Modal Testing and Analysis
by Ardalan Sabamehr, Ph.D.
What is Modal Analysis?
As structural analysis techniques continually derive and become increasingly complicated, the limits in the structural behavior become increasingly apparent. These limits are used and are prominently prevalent in the field of structural dynamics, namely in the case of vibration-based analyses of structures. Additionally, standard dynamic analyses of large-scale structure can be too sophisticated to be done by manual calculations and need to be approached differently.
In the last few decades, modal analysis has become a major method for evaluating the condition of structures. Furthermore, it is a reliable technique for optimizing and improving the dynamic characteristics of structures. Modal analysis (also called vibration-based system identification) is a technique used to extract the inherent dynamic behavior of a structure in the form of its modal properties (natural frequency, mode shapes and damping ratio).
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What is Modal Testing?
The dynamic responses are good parameters to assess the structure in order to increase the information known about the behavior and performance of structures. Vibration-based testing (also called modal testing) is an accurate method for extracting a structure's characteristics. This type of test is accomplished using a set of sensors to monitor the response of a structure. There are three different major types of modal tests depending on the type of excitation manner: Forced Vibration Test (FVT), Ambient Vibration Test (AVT) and Free Vibration Test.
Forced Vibration Tests (FVT), or Experimental Modal Analyses (EMA), are a traditional technique in the field of modal testing in which the structure is excited by a known, external input force. The response of the structure and known input are then used to extract the modal properties of the structure.
For Free Vibration Tests, the structure is induced by a brief impact for its initial displacement. No external forces have continuously applied the structure during free vibration, differentiating it from the FVTs.
Ambient Vibration Tests (AVT), or Operational Modal Analysis (OMA), are an innovative technique which relies solely on operational load as an input. This means that the excitation is not controlled and is typically considered to be a stationary Gaussian white noise random process. These are post-processed and analyzed to determine the modal properties of the structure in question. Significant research and experiments that have been done in the past 20 years show that AVTs are the most reliable technique for deriving modal properties.
Ambient Vibration Tests tend to be much easier to perform in the client’s point of view and can be done without the prevention of standard operations. Furthermore, since the structure is not excited, it does not have the risk of potential structural damage.
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