Nonlinear Material Behavior for RFEM 6

Isotropic | Plastic (Members, Surfaces/Solids)

Did you know? When unloading the structural component with a plastic material model, in contrast to the Isotropic | Nonlinear Elastic material model, the strain remains after it has been completely unloaded.

You can select three different definition types:

• Standard (definition of the equivalent stress under which the material plastifies)
• Bilinear (definition of the equivalent stress and strain hardening modulus)
• Stress-strain diagram: definition of polygonal stress-strain diagram
• Option to save / import the diagram
• Interface with MS Excel

Isotropic | Nonlinear Elastic (Members, Surfaces/Solids)

If you release a structural component with a nonlinear elastic material again, the strain goes back on the same path. In contrast to the Isotropic|Plastic material model, there is no strain left when completely unloaded.

You can select three different definition types:

• Standard (definition of the equivalent stress under which the material plastifies)
• Bilinear (definition of the equivalent stress and strain hardening modulus)
• Stress-Strain Diagram:
• Definition of polygonal stress-strain diagram
• Option to save / import the diagram
• Interface with MS Excel

Background information about nonlinear material models can be found in the technical article describing the yield laws in isotropic nonlinear elastic material model.

Orthotropic | Plastic (Surfaces, Solids) | Tsai-Wu

Do you already know the Tsai-Wu material model? It combines plastic and orthotropic properties, which allows for special modeling of materials with anisotropic characteristics, such as fiber-reinforced plastics or timber.

If the material is plastified, the stresses remain constant. The redistribution is carried out according to the stiffnesses available in the individual directions. The elastic area corresponds to the Orthotropic | Linear Elastic (Solids) material model. For the plastic area, the yielding according to Tsai-Wu applies:

All strengths are defined positively. You can imagine the stress criterion as an elliptical surface within a six-dimensional space of stresses. If one of the three stress components is applied as a constant value, the surface can be projected onto a three-dimensional stress space.

If the value for f_y(σ), according to the Tsai-Wu equation, plane stress condition, is smaller than 1, the stresses are in the elastic zone. The plastic zone is reached as soon as f_y(σ)  1. Values higher than 1 are not allowed. The model behavior is ideal-plastic, it means, there is no stiffening.

Isotropic | Damage (Surfaces/Solids)

Did you know? In contrast to other material models, the stress-strain diagram for this material model is not antimetric to the origin. You can use this material model to simulate the behavior of steel fiber-reinforced concrete, for example. Find detailed information about modeling steel fibre-reinforced concrete in the technical article about Determining the material properties of steel-fiber-reinforced concrete.

In this material model, the isotropic stiffness is reduced with a scalar damage parameter. This damage parameter is determined from the stress curve defined in the Diagram. The direction of the principal stresses is not taken into account. Rather, the damage occurs in the direction of the equivalent strain, which also covers the third direction perpendicular to the plane. The tension and compression area of the stress tensor is treated separately. In this case, different damage parameters apply.

The "Reference element size" controls how the strain in the crack area is scaled to the length of the element. With the default value zero, no scaling is performed. Thus, the material behavior of the steel fiber concrete is modeled realistically.

Find more information about the theoretical background of the "Isotropic Damage" material model in the technical article describing the Nonlinear Material Model Damage.