John Doyle
Ansys Employee

If your relaxation data is stress vs time, this curve fit can be done directly in Engineering Data.  Please refer to online docs Workbench=>Engineering Data Users Guide=> Material Data=>Curve Fitting=> Viscoelastic Test Data...

If you have storage and loss moduli as function of frequency, then you are correct.  The curve fit needs to be done in MAPDL.  Please refer to Section 6.2 of the MAPDL Material Reference Guide...  It would also help to review the documentation on 'TBFT,,,' command in the MAPDL Commands Manual.  This procedure will result in Prony constants in the time domain (alpha and tau values), but the input is storage and loss modulii in frequency domain.  The MAPDL curve fitting results (alpha and tau terms), in this case, would need to be manually copied into WB-Engineering data. 

If you have access to the Ansys Customer Portal, please refer to KM 2036139 for a worked out example in MAPDL.