Only linear behavior is valid in a modal analysis.

If you specify nonlinear elements, Mechanical APDL treats them as linear. For example, if you include contact elements, their stiffnesses are calculated based on their initial status and never change. For a prestressed modal analysis, the program assumes that the initial status of the contact elements is the status at the completion of the static prestress analysis. For more information, see Contact Behavior in Linear Analyses.

Material properties can be linear, isotropic or orthotropic, and constant or temperature-dependent.

Define both Young's modulus (EX) (or stiffness in some form) and density (DENS) (or mass in some form). Mechanical APDL ignores nonlinear properties.

If applying element damping, define the required real constants for the specific element type (COMBIN14, COMBIN37, and so on). For more details about damping definition, see Damping.