by KCStudly » Mon Sep 19, 2016 9:10 am
When I was in college we had this one grizzled old shop teacher (metallurgy) that used a startling technique to drill certain important life lessons into us. One of the things he used to be adamant about and yelled out frequently was, "test 'em like you use 'em!". While I can think of a scenario where maybe a folded edge might come loose and then be exposed to road wind or a snag where it might peel back in the same way that everyone finds so easy to make fail; I think the more common modes of failure are likely to be punctures (first, then maybe gouging tears and/or abrasion if the trailer continues to drive past whatever caused the puncture), and/or shear, such as in a flexure situation (wind loading on the front wall while underway trying to bow the panel inward, standing or snow loads on the roof, leaning against the wall while siting up inside or sleeping, or the dreaded grandson kicking).
Why do I say shear and not bending? Well, when you bend a panel like this the outer fibers stretch and the inner fibers compress, so between each micro layer of grain they are trying to slip past each other to varying degrees, so if the bond between canvas/glue/foam were to fail it would be in shear (call it cleavage if you like).
So I propose that, rather than tearing the canvas back in a peeling action, bending the whole sample back and forth would be a more reasonable test. This would alternately put tension and compression along the canvas to foam joint testing the shear strength of the glue joint. (Incidentally, when I did this to my canvas sample I was amazed at how far I could bend it with no apparent failure whatsoever.)
Also, when making test samples, go ahead and use the opportunity to test the technique of wrapping the corners. It will give the chance to see what minimum radius can be adhered to well, would be more representative to how the full scale build would be done, and would help reduce the temptation the tester has to peel the canvas up.
As to puncture resistance, if I were a betting man and had an ice pick to wield indiscriminately, I would guess and rank our typical building materials from best to worst as follows:
aluminum over thin plywood
aluminum directly over foam
thin plywood
epoxy/FG
PMF
Since there is such variation in the possible scenarios (blunt vs. sharp, angle of attack, force/speed, flexibility of offending object, etc.) it would be difficult to quantify what is reasonable performance. I expect that aluminum would be the hardest to repair and have it look as new.
KC
My Build:
The Poet Creek Express Hybrid Foamie
Poet Creek Or Bust
Engineering the TLAR way - "That Looks About Right"
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