Due to the creep behaviour of plastics, stress is heavily dependent on strain rate, so a value for Poisson's ratio is generally not specified. If you look at a typical plastic data sheet, you should see that some parameters are actually specified at a defined strain rate (e.g elongation before break is usually at 50mm/s - sometimes other rates are given.
As Pat suggested, talk to the material supplier and they should be able to help.
I tried talking to the material manufacturer, but the part that I am analyzing was made in the early-mid 80's and the company is unable to locate and provide the material data sheets.
I understand the creep-rate effects for a constant load, but for a one-time impulse-loading situation it seems there should be a poisson's rtion value applicable for polyurethane.
It will probably be called an apparent Poison Ratio. That's what they do with e mod which is also prone to creep.
Bayer Materials or Lenxis or whatever they call themselves now should have data on a similar grade. 20 years ago they literally published truck loads of technical data.
They invented PU and were always considered the market leader re technical service.
Regards
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