Characterizing the viscoelastic properties of blubber

[BioEstudent]

Hi All,
I am currently conducting research on whale blubber and hoping for suggestions as to what type of parameters and testing I could focus on to characterize this material. What parameters/tests are people generally interested in when looking to fully characterize a viscoelastic material? Thank you in advance!

[afreiden]

Your question is vague.

I find it useful to think about the process of characterizing a rubbery material as follows (I will assume no "damage" will occur in your material for your application):

Step 1: Choose a "hyperelastic" function. By definition this means that loading and unloading will be along the same path. Thus, finding the material constants for your hyperelastic function (e.x. initial shear modulus) will require that you perform tests (e.x. compression test) at an "infinitesimally slow" strain rate.

Step 2: Add on a "viscoelastic" function. Viscoelastic effects will cause the loading to "stiffen" and the unloading will no longer follow the loading path. You can find the material constants for your viscoelastic function by performing dynamic tests (e.x. Drop tests, DMA).

Step 3: Implement your completed material model into a finite element code. Now you can subject your material to various funky loads at various rates of loading and get good results -- now that your material model is "fully characterized," as you say.


In reality, this is all quite complicated.
Thus, Jorgen has created a tool called MCalibration that allows you to choose your material model (i.e. choose your hyperelastic and viscoelastic functions) and then MCalibration will take whatever test data that you have and "characterize" your chosen model, automatically. Some will fit the data better than others. MCalibration also exports to some of the commercial codes.

Alternatively, you can use the material models that are built-in to your commercial code.

Depending on the kinds of loads (tension? shear?), strain magnitudes, and strain rates that you expect your whale blubber to see in its application will determine the material model that is most appropriate and the kinds of tests and the number of tests that you ought to perform. That information would help experts in the forum (not necessarily me) answer your original question, for example.

Sorry for the longish post, I was on a roll

[IceBreaker]

afredien's comments are spot on.

Here are my 2 cents:

You can never "fully characterize" any material but the "hope" is by applying a few basic loading profiles and curve fitting all those stress-strain curves by a single strain energy potential, the material will behave reasonably well in complex loading. Of course, an appropriate failure criterion must also be chosen and a reasonable factor of safety taken into account. Uniaxial tensile test, compression (confined/unconfined), shear (regular/dual-lap), biaxial are just a few tests that are typically used in characterizing soft tissues. However, without knowing the microstructure, function and the application, the whole process can be off-target. In other words, the blubber may, rarely, if ever undergo compression in reality. So, is compression test really all that necessary to carry out? At what rates does the blubber typically undergo strains? So, at what rates should you conduct your uniaxial tensile test(s)? Should you simply go with hyperelasticity or should you pile on linear viscoelasticity (or quasi-linear or even nonlinear!)? Should you take the Mullins effect (hysteresis) into account?

These questions can't be answered without keeping the function and the overall goals in mind. And, of course, you can't incorporate everything in the first shot. It is prudent to start with the most simple stuff (like isotropic elastic -> bi/tri-linear isotropic elastic -> isotropic hyperelastic -> isotropic hyperelastic and linear viscoelastic -> isotropic hyperelastic and quasi-linear viscoelastic -> anisotropic hyperelastic and quasi-linear viscoelastic and so on.) The first two in the series may be quite inaccurate in characterizing the material but these material models will give you some sort of baseline values to do a sanity check on the results later on.

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