Nonlinear Material Behavior for RFEM 6

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

Are you familiar with 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 area is reached as soon as fy (σ) = 1; values greater than 1 are not allowed. The model behavior is ideal-plastic, which means there is no stiffening.

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. Further information about modeling steel fiber-reinforced concrete can be found in this technical article: KB | Determining Material Properties of Steel Fiber-Reinforced Concrete and Their Application in RFEM

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. This does not take the direction of the principal stresses 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 areas of the stress tensor are 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-reinforced concrete is modeled realistically.

You can find the background information on the "Isotropic Damage" material model in this technical article: KB 001461 │ Nonlinear Material Model Damage .