All things vibrate. Think of musical instruments, riding in a car with tires being out of balance, rattles in an airplane when the pilot is revving up the engines or the vibration under your feet when a train goes by.
Usually, vibration is bad and frequently unavoidable. It can cause gradual weakening of structures and the deterioration of metals (fatigue) in cars and airplanes.
Vibration is about frequencies. By its very nature, vibration involves repetitive motion. Each occurrence of a complete motion sequence is called a cycle. Frequency is defined as so many cycles in a given time period. One cycle per second is equivalent to one Hertz.
Individual parts have natural frequencies. For example, a violin string at a certain tension vibrates only at a set number of frequencies, It is why you can produce specific musical tones. There is a base frequency in which the entire string is going back and forth in a simple bow shape. Harmonics and overtones occur because individual sections of the string can vibrate independently within the larger vibration. These various shapes are called modes. The base frequency is said to vibrate in the first mode, and so on. up the ladder. Each mode shape has an associated frequency. Higher mode shapes have higher frequencies.
The most disastrous consequences occur when a power-driven device, such as a motor for example, produces a frequency at which an attached structure naturally vibrates. This event is called resonance. When vibration causes resonance in an object, destruction results unless it is designed to withstand the stress. A wine glass, for example, is not sound enough to withstand the resonance caused by the frequencies produced by an opera singer.
Engineers must design so that resonance does not occur during regular operation of machines. It is a major purpose of natural frequency (modal) analysis. Ideally, the first mode has a frequency higher than any potential driving frequency.
One result of the Natural Frequency (Modal) analysis is a series of restart files. These files are used by the linear dynamic analysis types that use the method of modal superposition: Dynamic Design Analysis Method (DDAM), Frequency Response, Random Vibration, Response Spectrum, and Transient Stress (Modal Superposition). Perform the natural frequency analysis first. If the dynamic loads have a significant effect on the natural frequencies, then use the Natural Frequency (Modal) with Load Stiffening analysis.
Although the results from a Natural Frequency (Modal) analysis include displacements, use these displacements only to visualize the mode shape. The magnitude of the displacements is relative to each other. The natural frequency is a theoretical result due to unspecified dynamic loads, so the results cannot contain absolute displacements.
After performing the modal analysis, perform a vibration analysis. Loads are applied to the model, and the displacement results have a physical value.
Modal analysis requires more degrees of freedom in the model than the number of frequencies (modes) being calculated. Simplified test models may not analyze if the mesh is coarse.