modeling polyurethane foam
modeling polyurethane foam
(OP)
Low density polyurethane foam is used to fill the space between a series of PCBs which are edge mounted to provide vibrational damping. How do you characterize the modulus for the reticulated foam at the resonant frequency of the assembly, in the 500 to 1000 Hz range?
Material is viscoelastic and may have a negative poisson ratio and appears to have much higher modulus at this frequecy than understatic conditions.
Material is viscoelastic and may have a negative poisson ratio and appears to have much higher modulus at this frequecy than understatic conditions.





RE: modeling polyurethane foam
RE: modeling polyurethane foam
cannot use springs in this heritage design,(changing design very difficult). I am attempting to provide a model for the assembly with the foam being the primary unknown. Using the static modulus of 30 psi does not give agreement with sine sweep measurements. Better results with about 500 psi, but no winner.
RE: modeling polyurethane foam
RE: modeling polyurethane foam
RE: modeling polyurethane foam
RE: modeling polyurethane foam
OT a bit.
Recall that polyurethane foam is an excellant thermal insulator - using it might produce undesired temperatures in your electronic components.
RE: modeling polyurethane foam
RE: modeling polyurethane foam
GBor--Deflection is small and therefore "nonlinearity" (whatever that means for viscoelastic materials) is probably not the issue. have run cases with a wide range of values for modulus but do not get agreement with test data on modal frequencies. Model geometry is known to be good.
RE: modeling polyurethane foam
Using a linear calculation will likely never match test data. Even with small deflection, viscoelastic material damping properties generally change pretty significantly.
Short answer: You are using the wrong material model. This foam may behave more like a 9th order Mooney Rivlin material or a neo-Hookean model. Or any of a dozen other proposed, very non-linear viscoelastic material. A simple, linear elastic modulus will not match test data if this is the case.
RE: modeling polyurethane foam
You may well need to get your materials tested to give data for a decent material model, which probably means uni-axial and equi-biaxial testing and possibly some variable strain rate stuff aswell.
I'm no expert on dynamics but couldnt you carry out a sine sweep on a simple block of insulating material at the desired frequency and then back calulate what the modulus should be. That might put you in the ballpark..
RE: modeling polyurethane foam
Even in a vacuum
1. A "hot" component may be touching the foam and get heat via conduction which will not leave the foam because of its own insulation and the lack of air to produce a convection heat transfer component.
Probably anything above 150 F should get serious consideration.
2. The other components that get heat radiation effects are also in a vacuum and therefore can only get rid of heat by conduction and re-radiation.
RE: modeling polyurethane foam
the locations dissapating the most energy get conductively coupled to the case and finally radiated to space
other components have come conductive path to case
the foam provides for more energy transport than a vacumm via radiation to the surrounding at similar temperature.
RE: modeling polyurethane foam
plans for such a material evaluation test are underway
However: a parametric fem for our geometry could not find agreement with the observed modes found in a sine sweep test -- only the foam modulus was varied
RE: modeling polyurethane foam