Hardboard tensile glue test

[JNCarr]

In the intarsia wall project, I will be gluing a lot of 3/16 hardboard panels to either Baltic birch or to other layers of hardboard (which in turn will be glued to BB ply).
There are two distinctly different surfaces on hardboard - a rough and smooth surface.
In PMing with a very knowledgeable member of the forum, he concurred with me that no sanding of the smooth surface should be needed, but I wanted to be absolutely sure for a project of this magnitude.

A tensile stress tester was fashioned as shown. Four samples were prepared each having 1 square inch of glue surface. The adhesive was Titebond 3 cured for 24 hours at 73 degrees and 55% humidity. Clamping pressure for the samples is about what I expect to achieve on the project, about 6 psi.
The samples were: 1) rough to rough, 2) rough to smooth, 3) rough to sanded smooth and 4) sanded rough to smooth. The opposing sides were untreated glued to Baltic birch.
The samples were secured in the tester and slowly increasing pressure was applied to the lever arm.
All failures occurred at the hardboard to hardboard interface and appeared to be cohesive, not adhesive failure. However, I suspect that there to be an interplay at the bond layer, since no hardboard near Baltic failures occurred.
In any event the average failure was 143 psi, the lowest being rough to smooth sanded. While 4 samples is not enough to qualify as a full blown statistical analysis, I'm confident that the results meet my needs for the wall -- no sanding needed.

 

[chris_goris]

Any modern adhesive designed for bonding wood will cause wood or substrate failure long before failing itself has been my experience. I think a larger issue you may have without knowing any specifics about your project would be weight versus glued surface area. Looks like you have a lot of weight going on there. Walls are never flat and that could cause voids possibly?. Humidity changes in solid wood versus an engineered substrate like hardboard and BB?. Again, I dont knwo how big this is or what its made up of, just my $.02 FWIW

 

[Wilsoncb]

The pieces will mostly be in sheer so I suspect it would take an even higher force in that direction.

 

[Henry W]

The pieces will mostly be in sheer so I suspect it would take an even higher force in that direction.

Agree - a shear test is the more applicable, but of course more difficult to construct.

This test and test data presented is considerably more data driven and superior to my typical "Well, I have glued many pieces and never had a failure."
I have also never done intarsia or a project on the scale that Joe is undertaking.

 

[JNCarr]

Any modern adhesive designed for bonding wood will cause wood or substrate failure long before failing itself has been my experience. I think a larger issue you may have without knowing any specifics about your project would be weight versus glued surface area. Looks like you have a lot of weight going on there. Walls are never flat and that could cause voids possibly?. Humidity changes in solid wood versus an engineered substrate like hardboard and BB?. Again, I dont knwo how big this is or what its made up of, just my $.02 FWIW

Yes, my experience also - I just wanted to be sure the smooth side would have a similar bond.
Regarding wall planarity and pockets during gluing, attached are a couple of pictures showing current plan of attack on that. Drywall will be removed, then a first layer of BB ply will be shimmed to be as close to perfectly planar as possible using a laser and strings. Second layer of BB will have channels hand routed under the middle of the hardboard subpanels. After glue is applied, the subpanels will be screwed on using oversized holes and fender washers to allow fine adjustment to previous panel. Once in place a vacuum will draw the subpanel to the BB ply. I'll be running tests on the needed CFM of vacuum since this will be a leaky system. Will probably use a venturi vacuum generator since they produce decent CFM at 15-25 mm/Hg.

Each subpanel will weigh between 20 and 60 lbs, so there's not a lot of sheer stress from weight. I am more concerned about the humidity changes as Chris points out. Am building the piece in my office at nom 73 deg and 50-60% humidity. The church is maintained about the same, so just have to run with it.