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Documentation

Plastic Materials

The plastic material model describes the material behavior after the onset of plasticity. At this point, the solid materials will undergo irreversible deformation when subjected to loading. For example, a solid metal being shaped by bending or pounding.

Most of the materials have an elastic and a plastic region separated by a yield point, after which the stress-strain curve becomes nonlinear. A typical stress-strain curve for steel is shown below:

stress strain curve plastic material
Figure 1: Stress-strain curve for a ductile material. (Source: Nicoguaro / CC BY)

The plastic material model has a wide variety of applications, such as sheet metal forming, metal forging, and crash analysis.

There are also applications where some plastic deformation is allowed in the design. In such cases, first run a simulation using a linear elastic behavior for the materials. If the stresses are greater than the yield strength, then an additional analysis is done with a plastic material behavior.

Important

A plastic material behavior can only be defined for the following analysis types:

Nonlinear static analysis;
Dynamic analysis;
Nonlinear thermomechanical analysis.

Defining Plastic Materials

To define a plastic material, follow the steps given below:

  • Navigate to the Materials tab, in the simulation tree. Change the Material behavior to Plastic. The button highlighted in Figure 1 is used to input the stress-strain data.
plastic material behavior setup
Figure 2: Changing the material behavior to plastic
  • After clicking on the highlighted button, the user needs to input data for the stress-strain curve. The figure below shows an example of a plastic aluminium stress-strain curve:
sample stress-strain curve for aluminum
Figure 3: Sample stress-strain curve for aluminum

One can define a series of data points over the stress-strain curve and determine their coordinates. These points can be manually input. Alternatively, it’s also possible to create a comma-separated .csv file in any text editor, containing the point coordinates in this format (no spaces after the comma): strain,stress. This .csv file can be uploaded to SimScale.

input stress strain curve in simscale
Figure 4: Table input for the stress-strain curve

Important

The first point of the series is taken as the yield point by the solver. Using Figure 2 as reference, this would be the structure of the .csv file:

stress strain input aluminium set up
Figure 5: Stress-strain curve for aluminium

To capture the stress-strain curve appropriately, make sure to add enough data points. Between two points, the values are linearly interpolated.

  • After saving the stress-strain curve, adjust Young’s modulus according to the stress-strain data. The Young’s modulus is given by dividing the stress and strain values at the yield point:

$$E = \frac {\sigma_{yield}}{\epsilon_{yield}} \tag {1}$$

After inputing the values from our example in equation (1), we have:

$$E = \frac {1.94e8}{2.75e-3} = 7.05e10 \ Pa \tag {2}$$

A wrongly calculated value for Young’s modulus may lead to a diverged solution.

Note

This model is not a suitable choice for brittle materials such as ceramics and concrete. It is only applicable for ductile materials, such as aluminum and steel.

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Last updated: June 30th, 2020

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