A Sigma-Epsilon diagram, also known as a stress-strain diagram, graphically represents the relationship between stress (σ) and strain (ε) for a material under load. This diagram helps to understand how a material behaves under different levels of stress and strain.

This diagram typically includes the following regions:

•Elastic Region (O-A): The initial linear portion where the material deforms elastically. Stress and strain are proportional, and the material returns to its original shape when the load is removed.

•Yield Point: The point where the material begins to deform plastically. Beyond this point, the material will not return to its original shape. There are 2 yield points: upper yield point (B) and lower yield point (C).

•Plastic Region (B-E): The non-linear portion where permanent deformation occurs.

•Ultimate Strength (D): The maximum stress the material can withstand.

•Fracture Point (E): The point where the material ultimately breaks.

Sigma-Epsilon diagram in PLE

Once the Material Non-Linear Model option is selected, the actual stress-strain relationship, defined using the input tables ISTROP and SIGEPS in Design Function 3.1, can be displayed in a Sigma-Epsilon diagram.

The output tables PIPEMAT and PIPEPLS in Design Function 3.1 will have an extra button in the table toolbar.

Having the graph on screen there is an extra button to export the underlying data to Excel:

Calculating material-linear, the diagram will show a straight line.
This also counts for anisotropic materials, because they are always calculated material-linear.

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H3105 (last modified: Sep 22, 2025)

See also:

Design function description

Input data table overview

Output data table overview

Design function errors, warnings and messages