Modes of vibration are associated with structural resonances.
The majority of structures will resonate. That is,under the proper conditions, a structure will vibrate with excessive, sustained motion.
Striking a bell with a hammer causes it to resonate. Striking a sandbag however, will not cause it to resonate.
Resonant vibration is caused by an interaction of the inertial and elastic properties of the materials within a structure.
Furthermore, resonant vibration is the cause of, or a contributing factor to, many of the vibration related problems that
occur in operating machinery.
These problems include failure to maintain tolerances, noisy operation, uncontrollability, material failure, premature fatigue, and shortened life.
To better understand a structural vibration problem, we need to characterize the resonances of the structure. A common
and useful way of doing this is to define its modes of vibration.
Each mode is defined by its modal frequency, modaldamping, and mode shape.
Mode Shape and ODS Contrasted
A mode of vibration is an inherent property of a structure.
A mode doesn’t depend on the forces or loads acting on the structure.
The modes will change if the structure’s material (mass, stiffness & damping) properties change, or its boundary conditions (mountings) change.
Mode shapes don't have unique values, and hence don't have units associated with them. However, mode shapes are unique.
That is, the resonant vibration level of one DOF relative to another is unique.
An ODS is quite different from a mode shape.
An ODS depends on the forces or loads applied to a structure.
It will change if the load changes. An ODS can have units, typicallydisplacement, velocity, or acceleration, or displacement per unit of excitation force.
It can be used to answer the question, "How much is the structure really moving, at a
particular time or frequency?"
Finally, while mode shapesare only defined for linear, stationary structural motion, ODS’s can be defined for nonlinear and non-stationary motion.
*Modes are only used to characterize resonant vibration.
*ODS’s can be defined for any type of forced or resonant vibration.