To fit a material law, a measurement must be loaded for the material. For this, check Save a Measurement under the Measurement tab of the current material law.

Material data can be loaded from a *.txt file with the Load… button.

The data must be formatted in two columns, the first column represents the strain values in percent, and the second row contains the corresponding stress values in GPa. Both columns must be separated by spaces.

Alternatively, data can also be entered by copy-and-paste or manually and save the data as a *.txt file with the Save… button.

To plot the data, click the Update Graph button.

To fit the material law, select the Fit Law Parameters tab under the current material law. Choose a material law from the pulldown menu:

The result is dependent on the start values for the fitting process. Therefore, the initial values can be user defined by checking Set Initial Parameters and Keep Initial Stiffness.

The quality of the initial values can be checked with Plot Initial Curves.

It is recommended to try several combinations of initial values to find good start parameters for the material fitting algorithm. To edit the plot settings, right-click on the plot and choose Edit Axis Settings.

The fitting algorithm stops if the error between the fit and the target (which is the measurement curve defined in the Measurement tab) is smaller than a threshold. This threshold can be defined in the Stopping Tolerance field.

If only a part of the measurement data should be used for fitting the material laws, a strain range can be selected with Restrict Strain Range. This is especially useful if a part of the data contains effects which are not simulated by the selected material law, e.g., damage effects when fitting a material law for plasticity.

The parameter fitting can be started by clicking Run Parameter Fitting. The fitting process will take several seconds, and the results are directly shown in the Fitted Parameters section and in the plot.

If the result is not satisfying, the Initial Parameters can be adjusted and the parameter fitting process restarted. Another way of improving the result is to reduce the Stopping Tolerance and force the fitting algorithm to search longer for a solution.

In the presented example the Initial Yield Stress has to be changed from 0.01 GPa to 0.04 GPa to achieve a good fitting result. In the figure below, you can see that the curve for the exponential plasticity model lays very well over the measurement data after the parameter fit.

With the button Set Parameters to Material Law, the fitted parameters are copied to the material law under the Material Law tab.

Finally, save the changes to the material database with the Save Database button. Now, the new material law is saved and can be used in ElastoDict.

Now you can select the material law (here MaterialLawName) of the newly created material Material_New. In oder to use the nonlinear material model, please make sure to check the according box (here: Exponential under Plasticity).