The nonlinear material behavior of concrete, reinforced concrete, and fiber-reinforced concrete can be depicted using the material models "Isotropic | Damage" and "Anisotropic | Damage".
Unlike other material models, the stress-strain diagram for the "Isotropic | Damage" and "Anisotropic | Damage" models is not antisymmetric to the origin. Thus, these models can represent the different behavior of concrete or reinforced concrete under compression and tension.
The models can also depict the continuous degradation of the material stiffness due to cracking. For this purpose, a smeared crack model is applied. The difference between the two models lies in the type of stiffness reduction.
- In the "Isotropic | Damage" material model, this is achieved via a scalar damage parameter.
- In the "Anisotropic | Damage" material model, the stiffness reduction occurs element-wise using a damage tensor.
Isotropic | Damage
The isotropic damage of concrete is characterized by a direction-independent degradation of material stiffness. Here, the stiffness is reduced uniformly in all spatial directions by a scalar damage parameter, as is typical for simple continuum damage models, also known as Mazars' damage model.
The direction of the principal stresses is not considered; instead, the damage occurs in the direction of the equivalent strain, which also captures the third direction perpendicular to the plane. The tension and compression regions of the stress tensor are treated separately. Different damage parameters apply in each case.
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 realistically represents the material behavior of fiber-reinforced concrete.
Anisotropic | Damage
The anisotropic damage of concrete is characterized by a direction-dependent reduction of material stiffness and is described using a damage tensor. This allows different stiffnesses in tension, compression, and shear directions to be depicted.
Physically, the cracks act as discontinuous weak zones with preferred orientation, significantly differentiating the normal and shear stiffness perpendicular and parallel to the crack plane, respectively. Consequently, the material locally exhibits orthotropic or transversely isotropic behavior.
Theoretical background on the 'Isotropic Damage' material model can be found in the technical article Nonlinear Material Model Damage. Detailed guidance on modeling fiber-reinforced concrete can be found in the technical article Material Properties of Fiber-Reinforced Concrete.
