High Frequency Flexible Modes The beam response was measured using a laser vibrometer with shaker excitation methods as well as with accelerometers more suitable for higher frequency ranges with low mass loading effects using impact excitation techniques. For the data in this paper either data set could have been used for the identification of the modes. The frequency response functions were collected at 34 measurement locations over a span of 3000 Hz and modal data was extracted using convention approaches available in LMS Test.LAB [13]. The next four flexible modes of the structure were extracted and were typical of the expected mode shapes for these modes. RIGID BODY MEASUREMENT POINTS LOW FREQUENCY MEASUREMENT POINTS HIGH FREQUENCY MEASUREMENT POINTS ALL MEASUREMENT POINTS Figure 3 – Overlay of All Measurement Points on the Test Structure One important item to note is that none of the DOF from the three separate tests contained any coincident measurement points; each contained a completely unique set of points. The collection of data points for each of the tests is shown in Figure 3 for the three separate tests along with an overlay of all the measurement points superimposed on the test structure. Each of the separate test data sets were correlated to the finite element model and very good overall correlation was observed for each of the sets. Then each data set was used in an expansion process as described in the theory section. The mode shapes were expanded and again correlated to the finite element model with essentially similar correlation results. The Modal Assurance Criteria was used for the correlation of the different data sets. The correlation of each of the different vector data sets is shown in Figure 4 along with the MAC; note that due to the large amount of data points obtained from the expansion process only the original collected data from each of the separate tests is shown superimposed on the finite element model. All of the data sets showed very good correlation with the model and, after expansion, each of the separate test data sets now has a common set of points to describe the model. In addition, the process of expansion has blended together data from different sources – optical, accelerometer, laser and ultimately any other types of modal data that could possibly be collected. 169
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