Modal analysis deals with how structure (such as plate, piping) vibrates in terms of its natural frequency. The analysis is done by applying a force (not in static sense). Since frequency and vibration mode are dynamic in nature, the force is applied by changing its rate of oscillation (sinusoidal). I n so doing the structure under study will respond to the excitation. Ur inetrest will focus on the behaviour of the amplitude in time (its response). What u will notice in such a condition is that the amplitude changes as the rate of scillation of the applied force is varied. And as the oscialltion gets closer to the natural frequency (resonant) the response magnifies (remember the Tacoma bridge?).
Modal analysis is best doneif both the operational and conditional data are available. Operational done are measure in the field using accelerometer (some simply refer to it as vibration probe. Weve use Bentley Nevada since our machineries are configured to the 3000 series. Fo cs pipe it has a magnet that u simply attach to pipe. For ss pipe, u may need to install a clamp to secure the probe) by getting both the amplitude (peaktopeak) and its corresponding frequency. The study will focus both on the natural and the resonant frequency. To measure the structure (in this piping) natural freq. u need to apply an excitation force. Normally its done by using an" impact" (not the cvn) test.
Given the field data (after reducing it to manageable level), u can just download it and input the number to the CII or Algor. Both software actually come short in performing an in-depth modal analysis using field data.
Peter Avitabile from University of Massachuset has delivered several seminar on this very subject in layman terms. He can be reached at uml.edu. he's also on the web. His lecture notes are very educational for those lacking experience in vibration study. i took the liberty of using some of peter matl.
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