Fiberglass overlaps , why so large?

Fiberglass overlaps. How large do they really need to be.

Over the years whenever I have done a build or a repair I have always done the overlap at around 50 to 75mm in width, most glass I use now has a coloured line 50mm in from edge.

Something that I have wondered about for a while is the Z Joint used on Duflex panels.



Apparently ATL say it has 90% strength because it is around a 15:1 scarf.

That being the case, why have I been doing such large overlaps all this time?

 

Do you limp wjen you walk ??

Some one has been pulling you leg !!
It might as well be a butt join because would be as stong as the grain in the wood thats all sorry i am a non believer !!
Mind you i reckon the world is flat so !!

 

Duflex panels are cored panels as you know, with various laminate schedules applied. These panels can be butted together, assuming you'll apply sufficient seam reinforcement to serve as the "bridge". In other words, it really doesn't matter if you scarf, lap or butt the panels, so long as your laminate is absorbing, dispersing and transmitting the loads.

 

The duflex literature states there is a weakening of the panel at the z-join similar to a weld between aluminum plates... I can't remember the figures off the top of my head but 17% reduction seems familiar, you'd have to check the data sheet... The further you overlap then the stronger it gets I would imagine, a good reason to overlap 50mm minimum IMHO...

 

sabahcat; as you say the 50mm lap seems to be the standard but what I can recall at least one or two classification codes that I have read only require 25mm overlap.
"petereng" from BD is the man to answer this question, he mentioned that as little as 3mm overlap is required to transfer the loads, all depends on your resin properties.

tunnels; ATL is a reputable company and has the data to back up their claims. The real world also supports this, there are many kit boats sailing around for many years that were build with just the glue in the Z joint no tapes at all.

 

Sabahcat, In the Aus standard(roughly transcribed) for lay up guide(my copy from 93) 5.3.2.lay up guide The edges of unidirectional reinforcements which are parralell to the fibres may be butted against each other, and the ends of the fibres shall be overlapped by not less than 100mm. All other adjacent pieces of reinforcement in a layer shall be overlapped by at least 30mm per 600gram/m2 of reinforcement. Butts & overlaps between reinforcement in each layer shall be offset with respect to the previous layer by not less than 150mm.

In regards to "tabbing" of other structural members the(from 3.7.4) "leg length shall be no less than 50mm and shall be tapered out at not less than 20mm per 600gram mass of glass reinforcement or 1 in 20 on thickness."

I reckon these are are a good default strategy but its kinda a to or in excess application as overlaps on a curved surface will "run out" & taper ending up bigger at times, some triax etc has the zero fibers cut back from the edges too for less "lump" in the overlap, kinda in line with the standard on uni fibers. Often hull deck joints & hull center line joins go well in excess of these & no place for skimping, it only takes one d head to contaminate a join with a greasy or damp hand to compromise the integrity so some extra overlap can help although much better to not happen

All the best from Jeff.

 

One thing I can relate from my designer is that 50mm (2in) laps are the sweet spot.

Another thing is that the larger the lap (75mm, 100mm), the *weaker* the end laminate. I know this is counter intuitive, but the results are from some structural engineering seminar my designer quotes in my boat building literature.

 

Two pieces of glass material laying one on top of the other can only be as strong as the resin system you are using ,50mm is the minumum overlap as a general rule of thumb be it polyester or vinylester resin BUT it needs to be covered with more glass , then it gains a lot of strength . For carbon fibre overlaps for girders the minimum we always used was 1 metre over lap again always covered with a 600 gram double bias glass !

 

50mm is the *maximum* overlap, according to new stress analysis studies that I can dig up and quote, if you need me to. This is using epoxy.

 

No, check with ATL
This is how it is done using their Zpress

Many on this forum have said you can simply butt join planks in a strip planked vessels as well, I have always done 8:1 scarfs, apparently wasting countless days of labour not to mention material.
And foam?
How strong is the grain in that, butt join is the norm
and ply gets the same treatment by some now as well.

Here is some educational material for the Non Believers
http://www.atlcomposites.com.au/atl_composites/products/composite_panels/duflex

 

That would be interesting, especially reference the application that overlap applies to, I can sense some logic there possibly to hull skins or such expected to experience deflections or such where excessive overlaps might cause "stiff spots" that might prove unfriendly to a structure, hopefully most composite boats have large factors of safety.
All the best.

 

Hey Sabahcat, mebee we're stuck in the past & need to catch up on some of this stuff.................

 

Scarfing is a waste of time on foam, as the compression is the only thing it does well. If the fibreglass wont hold the other components, the whole thing will fail.

The same goes for the majority of strip planking jobs, as you observed.
If you scarfed the planks ends - did you scarf the edges as well ? Of course not - you did Butt Joins! And they were never reached more than 50% of the timbers strength.

In the old days, they used to nail the strips to each other too, now we just use expanding foam.

Par takes exception at this statement, but one NA told me he just treats the strips as core material, because he cant control the quality of the glue job, and in many cases the timber.

Lets face it, for a little extra in the FG specs, you can ignore the timber strips as doing anything else but core material jobs.

Ply is a bit different - its a quality controlled multidirectional product that adds a lot of strength to the hull. You can save serious FG costs by using it. But likewise, it takes very little FG and epoxy to make an end to end join as strong as the rest of the sheet.

 

I don't like foaming polyurethane adhesives, though tests show it's stronger then the (usual suspects) substrate shear strength, so it works.

Again, you can butt cores or strips (if it's used as a core), the laminate carries the loads, while the core keeps the skins separated.

Look at the overlaps this way; think of them as a lapped scarf. The load bearer is the skin, so the scarf ratio needs only apply to it, not the core.

So if you have a 1/4" (6 mm) skin and it's overlapped 2" (51 mm), you have an 8:1 step or lapped scarf on that skin. If the laminate thickness is less then 1/4", then your scarf ratio is taller. Naturally, on thicker laminates, it would be wise to consider a taller ratio, though this may be dependent on panel loads.

 

I always use pencil grain timber for the core
I always treat the fore and aft grain of the timber core as strength, uni's if you like, in this direction, thats why I scarf.
Treating the timber core this way eliminates the weight of glass uni's and the resin needed for their wettout .
The uni or double bias that runs across the grain of the timber holds it together, which is why it is not scarfed in that direction

This makes for a lighter vessel.

More money on glass
More money on resin
More added weight for what benefit?