lines of what Doug is saying, a thick layer of epoxy not properly laminated, will likely be brittle. That is, it will deflect and break because it has an inadequate amount of reinforcement (resin to cloth ratio-wise). Further, pooled epoxy tends to kick hotter than epoxy that is spread out evenly throughout the fiberglass cloth (you know, just right!). It’s been my experience that resins that set hot tend to be more brittle, sometimes cracking in the pot, the proverbial dreaded smoking batch.
Makes perfect sense. Thanks