SimuTech Testing Services

Modal Testing & MAC

Modal Testing 2

Experimental modal analysis is a vibration testing technique for determining the natural frequencies, mode shapes, and damping ratios of a structure. During a modal test a hammer or electromagnetic shaker 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.

Experimental modal analysis provides structural dynamics information for finite element model (FEM) validation (correlation) and FEM refinement, failure investigation, vibration troubleshooting and diagnostics, and quality assurance. In over 30 years of service, SimuTech has tested steam turbines, gas turbines, pumps, blowers, compressors, foundations, buildings, optics, piping, and many more structures for industries including aerospace, automotive, military, electric power generation, research and development, tool and die, nuclear, petrochemical, air conditioning, and refrigeration. SimuTech’s repertoire of excitation techniques including impact (rap, bump, or bonk), electro-dynamic shaker (exciter), operating modal analysis, and multiple input multiple output (MIMO). SimuTech’s modal testing service offers state-of-the-art techniques at reasonable 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 such 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.

Recent modal test applications include flight qualification of helicopter modifications, diesel engine cooling fan failure investigation, mixed flow impellers, gas turbine blades, and medical x-ray equipment.

MAC Analysis



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.