What is Modal Analysis?

Effective modal analysis relies on a rigorous modal testing process involving detailed planning, thorough preparation, and precise execution. This includes selecting measurement locations, properly installing sensors and exciters, and fully calibrating the instrumentation.

Once prepared, testing begins with two key measurement types:

• Frequency Response Function (FRF): FRF is the process where an engineer records a building or structure’s reaction to an excitation force.
• Operational Deflection Shape (ODS): ODS records the actual movement that occurs in the real world.

After obtaining measurement data from the modal test, the next step is to validate its accuracy and reliability. This is primarily based on coherence, where the relationship between the FRF and ODS is examined and modal analysis performed. Here, natural frequencies, damping ratios, and mode shapes are identified.

Coherence, measuring the correlation between two signals, is key to data quality. It is the criteria that is used to see how accurate the modal analysis is. If there are high coherence values, then the modal testing is accurate; if coherence is low, the modal analysis is less accurate.

After completing and validating modal analysis, the next step is identifying the structural parameters governing a structure's dynamic behavior, including material properties, geometry, and boundary conditions. Accurate identification is essential to understanding how a structure responds to external forces.

Curve fitting and inverse modeling enable parameter values to be created by integrating theoretical models and modal analysis. Typically, this will take a few attempts to get the correct correlation between experimental and theoretical data.

The validation of modal models relies on two key methods: AutoMAC (Automated Mode Assurance Criterion) and CrossMAC (Cross-validation). AutoMAC checks consistency between measured data and simulated data from a finite element model, while CrossMAC compares measured data from various locations on the structure.

Both use statistical indicators to assess the accuracy of identified modes and determine if additional modes are needed.

Experimental Modal Analysis (EMA) measures vibrations using sensors like accelerometers while forces like vibrations are being applied to the structure. Modal hammers, shakers, or ambient vibrations create the external forces.

• Impact Testing: A modal hammer delivers precise impulses, and accelerometers record vibrations to determine modal parameters like natural frequencies and damping ratios.
• Shaker Excitation: Shakers apply controlled forces across various frequencies, which is ideal for analyzing large structures not suited for impact testing.
• Ambient Vibration Monitoring: This non-intrusive method uses environmental noise or existing vibrations to assess dynamic properties, offering simplicity, cost-effectiveness, and multi-location data capture.

Theoretical Modal Analysis (TMA) uses mathematical models, like Finite Element Analysis (FEA), to predict the dynamic behavior of structures without physical testing. FEA is a Computer Aided Design (CAD) program where the structure can be analyzed and tested in multiple different ways due to the infinite loading and boundary conditions available in the program.

As mentioned, FEA can be used to simulate situations that are likely to happen in the world and will carry out calculations on structural performance. Fortunately, this method can be combined with experimental modal analysis to make sure that any result has a strong foothold in real data. This ensures any designs or conclusions that are derived from FEA have a much higher chance of being accurate.

TMA offers several benefits over experimental methods, including:

• Non-destructive evaluation: Analyzes structures without causing damage.
• Comprehensive simulations: Models diverse conditions with precision.
• Cost efficiency: Cuts expenses from reduced physical testing.
• Time efficiency: Speeds up data generation and decision-making.

In aerospace, the vast majority of parts and components directly contribute to the safety of the passengers that will eventually travel on this mode of transport. Modal analysis ensures the safety of the wings, fuselage, and engine mounts. Airplanes specifically can come under turbulence, and knowing how all of the parts will react is vital to the safety of future passengers.

The construction sector also relies on modal analysis. Huge pumps and compressors that typically live in the basements of buildings are regularly tested to ensure proper function. Structures like bridges are also monitored on a regular basis to ensure that they remain structurally viable for purpose.

Ready to optimize and enhance your modal analysis operations? Contact us today to speak to an expert on how we can help you achieve accurate and efficient results with our multi-channel data loggers.