Experimental Modal Analysis Joe Spadola
What is modal analysis? 2 Modal Analysis is the process of characterizing the dynamics of a structure in terms of its dynamic properties. The dynamic characteristics include: Modal Frequency Modal Damping Mode Shape These characteristics can be obtained from analyzing the Frequency Response Function
Experimental Modal Analysis Motivations 3 Troubleshoot Noise and Vibration Problems Compare the frequency content of excitation with the natural frequencies of the structure Perform Structural Health Monitoring Cracks typically modify natural frequencies Validate Design Improvements Validate the effects of a structural modification with a new modal test Set up and Validate FE Models Compare modal frequencies Compare modal shapes Determine modal damping a required input to FE based modal analysis
Obtaining the Inputs to a Frequency Response Function 4 1. Apply random excitation to part 2. Measure time history of excitation 3. Measure time history of response at one or more locations Measure Input Excitation Apply Random Excitation Measure Response
Frequency Response Analysis 5 Typical results in an FRF include: Gain: the ratio of output to input Phase: lead/lag between output and input Coherence: measure of linearity between output and input
Frequency Response Analysis Modal Frequencies 6 The gain shows us modal frequencies and modal damping Modal frequencies are peaks of the frequency response function
Frequency Response Analysis Modal Damping 7 The gain shows us modal frequencies and modal damping Modal damping is determined from the shape of each peak Normalised Displacement FRF (Gain) 100 1 3 f n 10 Q Q 0 0.5 1 f n Un damped response Increasing Damping Dynamic Amplification where is the damping ratio 0.1 Critical Damping Frequency Hz
Frequency Response Analysis Mode Shapes 8 When energy is input into the system near a Fn, the structure vibrates in a distinct shape called a mode shape Mode shapes can be determined through FEA or experimentally Each natural frequency (peak in the FRF) has its own unique mode shape Example 1: cantilever beam First Bending mode at 90Hz First mode at 345Hz Second Bending mode at 570Hz Third Bending mode at 1570Hz Second mode at 1033Hz Third mode at 1820Hz
Structural Dynamics 9 Frequency Response Function with Gain (H1, H2), phase, coherence, auto and cross spectra. Waterfalls of FRFs to check for linearity or structural changes Custom FFT Filter to apply a transfer function defined as Gain and phase spectra Convolution to filter from an impulse response function Auto/Cross Correlation Hilbert transform to calculate the decay rate of an impulse response function Horizontal and vertical cursors on spectra to identify resonances
Experimental Modal Analysis glyph 10 The EMA glyph expects FRFs in : Gain Phase Analysis properties include: Frequency range List of natural frequencies Whether to account for residual modes Results are: The Regenerated FRFs in Gain and Phase A multichannel modal table containing the damping ratios FRF regenerated w/o residuals FRF regenerated with residuals
Operating Deflection Shapes glyph 11 The ODS glyph expects : A geometry UNV, BDF/DAT, INP, FE results files Nodal Displacements Time series FRF spectra Modal Table (from EMA) The mapping between nodes and measurements can be done via: The ODS Mapping Editor A text file Some Metadata #DOFIdentification.Node# and #DOFIdentification.Direction#
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Joe Spadola www.hbmprenscia.com Applications Engineer T: 213 215 4807 E: joe.spadola@hbmprenscia.com