Issue: Insufficient number of non-zero masses

Issue

Modal analysis fails with error message "Number of the non-zero numbers in the mass vector should be at least twice as big as it is" or "Too many eigenvalues wanted".

Meaning

The number of degrees of freedom with non-zero mass in the model is insufficient to compute the requested number of modes.

In any structure, the maximum number of eigenmodes is equal to the number of degrees of freedom with a non-zero mass value.

Moreover, depending on the selected resolution method, it is possible to compute only part of the eigenvalues.

• Lanczos and Subspace iteration methods require that the model has at least twice as many degrees of freedom (with mass) as the number of requested modes
• Polynomial and Iterative (ICGC) methods do not have such a restriction and can compute all eigenmodes

 

Solution

Try another resolution method

Lanczos and Subspace iteration methods may encounter numerical instability issues on particular structures.

Polynomial and Iterative (ICGC) methods are slower, but also more robust and might succeed where the other methods fail.

Reduce the number of requested modes in solver settings

The number of requested modes might simply be irrelevant for the problem at hand.

Consider carefully how many modes are really necessary to analysis the structure.

E.g. in a structure with 3 degrees of freedom with masses, it is simply not possible to calculate more than 3 modes.

Increase the number of degrees of freedom

This can be done simply by refining the finite element mesh.

However, this will help only if the refined members carry self-mass or distributed masses.

On a member without self-mass (material density set to zero or property modifier with mass coefficient set to zero) and without any distributed mass, adding intermediate mesh nodes will not affect the degrees of freedom that are relevant for eigenmodes.