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Alternative ply joint tests ...

Here I will be creating various test joints to simulate a side bottom joint in a small boat, and then stress test them in a variety of ways and report the results. For those of you unfamiliar with PL Premium polyurethane adhesives - they are construction adhesives related to 3M 5200 but much less expensive. See our PL Premium page for statistics and testimonial.

jointtest1.jpg (20172 bytes) 9/2/02

The first material I am using is 1/4" exterior luan plywood. (3 ply - 1 thick inner ply and 2 thin outer plies.) Partly because many small home built boats are made from it, and partly due to it being cheap and available. The ply is cut into 8" squares, and the joints are made at a 15 degree angle. (To simulate average small boat flare.)

jointtestbatch.jpg (16905 bytes) I'm testing 6 joints to start with. 
1. Epoxy and Glass inside and out.
2. Chine log and bronze ring nails and PL Prem. & Titebond II (for log/side joint)
3. Same as 2 except all PL Premium glue used on both panels.
4. PL Prem. fillet  inside & PL Prem. and mesh drywall tape outside
5. PL Masonry fillet inside & PL Mas. and mesh drywall tape outside
6. PL Prem. bead inside & PL Mas. and mesh tape outside (Then a PL Mas. fillet is spread over the inner bead after it cures.)

For details on the construction of these panels go to this page.

Stress testing the assemblies:

jointtest19.jpg (14326 bytes) #2 Since the PL Masonry is required to cure for a week before fully set the first stress test is applied to the #2 chine log joint. A test showed me that I could max out my 300lb bathroom scale using my drill press as an arbor press. 

With a weight distribution board cover in wax paper to reduce friction, I centered the chuck above the joint. I figured with my eye on the scale, if I stopped the head travel when the joint failed I'd know both the force and deflection. 

jointtest20.jpg (22601 bytes) I was surprised to find that it only took 80 lbs of pressure to extend the 3" travel of the drill head. You can see the amount of flex in the ply panels. There was no sound or sign of failure.
jointtest21.jpg (16986 bytes) When I released the pressure, the joint assembly rebounded to within 1/8" of its starting position.
jointtest22.jpg (11594 bytes) In the "close the book" configuration, the 3" travel bent the test panel to a 90 degree angle (from the original 105 degrees) at 37 lbs of pressure. I decided to attempt to cause failure of the panel by hand.
jointtest23.jpg (8768 bytes) Placing the assembly on the floor and applying hand pressure, the angle of deflection before failure was not easily guaged, but failure occurred at 50 lbs at an angle smaller than 90 degrees. The part that failed was the chine log to side joint - which is the Titebond II part of the joint. There was a crackling and then a slow opening of the joint as the nail withdrew.
jointtest24.jpg (47039 bytes) However if you look closely you'll notice it was wood fibers that fractured and the ring nails pulled out almost as if they were pulled with a claw hammer. Interesting that the PL part of the joint didn't seem to give at all. 

The fact that the joint was able to flex as much as it did (over 20 degrees) before failure is interesting. We'll see how it compares to other joints.

jointestchine3a.jpg (18898 bytes) #3 The fact that the first test failed at the Titebond II joint led me to clean up the chine log, rotate the "side" piece around and re glue it with PL Premium. (I cheated and tacked it with 3/4" pneumatic brad nails instead of ring nails, but I'm mainly testing the PL joint.)

In the "tent" position this piece flattened out as far as the first piece did without failure.

I'm compensating for the limited travel of the drill head by propping up the scale and pre bending the test piece as I load it into position.

jointestchine3b.jpg (16195 bytes) Using a bevel gauge I know this piece flexed to approximately a 60 degree angle at about 80 lbs of pressure before it failed with a rather loud CRACK.
jointestchine3c.jpg (10176 bytes) The test piece and the bevel gauge in the background shows the amount of flex that occurred before failure. The flex was around 45 degrees before failure.
jointestchine3d.jpg (14689 bytes) What failed was the exterior ply of the plywood and in some places the factory glue joint between the outer and inner plies.

#1 The epoxy glass joint handled the tent test without failure though it emitted crackling sounds. It required greater propping (pre loading) then the others - continued to crackle but did not fail. Interestingly the assembly seemed to be absorbing the downward pressure during the press travel as the scale never read more than slightly above 40 lbs. With the cracking I assume there was a relaxing of tensions in the piece as the scale reading gradually went down.

jointestepox1.jpg (15686 bytes) jointestepox2.jpg (17646 bytes) jointestepox3.jpg (16134 bytes) jointestepox4.jpg (27451 bytes)

Since I could not produce failure on the press, I placed the joint on the floor and stood on it. First in the "tent" position (no failure) then in the "book close" position. With my full weight (180 lbs) the ply ruptured at the edge of the joint.

jointtestPLCinout1.jpg (20049 bytes) #4 #4 is PL Premium fillet on the inside and PL Premium and mesh tape outside.

I did the usual spring pre load - which flexed the joint panel slightly and applied 20 lbs of pressure.

jointtestPLCinout2.jpg (19433 bytes) The joint failed so quickly I was unable to see exactly how much pressure had been applied but it was around 30 lbs.
jointtestPLCinout3.jpg (19015 bytes) The inner PL Premium fillet failed - the wood ruptured in places and the glue/wood line in others.
jointtestPLCinout4.jpg (14455 bytes) I went ahead and tried the "book close" position and it also failed quickly with about 30 lbs of pressure.

jointestPLMinout1.jpg (19269 bytes) #5 #5 This joint has PL Masonry fillet on the inside and PL Masonry and mesh tape on the outside. The preload pressure on the "tent" test was minimal... 3 lbs.
jointestPLMinout2.jpg (15407 bytes) The joint was so flexible it required little to distort it. About 8 lbs here. But other than a slight ridge  along the outside of the joint there was no sign of failure.
jointestPLMinout3.jpg (18057 bytes) In the "book close" test it displayed pretty amazing flexibility. 
Still, only about 10 lbs.
jointestPLMinout4.jpg (15053 bytes) It also required 10 lbs of pressure to fully flatten the joint.
jointestPLMinout5.jpg (12675 bytes) Still it would spring right back.
jointestPLMinout6.jpg (15013 bytes) This is bending the joint at a backwards angle, like opening a book beyond the flat point. Still no failure.
jointestPLMinout7.jpg (12678 bytes) After all this, this shows it's deviation from its starting angle. 
jointestPLMinout8.jpg (18971 bytes) The ridge along the apex of the joint shows some fibers from the mesh coming through.


jointestPLMnbead1.jpg (15723 bytes) #6 #6 This joint is the same as the above except it has a bead of PL Premium on the inside, covered by a fillet of PL Masonry.
jointestPLMnbead2.jpg (31106 bytes) This joint behaved similarly to joint #5 except that in the "tent" test the pressure went up to 20 lbs, and then slowly returned to 8 lbs. My assumption is that the PL premium held rigidly at first requiring more pressure to flex, then it failed and the PL Masonry took over. 

In the position of "opening a book beyond the flat point" it seems that inner bead of PL Premium caused the PL Masonry to stretch over it.

After considerable flexing the internal fillet ruptured revealing the mesh tape fibers had been pulled into the interior of the joint, but had not broken as they did in joint #4.  The outside of the joint remained sound, and apparently even water tight. It continues to hold tenaciously to the plywood and function as a hinge.

Someone commented that this glue might be useful for forming hinged panels of a folding boat. 

Since these types of joints are very difficult to get to fail in these flex test, I will see if I can devise a pulling system to measure shear resistance.


I sent out this report to a bunch of boat builders and designers on the web with a plea for help interpreting these results.

If you only consider how much force is required to first distort and then rupture a joint, epoxy is the clear winner with the PL Premium and chine log the runner up. The worst performer was the PL Premium fillet inside and PL Premium and mesh tape outside. It failed with very little effort. 

    The PL Masonry joints are harder to interpret because they were so flexible they were near impossible to get to rupture, requiring repeated extreme flexing, as in {"fully closing a book" or "opening a book past flat." The tricky part is that the forces necessary to flex these joints was low - in the 10 lb range, as opposed to the epoxy which required 180 lbs, and chine logs 80 lbs. However both epoxy and chine log joints held so well that the plywood fibers ruptured. This reflects all modern glue advertisements of "the glue joint is stronger than the wood!" Which seems like a good thing.
    However, the PL Masonry flexed so much it absorbed all the force and didn't pass it on to the wood, and for the longest time, neither the glue joint nor the wood failed, returning repeatedly to its pre stressed state. After repeated extreme flexing I was able to get the glue to fail, but only by asking it to bend in ways that it could never bend while incorporated into a boat hull.
    I imagine there are up sides and down sides to this. These joints could potentially withstand great stress and stay water tight, and not pass the stress on to the wood that is joined.  Someone mentioned they might be candidates for creating hinges on a folding boat. Anyone who has ridden in one of those polymer folding "port a boats" knows how strange it feels to have a hull flex under you. It takes a little getting used to. Yet it doesn't necessarily mean its not strong. On the Portaboat web site they show photos of loading 600 lbs of concrete in their folding boat and dropping it from 21 ft.  http://www.porta-bote.com/   I do not mean to imply that a boat made with PL joints will feel as flexible as foldboats. After all a fold boat is made from sheets of plastic not plywood.

James Wharram imitates the Polynesians by lashing his hulls to the amas to allow flexing at sea.

    I'm needing help to make sense of this data. As usual, all ideas, opinions and theories are welcome.... 

Feel free to email me your in put.

Back to Methods and Materials When all panels have been tested I'll create some kind of chart or table to display results.

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