Examination of the application and limitations of structural mode extraction via force apportionment
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This paper will discuss the use of force apportionment to isolate modes being excited by the sine-dwell technique. The effectiveness of the apportionment technique can be determined by examining the structural response as measured by laser vibrometry.
First, the structure is investigated using impact-test-based modal extraction methods. Approximate mode shapes are determined by examining the phase resonance indicator function for the resonance responses at a number of reference points. By comparing condition numbers of submatrices of the approximate modal matrix, one can select the best positions for force application. The apportioned forces for a given mode are arrived at by requiring that the input energy excite only the mode of interest while the net amount of work on adjacent modes is zero.
This method is illustrated on a 24 in. x 1.5 in. x 0.375 in. steel beam. The fourth bending mode is to be separated from the first torsional mode which is 26 Hz below the bending mode. The apportioned forces are applied and laser scans are acquired of the "modal" response. The laser allows detailed investigation of the deviations of the response from the theoretical fourth mode response. The scans reveal that the force apportionment technique used in this test case fails to reliably extract the theoretical modal response of a beam.
A finite element model of the beam is created to verify that the apportionment technique works. Applying an apportioned force vector to the model shows that the method is capable of isolating the mode of interest. The interactton of the electrodynamic shaker, stinger, and force transducer with the structure is investtgated as a possible explanation for the failure of the technique in experimentatton. It is found that there exists axial and rotatory coupling which can influence the structural response of the test specimen and decrease the reliability of the apportionment technique.
- Masters Theses