3-ply plywood

I did some crude experiments some time ago, with sheathing the plywood I am using to build my boat and have been meaning to put the results on the site and was reminded of this in today’s thread on lauan plywood.

My plywood is 4mm 3-ply, the outer layers are about 1mm each and the core about 2mm. It was described as Far Eastern marine ply, stamped BS1088.

When I got it, it was obvious that it was very much stiffer parallel to the face grain than perpendicular.

I was wanting to assess how to improve stiffness at least weight/cost.

I cut strips parallel and perpendicular to the face grain and used various treatments one side only and also on both sides giving 16 strips in total.

They were:-
a) untreated
b) 3 thin coats of unthickened epoxy
c) 1 layer 81grm/m^2 plain weave one side only
d) 2 layers of 81grm/m^2 plain weave one side only
e) 1 layer of 162grm/m^2 open weave one side only
f) 1 layer 81grm/m^2 plain weave both sides
g) 2 layers of 81grm/m^2 plain weave both sides
h) 1 layer of 162grm/m^2 open weave both sides

all the cloths were finished with peel ply which was removed prior to testing. Where only one side was clothed the strip was tested with the cloth in tension.

I tested the strips for stiffness before adding the treatments and the average stiffness parallel to the face grain was approximately 7 times the stiffness perpendicular to the face grain. The strength was about 2.75 greater parallel to perpendicular.

The results for the improvement parallel to the face grain were


treatment _______strength____________stiffness
a)__________________ 1 _________________ 1
b)__________________ 1 _________________ 1
c)__________________ 1.1 _______________ 1.2
d)__________________ 1.1 _______________ 1.2
e)__________________ 1 _________________ 1.2
f)___________________ 1 ________________ 1.75
g)__________________ 1.3 _______________ 2.15
h)__________________ 1.1 _______________ 2.35

The results for the improvement perpendicular to the face grain were

a)_________________ 1 _____________ 1
b)_________________ 1 _____________ 1.1
c)_________________ 1.7 ___________ 2.1
d)_________________ 2.3 ___________ 2.5
e)_________________ 2.4 ___________ 2.45
f)_________________ 2.8 ___________ 2.7
g)________________ 4.35 __________ 4.4
h)________________ 4.3 ___________ 5



Given I only used 1 sample per spec. it is subject to random variations that a proper testing system would resolve but I think the results are reasonably consistent and do give an idea of what is happening.

Where plywood spans in 2 directions the spacing of its supports will determine which direction is critical for failure.

My conclusions are
1) that epoxy coating on its own provided negligible benefit in terms of strength/stiffness.
2)Where the stress parallel to the face grain is critical, sheathing one side is ineffective and sheathing both only improves stiffness with not much better strength.
3)Where the stress perpendicular to the face grain governs e.g. a multi-chine canoe where the ply tends to span between the chines, significant improvements in strength and stiffness can be achieved.

Once you go to thicker plywood with 5-ply the difference between parallel and perpendicular will be greatly reduced so the above analysis probably would not apply. Similarly for 3-ply with thinner outer layers and a thicker core.

It was comforting to see how far you had to bend ply before it went bang.

 

What exactly are your definitions of stiffness and strength? I don't want to make assumptions.

Great data set, thanks. Echoes the data from the Gougeon Brothers docs.

 

nice work. One of my engineering professors used to say one simple test is worth a thousand expert opinions.

 

In this case stiffness was 1/deflection under the same load i.e. the hammer
Strength was the load at which it snapped or folded. I was surprised how "plastic" it was. I applied the load gradually so a sudden impact might be different

I had meant to test with the treatments on one side only and the cloth in compression. It would be interesting to know what improvement, if any, it would achieve, to give an indication of the benefit to just coating the outside of the hull, as usually the worst load condition would have tension on the inside and compression on the outside.

If anyone is planning on a build of your own I would encourage you to do your own testing as I am not sure the ply I am using is typical. As you can see there is nothing sophisticated or expensive used in the test.