Did you already know? Unlike other material models, the stress-strain diagram for this material model is not anti-metric with respect to the origin. You can use this material model, for example, to simulate the behavior of steel fiber-reinforced concrete. For detailed notes on modeling steel fiber-reinforced concrete, see the 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 by a scalar damage parameter. This damage parameter is determined by the stress distribution defined in the diagram. The direction of the principal stresses is not taken into account; rather, damage occurs in the direction of the reference strain, which also encompasses the third direction perpendicular to the plane. The tension and compression areas of the stress tensor are treated separately, with different damage parameters applying to each.
The “reference element size” controls how the strain in the crack region is scaled to the length of the element. With the default value of zero, no scaling occurs. This ensures that the material behavior of steel fiber-reinforced concrete is close to reality in terms of modeling.
You can read the theoretical background on the “Isotropic Damage” material model in the technical article: KB 001461 │ Nonlinear Material Model Damage .
