How the vibration testing is used to detect damage in aerospace components using modal parameters?
What will be the accuracy, if only natural frequencies are used in damage detection?
Are natural frequencies upto 10th mode enough to detect damage and estimate severity?
Actually, the number of modes that are identified is not the primary parameter. I would think more in terms of firstly, having an a priori knowledge of where the defectts can predomintently appear. And then, and only as a second step, select (by proper excitation) only a handful of modes for which the generalized stiffnes (or the modal strain energy, if you will) is significantly modified (reduced) by a localized damage. Natural frequencies can noramlly be measured with reasonably good accuracy (1% or better is largely feasible) , meaning any defect that modifies the strain energy by 2% or more can in principle be deteted. That's for the principles. In reality, things are certainly not easy and will require lots a trial and error and above all, engineeering judgment :-).
Hi
I am no expert neither on aerospace, nor on Structural Health Monitoring (SHM).
However, I do know that modes and damping are weak damage indicators, ie damage needs to be severe before it truly shows.
Therefore, in SHM, one tend to use point to point transfer functions instead as these readily show whenever the relative motion changes.
The points are situated next to each other, so for a line of, say four points #1-#4, one would look at transfer functions for #1/#2, #3 and #4/#3. Points can be placed also across joints.
The approach is pretty straightforward. Just plot the transfer functions as a function Frequency and of time to detect changes,
I've used both simple comparison of autospectra when impacting on structures as well as FRFs. The latter is more precise. The former is HillaBilly but way faster when looking for problem areas as one then can move excitation around more easily.
Hope this helps.
Sincerely
Claes
Well, I see the question is proposed long time ago, however I would say a damaged component generally offers higher dissipation rate (damping). The reason can simply explain as a crack opens and closes regularly. Thus, you will see a shift of natural frequencies in a damaged one compared to the healthy structure. Of course, it is a fundamental symptom. For a detailed health monitoring to figure out the types of failure, more symptoms could be studied.
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