Experimental modal analysis
Experimental modal analysis is a vibration testing technique for determining the natural frequencies, mode shapes and damping ratios of a structure. During modal testing, an instrumented hammer, electromagnetic shaker or piezoelectric actuator is used to excite vibration of the test article. The vibration amplitude and phase (response) to the excitation is measured at multiple locations and frequency response functions are calculated. Curve fitting techniques are used to extract the natural frequencies, mode shapes and damping ratios of a structure. These modal parameters may be used for finite element model validation (correlation) and refinement, failure investigation, vibration troubleshooting and diagnostics, and quality assurance. Some applications for modal testing include aircraft components, turbine blades, impellers, automotive structures, buildings, optics and piping. SimuTech performs modal test services both in its laboratory and on-site at customer facilities. Experimental modal analysis provides structural dynamics information for finite element model (FEM) correlation and refinement, failure investigation, vibration troubleshooting and diagnostics and quality assurance. With over 30 years of service, SimuTech has tested steam turbines, gas turbines, pumps, blowers, compressors, foundations, buildings, optics, piping, and many more structures for wide variety of industries. Industries served include aerospace, automotive, military, electric power generation, research and development, tool and die, nuclear, petrochemical, air conditioning and refrigeration. SimuTech’s repertoire of excitation techniques include hammer impact (rap, bump, or bonk), electro-dynamic shaker (exciter), operating modal analysis, piezoelectric actuators and multiple input multiple output (MIMO). SimuTech’s modal testing service offers state-of-the-art techniques at competitive prices with a strong commitment to customer service.
Experimental modal analysis is one of the principal techniques for validating structural FE models. Most analysts will agree that applying boundary conditions and determining system properties which pose the greatest modeling challenge. Correlation of experimental and analytical natural frequencies can verify that modeling data such as geometry and material properties are accurate and that assumptions about boundary conditions are valid. Modal Assurance Criterion (MAC) is used to quantify the correlation between experimental and finite element modal analysis results. Modal testing can also determine the modal damping ratios that are crucial for dynamic stress and fatigue life calculations.
Modal testing applications
Recent modal test applications include flight qualification of helicopter modifications, mixed flow impellers, gas turbine blades and an optical table.
The modal assurance criterion (MAC) is a technique for quantifying the comparison of mode shapes from two sources. A MAC value greater than 90% generally indicates highly correlated modes. SimuTech engineers often use MAC to compare the mode shapes from two test articles or to correlate the results from a modal test with those from a corresponding finite element analysis.
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