It's mentioned that group damping can be applied to universal link elements. For stiffness-proportional damping, does it proportion to the linear properties of stiffness or nonlinear properties of stiffness?
All elements, not just universal link elements, can have group damping applied.
If "Damping Matrix Update" → "No", the damping is proportional to the stiffness of the linear property.
If "Damping Matrix Update" → "Yes", it is divided into the following categories depending on the type.
1. Force Type Universal Link: Proportional to the stiffness of linear properties only. The stiffness of the nonlinear property is only reflected in the nonlinearity of the element.
The nonlinearity of the Force Type Universal Link element is treated as the difference between internal and external forces (unbalance force), and the stiffness of the Universal Link comprising the structure is not updated. Therefore, even if the instantaneous stiffness proportional type is selected, the stiffness is used as the initial stiffness when constructing the damping matrix.
2. Element Type Universal Link: The updated stiffness is used to construct the damping matrix. However, in this case, the initial stiffness is a linear property.