I tried to curvefit viscoelastic material data, but the fit I get is very different than the experimental data I supply. I tried curvefitting with several terms, but without luck. I only readin data for shear. I’m probably doing something wrong when curvefitting, but I don’t know what. Could you please have a look and try to create a good curvefit with the data I attach? The data 0.0001 100e6 .05 43e6 0.1 22e6 .15 15e6 0.2 11e6 .25 10e6


January 25, 2023 at 7:34 amFAQParticipant
In this case, the guess that you are providing is likely sending the algorithm off in the wrong direction. ANSYS curve fitting is using the LevenbergMarquette algorithm to perform a non linear regression on the data. One of the problems with LevenbergMarquette is that it does not guarantee that the minimum energy condition is satisfied, that the minimum obtained using the algorithm may be a local minimum rather than the global minimum. Because of this difficulty, the judgment of the analyst is required to determine when “good enough” has been found. For the data that you supplied, visual inspection implies that Ao=1e7, A1=9e7 and tau1=.05 should give a good approximation using a 1 term Prony series. The logic here is that the Prony series will look like G = Ginfinity + G1 * exp(time/tau1) where Ginfinity would be the value of the modulus at infinite time, G1 would be the amount of decay (in this case the difference between the modulus at zero time and the modulus at infinite time), and tau1 would be the time constant where 5 time constants represents the required time. To see this, consider the following two cases. Guess 1 assumes that the default values of 1 for all constants can be used for the curve fitting. Figure guess1a.jpg shows the setup screen from the curve fitting GUI and guess1b.jpg shows the resulting plot comparing the experimental data with the curve fit. Clearly these resultsare not acceptable. Guess 2 assumes that the initial values for the curve fit will be Ao=1e7, A1=9e7 and tau1=1, as shown in Figure guess2a.jpg. The resulting plot comparing the experimental data with the curve fit shows good agreement. The conclusion is that for non linear curve fitting, the analyst must take care to insure that the progress of the curve fitting algorithm will be in the direction of the global minimum. That poor choice for the initial guesses for the series constants will lead to poor correlations with the experimental data. Unfortunately, the ability to make good choices come from experience, and a lot of that expericence will come from making poor choices. Some suggestions can be provide. Selecting Ao as the infinite modulus will be a good choice. Selecting Ai equal to the total decay divided by the number of terms in the Prony series will be a good choice. Selecting time constants that span the entire range and that are at least a decade apart will be a good choice. Fewer terms in the series, rather than more terms in the series, is probably a good choice. One final word. The ANSYS auto time stepping algorithm does not survey the viscoelastic behavior in determining whether the time step size should be increased or decreased. At least for now, it is the analyst’s responsibility to set the incremental time step size for the problem. Experience suggests that the number of substeps should be no greater than 1/10 of the currently active Prony time constant.

Introducing Ansys Electronics Desktop on Ansys Cloud
The Watch & Learn video article provides an overview of cloud computing from Electronics Desktop and details the product licenses and subscriptions to ANSYS Cloud Service that are...
How to Create a Reflector for a Center HighMounted Stop Lamp (CHMSL)
This video article demonstrates how to create a reflector for a center highmounted stop lamp. Optical Part design in Ansys SPEOS enables the design and validation of multiple...
Introducing the GEKO Turbulence Model in Ansys Fluent
The GEKO (GEneralized KOmega) turbulence model offers a flexible, robust, generalpurpose approach to RANS turbulence modeling. Introducing 2 videos: Part 1 provides background information on the model and a...
Postprocessing on Ansys EnSight
This video demonstrates exporting data from Fluent in EnSight Case Gold format, and it reviews the basic postprocessing capabilities of EnSight.
 ANSYS ACCS: Simulation of a Composite Rib Using ANSYS Composite Cure Simulation Tool
 Hyperelastic Simulations
 ANSYS Mechanical: Delamination Analysis using Contact Debonding
 Does it make sense to use viscoelastic material in static structural since it requires the calculation of strain rate?
 How does the analysis interpret the time beyond the shear relaxation test data? Will it be a linear behavior. Say, I have shear relaxation data for 10 minutes, and I set my analysis to run for a time of 20 minutes.
 When the material data sheet of a polymer reports both the Tensile and the Flexural Modulus, which value may be used in place of Young’s Modulus?
 Why the anisotropic stiffness matrix in Engineering Data highlighted in yellow?
 How to investigate fracture mechanics parameters for a thin geometry?
 Question: What is the difference between PLNSOL, EPPL, EQV and PLNSOL,NL,EPEQ?
 What is the slope of the multilinear hardening plasticity curve beyond the last user defined stress/strain value?
© 2023 Copyright ANSYS, Inc. All rights reserved.