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[StrangeBrew]

PS the boards I am building now have wooden cores of a density equivalent to 1 pound eps, and wooden skins which take the  majority of the load, they are sandwich  structures.

PPS I have to add that I was the first as far as I know to use the FLAT BOARD system which some people here have adopted and which is so supposed to be so 'hush hush'  that it isn't to be spoken about. . . . . . . we make flat panels which we called 'SANDWICH BOARDS' from day one in 1995

And they are composites too. . .. . wouldn't get far without epoxy resin glue. Wood and resin composite, sometimes wood resin and cloth.

OK ?

let's remember as a moderator to keep things in moderation.

the lattices you use are elegant roy. I admire the time it takes to put them together. They don't absorb energy in the same way a low density core does under the skin though, nor do they have very little resistance to shear, which is what makes the "composite" construction people often refer to here effective; flexy, light and resilient. It doesn't work as soon as the core resists shear. Then again, I don't know...maybe your lattices aren't actually locked up on the inside; are they? If they aren't, do your boards make noise when they flex and the pieces of wood rub against one another? That'd actually be pretty cool...

I'll back you that if the wood had to bear a significant structural element, that yes; butt joints = bad. For your purposes, this is obviously the case. For the most part, in the construction being described by people here, skin thickness RARELY ranges over 3mm. I have built skins with butt joints out of wood and corecell. With proper attention to the adhesive used to connect the pieces, I have experienced zero failures at the butt joints...and that includes a few kiteboards that routinely get landed on from 15ft up.

oh yeah....I think the ski industry was on to pressing curves in to flat material QUITE a while before you even started growing a beard....

[RoyStewart]

let's remember as a moderator to keep things in moderation.

Quite so, which is why I had to object to Kits comment that my boards are just 'lumps of wood' and not properly engineered.. . . that sort of flaming is unnecessary.

the lattices you use are elegant roy. I admire the time it takes to put them together. They don't absorb energy in the same way a low density core does under the skin though, nor do they have very little resistance to shear, which is what makes the "composite" construction people often refer to here effective; flexy, light and resilient. It doesn't work as soon as the core resists shear. Then again, I don't know...maybe your lattices aren't actually locked up on the inside; are they?

The lattice core is extremely flexible without the skins, and the finished structure is excellent for flexible boards. . . it is optimal for flexible longboards, your eps skinned boards are better for flexible shortboards. both systems work.

Weight is a red herring in this discussion, let's not go there. 

 

I'll back you that if the wood had to bear a significant structural element, that yes; butt joints = bad. For your purposes, this is obviously the case. For the most part, in the construction being described by people here, skin thickness RARELY ranges over 3mm. I have built skins with butt joints out of wood and corecell. With proper attention to the adhesive used to connect the pieces, I have experienced zero failures at the butt joints...and that includes a few kiteboards that routinely get landed on from 15ft up.

Yes I can see that.   At 3mm the core is taking a fair slice of the load but it seems that it's ok provided that the glass fails before the core, that would be the critical issue. 

oh yeah....I think the ski industry was on to pressing curves in to flat material QUITE a while before you even started growing a beard....

Lol I was also pressing curves into flat boards before I grew a beard. . .. ..  main point though is that the structures I make are sandwiches.

.

[Kit Sidwell]

We are getting somewhere!

Sorry if you thought that was a dig at your boards Roy, it sincerely was not!
It was merely a way of making the distinction between a sandwich panel that nears the accepted optimal configuration, and any other structure that resembles a sandwich panel but has the balance between glass and core further away from optimal.

Whether your boards are sandwich constructed or not is not really my point, the point is that you are using the wood to dictate the dynamics of your boards (which I admire), not so much the glass skins!

I finally feel like we are all saying the same thing....
Butt joins are bad in wood panels.
In sandwich cores that are commonly used, they are not relevant.
In sandwich cores that are not optimally balanced they do matter, but in that circumstance the wood is dictating the mechanics of the panel, not the sandwich structure.

By the way, I got a chance to see one of your boards first hand!
Absolutely beautiful!
I have never said otherwise

From the start i've been saying that i'm interested clarify whether butt joins are to be avoided or not in the boards i've been making.
Heres a serious butt join - this board has been surfed hard for 1 1/2 years, no dings, up to head-and half high west coast power - no probs at all.

I realised I have done butt joins after all!

[paul cannon]

they will fail on the joint with enough force
but it all seems pretty random
others have failed in other places
the joint failed in compression whilst jumpin on the board
 it is not flexible like the the rest ofthe structure
it has no give and is brittle
but i dont know really
the rail has to fail first tho
thats where id be avoiding butt joins

[paul cannon]

its true
roys been doing the flat and flex thing for longest
best to keep that chat low key(pm) at this stage tho please roy
time for everything
nows not the time
all credit will given where due roy
ive said that to you

[soulvoid]

Thinking about it some more after talking to Kit I realised that if the glass fails first prior to the butt join failing, and if the butt join is as strong as the wood until it is very close to the point of failure then it's likely that the butt join isn't weakening the structure in that case. Does that make sense ?

If on the other hand the glass and wood failed together or the wood failed first then the butt join would have weakened the structure. Which way it goes will depend upon the relative thicknesses of the glass and the wood.

So the question I have is this: We know that a butt join weakens the wooden panel, but how suddenly does this  occur ? Does the weakness occur very close to the point of failure or is the join weaker at lower loads as well ?  I imagine that the butts fail suddenly it's not as if they stetch much prior to failure.

I think the general idea of the sandwich construction is that the glass should take the majority of the load so you never get to the point where the core fails. So you should ideally never get to the point where the butt joint fail. The more load you put on the core, the more likely the core will fail.

I would expect a butt joint to fail suddenly and not stretch much. However, looking at the resin research modulus it's lower than the modulus zfennel used for the balsa in his equation. I've also found references to balsa having significantly higher modulus than this, variations from 0.3 to 1.1 x10^6psi... Also if you look at the table here=http://en.wikipedia.org/wiki/Young%27s_Modulus, the carbon fibre composite is 7-9 times stiffer than the glass fiber laminate suggested by zfennel (and about 15 times stiffer than pine). So I start to question the accuracy of those modulus used. But if the resin is actually more flexible than the balsa it could possibly create a weak hinge if the load on the core is substantial. So I may be wrong about my assumption that epoxy is stiffer than balsa. But then again why is epoxy considered to be more brittle than balsa? It would be nice if there were a trustworthy source for the material properties.

One thing that would be nice would be to either make a derivative of the function for panel stiffness on c (or t/c) and figure out where the ideal ratio of core to laminate thickness is. Maybe add the weight to the equation too. Or just make a small program that will graph it... :-)

The sandwich handbook also has formulas for stresses internally in the sandwich. Could be interesting for the butt joints...

One other thing, I wonder how much of the load is distributed in the second full board sandwich with Roys construction. If the core allow the skins to flex independently they will carry a substantially higher load each. On the other hand, a very flexible EPS core will have a similar effect.

[paul cannon]

i guess the easiest way it to join a couple of balsa boards and bend them and see where it breaks

[afoaf]

six pages...

[RoyStewart]

  I just got some pictures back today of an 11 year old balsa board, with butt joins. It was surfed a lot when I knew it and it looks like it has been  since. 

 The board is only 44 mm  thick, with 4 layers each of   11mm, it's a solid balsa multi directional laminate, no chambering, glassed with WEST system and a single layer of 6oz glass.
 
 Anyway, it looks like the butt joins are still sweet

[paul cannon]

nice board roy
that looks like it would go good in the rivermouth