Page 308 - Finite Element Modeling and Simulations with ANSYS Workbench

Structural Vibration and Dynamics 293

Modeling tips: Note that modal analysis can be run as constrained, unconstrained
or partially constrained. Also note that symmetric structure may have asym-
metric modes, and thus, it is not recommended to take advantage of symmetry
to simplify models for modal analysis. In the following, we are going to show
a result comparison between an unconstrained model and the fixed model.
For the unconstrained model, the first six mode shapes obtained from simula-
tion are rigid body modes that allow the structure to move freely. They are not
considered as structural modes. The comparison indicates that a free floating
guitar has a different set of natural frequencies and mode shapes from that of
a fixed guitar. In general, constraint conditions have an effect on the vibration
characteristics of a structure and should be considered carefully when setting
up a model.

The first four structural modes of the unconstrained guitar model.

The first four structural modes of the fixed guitar model.

8.8 Summary
In this chapter, we first reviewed the equations of motion for both single DOF and mul-
tiple DOF systems and discussed how to compute the mass and damping matrices in the
FE formulations. Then, we discussed the methods for solving normal modes, harmonic
responses, and transient responses for structural vibration and dynamic problems. The
advantages and disadvantages of the direct method and modal method are discussed.
Several examples of vibration and dynamic analyses are also discussed to show the appli-
cations of the FEA in vibration and dynamic analyses. The vibration modes of a guitar are
analyzed using ANSYS Workbench.