Laminations

[Pat B]

Jax, I think it is both. By swapping ends with the pieces of the wood the grain crisscrosses each other making that stronger than the grain running the same and the glue up adds more strength. 

[jaxenro]

I would agree and I wonder if this is where a tri-lam would be stronger with three different laminations? Plus the fact that by using dissimilar woods with different grain patterns you are adding another factor

[Dances with squirrels]

Simply put, yes, a laminated piece is stronger... but we should be careful how we word it. "A sum of its parts" is likely taken to mean something different than "same wood, same dimensions". The former puts me in mind of say, a trilam's three lams laid on top one another prior to glue up, which would bend MUCH easier than than they will after being glued together, and the latter puts me in mind of a board, which would show a difference, but less of one.

As far as building a recurve(or reflex) by sawing a kerf into the end of the limb, I've done it on backed bows and selfbows, and I really like the technique and results. I generally cut the kerf  10-12" deep, and fill the void with a parallel or tapered lam, depending on the limb action I want, but even the parallel lam helps strengthen its area relative to the remaining limb and it can be tillered to work while this now new 'trilam section' maintains its shape due to the effects of the two glue joints.

[Pat B]

I've built selfbows, backed bows and 1 tri-lams, the tri-lam wasn't a very good bow.  For me, one reason to build a tri-lam is to be able to build it with an extreme profile. Easier to made radical bends with thinner layers and the glue up holds it all together in it's profile.
I prefer a selfbows but I can get better performance from a backed bow but that's not always my goal. I'm in the process of building myself a straight limb osage selfbow. A simple workhorse.
If your wood supply is limited, a tri-lam would be practical but for me, it's not worth the trouble. 

[jaxenro]

I think to test if the laminations provide a performance or strength benefit it would probably be necessary to construct several bows as close in design as possible both laminated and self for comparison but that isn't easy to do

[Badger]

  I have several thousand bows under my belt and I think that would be a sufficient number to say laminated is stiffer. Oddly I have very few laminated straight bows that I have done. A laminated bow is thinner for the same profile and the same draw weight in most cases. Some woods tend to come much closer without being laminated such as black locust and ipe, sometimes osage. If you take the same board and split it in half and self back one of two bows it would tell you all you need to know. For the test I would do it as a straight bow.

  Make both bows a pyramid of identical length, width and thickness and see if the draw weight is the same.

[willie]

Answer his question if the rest of us are wrong.

Ok, PD I will try to clarify, even if you are talking about something different than the OP.

If "his question" was....

So are three stronger than two?

Then yes, as there are benefits to minimizing defects in the materiel, which is what we do when we swap ends, make multiple lams or build glulams and I-joists. Making a limb with minimized defects  bend further than a comparable limb with the original sized defects, might be called stronger in a sense. More bend stores more energy.

if "his question" was..... 

Is a laminated piece of wood stiffer than a solid piece of the same dimensions?

then, No. The stiffness would not change unless the glue itself had some extremely superior properties.
There are of course, those claims made for Perry Reflex, but I do not think the OP was asking about prestressing the pieces before glue-up.

 

[gfugal]

Thats a good distinction Willie: stiffer vs stronger. I have a hard time believeing laminations will somehow magically change the properties of the wood. Maybe the glue's properties are superior, but as far as "stiffness" is concerned i doubt it. I believe i was reading that epoxy has a MOE of around 4 Gpa which is well bellow the majority of most woods. However, there are a couple of things i see going on that may account for improved strength.

1) laminations make it so defects aren't isolated in one area. The bow is only as strong as its weakest link, and if there is a defect it's most likely a defect that effects the sourounding material aswell. however if you were to add an independent material or just filp the same material then the defect is separated and more evenly dispersed. Likewise, diferent grain orientation can provide strength and stability to other plains of stress. For example i heard once that laminating siyahs with alternating grains may increase its stiffness against lateral movement

2) The glue in laminations are usually stronger than the natural adhesives in the wood its self. Therefore i imagine there is some benefit too the integrity in that regard too. Plus any perry reflex will result in more stored energy then if left natural. It's possible even a straight proffile without any.
 induced reflex will store more energy than the natural profile due to better adhesion preventing unseen damage or something. Who knows.

3) using two materials with different strengths such as one material good in tension with another thats good at compression will result in a synergistic product that truely is better than either materials could have been on their own.

Its intersting to note Badger's findings that laminates tend to be thinner than their self bow counterparts under similar designs and draw weights. This brings up some queations of my own. Were these laminates of a single material or differing materials? If differing then i could see how something like a synergystic relationship may account for its apparent greater stiffness. However, If they were the same material what is happening here? Is it just because it increases the strength, thus a more stressfull thinner design will more likely survive in a laminant vs a self bow. That way instinces of stiffer specimens of that species will more likely survie. Thus increasing the number of thinner bows for comparison? This is all interesting to consider.

[PatM]

then, No. The stiffness would not change unless the glue itself had some extremely superior properties.
There are of course, those claims made for Perry Reflex, but I do not know if it hold true if the limb is not reflexed during glueup.

  You seem to say no and you're not sure at the same time here.

[willie]

and you're not sure at the same time here.

thanks, for pointing that out Pat, went back and fixed that.

[PEARL DRUMS]

I'm only guessing the shear strength of the glue line is stronger than the wood and adds a stiffness wood alone cant achieve, thus making a thinner bow. But what do I know? I'm just a simple high school graduate (barely) that works in manufacturing. 

[Knoll]

heh, buddy . . . don't denigrate us manufacturing guys!   

I believe gfugal's points (1) and (2) and Chris' post are correct.

[PatM]

In a beam you also have the option of having the laminations vertical.

[bradsmith2010]

 

[jaxenro]

I could see why lamination's are stiffer (measured by the force it takes to bend two identically sized pieces a set amount) due to the different grain orientation of the different pieces. Assuming you used hickory for all the lamination's then one piece would bend a certain way depending on the grain orientation and the varying density that causes in the wood. Isn't this why one limb bends differently from the other and you can't just machine them both exactly the same and it is magically in tiller? Assuming the glue line is at least as strong as the wood being glued, and it is usually stronger, than two pieces with grain across each other would be stiffer than one piece as theoretically any weakness in one piece that allows it to bend easier would be offset by the other piece being glued to it.

I could also see why lamination's are stronger (measured by the force it takes to break two identically sized pieces) due to the different grain orientation of the different pieces. Wood is an organically created product and contains hard and soft areas within it. Even two boards cut from the same tree can contain slightly different properties when it comes to bending and breaking. What the lamination can do is strengthen a weak area by having a crosswise grain pattern on it

That said it doesn't translate that a laminated or backed bow shoots better or is better. The stiffness may work against it's performance. And an incorrect tiller or profile on the finest hickory backed yew bow will probably not shoot as good as a self bow made from meaner wood that is correctly tillered and profiled

I think that is why wooden bows made from minimal or no lamination's are so fascinating is so much depends on the skill of the bowyer to bring out the qualities in each piece of wood. It is why I love looking at character bows so much, especially yew, to see the skill in working with a challenging piece of wood