چهارشنبه ۸ دی ۱۳۸۹ -
Modal testing is a form of vibration testing of an object where the natural (modal) frequencies, modal masses, modal damping ratios and mode shapes of the object under test are determined. A modal test not only consists of an acquisition phase, but also of an analysis phase as well. The complete process is often referred to as a Modal Analysis or Experimental Modal Analysis. There are several ways to do modal testing. The most widely used are Impact Hammer modal testing and Shaker modal testing. In both cases energy is supplied to the system with a known frequency content. Where structural resonances occur there will be an amplification of the response, clearly seen in the response spectra. Using the response spectra and force spectra, a transfer function can be obtained. The transfer function (or frequency response function (FRF)) is often curve fitted to estimate the modal parameters; however, there are many methods of modal parameter estimation and it is the study of much research. Correlate Theory with Modal Testing Reality: Model correlation and updating are performed to achieve good agreement between analysis results and modal test data. Both qualitative (mode shape appearance) and quantitative (cross-orthogonality, modal assurance criteria, etc…) techniques are used to assess correlation accuracy. Finite element parameter sensitivity methods are used to accelerate the correlation process by identifying optimum changes that minimize test-analysis errors.