Poisson's ratio for a specific plywood

[EdClymer]

Greetings

Can anyone suggest a value for Poisson's ratio for plywood to this specification.

18 mm thick, five ply, eucalyptus with phenol-formaldehyde glue in core and on face and back.
The Modulus (Em) is given as 3000 MPa (class E30).

Made in China, marked Q grade.

I am trying to determine deflection of small panel constructed of this plywood with a GRP (FRP) skin.
(Modulus and Poisson's ratio are required for FEA input).

Regards

Ed Clymer
Resinfab & Associates
England

 

[SwinnyGG]

A property like that for plywood is going to have a lot of variability, and is going to be different depending on many factors such as how dry the veneer is, the specific shear strength of the glue, the percentage of glue used in the layup, etc etc.

Published values range from 0.2 to 0.4, which obviously is a wide range.

 

[Ron]

Bracket the Poisson's ratio values from 0.15 to 0.45 and consider the worst case. You might find that even though it is required input, it might have limited influence.

 

[EdClymer]

My thanks to jgKRI and Ron for their input and advice.

My understanding of how FEA works is limited; but I am assured, that from Modulus and Poisson's ratio the distortion of the mesh is calculated (and hence
displacement is calculated). I have today run a test - which confirms that displacement varies directly with Poisson's ratio (between 0.15 and 0.3).

Historically we have used 0.3 (for Poisson's ratio) but it has always been used with/for APA marine grade (hardwood) plywood - without apparent problem.

I am looking at deflection of plywood panels above joists - which support a flat bottomed plastic (HDPE) tank - and base disc of tank (very low Modulus)
follows deflection of panel.

I believe that this lack of full support is damaging to tank welds. The joists are supported by mass concrete plinth, but tank and contents (3 m diameter) is
23 tonne.

The Modulus for the 18 mm plywood is given as 3000 MPa - which is way to low. However, the plywood is cheap.

I just thought that your guess would be better informed than mine.

Regards








Ed Clymer
Resinfab & Associates
England

 

[chicopee]

Made in China plywood reminds me of the case dealing with the Chinese made defective sheet rock.

 

[EdClymer]

chicopee

Thanks for your comment.

In the UK, we use high-quality Polypropylene sheet to fabricate
and weld tanks and vessels for the storage of strong chemicals.

Chinese Polypropylene sheet, of low quality and variable properties,
is being sold on the UK market. It is of course cheap.

This sheet is completely unsuitable for chemical storage duty.

Regards



Ed Clymer
Resinfab & Associates
England

 

[racookpe1978]

Environment matters: A shipping box may be exposed to severe wet, water, spray and sunlight (bending and twisting too!), but only for a short time. Something else may even be painted or covered with plastic or a solid film (linoleum or cover), but be required to withstand kitchen or bath or warehouse conditions for years and years.

Under a roof gets 110-150 degrees F for years, but no direct sunlight.

How long does the plywood need to be that strong that somebody is paying for an FEA model analysis?

 

[EdClymer]

racookpe1978

Good words and quite correct.

To answer your question - at least 20 years.

The plywood in question is over-laminated with FRP to form
a bunded area to a kiosk containing two tanks (20 m3 each) of ferric
chloride (an evil acid comprising of hydrochloric acid with added iron).

Tanks are from welded HDPE; designed and constructed to EN12573-2.

Fifty-four sets were manufactured, a group of seven in one English county have
developed drip leaks from tangential branches.

HDPE (and all other plastic) tanks need full under-bottom support. The original
data gave a wide pitch for the joists under the plywood - which allowed local
unacceptable deflection (and very localised high strain level to the tank)

Further work and investigation have indicated that 2018 temperatures were as high
as 37°C (98°F) for a sustained period of time (three months) - compared with a
design temperature of 20°C (68°).

Currently, we are investigating this aspect.

Regards





Ed Clymer
Resinfab & Associates
England

 

[dhengr]

EdClymer:
I think you are barking up the wrong tree. In the real world of Theory of Elasticity, Poisson’s Ratio applies to homogeneous, isotropic materials, like various metals, an ideal material system. And, less so to materials like concrete (less homogeneous and still fairly isotropic) and much less so to wood which is slightly homogeneous in each direction, but certainly not isotropic in all directions. With the FEA software which you admit you don’t fully understand, you are using a surgeon’s scalpel to do the job of a cross-cut saw or an ax. The wood is hardly isotropic in itself, and the cross banded and bonded plys of the plywood are certainly not isotropic, so I can’t imagine that a Poisson’s Ratio has any meaning. You don’t get some uniform change of material volume, or uniform change of strain in a lateral direction, due to stress/strain within the elastic limit in a perpendicular direction. At best, you get some cracking and splitting in various y-direction plys due to stresses in the perpendicular x-direction, and the more so wherever there is a defect/flaw in one of the y-direction plys, and this leads to stress/strain concentrations, at the splits, in x-direction plys. I would run a few tests on the software, as JgKRI and Ron suggested to see how sensitive it is to the Poisson’s Ratio, and let that guide you, if you insist on using some fancy software. I’d do this plywd. sheathing on 2x joists deflection problem by hand calcs. and be done with it. You might reduce the joist spacing or double the sheathing thickness based on the results.

I think your real problem has more to do with your HDPE welding techniques and the location of any welds in a high stress/strain region of the whole system. Is the HDPE bonded to the plywd? The mech. props. of the HDPE should allow it to stretch and be conforming/compatible with the plywd., maybe/except at any locations where you get a large wood ply split due to cross grain stresses in the wood plys. My goodness, the mech. props. of the HDPE will/should allow it to hang like a hammock and carry a tension load, as long as the primary load is taken by the plywd. But, depending on the layup of the fibers it might not allow it to stretch in a concentrated fashion at a wood ply crack. And that may certainly be true where you over-cooked a weld joint, assuming heat welding, or didn’t apply enough solvent and pressure to the joint.

 

[JackWood]

"Made in China, marked Q grade." it's not good I guess.
I’d like to share my story. I needed plywood for my boat, at first I ordered plywood from the first company available on the Internet. After 2 months, the plywood began to crack and turn black, although I ordered it specially water-resistant. As a result, I had to redo everything. I read a bunch of reviews this time and decided to order plywood here www.plyterra.com And for good reason! I really liked the quality of plywood, it’s been a year of use, and plywood still looks like it is new. In general, very high quality plywood from this company. I advise everyone!

 

[Guest090822]

Buy American, that is U.S.!

 

[EdClymer]

EdClymer (Mechanical)(OP)

My thanks to JackWood and TheRick109 for their contribution to my original question.

I was trying to find out why seven out of fifty-four 20m3 each HDPE tanks developed a leak - mostly in the same position.

I took the view, that it was due to lack of support as the (Chinese) plywood between the joists was deflecting.

After a bit of a struggle; it was proven, that deflection was the cause (allowing the bottom plate to move relative to
the shell and particularly the branch in the shell.

Clearly, the plywood was of very inferior quality, as (more recently) was the Polypropylene sheet (used on another job) and the
EPDM sheet ditto (with adulteration and imaginary properties). Things that these three materials had in common was - they all originated in
China and all were very cheap.

As Rick109 says, buy clearly marked APA plywood, or BS 10#88-1 plywood - expensive? but very good, predictable quality.

The comment of dhengr is simply wrong: the case was proven to a UK High Court Judge - who found the tanks (and the welding) not to be at fault.

Ed Clymer
Consultant plastics Engineer
Resinfab & Associates
England





Ed Clymer
Resinfab & Associates
England

 

[charliealphabravo]

Comments from dhengr and others with respect to the significance of poissons ratio in the FEA modeling of wood are not off base. I learned very early that if a property cannot be determined with precision, then other precisely determined and controllable properties cannot depend on that property. Alternatively, if a property is not measurable, controllable, or predictable, or is widely variable, then established design methodologies will prevent the property from controlling.