Hello,
My first post here. My apologies if Im asking an exceedingly foolish question. My apologies also, in advance, for the length of this post.
I have been reading a great deal about FEA of viscoelastic materials over the past week or so. This (PolymerFEM website and forum) is clearly a great resource! However, I have not really come across any threads (yet) that address several aspects of the situation Im looking at.
Specifically, material-wise, Im looking for a viscoelastic material that displays good damping characteristics for our application BUT also has relatively high stiffness for good transduction from the viscoelastic material to a (much stiffer, non-viscoelastic) material in contact with it. (Im seeking a high-|E*| tan(delta) material, in some terminology appearing in the literature.) Added complications are that the transduction will occur at relatively high frequency (250-500+ kHz) and across a very small time interval - 10s to 100s of micro-seconds, i.e. 1e-5 to 1e-4 seconds, a good thing is that the strains are very (very) small also - as small as 1e-10 and certainly no larger than 1e-8. Basic (potentially naive) general question: Can materials exhibit viscoelastic behavior - in particular, damping - at such high frequencies with such small strains across such small time intervals?
As a candidate material, Ive been looking at data for PMMA. (A number of posts here address this material explicitly.) Ive found a few references that had some Prony series data, some of which covers quite a few decades of time constants. However - and here comes, potentially, my first exceedingly foolish question (if the one in the preceding paragraph was not the first such question) - given the short time period of interest to me, wouldnt I need Prony series terms with relatively small time constants? I.e., ones close-ish to my time interval of interest? Im certainly not interested in what might be happening at 1e2 (or even 10 or 1) seconds, let alone, 1e6+ seconds.
I did a run last night with a 3-term Prony series that had time constants of ~0.5, ~25, ~250 s. I didnt have shear relaxation moduli so (in Ansys) I selected Prony Volumetric Relaxation. The results appeared a bit odd to me. Not only did the ringing that I want to see disappear (or at least be mitigated) NOT decrease, the amplitude of the ringing increased (I cant tell about duration since the simulation cut-off time is early in the ringing) in comparison to the identical model with the Prony specification removed, i.e. just density, Youngs modulus, and Poisson ratio for PMMA carried over.
I did not specify hyperelasticity etc. Thats another question: Do I definitely need to? At these frequencies with these strains and this time interval, it didnt seem necessary.
Also, I noticed the execution (cpu) time was nearly 11X greater with the 3-term Prony series. Yikes! Does anyone know a rough rule-of-thumb for how much additional FEA cpu time is required per additional term in the Prony series? Given my short time interval etc., should I be able to get away with using only a single term perhaps?
I believe I have now well-exceeded common-courtesy in terms of post-length. Sorry about that.
Thank you all (or anyone) for your time and assistance and good day,
David
