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.
