Okoume shear strength test

I'm building a wing for a 32 foot catamaran, and wanted to test some okoume plywood for shear strength in the orientation in which it will be resisting shear in the design. The test coupon is 1/8" x 1" in cross section. What is your guess for the load at which the deformation is permanent?

I'd put a picture here of the test rig, but it has apparently not been approved yet.

 

Welcome to the Forum TWG.

If the photo above is not (part of) the test rig, then what is missing?
I presume a tensile force will be applied to the test sample using shackles attached to the holes in the sample?

 

Nothing missing, actually. I managed to get the picture to post. And yes, tension will be applied at the holes.

 

Unless the jig is restrained for transverse movement, I would expect the glue line to fail (peeling).
If my maths is right failure at 500N.
Permanent deformation? Try two pieces.
Regards Alan

 

Here is the data. I digitize it.

 

I assumed a shear strength of 6.1 MPa (F14) versus your 5.5 MPA.
In your table, I am surprised how the longitudinal bending strength drops offs with increasing thickness:
6 mm 41 MPa
12 mm 29 MPa
18 mm 25 MPa
I would have expected these numbers to be similar with increasing thickness?
Alan

 

It is not MY data but I trust it as it has EN and DIN standards. I think this is the original. Otherwise go search the web.
Plywood data sheet - World Panel Products Inc - PDF Catalogs | Documentation | Boating Brochures https://pdf.nauticexpo.com/pdf/world-panel-products-inc/plywood-data-sheet/22265-80271.html

 

By the way, I am not a structure engineer, but I did a structures unit back in the 80s. So a little bit of knowledge can be a dangerous thing!
Well, I discovered something today. The Australian and New Zealand standard for Structural Plywood Characteristics have been changed (at least once).
Overall the numbers have been lowered. Here is a link to the current Australian/New Zealand "F" stress grades (page 34):
https://www.ewp.asn.au/wp-content/u...uctural-Plywood-LVL-Design-Guide-v5-FINAL.pdf
In my mind, F8 is your cheap (marine) plywood, F11 petty good plywood and F14 would be your top of the range (Hoop Pine in Australia).
My updated shear failure estimate would now be about 400N (based on the new standards).

And I still don't understand the bending strength and bending MOE numbers in that table you posted.

Regards Alan

 

There lies the confusion. You are using Australia/New Zealand Standards. The data I have posted uses the British Standard and DIN (German) standard. Table shows it uses the EN and DIN method of test. There are other standards but let us not deviate much from the topic.

As far as I know, there are eight stress grade used for structural plywood. F8 to F15 being the most common, and F17 to F34 are the higher grades. I have read somewhere that Okoume 1088 although classified as structural plywood would have to be recertified to pass EWPAA standard.

While the Modulus of Elasticity in data in bending (EN 310 method) seems to be counterintuitive, bear in mind that plywood is a composite of veneers, thus the fibers are arranged perpendicular to each other. It is identified by the stress and ID code. The number of veneers appears to be of odd numbers. That means in the longitudinal and transverse arrangement, one will have more fibers/veneer running in that direction. Look closely, in the MOE longitudinal bending, the value goes down, while in the MOE transverse, the value goes up. The more the veneers are arranged in that direction, the higher the stress value. It can’t be equal as the veneers are odd numbers. Even in woven fiberglass cloth, the longitudinal and transverse modulus are close but almost impossible to be the same.

 

Thanks! That's a great design guide.
I'll do the test tomorrow, by which time the glue will have cured sufficiently. I think that the aspect ratio of the test coupon is such that peeling won't occur, but we will see... it always fun to break stuff.

For tensioning, I made a hanging basket into which I can put one cat, or two cats, or one dog, or one dog plus one cat... on up through a couple small humans. That should provide enough mass to test to failure, given some gravity.

 

Yes. In fact, 3mm Okoume is often called "bending ply" partly because it is thin. In transverse bending, with just three layers, essentially all of the stretching occurs in the weak direction in the top and bottom layers, so it bends really easily. In the longitudinal direction, however, I'd expect it to be stiffer (relatively) in bending than even the 6mm ply.

 

Looking through my saved Internet searches I found a test rig design for shear testing that is doable but would require a bit more force than 400N or 500N.
Refer to page 19 of the attached pdf.

Regards Alan

 

Do it properly. There is a specified shape for testing. Measure the width and thickness of the necked part, clamp one end to an A frame and hang weight on the opposite end. Tensile strength formula is Force/Area. Whatever unit you are using it will boil down to a unit of measure. Depending on the width and thickness of the test piece, you can calculate in advance how much weight (force) you need apply.

Unfortunately, this crude test will only tell you the ultimate strength (to failure). If you take note at which the test piece cracks, that would be the yield strength. If you have a means to measure the length it stretches for every load applied, you will be able to plot the stress/strain curve and derive the Modulus of Elasticity (MOE). It is the theoretical straight line riding on the back of the stress/strain curve.

 

Can you express in engineering term or mathematical formula what you are expecting so that we are on the same page so that my answer is precise.

 

For the in plane shear test there are several crude methods. 1. you can glue a strong piece on both sides of the specimen and do the test by clamping one end and attaching weight on one end. This is the most inaccurate as the specimen tends to rotate. 2. You can do the test as described above but orient the test piece at 45 degree angle. 3. The last method is the notched shear test. Will have to search the net for the proper shape of the specimen.