Avoid mass loading in vibration test with Laser Vibrometry
Dynamic simulation models are designed to represent the real vibration behavior of a later product or component. Since reality is infinite complex, we all have to make assumptions, simplifications and are usually lacking correct parameters for (joint) damping. That's why we perform an experimental modal test to validate our models and make them capable to make predictions based the validated model.
The modal test brings other challenges: besides all the questions of having consistent boundary conditions (What is an actual free-free support?) mass-loading is always an issue when not working with optical sensors. The equation of motion is changed when adding sensor masses.
How becomes mass-loading obvious in the results?
The eigenfrequencies will typically lower than predicted. This can of course have other reasons in the design of the model parameters.
The MAC (Modal Assurance Criterion) shows low diagonal values and modes are missing in the test results. Due to mass-loading deflection shapes get distorted and don't match the results of the simulation.
In model-updating using a sensitivity-based approach material density parameters are suggested to be altered at the sensor locations. Changing the parameters based on the mass-loaded results will end up in a wrong model.
The remedy - optical vibration measurement
A scanning Laser Doppler Vibrometer (SLDV) is the the tool that completely avoids this negative influence. It's a no-brainer that lightweight structures are prone to change eigenfrequencies when loaded. Thus, SLDV are widely used in the field of microstructures, biological or aerospace structures. But also, if you think of a sample designed from thin sheet metal with low damping the influence of mass-loading especially on locals modes relevant for the acoustic behavior get obvious.
You find some experiments and explanations on mass-loading in modal testing in the links below.
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Application fields of non-contact vibration measurements
Being non-contact and thus independent from "physical" (of course laser interferometry is pure physics) sensors a wide field of applications opens up for SLDVs. You find a good introduction again to modal testing, but also to durability assessments and NDT in a video tutorial derived for the 2022 SEM IMAC conference.