Introduction - Ansys Bergstrom-Boyce Model
This tutorial is the third in a series on how to calibrate a temperature-dependent viscoplastic material model to experimental data for polyether ether ketone (PEEK). The first part on this series covered the temperature-dependent Three Network Viscoplastic (TNV) model, the second part covered the temperature-dependent Abaqus Parallel Rheological Framework (PRF) model, and in this article I will cover the temperature-dependent Ansys Bergstrom-Boyce (BB) model.
Step 1. Calibrate a Material Model to the Data at Each Temperature
MCalibration can quickly calibrate the Ansys Bergstrom-Boyce (BB) material model to any data set. As is shown in our introductory tutorial to MCalibration, most of the time you just need to read in the experimental data, select the material model you want to use, and the click run calibration. In this case, if I perform those steps for the room temperature data then I get the following results.
If you look at the predicted dashed lines closely you will see that the predicted Young’s modulus is significantly lower than the experimental Young’s modulus. This sometimes happens when working with a material model that is not quite accurate enough to capture all experimental behaviors. The Bergstrom-Boyce (BB) model is a two network viscoplastic material model that was designed for soft elastomer-like materials, and it therefore is not always accurate for thermoplastics. As we have seen in the previous 2 parts of this series, a three network representation is typically needed for thermoplastics.
One way to improve the Bergstrom-Boyce model predictions at small strains is to add one additional load case with information about the target Young’s modulus at small strains. The results from this alternative calibration approach are shown in the following figure.
In this case the predicted stress-strain response is really accurate at small strains, but the unloading modulus is too stiff. As mentioned, this is a limitation of the Bergstrom-Boyce model when used for thermoplastics.
Step 2. Export the calibrated material model parameters
After the calibrations have finished, export each calibrated material model into Ansys APDL dat-file format.
Step 3. Combine the Calibrated Material Models into a Temperature-Dependent Model
The exported material model parameter files can be combined into a single temperature-dependent material model by:
- Opening the Material Model dialog box
- Click on “ANSYS-Template”
- Then clock on the “Temperature-Dependent BB” button.
This will bring up a file selection dialog box in which you should select the parameter files that were exported in Step 2. Click “Open” and then “Save” to select the new temperature-dependent material model.
The last step is to specify the actual temperatures that were used for the experimental tests. In this case the temperatures were: 296 K, 373 K, 423 K, and 474 K.
That is it. You now have a calibrated temperature-dependent Ansys Bergstrom-Boyce model that you can use!