Guidelines for Plasticity, Damage and Failure Models
The convergence for nonlinear materials can be bad if the simulation settings are not set up properly. Therefore, this section contains some best practices for these simulation types.
Boundary Conditions
Don’t use Force boundary conditions, if possible:
With Force boundary conditions, it is possible to choose the applied load too high for the materials in the structure: For plastic materials and for the damage models, the maximal stress is limited by the material model. Therefore, applying a load which is higher than this maximal stress leads to unsolvable equations.
If it is important to use force boundary conditions (as e.g., in a cyclic load case, where the stresses should decrease to zero again), we recommend
- Do a first simulation with Path Boundary Conditions, to check out the maximal allowed stresses for the structure, or
- Check the materials carefully to estimate the maximum allowed load.
Nevertheless, even if the maximum allowed load is not reached, the convergence might be slow. For example, when using the exponential hardening model, a load slightly lower than the maximal allowed load might lead to strains near to infinity (which is also not realistic).
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Damage models vs. failure models
The main difference between the damage models and the failure model is that the failure model decreases the stiffness suddenly, which introduces a discontinuity into the stress-strain relationship. The damage models lead to a gradual reduction of the stiffness. Thus, the damage models introduce no discontinuity, this means that the simulation is more stable and usually faster. Nevertheless, this gradual reduction of stiffness can be arbitrarily fast – this means that nearly the same results as with the failure model can be achieved. Additionally, the damage models can be combined with other models like e.g., plasticity.
The main advantage of the failure model is that it is easier to set up, since it contains less parameters. Additionally, the damage models are not available for the transverse isotropic case. Altogether, it is strongly recommended to use the damage models instead of the failure model, whenever possible.
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