Learning Forum

[Julien Baba]

Hello,

I have a viscoelastic material with 2 representations, but I'm not sure if those two are equivalents for harmonic analysis, I would need some theoretical help here because I can't find an answer to my question:

My viscoelastic material has been characterised by DMA measurements by my supplier. The mastercurve has been fitted to my data and I have the shift factors. A material engineer made 2 representation of that material, in an ANSYS material card format :

• I have my first Ansys material card which is frequency dependent and temperature dependent, based on the mastercurve and shift factors. Elastic modulus define with a table TB,ELASTIC (to sweep though temperature and frequency) and the beta damping defined by a table TB,SDAMP (same as elastic modulus).
• The second material card is a Prony shear coefficient table fitted from the master curve (with the same DMA dara) with an initial modulus and a WLF shift function defined with parameters.

The first representation can be used for all type of analysis (Modal,  Mode superposition, Full harmonic,)

But the second representation, with the Prony coefficients, it can only be used with Full harmonic analysis, as long as Prony coefficient incude complex moduli (loss and storage),if I recap from ANSYS trainings in Non-linearities section, if I do no mistake (correct me if I'm wrong).

 

So my question is does the 2 material representations equivalent in a material point of view ?

In addition, if I have a geometry with different materials, including viscoelastic material, if I want to know the mode shapes and eigenvalues of my model, I need to use the first representation with elastic and damping temperature-dependant to know my mode shape ?

Thanks for the help

 

[John Doyle]

If you are trying to represent the frequency dependent stiffness response of a viscoelastic material in a  harmonic analysis, you can define it as a Prony Series in the frequency domain.  If you are trying to represent the rate dependent stiffness response (relaxation) in a nonlinear structural analysis, you can define it as a Prony Series in the time domain.

Please refer to Section 4.8 of the MAPDL Material Reference Guide for more details.