After an eigenfrequency step has been used to find the eigenvalues of a model, ABAQUS/Standard automatically calculates the participation factor, the effective mass, and the composite modal damping for each mode so that these variables are available for use in subsequent linear dynamic analysis.
The “generalized mass” associated with mode is
ABAQUS/Standard allows the user to choose between two types of eigenvector normalization: the eigenvectors can be scaled so that the largest entry in each vector is unity, or they can be normalized so that the generalized mass for each vector is unity (). The choice of eigenvector normalization type does not influence the results of subsequent modal dynamic steps. The normalization type determines only the manner in which the eigenvectors are represented.
The participation factor for mode in direction i, , is a variable that indicates how strongly motion in the global x-, y- or z-direction or rotation about one of these axes (indicated by i, , 2, , 6) is represented in the eigenvector of that mode. It is defined as
For coupled acoustic-structural eigenfrequency analyses, an additional acoustic participation factor is computed for each mode as outlined in Coupled acoustic-structural medium analysis, Section 2.9.1.
The effective mass for mode associated with kinematic direction i (, 2, , 6) is defined as
For coupled acoustic-structural eigenfrequency analyses, an additional acoustic effective mass is computed for each mode as outlined in Coupled acoustic-structural medium analysis, Section 2.9.1.
ABAQUS/Standard provides an option to define a composite damping factor for each material. These are assembled into fractions of critical damping values for each mode, , according to